Lecture 1 & 2
Steel StructuresSteel structures are assembly of structural
steel shapes joined together by means of riveted, bolted or welded
connections.Concrete structures are easily joined together by means
of monolithic construction.Steel construction is used for every
type of structure.There are two main categories of steel
structures.i) Framework/skeletal systems ii) Shell
systemsFramework/skeletal systems: Main load carrying elements are
one dimensional or line elements forming two dimensional or three
dimensional frames.e .g: industrial buildings, railways bridges ,
towers , poles ,multi-storey buildings, all trusses and rigidly
connected components etc.Shell systems: Main load carrying elements
are sheets and plates besides some skeletal members etc.e.g: gas
tanks , tanks , reservoirs , bin and bunkers , blast furnaces , air
heaters , large diameters pipes and all other plate and shell
structures etc.Features of steel structures: Freedom of expression,
creativity , easy extension , easy fixing of facade , easy and
efficient fabrication , express function , large span , no limit of
architectural design , recycling is possible , slender columns ,
more space , transparency , visible connections , weather
independent construction. Merits of Steel
Constructions:1-Reliability: (i) Consistency and uniformity in
properties (ii) better quality control being factory manufactured.
(iii) large ductility and elasticity (iv) because of the truly
homogeneous and elastic material. (v) steel satisfies most of the
assumptions involved in the derivation of the analysis and design
formulas and hence the results obtained are reliable.2-Industrial
Behavior: (i) Rolled steel section are manufactured in the factory.
(ii) members may be cut and prepared for assembly in the factory
and therefore adaption to prefabrication is possible. (iii) only
joining of the components is carried out at the site by installing
rivets , bolts or welds.3-Lesser Construction Time: (i) Due to
industrial nature , the progress of work is very fast making the
structure economical. (ii) The structures can be put to use
earlier. (iii) The reduction of labour cost and overhead charges
and the benefits obtained from early use of the building contribute
to the economy.4-High Strength and Light Weight Nature: (i) High
strength per unit weight. (ii) Lesser dear loads. (iii) When dead
load reduces, the underneath members become still smaller due to
lesser load acting on them.(iv) Important for long spans bridges ,
tall buildings and structures having poor foundation
conditions.Factor (C) is defined as the ratio of the density (r) of
the material to the stress (f) it can carry and is one of the least
for steel. Aluminium 1.1x10-4,Steel 2.3x10-4, Wood 4.5x10-4
,Concrete24x10-4.5-Uniformity,Durability and Performance: (i) Long
life of the structure. (ii) Homogeneous and durable material. (iii)
It satisfies the basic assumptions of most of the analysis and
design formulas. (iv) If properly maintained by painting ect; the
properties of steel do not change appreciably with time. (v) Hence
steel structures are more durable.6-Elasticity: (i) It follows
Hooks law up to high stresses. (ii) The stress-strain diagram
remains a straight line. (iii) The steel sections do not crack or
tear before ultimate load and hence the moment of inertia can be
definitely calculated.7-Ductility and Warning Befor Failure: (i)
The property of a material by which it can withstand extensive
deformation without failure under hight stresses is said to be its
ductility. (ii) Mild steel is a very ductile material (25 to 30%
elongation in standard tension test specimen after fracture). (iii)
Visible deflection in case of overloads.(iv) Extra loads may be
removed from the structure to prevent the collapse. (v)Even if the
collapse occur, time is available for the occupants to vacate the
building.8-Addition to Existing Structures: (i) Additions to
existing structures are very easy to made.(ii) Connections between
existing and new structures can be employed very effectively.
9-Possible Reuse: Steel sections can be reused after a structure is
disassembled.10-Scrape Value: Steel has a scrap value even though
it is not reusable in its existing form.11-Water Tight and Air
Tight Construction: (i)Steel structures provide completely
impervious construction. (ii) Reservoirs , gas pipes , oil pipes
etc. are preferable made from structural steel.12-Long Span
Construction: (i) High rise buildings, long span bridges and
transmission towers are made from structural steel. (ii) Industrial
buildings up to a span of 90m can be designed by plate girders or
trusses. (iii) Bridges span up to 260m are made with plate girders.
(iv) For through truss bridges, spans of 300m have been
used.13-Temporary Construction: (i) For temporary structures, steel
construction is always preferred. (ii) Army constructions during
war are mostly made of structural steel. (iii)The structures may be
disassembled by opening few bolts, component parts are taken to the
new places and the structure is easily reassembled.Demerits of
Steel Construction1-High Maintenance Cost and Corrosion:(i)Periodic
paints are required to avoid from corrosion. (ii) This require
extra cost and special care. (iii)The use of weathering steel, in
suitable design conditions tends to eliminate this cost. (iv) If
not properly cared thickness loss may be 1 to 1.5mm per year.
(v)35% weight loss during specified life and failure under extra
loads.2-High Fireproofing Costs: (i) Although steel members are
incombustible,their strength is tremendously reduced at
temperatures prevailing in fires. (ii)At about 400 oC, creep
becomes more pronounced. (iii) Creep is defined as plastic
deformations under a constant load for a long period of time.(iv)
Steel is an excellent conductor of heat and may transmit heat from
a burning compartment of a building to start fire in the other part
of the building.(v) Extra cost is required to properly fire proof
the building.3-Susceptibility to Buckling: (i) Buckling is the type
of collapse of the members due to sudden large bending caused by a
critical compressive load. (ii) Steel sections consist of a
combination of thin plates. (iii) Overall steel members dimensions
are smaller than reinforced concrete members. (iv) If these slender
members are subjected to compression, there are greater chances of
buckling. (v) Sometimes steel , when used for columns , is not very
economical because considerable material has to be used merely to
stiffen the column against buckling.4-High Initial Cost/Less
Availability: (i) In few countries like Pakistan, steel is not
available in abundance.(ii)Its initial cost is very high as
compared with other structural members. (iii) These factors has
resulted in the decline of steel structure in
Pakistan.5-Aesthetics: (i) For certain buildings, the steel form is
architecturally preferred. (ii)For the residential and office
buildings, the steel structure without the use of false ceiling and
cladding are considered to have poor aesthetic appearance. (iii)A
considerable cost is to be spent on such structures to improve the
appearance.Cladding is the covering of metal , concrete plastic or
timber put on the surface of a structural member to completely
encase it.Specifications: The adequacy of a structural member is
determined by a set of design rules called specifications.
Types of Loads1-Dead Load: It retains the magnitude and point of
application throughout the life of the structure and is denoted by
D.It is estimated by multiplying the volume of the structure with
the standard density of the material. It constitutes a bigger part
of the total load on the structure.2-Live Load: It is the load of
the persons occupying the structure and their belongings. It is
denoted by L.Its magnitude and point of application changes with
time .The typical values for common occupancy types are given
below;
3-Self Load: It is the type of the dead load , which is the self
weight of the member to be designed.4-Imposed/Superimposed Load:
This is the term used for all external loads , leaving the selfload
, acting on the member to be designed . This includes live load ,
wind load , earthquake load etc. Part of dead load may also act as
imposed load.5:Service Load:The maximum intensity of load expected
during the life of the structure depending upon the certain
probability of occurrence is called as service load.6-Factored
Load:Service load increased by some factor of safety or overload
factor is known as factored load.Mechanism of Load Transfer1-The
gravity load passes from top to bottom through all the members of
the structure until it reaches the underneath soil.2- The load acts
at floor finish , goes to the underneath slab and transfers to
beams and walls.3- This is then accumulated in the columns , moves
to the foundations and dissipates in the soil.4- This terms members
and supports are defined relative to each other.5- For roof slabs ,
beams and walls are supports.6- For the beams, columns are acting
like supports and for the columns , foundations are acting as
supports.7- Similarly , the underneath soil acts as support for
foundations.8- The load path is only in one direction.9- The loads
of supports can not act on the structural members.