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1. Basic R.C. Structures 1. Reinforced Concrete (R.C.) There are various types of construction materials, among which reinforced
concrete is the most common one used in Hong Kong. Reinforced concrete is
composed of concrete and reinforcement, where reinforcement is in the form of steel
bar. 1.1 Concrete Concrete is a mixture of cement, aggregate and water. Concrete sets to a
rock-like mass due to the chemical reaction (hydration) which takes place
between cement and water, resulting in a paste or matrix which binds the other
constituents together.
The quality of a concrete is denoted by its compressive strength. For example,
a concrete that has a compressive strength of 40 MPa is known as Grade 40
concrete (or G40).
Plain concrete is strong in compression but weak in tension. The actual ratio
varies but roughly the compressive strength is about ten times of the tensile
strength.
If a plain concrete beam is bent, the upper part of the beam will be set in
compression while the lower part will be in tension.
It can be expected that the beam will fail in tension at a relatively small loading.
If this weakness in tension is reinforced in such a manner that the tensile
resistance is raised to a similar value as its compressive strength, the reinforced
beam will be able to support a load ten times that of the plain concrete beam.
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1.2 Reinforcement Reinforcing bars (rebar) are made of high carbon steel. They are manufactured by
hot rolling and therefore also known as Hot rolled steel bar. Two types of rebar
are used for construction: 1.2.1 Plain round steel bar A plain round steel bars is made of mild steel. It is classified as Grade 250. It
means that the characteristic yield stress shall not be less then 250 MPa. Common
nominal sizes (bar diameter in mm) are: 10, 12 and 16. Plain round steel bars are
Loading a plain Concrete Beam
Load
The beam will crack
Compression in upper fibres
Tension in lower fibres break at low loading
simply supported plain concrete beam
Loading a Reinforced Concrete Beam
Load
The reinforcement
Compression in upper fibres
Tension in lower fibres helps to resist tension
Simply supported R.C. beam
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named with a prefix R. For example, R10 denotes a plain round steel bar of 10
mm diameter.
1.2.2 Deformed high tensile steel bars A deformed high tensile steel bar is rolled on the surface with ribs to increase the
bond strength with concrete. It is classified as grade 460. Common nominal sizes
are: 10, 12, 16, 20, 25, 32 and 40. They are named with a prefix T. (Formerly it
is name as high yield steel bar with a prefix Y.) 2. Reinforced concrete structures
A typical reinforced concrete building
Superstructure
Substructure
Beam
Wall
Column
Floor slab
Foundation
Plain round steel bar Deformed high tensile steel bar
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2.1 Major structural elements of a R.C. framed building Columns are vertical members which carry the loads from beams and transfer
the loads down to the foundations.
Beams are horizontal member supporting floor slabs. They are further
subdivided into main beam and secondary beam.
Main beams span between columns and transfer the loads placed upon them to
the columns.
Secondary beams span between main beams and transfer their loadings to the
main beams. Their primary function is to reduce the spans of the floor slabs.
Floor slabs provide platforms on which people can circulate and furniture can
be placed.
Walls wall can be classified into external walls and internal walls:
External walls are the envelope of the building to exclude rain, wind, sunlight,
etc.
Internal walls (partitions) are used to subdivide the floor space in a storey of
different uses.
Some walls also help to take loadings.
Typical R.C. Framed Structure
Main beam Span
Secondary beams
Column
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2.2 R.C. Beam When a beam is subjected to loading, the upper part pf the beam will be in
compression while the lower part is in tension.
Therefore, main reinforcements should be placed at the bottom side to resist
tension.
Top reinforcements are often used as carriers for links.
For heavily loaded beams, top bare also help resist compressive stress.
Even though compressive and tensile strengths of the beam are not exceeded,
cracks may still appear in the web of the beam near the supports. These cracks
are in fact shear failure lines at an angle of approximately 45 to the horizontal, and sloping downward toward to the supports.
Shearing stress may be resisted by bent up bars at 45 to the horizontal and positioned to cut the anticipated shear failure plane at right angles. These are
in fact the main bars from the bottom of the beam which are no longer required
to resist tension which can be bent up to the top and carried to the support.
Crack Pattern of Loading a R.C. Beam
P P
Flexural cracks
R R
Shear crack
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More often, links (also called stirrups or binders) are provided in close centres
about the position at which shear is likely to occur to replace the bent up bars.
Several stirrups may cut the shear plane and therefore the total area of steel
crossing the shear plane is sufficient to offer the tensile resistance to the shearing
force. Even where shear resistance is not required, nominal links are provided in
beams for hanging up the top bars. They also help to minimize shrinkage
cracks of the concrete.
For cantilever beams, main bars should be placed at the top as tension is
appeared at the top of the beam.
Typical R.C. Beam Reinforcement
Main bars 01 Top bars 02 Link 03
A
A
02 02
01 01 01
03
Section A-A
Links at close centres Links at nominal centres Links at close centres
Typical Reinforcement Details of Cantilever Beam
Fixed support
Main bars
Bottom bars Link
Typical R.C. Beam Reinforcement
Main bars 01 & 02
Top bars 03 Bent up bar 02
Link 04
B
B
A
A
02 03 03
01 01
03 03
02 01 01
04
Section B-B
Section A-A
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2.3 R.C. Column Since concrete is strong in compression, it may be concluded that no reinforcement
will be required in columns provided the compressive strength of the concrete is not
exceeded. However, rebars are often added in a column. The major reasons are to
increase its compressive capability of the column and to resist bending. Bending
may be induced in columns in the following conditions: Buckling of slender column when subject to axial load (a column is considered
to be slender if the ratio of effective height to thickness exceeds 12). Reaction to beams upon the column, as the beam deflects it tends to pull the
column towards itself thus inducing bending in the column. Wind loading acting on high-rise buildings, the columns on the windward side
may be subject to tension. The minimum number of the main bars in a column should not be less than four
for rectangular columns and six for circular columns.
Buckling of Slender Column
Wind Loading acting on High-rise Building
Reaction to beam upon the column
Wind
P P
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To prevent the slender main bars from buckling due to compression and hence causing spalling
of the concrete, links shall be provided at
suitable centres as restraints. 2.4 R.C. Slabs Slabs are spanning members and they behave very similar to beams.
Main bars should be placed at the bottom side of the slab to resist sagging.
Normally, top reinforcement is not required except for heavily loaded slabs.
However, a hogging moment will occur above supports which necessitating top
reinforcements.
For cantilever slab such as canopies, the main bars shall be place near the top of
the slab to resist top tensile stresses.
The slab thicknesses of most domestic buildings are about 100 mm to 150 mm.
Typical R.C. Column Reinforcement
A
main bar
A
link
Section B - B
Main bar link
B B
Section A - A
Canopy
Main bars (at top for cantilever structures)
Main bars of slab Beam
Top reinforcement (to resist hogging over support)
Distribution bars
Slab
Typical Slab Reinforcement
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There are three basic forms of floor slab systems: Beam and slab system Flat slab system Ribbed slab system 2.4.1 Beam and slab system In this system the slab are supported on beams.
Most beam and slab systems are designed to span in one way but some are in
two ways.
2.4.2 Flat slab system A flat slab is a slab supported on columns directly without beams.
In fact, a fact slab is divided into columns strips and middle strips while the
reinforcement are concentrated in the column strips.
Therefore, a column strip can be viewed as a beam with the same thickness as
the slab.
Structurally, flat and shallow beams are inefficient but have the advantage of
giving a clear ceiling height. Moreover, the most labour intensive element,
beam, is eliminated which makes the construction much easier.
Flat slabs are normally designed to span in two ways. Sometimes dropped
Beam and slab system
Beam
Slab
Slabs
Beams
Columns
Beam & slab spanning one-way Beam & slab spanning one-way
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panels are added at the supports to resist punching shear around the column
heads.
2.4.3 Ribbed slab system (Waffle slab) A ribbed slab is also known as waffle slab or honeycomb floor (or roof).
It is cast over lightweight moulds or pans made of glass fibre, or polypropylene.
Ribs in both longitudinal and transverse directions are formed in close centres
and tied with each others.
Ribbed slabs can resist great bending moment in both longitudinal and
transverse directions.
They are used for large span slabs and require less concrete and less
reinforcement than other slab systems.
Column strip Middle strip
Drop panel
Flat slab (Two ways slab) Flat slab with drop panels Columns
Slabs
Waffle slab
Slab
Ribs
Column
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2.5 R.C. Walls 2.5.1 External walls An external R.C. wall should contain a mat of reinforcement in each of two faces.
The horizontal reinforcement of each mat should be evenly spaced in the outer layer
to contain the vertical compression bars. Additional horizontal restraining links
should be provided. Minimum thickness of a R.C. wall as an external wall is 100 mm. 2.5.2 Shear walls Shear walls are thick R.C. walls to increase the lateral stability of a building.
Strong wind tries to cause a high-rise building frame to sway. Shear walls are
ideally suited for bracing tall buildings because of their very high in-plane
stiffness and strength. Therefore, shear wall is also known as wind wall. Shear walls also carry gravity loadings. They replace part of or even all
columns. Shear walls are arranged as external walls and partitions. Grouped
shear wall may also be used to form services cores, elevator shaft and stairwells.
Restraining link
Vertical bar Horizontal bar
Typical wall reinforcement (plan view)
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2.5.3 Non-load bearing walls Most partitions are non-load bearing.
They may be made of reinforced concrete, brick or block.
Non-load bearing R.C. walls contain only minimum steel.
Bricks are made of hard well-burnt clay while block are made of cement sand
mortar or concrete.
They are bond together by cement sand mortar of approx 1:3 (by volume) to
form a wall. Brick or block walls are seldom used as loading walls in HK.
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3. Basic construction process of R.C. structures Namely, the basic construction process of R.C, structures are:
Formworking
Steel fixing
Concreting 3.1 Formwork Concrete when first mixed is a fluid. Formwork is the temporary moulds for casting
concrete members where the fresh concrete can be placed to retain its shape, size and
position as it sets. 3.1.1 Formwork materials A lot of materials can be used as formwork materials, such as timber, steel,
aluminium, glass reinforced plastic, etc. (Only timber formwork will be discussed here.) Timber plank
Various sizes of timber planks are available but the most common one used in
Hong Kong is 2 x 4 (50 mm x 100 mm) planks. Plywood
Most common type of plywood used in Hong Kong is 4 x 8 x 3/4 (1.2 m x 2.4
m x 19 mm) 7 plies plywood.
Some plywood is resin coated (sealed plywood). It is more expensive but the
board life is extended (typically 5 to 10 reuses) and it gives good concrete finish. Advantages if using timber as formwork material
Timber can be easily cut and fabricated into different sizes and shapes.
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Using timber to form formwork is more flexible and is economical in small
projects.
Not much machinery is needed. (Basically one or two carpenters can complete
the work with simple hand tools, such as hammers, hand saw and nails.) Disadvantages if using timber as formwork material
Constructing timber formwork is a labour intensive work. It is uneconomical
in large scale projects.
Mechanization usually cannot be employed.
The reusability of timber formwork is low, only two to five times.
Considerable amount of timber waste produced which increase the cost to treat
the solid waste.
Considerable amount of hardwood consumed which destroys our rain forest. 3.1.2 Soffit form Bearers of suitable size shall be placed on top of the U-heads / prop cap plate.
Bearers shall be fixed by wedges / nails centrally to prevent eccentricity and
dislodgement.
Joists shall be placed at suitable centres on top and at right angle to the bearers.
Plywood can then be placed on top of the joists and fixed by nails.
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3.1.3 Column Form
Column forms are commonly formed by using four plywood panels backed with
vertical studs.
To resist lateral hydrostatic pressure caused by the wet concrete, clamps shall be
placed at suitable centres. 3.1.4 Beam Form This is basically a three sided box supported and propped in the correct position
and to the desired level.
The beam formwork also has to resist lateral hydrostatic pressure of the wet
concrete, sufficient braces shall be provided.
Clamps
Studs
Plywood
Plan Front view
Timber Column Formwork
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3.1.5 Wall formwork Wall forms are constructed from the following basic parts:
plywood panels to retain concrete until it hardens,
studs which supports the plywood panels,
wales to support the studs and align the forms
ties resist lateral hydrostatic pressure of wet concrete
Vertical waler wall formwork
Tie
Double wales
Studs
Plywood
Kicker
Beam formwork
Beam side plywood
Joist
Raking strut
Stud
Kicker
Bearer
Falsework
Slab soffit form
Front view Section A-A A
A
Beam soffit plywood
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3.2 Steel Fixing Steel bars are cut to correct lengths, bent to desired shapes and then fixed to positions with steel wires. 3.2.1 Reinforcement Schedules and detailing Reinforcement on detail drawings is annotated by a coding system to simplify
preparation and reading of the details, for example: Legend Typical R.C. Beam Bending Schedule Member Bar
mark Type and size
No. of mbrs
No. in each
Total No.
Length of each bar
Shape
Beam1 1 T20 3 2 6 2600
2 T16 3 1 3 1400 straight 3 T10 3 2 6 2300 Straight 4 R10 3 16 48 1000
5R10-04-100
total number of bars in the group centre to centre spacing
plain round steel bar type diameter in mm bar mark number
Typical R.C. Beam Reinforcement Details
6R 1004-2002T 1003
5R 1004-100 5R 1004-100
2T 2001 & 1T 160202 02
03 03
0101 02
03 03
01 01
A
A B
B
Section A-A Section B-B
250
150
250
2300
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3.2.2 Hooks and Bends To prevent bond failure hooks or bends can be used at the ends of bars. 3.2.3 Bending Radii (r) Too small a bending radius will weaken the steel
Too large a bending radius may cause problems such as:
- lack of anchorage,
- create difficulties in keeping other steel bars in the correct place. Standard bend radii (r)
Grade 250 bars - 2 for all bars Grade 460 bars - 3 for bars up to and including 20 mm diameter - 4 for bars over 20 mm diameter
Bend sand Hooks for R-bars Bends and Hooks for T-bars (Source: R. Chudley)
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3.2.4 Lengthening bar are commonly lengthened by lapping
lap lengths range from 20 - 120 , which depends on - bar type - concrete strength - whether the bar in tension or compression state
3.2.5 Concrete Cover Concrete cover is the thickness of concrete measured from the concrete surface
to the outer face of the reinforcing bar.
The concrete cover provides corrosion protection and fire protection to the steel.
Normally, concrete cover is between 20 to 40 mm.
In very severe condition concrete cover may be increased to 100 mm. 3.3 Concreting Instead of batching and mixing on site, most concrete are produced in central
plants and delivered to site by mixer trucks. This type of concrete is called
ready mix concrete.
The concrete can then be further distribute to its destination by the following
means: - Chutes - Hoist and wheel barrows - Crane and skip - Pump and pipelines
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3.3.1 Chutes The truck mixer chute is the initial means of delivering concrete on site, either to
another method of distribution.
Sometimes the concrete may be distributed directly to its final position,
provided:
- truck access to within chute radius is available;
- the structure is below truck tray level.
The concrete shall not be allowed to fall freely exceeding two metres because it
may cause segregation of the concrete. In this case a long chute shall be used.
3.3.2 Hoists In case the concrete has to be lift to the upper floor level, hoist may be use.
However, further distribution of from the hoist to the placing points relies upon
wheel barrows which is very slow and labour intensive. 3.3.3 Crane and bucket If tower crane is available, the use of crane and bucket is and efficient mean of
concreting.
Hoist and wheel barrows Crane and bucket
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Buckets of 0.5 to 2 m3 capacity are most commonly used on building sites. 3.3.4 Pump and pipelines Concrete pumps and pipelines are perhaps the most widely used method of
moving concrete on building sites.
The ready availability of mobile pumps, and their relative reliability, makes
them an efficient and economical means of transporting concrete, even on quite
small sites.
Moreover, concrete for high-rise building is normally very suitable for pumping
because most high-strength concrete has high cement content and small
maximum size aggregate.
A wide range of pump types are available, generally trailer or truck mounted.
Fixed pumps generally have the highest pumping capacity and are the usual
choice for major projects. 3.3.4.1 Pumping Operation
Pipelines must be adequately supported and fixed in position since quite
substantial forces can be generated as the concrete is forced along the lines.
Before actual pumping of concrete, the pump and pipelines must be lubricated
by pumping through the pipes with a cement slurry or mortar.
Once commenced, concrete pumping must be continuous to avoid blockages in
Pump and pipelines
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the pipeline.
After pumping is completed, the pipelines must be cleaned out as soon as
possible to prevent mortar residue which will increase pipe friction and cause
blockage. 3.3.5 Compaction Concrete must be fully compacted
when placing to achieve its potential
strength.
Compaction is done on site with probe
vibrators.
In condition when compaction is
difficult to carry out, self-compacted concrete (high workability concrete of
slump >150 mm) may be used.
Reference
Construction Technology Vol. 2 (1991), R. Chudley, Longman Civil Engineering Construction IV, S.A.R. Jurfi & R.J. Wellmen, Hong Kong Polytechnic. Advanced Construction Technology 3rd edt (2000), R. Chudley, Person Education Ltd. Tall Building Structural Analysis and Design (1991), G.S. Smith & A. Coull, John Wiely &
Sons, Inc. Formwork, a practice guide (1997); P.S. McAdam and G.W. Lee; E & FN Spon. Guide to concrete construction (1994), Cement and Concrete Association of Australia and
Standard Australia
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2 - Earthwork 1 Site Formation
Site formation works include:
site clearance reshaping of land profile - cut and fill road works road drainage works and laying of services & utilities 1.1 Site clearance
This may involve:
felling trees, grubbing out of bushes and stripping, demolition of existing building 1.1.1 Felling trees:
Chain saws can be used for cutting down trees. The roots of trees and shrubs which have been cut down shall be grubbed up. Tree roots can be removed by backactors or rippers. For deep tree roots, blasting may be necessary. 1.1.2 Stripping:
About 300 mm of the top soil will contain plant life and decaying vegetation and should be stripped off.
Stripping can be best performed by bulldozers. Topsoil is unsuitable for backfilling but will be valuable for finishing off
embankments and general areas to be grassed.
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2 Reshaping of land profile - cut and fill Very few sites are level and therefore re-shaping works have to be carried out before
any construction work can be taken place. A site may be leveled by: cutting, filling, or both.
Cut: - It has the advantage of giving undisturbed soil over the whole of the site but would increase the cost of disposing the spoil
Fill:- Filling materials must be available and compaction must be properly performed to prevent settlement.
Cut and fill: - if properly carried out, the amount of cut will be equal to the amount of fill.
Leveling slopping sites
original ground level
cut
battered face
formation level
fill
original ground level
formation level
original ground level
fill
cutformation level
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3 Bulk Excavation There are various kinds of excavation plants. Suitable choices of plants increase the
efficiency and reduce the cost. 3.1 Backactor / Backhoe Designed for digging below track level Also digs above track level but in reduced efficiency. Suitable for trench and bulk excavating. For trench excavation, using a bucket width equal to the trench width can be
very accurate with a high output rating. 3.2 Loader Shovel Designed for loading loose materials such as aggregate and loosened soil. Other tasks: spreading soil and rough grading Require a level working platform when operating. 3.3 Bulldozer Mainly designed for excavating, spreading or pushing soil from one position to
another.
Excavation is carried out by lowering the mould board or blade into the soil and pushing the soil in front of the machine.
Other tasks: clearing vegetation
stripping topsoil
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excavating and opening up pilot roads
maintaining haul roads
as tractor for towing other plants 3.4 Face Shovel (Front Shovel) Designed for digging above track level. Extensively used in quarries and pits and on construction sites and is useful in
excavation blasted rock in cuttings, etc. 3.5 Multipurpose Excavator Both backhoe and loader shovel attached in one machine. Multi-function. Useful in confined site. The two components cannot be operated at the same time. 3.6 Road Lorries For hauling on public roads. Sizes up to about 38 tonnes gross vehicle weight. Loaded by other plant but unloaded by side or rear tipping. 3.7 Unlicensed Lorries They are often old and with no license. Caution! Running on or traversing public roads are illegal.
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3.8 Dump trucks and dumpers. Vary in size from 1 to about 80 tonnes capacity. Large capacity machines are generally used in large mines or quarries. The speed of tipping is increased over a road lorry by the absence of a tailgate. Small dumper units are available for work on small sites and commonly have
the load carried in front of the driver. 4 Rock Excavation The techniques of breaking and excavating rock or other hard material depend on the
type of material, the quantity involved and the conditions on site. Such techniques
include:
4.1 Pneumatic breakers Pneumatic breakers may be used to break the rock into small fragments. The power
supply to this type of breaker is from an air compressor. Small pneumatic breakers
are hand-held while giant breakers are hung on excavators.
4.2 Drill and blast Holes of 25 150 mm can be formed in rocks by rock drilling methods. The rocks can then be further broken down by explosives. There are basically two methods of producing holes in rock. These are:
Rotary-percussive drilling Rotary drilling
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4.2.1 Rotary-percussive drilling For medium to hard rock, the rotary-percussive drilling method is often favoured
because the rig is light and provides good rates of penetration up to 50 m deep and
150 mm diameter. The method is used for blast holes, rock anchors, grouting holes,
etc. In rotary-percussive drilling the drill bit is supplied with both a percussive and a
relatively slow rotary action. The broken rock fragments are flushed out with either
compressed air or water. Various types of bits are available for different conditions:
4.2.2 Rotary drilling For larger diameters boreholes, or when boring to greater depths and accuracy are
required, or where soil or soft rock are encountered, rotary drilling methods are
preferred. Rotary drilling relies on a high feed thrust applied down the drill stem, to force the
edges of the bit into the rock surface. High torque and rotation of the drill shaft then
cause cracking and chipping, and rock fragments are broken away. Flushing of the
drill hole may be carried out with water or compressed air. Various bits are
available for different conditions:
4.2.3 Blasting Blasting is a specialist operation not to be discussed here.
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5 Filling 5.1 Fill material Earthworks fill material may consist of soil, rock, or inert construction and
demolition material. (Inert construction and demolition material shall mean rock,
rubble, earth, soil, concrete, asphalt, brick, tile and masonry generated from
construction and demolition works.) Fill material shall be capable of being compacted to form stable areas of fill. Fill
material shall not contain any of the following.
Material susceptible to volume change, such as marine mud and swelling clays
Peat, vegetation, timber, organic, soluble or perishable material
Dangerous or toxic material or material susceptible to combustion,
Metal, rubber, plastic or synthetic material
The different types of fill material shall have the particle size distributions within the
ranges stated in Table 6.1.
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5.2 Compaction Earthwork fill materials when deposited are normally loose and bulked. It is
therefore necessary to compact the materials so as to prevent softening, dislodgment
and settlement of the earth. Fill material shall be compacted in layers to a stable condition. The thickness of
each layer shall be 150 mm to 300 mm which depends of the capacity of the
compaction plant used. The amount of compaction attained is measured by dry
density of the fill. Generally, the fill material shall be compacted to obtain a
relative compaction of at least 95% of the maximum dry density of that material. Compaction plant 5.2.1 Vibrating Rollers Beside the weight of the machine, vibration greatly improves compaction
performance.
They are suitable for compacting granular soil Various size available:
manually guided tandem roller (1 tonne baby roller) up to 20 tonnes vibrating roller
5.2.2 Vibrating plate
manually guided for light compaction useful in utility trenches, confined space
and awkward situations
maximum compaction thickness: 150mm
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5.2.3 Power Rammers manually operated suitable for compacting soil in narrow
trenches and around foundations
effective compacted depth about 200mm 6. Trenching 6.1 Impact to existing utilities According to Hong Kong statutory requirement, all utilities must be laid not less than
0.9 m below ground surface. Normally, small diameter utilities pipes and lines are
laid under pedestrian payment while large diameter pipes are laid under carriageway. It must be noticed that there may be a lot of utility pipes and lines exist under the
ground. Breaking of these utilities not only causes inconvenience to citizens, incurs
expenses for reinstatement but also is dangerous especially for electric cables and gas
pipes. Therefore, these pipes and lines must be located and their alignments clearly
marked on the ground before digging. As-built drawings are available from utility companies and government departments.
However, total reliance should not be placed on them, as some of these drawing are
not accurate. High voltage cables and gas pipes shall be located by magnetic
detector to ascertain their alignment and depth. If under ground utilities are encountered while, they shall be carefully unearthed,
preferably by hand, to prevent damages. Once exposed, suitable supports such as
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props or hangers shall be placed to prevent excessive deflection of the utilities. 6.2 Digging For paved area, both sides of the trench shall be cut by a diamond disc or an abrasive
disc then the paving further broken down by pneumatic breakers.
For digging trenches, backhoe would be the best choice. A backhoe using a bucket
with width the same as the trench could result an accurate and efficient work. 6.3 Timbering The most important factor in trench excavation is the stability against the collapse of
the trench sides. Caution, even a trench of 1.2 m deep can kill! 6.3.1 Trenches remained unsupported For trenches of less then 1.2 m and in firm soils, they may remain unsupported
(self-supported). 6.3.2 Trenches of depth between 1.2 to 2 m and in firm soils These trenches shall be excavated progressively and poling boards shall be installed
immediately after digging.
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6.3.3 Trenches in weak soils or deeper than 2 m
For deep trenches or in weak soils, runners, or more often sheet piles, shall be driven
along both sides of the trenches before excavation.
Typical timbering in firm soil (Source: R. Chudley)
Typical timbering in weak soil (Source: R. Chudley)
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7 Slope Stability and Protection 7.1 Common factors affecting slope stability
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Various Methods of Stabilizing Rock slopes (Source: G.E.O)
7.2 Slope protection methods The stability of slope can be increased by the following methods:
a. cutting back of steep slopes
b. re-compaction of existing loose fill slopes
c. rock scaling, removal/trimming of rock masses
d. installing soil nails, rock dowels and rock bolts
(details to be discussed in the topic prestressed concrete)
e. dentition to rock joints
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(Source: G.E.O.)
(Source: G.E.O.)
f. surface protection
vegetated Surface (hydroseeding)
shotcrete surface
chunam Surface
Stone-pitched surface
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(Source: G.E.O.)
g. ground water control methods surface channel
raking drain
weepholes to release the hydrostatic pressure h. retaining walls 7.3 Routine maintenance inspections of slops Typical features of slope that require maintenance are illustrated in the figure below.
As a minimum, it is recommended that routine maintenance inspections are carried
out to ascertain the need for basic maintenance work items, including:
a. clear accumulated debris from drainage channels and slope surface
b. repair cracked or damaged drainage channels or pavement
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c. repair of replace cracked or damaged slope surfacing
d. unblock weepholes and outlet drain pipes
e. repair missing or deteriorated pointing masonry walls
f. remove any vegetation causing severe cracking of slope surface cover the
drainage channels
g. re-grass bare slope surface areas
h. remove loose rock debris and undesirable vegetation from rock slopes or around
boulders
i. investigate and repair buried water-carrying services where signs of possible
leakage are observed
Slope Maintenance Inspections (Source: G.E.O.)
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Possible modes of failure in retaining walls
8. Retaining Walls A retaining wall is a structure designed to maintain a difference in the elevation of the
ground surface on each side of the structure. 8.1 Possible modes of failure of retaining walls
overturning overstressing of the material in the wall forward sliding settlement circular slip 8.2 Releasing of hydrostatic pressure Except for retaining walls which are designed to resist water pressure, e.g. basement
wall, hydrostatic pressure should not be allowed to build up behind the walls. The hydrostatic pressure can be released by including a subsoil drainage system
behind and / or through the wall. Adequate weep-holes, filter layers and preferably
back drains behind the wall should be provided, together with adequate channels and
paving at the top and toe of the wall to prevent infiltration of water into the back of
the wall.
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8.3 Type of retaining walls
Types of Retaining Walls (Source: GeoGuide 1)
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Mass Concrete Retaining Wall (Source: GeoGuide 1)
8.4 Gravity Retaining Walls Gravity retaining walls rely upon their own mass together with the friction on the
underside of the base to overcome the tendency to side or overturn. 8.3.1 Mass concrete retaining walls Mass concrete retaining walls are one of the simplest forms of retaining wall and are
usually trapezoidal in cross-section. They are particularly suitable for retained
heights of less than 3 m. They can be designed for greater heights, but it is not
economic. 8.3.2 Crib walls Crib walls are assembled of individual prefabricated units to form a series of crib-like
structures containing suitable free-draining granular infill. The crib units together
with the infill are designed to act together as a gravity retaining wall. Low crib walls may be built vertical. Walls higher than 2 m are usually built to a
back batter and with a tilted foundation to improve stability and even out ground
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Crib Wall (Source: GeoGuide 1)
bearing pressures. Crib walls are very sensitive to difference settlements and problems may arise for
walls which are higher than about 7 m. They are generally not suitable to be used
on ground which is liable to settle, nor should they be used for supporting heavy
surcharge. 8.3.3 Gabion walls Gabion walls are made up of rows of orthogonal cages or baskets (gabions) which are
filled with rock fragments and tied together. Their permeability and flexibility make
them particularly suitable for use at sites which are liable to become saturated and
where the foundation is composed of relatively compressible materials. Hence,
gabion walls are widely used in river works. They are also used as retaining walls
on dry land, especially in rugged terrain. Gabion walls are relatively simple to construct. Where suitable rock is readily
available, the use of gabion walls is particularly attractive for reasons of economy
and speedy construction. A variety of cage sizes can be produced using suitable
materials to suit the terrain. The gabions are normally in modules of 2m x 1m x 1m.
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Detail of Hexagonal Woven-mesh Gabions (Source: GeoGuide 1)
Typical Gabion Walls (Source: GeoGuide 1)
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Typical Drainage Scheme of R.C. Retaining Walls (Source: GeoGuide 1)
8.3.4 Reinforced Concrete Retaining Walls A reinforced concrete retaining wall resists earth pressure from the back. The earth
rests on the base slab of the retaining wall which provides part of the stabilizing
weight. The wall and the earth acts together as a semi-gravity structure. A shear
key is sometimes provided below the base of the retaining wall to improve sliding
resistance. The following are the main types of wall:
1. L-shaped or inverted T-shaped cantilever retaining walls, which have a vertical or
inclined slab monolithic with a base slab. The stem is subject to bending and
the economical height is about 8 m.
2. For greater heights, the stem of the retaining wall may be braced with
counterforts at the back in suitable spacings. A counterfort is a triangular slab
or a tapered beam in vertical tied to the stem of the retaining wall.
3. A buttressed retaining wall is similar to a counterfort retaining wall but with the
triangular slabs placed in the front of the wall. However, due to its out looking,
buttressed retaining wall is seldom used in urban areas.
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Types of R.C. Retaining Walls (Source: GeoGuide 1)
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Soldier pile wall (Source: S.A.R. Jufri)
8.3.5 Bored pile or caisson wall Cantilevered retaining walls in Hong Kong are sometimes formed by constructing a
row of bored piles. (Hand-dug caissons had been wildly used in the past but were
banned in 1980s.) The wall formed by this method are called soldier pile wall.
Piles of soldier pile wall are normally contiguous (i.e. they are touching each other or
adjoining). Adequate weepholes shall be installed for the releasing of hydrostatic
pressure behind the wall.
Reference: Modern Construction and Ground Engineering Equipment and Methods (1994) 2nd edition,
Frank Harris, Longman. Guide to retaining wall design (GeoGuide 1) 1993, Geotechnical Engineering Office. Laymans Guide to Slope Maintenance 1997, Geotechnical Engineering Office. Trenching Practice (CIRIA Report 97) 1992, D.J. Irvine & R.J.H. Smith, Construction
Industry Research and Information Association
Weep holes in soldier pile wall (Source: R. Chudley)
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Blinding concrete layer Reinforcement of pad footing
3 Basic Substructures Substructures are structure below ground. Foundations and basements are the
most common type of structures.
1 Shallow Foundation A foundation is structure designed and constructed to be in direct contact with
and transmitting loads to the ground
Shallow foundations are found at a depth of less than 3 m below the finished
ground level.
1.1 Pad footing A pad footing is an isolated foundation
to spread and transfer a concentrated
load to the earth.
The plan shape of a pad footing is
usually square, but if the column is
close to the site boundary it may be
necessary to use a rectangular plan
shape of equivalent area.
Pad footing (Source: R. Chudley)
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Strip foundation (Source: R. Chudley)
(Blinding concrete is a layer of non-structural concrete of about 50 mm thick
laid on the earth. It functions are to provide a flat and clean platform for steel
fixing, formwoking and to prevent contamination of the fresh concrete by the
earth.)
1.2 Strip Foundation A strip foundation is a foundation
providing a continuous longitudinal
ground bearing.
Strip foundations are used to transfer the load from a wall, or from a
succession of closely spaced piers or columns, to the ground.
They consist of a continuous ribbon-shaped strip formed of reinforced
concrete. Main bars are placed transversely to resist bending while
longitudinal bars are used for the continuity of the strip foundation and to
bridge soft spots in the soil.
Strip foundation
Column
FRONT VIEW
PLAN
Blinding concrete
Strip foundation supporting closely spaced columns
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Solid slab Raft (Source: R. Chudley)
1.3 Raft foundation A raft foundation is a foundation continuous in two directions, usually
covering an area equal to or greater than the base area of the structure.
The structure moves together with the raft foundation when ground
movements occur such that cracking or damage can be prevented.
A raft foundation is also called a floating foundation.
Raft foundations are useful in the following cases:
1. where buildings have to be erected on soils susceptible to excessive
shrinkage, swelling or frost heave;
2. where differential settlements are likely to be significant.
3. for structures where the column loads and/or soil conditions are such that
the resulting footings occupy most or the site.
1.3.1 Solid Slab Raft Solid slab rafts are suitable for lightly loaded structures such as small houses.
A solid slab raft consists of a reinforced concrete slab, usually slightly larger
than the area of the building. Reinforcement in the form of a mesh fabric is
provided on both the top and bottom faces of the slab.
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Solid slab raft with edge beam (Source: R. Chudley)
Beam and slab raft with downstand beams (Source: R. Chudley)
1.3.2 Beam and Slab Raft There may be variations in ground stiffness and cause differential settlement.
The solid slab raft may be further reinforced with:
The slab is stiffened under the peripheral walls with edge beams.
The beams and slab raft provide stiffness and prevent the distortion of the
building.
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1.3.3 Cellular Raft This type of foundation consists of two reinforced concrete slabs linked by
internal walls which divide the void into cells. The walls help to spread the
load over the raft. Openings can be formed in the cell walls allowing the voids
to be utilised for the housing of services, as storerooms or for general
accommodation.
With cellular raft foundations, the columns shall be positioned at the
intersection of the internal walls.
The cellular raft foundation provides a great stiffness against the differential
settlement.
Cellular Raft foundation (Source: R. Chudley)
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Balanced base footing (Source: R. Chudley)
1.4 Combined foundation Rectangular footing is used for two closely spaced columns. Balanced base footing is used for
eccentrically load column. This often
happen in perimeter column where the
footing is limited by site boundary.
Shallow foundations
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2. Shallow basement A basement can be defined as a storey with a floor which at some point is more
than 1.2 m below the highest level or ground adjacent to the outside walls.
The structural walls of a basement below ground level are in fact retaining walls
which have to offer resistance to the soil and ground water pressures as well as
assisting to transmit the superstructure loads to the foundations.
Considerations of basement construction
Excavation methods.
Surface and ground water control
Lateral stability of basement excavation.
Stability of adjoining building.
2.1 Surface and ground water control For basement construction, water may come from the rain or the infiltration of ground
water when excavated below the ground water table. Problems caused by ground
water are:
Water logging of the ground which may restrict the carrying out of works.
Reduction in the shear strength of the soil which may lead to collapse of
excavation side.
Overloading or collapse of the temporary support to the excavation caused by
hydrostatic pressure.
Consolidation or loss of ground under adjacent structures due to dewatering
which would cause settlement.
2.1.1 Surface water control
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sealed with
excavation
soil cement
poling boardor
sheet piling
Intercepting surface water
Open sump pumping (Source: CIRCA)
A submersible pump
Adequate surface channels, usually in the form of U-channels, should be placed in
suitable locations on the site to drain away the rain water. If the channel is laid
around the perimeter of the site, it is termed as garland drain.
Where poling boards or sheet piling are provided
as support for excavation, they should be
extended 200-300 mm above existing ground
level and the edges sealed with soil cement. This
also effectively prevents the surface water from
entering the excavation.
2.1.2 Dewatering Suitable dewatering outside a cofferdam reduces the hydrostatic pressure acting on
the cofferdam, but significantly draw down of ground water table would cause
settlement in surroundings
2.1.2.1 Open Sump Pumping An open sump should be excavated below the formation level of the excavation and
preferably be sited in a corner position. The water seeped into the excavation is led
into the sump, either by sloping the ground towards it or by using shallow garland
drains that feed the water into the sump. The water can then be pumped away by a
submersible pump.
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2.1.2.2 Wellpoint System This method is suitable for lowering water in non-cohesive soils, e.g. sand or gravel
soils of average permeability
Before excavation, a series of small diameter wells are jetted (or drilled) into the
ground in suitable positions and at predetermined centres e.g. from 600 to 1800 c/c.
The wellpoints are connected in series to pumps with header pipes.
Wellpoint (Source: R. Chudley) Header pipe of well point system
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Ring wellpoint system (Source: CIRCA)
The wellpoints may be arranged as a ring system enclosing the area to be excavated,
or as a progressive system alongside a long trench or similar excavation (to one or
both sides according to the width of the excavation).
The standard equipment will lower the water level up to a depth of between 5 and 6 m
under average conditions. Where the depth of excavation exceeds 6 m then a multi-
stage wellpoint system is required.
Progressive wellpoint system (Source:CIRCA)
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Multi-stages wellpoint system (Source: R. Chudley)
Deep bored well (Source: R. Chudley)
2.1.2.3 Deep-Bored Well System This system can be used as an alternative to
a multi-stage wellpoint dewatering
installation where the ground water needs to
be lowered to a depth greater than 9 m or
where a suction pump cannot be used.
Large diameter boreholes or wells are
formed by sinking a 300 to 600 mm
diameter steel lining tube into the ground to
the required depth and at spacings to suit the
subsoil being dewatered. A submersible
pump is set at a suitable depth to extract the
water. The annular space is filled with a
suitable media such as sand and gravel to act
as a waterway as the outer steel lining tube
is removed.
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Open excavation (Source: R. Chudley) Open excavation
3 Lateral stability for basement excavation There are various kinds of methods for basement construction and maintaining
the stability of the ground. The choice depends on the nature of ground
condition and the depth of basement.
3.1 Open Excavations
Temporary support is often needed to the sides of the excavation for stability. These
temporary members can be intrusive when the actual construction works of the
basement is carried out. One method is to use battered excavation sides that cut back
to a safe angle of repose thus eliminating the need for temporary support.
This method is suitable for shallow basement only because the extra volume of
soil needed to be excavated increases rapidly with depth increased. Another
limitation is that large amount of free space around the site must be available.
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Driving of sheet piles Sheet pile cofferdam
3.2 Sheet piling cofferdam The term cofferdam can be defined as a structure, usually temporary, built for the
purpose of excluding water or soil sufficiently to permit construction to proceed
without excessive pumping, and to support the surrounding ground.
There are a lot of methods and materials for forming cofferdams. Among which steel
sheet piling cofferdam is the most common on Hong Kong. It has the following
advantages:
a. Steel sheet piles have high structural strength
b. They can be driven deep into most types of ground.
c. Cofferdams can be constructed to a depth of about 15 m below existing ground
level.
d. The sheet pile interlocks provide an almost completely watertight enclosure.
e. The sheet piles can be withdrawn and reused.
Sheet piles are normally driven into the ground by drop hammers or vibration
hammers to form an enclosure prior to excavation. To ensure that the sheet
piles are pitched and installed vertically a driving trestle or guide frame is used.
When excavation is taken place inside the cofferdam, adequate support must be
provided for the lateral stability.
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3.2.1 Cofferdam supported with Raking Struts This method is suitable for a depth up to 5 m. After the sheet piles has been driven
around the perimeter of the site to form an enclosure, the centre of the basement is
excavated down to the formation level but leaving a wedge of soil at the perimeter to
support the cofferdam. Raking struts are installed to support the cofferdam. Finally
the wedge of soil is trimmed away.
raking strutssheet pile wall
battered slope
base slab
waling
base slab
Stage I Stage II Stage III
Cofferdam supported with Raking Struts
wedgeof soil
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3.2.2 Cofferdam supported with strut and waling In deeper excavation, the sheet pile cofferdam can be supported with layers of
bracing frame. Each bracing frame is formed with struts and walings. For wide
cofferdams, king posts (vertical supports) are installed to support the bracing
frames. This method is suitable for excavations up to about 10 m deep.
Work sequence of basement construction (Bottom-up method):
1. Sheet piles are driven into the ground in predetermined location to enclose the
area to be excavated. Meanwhile, piles are installed at suitable positions.
2. The earth inside the cofferdam is excavated to about 1 m below the first bracing
level.
3. The first bracing frame (struts and walings) is installed by welding to support
the cofferdam.
4. The processes of excavation and bracing frame installation are repeated until the
desired depth is reached
5. The pile caps and the base slab of the basement are constructed.
6. The construction of the basement is continued upward until the lowest bracing
frame is lowest bracing frame is encountered.
Cofferdam supported with struts and walings Cofferdam supported with struts and walings (Source: R. Chudley)
king post
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7. The cofferdam is shored to the basement wall by short struts, and the original
struts are then removed.
8. The above process is repeated until the basement is constructed to the ground
level.
9. The space between the basement and the cofferdam is backfilled with soil and
compacted in layers. The short struts are removed progressively and finally the
sheet piles are withdrawn.
Bottom-up basement construction
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Cofferdam support with ground anchors
3.2.3 Cofferdam support with ground anchors After the sheet piles have been driven around the perimeter to form an enclosure,
the centre of the cofferdam is excavated to about 1m below the first bracing
level.
Holes at suitable spacings and in the same level are drilled into the ground at an
inclination of 30-45 below the horizontal penetrating through the sheet piles. Prestressing wires are inserted in to the holes and the ends are grouted with
cement grout.
Walings and anchorage heads are installed and the wires are prestressed to hold
the cofferdam.
The process is repeated for the subsequent bracing levels until down to the final
formation level.
This method is suitable for wide and deep basement. It also provides a clear working
area within the cofferdam. However its use is often limited by the site boundary.
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4 Precast Concrete Precasting means casting a concrete member at a place other than where it will be
used and then moving it to the place where it will be installed.
1. Precast yard Most precast units are produced in factories or casting yards. Fundamental factors
that contribute to the success of a factory/casting yard for the precasting include:
proximity to the place where the precast unit will be installed
good access such as road, rail or pier
sufficient area for the storage of materials, bending and fabrication of steel
reinforcement, casting, curing and storage of finished products
availability plants such as batching plants and lifting facilities
availability of materials and labour supply
2 Techniques to improve the production 2.1 Fabrication of reinforcement
Reinforcement fixing is labour intensive on site. For precasting, mechanization is
possible for the fabrication of reinforcement because of mass production. Cutting,
bending and fixing the reinforcement can have high degree of automation. The
reinforcement can also be fixed by spot welding. (Normally, welding for T-bars is
not permitted on site as poor temperature control on welding lowers the strength of
high tensile steel.)
Automatic cage fabrication machine
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Steel Mould for Precast Staircase
2.2 Prestressing
The technique prestressing greatly improve the strength of precast units. Usually,
pre-tensioning is used for precasting. Sometimes, post-tensioning is also used for
non-standard units or where curved tendons are required.
2.3 Concrete moulds
Steel moulds are usually used for precasting which have the following advantages:
easy assembling and demoulding
durable - can be reused up to a
thousand times for percasting works
hard and smooth surfaces of the
moulds can be cleaned easily and
give good concrete finishes
2.2 Compaction of concrete
Centrifugal Spinning Hydraulic Pressing 2.2.1 External vibrators - which mounted on the moulds reduce the labour works
for compacting the concrete.
2.2.2 Hydraulic pressing - which can be employed to compact low slump concrete
of small precast units, such as paving blocks and concrete drainage pipes.
The units can also be demoulded immediately without breaking.
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2.2.3 Centrifugal spinning - In the production of some precast concrete pipes and
prestressed tubular pile (e.g. Daido Pile), the centrifugal spinning process
effectively compacts the zero slump concrete. It produces a uniform hollow
tube without the need of a void former.
2.3 Accelerated curing
An increase in the curing temperature of concrete increases its rate of development of
strength. It reduces the curing time hence reduces the cycling times of concrete
moulds and prestressing bed.
2.3.1 Steam curing (at atmospheric pressure and below 100C) Steam curing is normally applied in special chambers or in tunnels through which
the concrete members are transported on a conveyor belt.
Alternatively, portable boxes or plastic sheet covers can be placed over precast
members; steam is supplied through the connections of flexible hose.
Plastic sheet cover for steam curing Autoclaving 2.3.2 Autoclaving (high pressure steam curing)
Precast units are placed into an autoclave (a pressure vessel) and steam of high
pressure and temperature (about 177C and 0.8MPa above atmospheric pressure) are applied.
Usually the 28-day strength on normal curing can be reached in about 24 hours.
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3 Handling of precast units Since precast concrete unites are bulk and heavy, lifting equipments are required for
the lifting. Lifting fittings should also be cast into units for easy handling.
3.1 Lifting fittings
lifting hooks
threaded sockets lifting plates
lifting hook
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3.2 Lifting devices
mobile cranes derrick
tower cranes Launching Girder 4. Application and installation of precast units There are various applications of precast concrete, and the precast units can be
installed by different methods.
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Dowel and sleeve connection
precast column sleeve
grout hole
dowel
filled with dry pack or grout
foundation
4.1 Column to foundation connection 1.1 Pocket connection
A pocket is reserved in the foundation.
The column is set into the reserved
pocket in the foundation and the spaces
between the column and the socket is
filled with cement grout.
1.2 Bolting connection
The main bars of the precast column are connected to the steel base plate / channels
by welding. The precast column can then be connected to the foundation by bolting.
1.3 Dowel and sleeve connection
Grout-sleeves are cast into a precast unit.
The sleeves fit over reinforcement
projecting from the mating part.
The sleeves are grouted and the gap
between the units is filled with dry-pack
or non-shrink grout
Pocket connection
Column-foundation bolting connection (Source: R. Chudley)
Column-foundation bolting connection
precast column
column reinforcement welded to channel steel channel
holding down bolt grout or dry pack
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Beam to Column connection by Bolts and Brackets (Source: R. Chudley)
4.2 Beam to column connection 2.1 Simply supported joint and hinge joint The beam usually sits on a corbel or the column head. For a heavy structure, it is
important to place a resilient pad, which is commonly called a bearing, between the
two structural components to transfer the load uniformly so as to prevent localized
stress. If the horizontal translation is restrained, say by a dowel, it becomes a hinge
joint.
Simply Supported joint and hinge joint Beam-column dowel sleeve connection
2.2 Bolting Connection For bolting connection, steel brackets have to be shop welded to the main bars of the
precast units. The precast units can then be connected to the main structure on site
rapidly by bolting.
Simply supported
Hinge
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2.3 Exposed reinforcement connection The exposed reinforcements of the precast column and the precast beam are
lapped together.
The joint is then completed with insitu concrete.
The joint provides good moment resistance.
It is also called composite moment connection.
3. Column Splicing 3.1 Welding connection A steel shoe is fixed to the end of each precast column by welding to its main
reinforcements. The columns are butted against each other and the joint is
completed by butt welding.
Welding Connection
Joint completed with insitu concrete
Precast column
Precast beam
Exposed reinforcements of beam and column
Beam column exposed reinforcement connection
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Column Splicing with insitu concrete
3.2 Dowel and sleeve connection Grout-sleeves can be cast into a precast unit, then the sleeves fit over reinforcement
projecting from the mating part. The sleeves are grouted and the gap between the
units is filled with dry-pack or non-shrink grout.
Grouted sleeve connection 3.3 Insitu concrete connection The exposed reinforcements of the columns are lapped together.
The joint is completed with insitu concrete.
It provides good moment resistance.
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4. Precast Slab 4.1 Planks and blocks
A precast slab can be formed by placing long planks at suitable centres
supported on main beams or loading bearing walls
The intermediate spaces are then filled with smaller block units to complete the
slab.
Normally, a structural topping is not required but the upper surfaces of the units
are usually screeded to provide the correct surface for the floor finishes.
This method eliminates the requirement of falsework during the construction
period.
Planks and blocks
4.2 Hollow core slab
Hollow core slabs can be used for most
building floor or roof systems.
The voids greatly reduce the dead load of
the slab and the material cost.
The assimilated I-beam sections
provide efficient moment resistance.
Prestressed hollow core slabs are available. This means long spans, shallow
depth and the ability to carry heavy loads are easily accommodated.
Hollow core slabs may be simply supported on beams or load bearing wall.
Hollow core slab
Assimilated I-beam Prestressed tendon
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To resist hogging moment at the support, steel dowel are can be provided.
4.2 Double Tee Slabs
Double tee slabs are prestressed.
Double tee slabs can be used for most applications requiring a long span floor or
roof system (10m to 30+ m) and/or additional load carrying capability.
Dowel
Filled with cement grout
Hollow floor installation
Min. 75mm
Double Tee Slabs (Source: CPCI)
Steel bracket welded to the main reinforcements
Steel bracket welded to the main reinforcements
Min. support width 150 mm
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5 External Wall Faade Panel is the most widely used precast concrete wall in Hong Kong.
Various installed methods had been used.
The prevailing installation method:
Erection of the faade panel with temporary plumbing guide
Fixing of reinforcement of adjoining walls lapping with the dowel of the faade
panel
Shuttering of wall formwork and casting of concrete
Fabric reinforcement for wall construction after installing precast faade
Fixing of Facade Panel (Source: City University)
Building structural frame
Cast with insitu concrete
Joint filled with cement grout
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6 Composite Construction / Permanent Formwork Precast units are placed underside to serve as formwork for concreting. They also
become integral parts of the permanent structure. The major advantage of using
permanent formwork is that it eliminates or minimizes the temporary works such as
formwork and falsework. In addition, there is no need for stripping.
5.1 Composite Floor Slab
Solid Planks for Composite Slab
5.2 Composite Beam
Shell beam & Precast Slab
precast slab
binder
top reinforcement Insitu concrete
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6. Advantages and disadvantages of Precast Concrete 6.1 Advantages of precast concrete
The units can be mass-produced and are therefore cheaper.
Cost of formwork can be reduced.
Easier to fix reinforcement and place concrete which to be done on ground and
under cover.
Units can be cured by accelerated techniques.
The quality of units can be strictly controlled.
Units can be cast before the site becomes available hence the construction time
can be reduced.
Temporary supports such as falsework and scaffolding are reduced to minimum.
Precast units can be structurally load tested if required.
Precast units can be pre-tensioned.
Precasting produces less construction waste than insitu works and therefore more
environmental friendly.
6.2 Disadvantages of precast concrete
Uneconomical if only a small number of units are required.
Waterproofing of joints may be expensive.
The transportation of long units may be difficult.
Cranes may be required to load and unload the units on site.
Reference: Construction of Prestressed Concretes 2nd Edt., Ben C. Gerwick, Jr. (1993), Wiley Inter.
Science. Modern Prestressed Concrete Design Principles and Construction Methods 4th Edt., James R.
Libby (1990), Van Nostrand Reinhold. Post-tensioning in Building, VSL Construction Technology Vol. 3 2nd Edt., R. Chudley (1991), Longman. Civil Engineering Construction IV Vol. 4, S.A.R Jufri & R.J. Wellman (1992), Hong Kong
Polytechnic. Precast Concrete Material, manufacture, properties and usage, M. Levitt (1982), Applied
Science Publishers Recommended Practice for Erection of Precast Concrete, PCI Erectors Committee (1983), PCI http://www.cpci.ca/
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5 Structural Steelwork 1. Structural Steel Sections
Structural steel sections can be classified into various groups according to their
shapes.
A steel section is designated by the serial size in millimeters (mm) and the mass
per unit length in kilograms per metre (kg/m), i.e. h x b x mass.
For example, a universal beam of 920.5mm x 420.56 mm at 388 kg/m is
designated 914 x 419 x 388 beam.
1.1 Universal Beam (UB)
Universal beams are rolled with parallel flanges.
The depth of section is about double of the width
of the flanges.
Universal Beam
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1.2 Universal Column (UC)
Universal Columns are rolled with parallel flanges.
Generally, the depth of the section and the width of the
flanges are approximately equal.
1.3 Joist
Joists are rolled with 8 tapered flanges.
They provide a useful range of sections smaller than those
in the universal beam
1.4 Channel Section
There are two types of channel sections:
Parallel flange channels
Taper flange channels with 5 tapered
flanges.
1.5 Structural Tee
Structural tees may be cut from
universal beams or universal columns.
They are designated by their nominal
width of flange, depth of stalk and mass
per metre, i.e. b x A x mass.
1.6 Angle
Angles are designated by their nominal leg lengths and thickness (in mm),
e.g. 50 x 30 x 5
Universal Column
Joist
Tapered flanges Parallel flanges Channel
Cut from UB Cut from UC Structural Tee
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Sand Blasting (Source: S.A.R. Jufri & R.J. Wellmen)
There are two types of structural
steel angles :
a. equal leg angles
b. unequal leg angles 1.7 Hollow Sections
Available hollow sections are round, square and rectangular.
Hollow sections are specified by their size and thickness.
2. Fabrication Fabrication is carried out in a fabrication workshop, where the steel sections undergo
the following stages of treatments:
1. The steel sections are first cleaned to remove dirt, mill-scales and any corrosion
by sand blasting. They are then painted with a priming coat of paint within 2
hours.
2. The sections are cut to the correct length by sawing or cropping.
3. Holes are drilled or punched on the workpiece for bolted connections.
Sometimes the edge of a workpiece is machined for welded connections.
4. Jointing accessories (fittings), such as angle cleats, plates, bases, etc., are
manufactured by drilling, punching and cropping machines at the same time.
5. To reduce the site works, the main components and the fittings are then
assembled into modules. The size of each module should be convenience for
lifting and transportation.
6. The components are then transferred to the dispatch bay to await transport to
site.
Equal leg angle Unequal leg angle
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3. Connection
Steelwork can be connected by bolting or welding
3.1 Bolting connection
3.1.1 Black bolt
Black bolts are made of mild steel, unpolished and the least expensive.
They are used in clearance hole, i.e. the hole diameter is 2 mm larger than
the bolt, or 3 mm larger if the bolt diameter is 24 mm or above.
3.1.2 High strength friction grip (HSFG) bolt
HSFG bolts are made of high tensile steel.
They are used in clearance holes as black bolts.
Bolts are always tightened to a predetermined shank tension.
This enables shear loads to be transferred by friction between the
interfaces.
HSFG bolts may be tightened by three methods:
a. Torque control - e.g. by torque wrench
b. Part turn method
c. Direct tension indication
H.S.F.G bolt
waist
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3.2 Welding Connection 3.2.1 Methods of welding
Gas welding
In gas welding, an oxy-acetylene flame provides the heat needed to melt
the steel interfaces and the weld metal.
Metal-arc welding (Electric-arc welding)
A metal filler rod is connected to an electrode of a power supply while the
work piece is connected to the other electrode.
When the metal filler rod is placed near the work piece, an electric arc is
formed which heats and melts the interfaces and the end of the filler rod.
3.2.2 Types of Welds
Fillet Welds
Fillet welds are used to join
plates at an angle (usually
90) to each other.
Metal-arc welding equipment Gas welding equipment
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Welded Bloom Base (Source: R. Chudley)
Butt Weld
Before butt welding, the ends of the work pieces have to be machined to
receive the weld.
The work pieces are then butt against each other and are welded together.
4. Erection 4.1 Connection of steel column to foundation
In base connection, a steel base plate is required to spread the load of the
column on to the foundation.
The base plate and column can be connected together by using cleats or by
fillet welding.
Gusset plates can be used to increase the stability.
The base plate is then fixed to the foundation by holding down bolts.
Butt weld (Source: S.A.R. Jufri & R.J. Wellmen)
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Connection of steel column to foundation (Source: R. Chudley)
4.2 Beam to Column Joints
4.2.1 Double web cleats connection
The UB and the UC are connected by bolting with two web cleats.
The joint allows some degree of rotation and is considered as a semi-rigid
joint.
Welded Gusset Base (Source: S.A.R. Jufri & R.J. Wellmen)
Double web cleats connection
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Welded connection rigid joint
4.2.2 Header plate / End plate connection A header plate /end plate is shop welded to the end of the universal beam.
The connection is completed by bolting on site.
The size of the header plate may be smaller, the same or larger than the
section of the UB and the rigidity of the joint increases with using larger
end plate.
4.2.3 Welded connection
A fully rigid connection, which gives the greatest economy on section, can
be achieved by welding the beam to the column on site.
The quality control of site welding is difficult and the testing of weld
quality is expansive, on site welding is rarely used except for very large
scale projects.
Header plate connection semi-rigid joint
HEADER PLATE
SHOP WELDED
End plate connection rigid joint
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Double web cleat connection (Source: S.A.R. Jufri & R.J. Wellmen)
4.3 Beam to Beam Joints
The top flange of the secondary beam is cut away (notched) so that the tops
of both beams are all level with one another, ready to receive the floor or
roof decking.
The secondary beam can then be connected to the main beam by web
cleats, header plate or to the stiffeners of the main beam.
Header plate connection Stiffener connection
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Column with equal sections
4.4 Column Splices
For the connection of equal sections, fish plates are used for the splicing.
For the connection of unequal sections, a cap plate is shop welded to the
top of the lower column.
The connection is then completed by bolting with web cleats or site
welding.
Column with unequal sections
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6. Fire Protection
Though structural steel does not promote spread of fire, it does not behave well
under fire conditions.
If steel is heated to 550C, it will lose most of its useful strength. 6.1 Methods of fire protection
According to Code of practice for fire resisting construction, Building Authority,
fire protection to structural steel can be classified into:
Hollow protection - means there is a void between the protective material and
the web of the steel section, such hollow protection to columns should be
effectively sealed at each floor level.
Solid protection - means casing which is bedded close to the steel without any
intervening cavities and with all joints in that casing made full and solid.
6.2 Fire Protection Materials
Concrete not inferior to grade 20
Solid bricks of clay, concrete or sand lime
Plaster (Portland cement plaster, Portland cement-lime plaster or gypsum
plaster) on metal lathing
Gypsum plaster on gypsum plaster board
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6.3 Example of fire protection
6.3.1 Solid protection
Solid brickwork encasement to column
Concrete encasement to Column
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6.3.2 Hollow protection
Hollow protection using plasterboard and plaster
Hollow protection with metal lath and plaster coating
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6.3.3 Sprayed vermiculite-cement
Vermiculite is natural mineral that expands to about 10 times when heated to
200-300C. The expansion process is called exfoliation. A mixture of exfoliated vermiculite and Portland cement is sprayed onto the
steel structure.
Wire reinforcement is tied around the steel before spraying if required.
The advantages of sprayed vermiculite-cement are that it is lightweight and can
be applied to any configuration of steel.
Sprayed vermiculite-cement encasement
6.3.4 Intumescent Coating
Intumescent paints are plastic polymers containing nitrogeneous catalyst, which
When exposed to extreme heat, will expand by as much as 50 times to form a
thick carbonaceous foam.
This foam insulated the treated steelwork against the heat of the fire.
The advantages of intumescent coatings are that they are light in weight, available
in many colours as decorative paint and simple to apply.
The main disadvantage is its high cost.
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7. Composite Construction
Composite construction consists of using two different or similar structural materials
which interact together in resisting the external loadings. The number of
combinations is almost endless: steel and concrete, timber and concrete, precast and
cast in-situ concrete, timber and steel ..etc.. (In this section, only composite
construction of steel and concrete is considered.)
7.1 Composite structure versus non-composite structure For non-composite situation, the load will be shared between the two sections.
Slip will occur between the two contact surfaces.
For composite construction, the two sections are connected such the combined
sections will act as a single unit to resist the applied moment.
It is clear that the composite section is more structurally efficient than that of the
non-composite section.
If both sections are of same material, the composite beam deflection would be
only 25% of those of the non-composite beam.
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7.2 steel concrete construction, In Steel-concrete composite construction, shear connectors are used to connect the
two materials so that they can interact with composite action.
The function of the shear studs are:
To transfer the shear stress between the steel and the concrete, thus limiting the
slip at the interface so that the two materials can act as a unit.
To prevent an uplift between the steel beam and concrete slab, i.e. to prevent
separation of the steel and concrete at right angles to the interface.
7.2.1 Composite beams
Composite beams comprise steel beams, usually of I section which are designed to
act compositely with concrete by use of shear connectors.
Composite beam
Shear stud
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7.3 Composite column Concrete is placed around the steel core to provide a composite column, or
Square or round steel sections with infill concrete.
Steel-concrete composite columns
Steel-Concrete Composite column
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7.4 Composite slabs Composite slabs comprise underside profiled steel decking and reinforced concrete
topping. The steel decking serves to take tension and acts as the permanent
formwork for concrete casting.
Profiled Steel Decking
Reference: Construction Technology Vol 2 (1991), R. Chudley, Longman Civil Engineering Construction IV, S.A.R. Jufri & R.J. Wellmen, Hong Kong Polytechnic. Advanced Construction Technology 3rd Etd (2000), R. Chudley, Pearson Education Ltd. Tall Building Structural Analysis and Design (1991), G.S. Smith & A. Coull, John Wiely & Sons, Inc. Materials for Civil and Construction Engineers (1999), Micheal S. Mamlouk and John P. Zaniewski, Addison-Wesley Longman, Inc.
Profiled steel decking
Top layer of
Composite Slab
slab reinforcement Insitu concrete
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6 Safety and Health in Construction 1. Introduction The construction industry has a poor record in safety. No matter counted in terms of
number of accidents, or in term of accident rate, the construction industry is one the
most dangerous industry in Hong Kong. All site personnel should therefore aware the
potential hazards on site and be conversant the preventive measures. 3. Potential hazards of various construction activities and preventive measures 3.1 Hazardous nature of construction site
a. Variable nature of construction sites (a construction site changes everyday).
b. Construction activities involves different trades, they have different characteristics
but work in the same area and influence each other.
c. High turnover of labours so that to build up a general safety consciousness on site
is difficult to carry out.
d. Large working frontage within a construction site where unsafe conditions often
exist.
e. Requiring to work on high level, under poor working conditions or in places where
access is difficult to provide.
f. Requiring to handle very bulky or heavy materials (e.g. soil, concrete, timber, steel,
prefabricated components, etc.)
g. Temporary nature of site facilities and site works (e.g. electricity, scaffoldings). 3.2 Working at height 3.2.1 Risks of working on scaffolds/working platforms:
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fall of persons from height struck by objects falling from scaffolds or working platform collapse of scaffolds or working platform. 3.2.2 Prevention of falls: a. Take adequate steps to prevent any person on a construction site from falling a
height of 2 m or more.
b. Adequate steps include the provision, use and maintenance of :
working platforms, guard-rails, barriers, toe-boards and fences; coverings for openings; gangways and runs.
3.2.3 Safety requirements for working platforms and gangways Width of working platforms,
gangways and runs
not less than 400 mm not less than 650 mm for gangway or run used for
movement of materials
Construction of working
platforms gangways and
runs
close boarded or planked (a working platform, gangway or run -
a. consisting of open metal work having
interstices none of which exceeds 4000 mm2 ;
b. the boards or planks forming it are secured to
prevent movement and the space between
adjacent boards or planks does not exceed
25mm
need not be closely boarded or planked if there is no
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risk of persons below it from being struck by
materials or articles falling through the platform,
gangway or run)
boards or planks forming platforms etc.:
- of sound construction, adequate strength and free
from defect
- not less than 200 mm in width and not less than 25
mm in thickness, or not less than 150 mm in width
when the board or plank exceeds 50 mm in
thickness
- not protruding beyond its end support to a distance
exceeding 150 mm
- rest securely and evenly on its supports
- rest on at least 3 supports
Coverings for opening so constructed as to prevent the fall of persons,
materials and articles
clearly and boldly marked as to show its purpose or securely fixed in position
Height of toe-boards not less than 200 mm in height
(toe-boards are not required for stairs)
Height of guard rails The height of a guard-rail above any place of work on a
working platform, gangway, run or stairway shall be:
top guard-rail: not less than 900 mm and not more than 1150 mm
intermediate guard-rail: not less than 450 mm and not more than 600 mm
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3.2.4 Use of safety Harness a. In special circumstances where it is impractical to provide the above means of fall
prevention, safety nets and safety harnesses/belts shall be provided.
b. Wear safety harness or belt and attach it to a suitable anchor point; the lanyard shall
be left with the minimum free length.
Proper Installation of Scaffolding / Working Platform
Safe Use of Ladder
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Full Body Harness (Recommended) Safety Harness and Safety Belt (Source: Works Bureau)
3.3 Using of electricity Quite a number of tools an
