Design Example: Trench Fill Strip Footing.The internal
load-bearing wall for a four-storey ofce block is to be supported
on a strip foundation. Borehole investigations produced the
consistent soil proles shown in Fig. 11.13.
Soil analysis shows that the sand ll is an unreliable bearing
strata. The weathered sandstone has net allowable bearing pressures
of na = 400 kN/m2 for strip footings and na = 550 kN/m2 for pads,
both with a maximum of 20 mm settlement. The sandstone bedrock has
a net allowable pressure of na = 2000 kN/m2for pad foundations.By
inspection of the soil prole and analysis in Fig. 11.13, the strip
will be founded in the compact weathered sandstone. The relatively
even distribution of the loading will not lead to unacceptable
differential settlements and, as the sides of the excavations do
not collapse in the short-term, mass concrete trench ll footings
have been selected as the most appropriate foundation type.
Fig. 11.13 Borehole log for Design Examples 1, 2 and 4.
LoadingsThe loadings from the four-storey structure have been
calculated (as working loads) as follows.
Size of base (normal method)The foundation surcharge is
considered small enough to be neglected. The minimum foundation
width is given by
In many instances this approximate method is satisfactory.
Where the new foundation surcharge is large, or the allowable
bearing pressure is low, the following method should be used.
Size of base (allowing for foundation surcharge)Dead load from
new surcharge
Imposed load from new surcharge
The weight of the new foundation is taken as approximately equal
to the weight of soil displaced, and thus isexcluded from the above
loads.
The net bearing pressure is
In this case the existing surcharge sS = 0.
As may be seen, the normal method value of B = 0.71 m in this
example is sufficiently accurate for all practical purposes.Final
selection of foundation width must take into account the width of
the wall, together with an allowance for tolerance. It should also
try to suit standard widths of excavator buckets which are in
multiples of 150 mm, e.g. 450 mm, 600 mm, 750 mm, etc. In this case
a width of B = 750 mm would be appropriate, as shown in Fig.
11.14.Actual net bearing pressure (ignoring foundation
surcharge)
The actual net bearing pressure beneath the strip footing may
now be calculated, if required.
Fig. 11.14 Trench ll strip footing design example.
Design Example 3: Reinforced Strip Foundation.The load-bearing
wall of a single-storey building is to be supported on a wide
reinforced strip foundation.A site investigation has revealed
loose-to-medium granular soils from ground level to some
considerable depth. The soil is variable with a safe bearing
capacity ranging from 75125 kN/m2. Also some soft spots were
identied, where the bearing capacity could not be relied upon.
The building could be supported on ground beams and piles taken
down to a rm base, but in this case the solution chosen is to
design a wide reinforced strip foundation capable of spanning
across a soft area of nominal width.
To minimize differential settlements and allow for the soft
areas, the allowable bearing pressure will be limited to na = 50
kN/m2 throughout. Soft spots encountered during construction will
be removed and replaced with lean mix concrete; additionally, the
footing will be designed to span 2.5 m across anticipated
depressions. This value has been derived from the guidance for
local depressions given later on raft foundations. The ground oor
slab is designed to be suspended, although it will be cast using
the ground as permanent formwork.
Loadings
If the foundations and superstructure are being designed to
limit state principles, loads should be kept as separate unfactored
characteristic dead and imposed values (as above), both for
foundation bearing pressure design and for serviceability checks.
The loads should then be factored up for the design of individual
members at the ultimate limit state as usual.
For foundations under dead and imposed loads only, factoring up
loads for reinforcement design is best done by selecting an average
partial load factor, P, to cover both dead and imposed
superstructure loads from Fig. 11.22 (this is a copy of Fig. 11.20
Reinforced concrete strip design conditions.).
Fig. 11.22 Combined partial safety factor for dead + imposed
loads.
From Fig. 11.22, the combined partial safety factor for
superstructure loads is P = 1.46.
Weight of base and backll, f = average density depth = 20 0.9 =
18.0 kN/m2
This is all dead load, thus the combined partial load factor for
foundation loads, F = 1.4.
Sizing of foundation widthNew ground levels are similar to
existing ones, thus the (weight of the) new foundation imposes no
additional surcharge, and may be ignored.
The minimum foundation width is given by
Adopt a 1.2 m wide 350 mm deep reinforced strip foundation,
using grade 35 concrete (see Fig. 11.23).
Fig. 11.23 Reinforced strip foundation design example loads and
bearing pressures.
Reactive upwards design pressure for lateral reinforcement
design
Lateral bending and shear b = 1000 mm.
Thus vu < vc , therefore no shear reinforcement is
required.
Loading for spanning over depressionsWhere a local depression
occurs, the foundation is acting like a suspended slab. The
ultimate load causing bending and shear in the foundation is the
total load i.e. superstructure load + foundation load, which is
given by
Longitudinal bending and shear due to depressionsUltimate moment
due to foundation spanning assumed simply supported over a 2.5 m
local depression is
Width for reinforcement design is b = B = 1200 mm.
Thus vu < vc = 0.49 N/mm2, therefore no shear reinforcement
is needed.Depression at corner of buildingThe previous calculations
have assumed that the depression is located under a continuous
strip footing. Thedepression could also occur at the corner of a
building where two footings would meet at right angles. A similar
calculation should then be carried out, to provide top
reinforcement for both footings to cantilever at these corners.
Fig. 11.24 Reinforced strip footing design example
reinforcement.Strip Footings - Typical Examples.Strip footings are
commonly used for the foundations to load-bearing walls. They are
also used when the padfoundations for a number of columns in line
are so closely spaced that the distance between the pads is
approximately equal to the length of the side of the pads. (It is
usually more economic and faster to excavate and cast concrete in
one long strip, than as a series of closely spaced isolated
pads.)
They are also used on weak ground to increase the foundation
bearing area, and thus reduce the bearing pressure the weaker the
ground then the wider the strip. When it is necessary to stiffen
the strip to resist differential settlement, then tee or inverted
tee strip footings can be adopted. Typical examples are shown in
Fig. 1.6.
Fig. 1.6 Strip Footings - Typical Examples.
Reinforced Concrete Pads and Strips.IntroductionThese pads are
used in similar locations to those of the mass concrete pad, but
where the reduction in cost of mass concrete exceeds the cost of
the additional labour and materials.
These extras would include providing the reinforcement and any
extra shuttering, blinding, or working space which may prove
necessary for the reinforced solution.
The plan size and shape is determined from the vertical load and
allowable bearing stress in conjunction with any physical
requirements. The depth and amount of reinforcement is determined
from the resulting bending moments and shear force considerations
(see Fig. 11.20) or from past experience. The experience basis is
often used where reinforcement needs are related to variable ground
for a familiar location and use or where there is a need to cater
for a number of time-related variations in differential
settlement.
1 Design decisions and Sizing up of the design Design decisions
The decision to reinforce a concrete foundation of this type
usually follows the realization that the ground conditions are
variable and/or deep trench ll is...
2 Design Example 3: Reinforced strip foundation The load-bearing
wall of a single-storey building is to be supported on a wide
reinforced strip foundation. A site investigation has revealed
loose-to-medium granular soils...
3 Design Example 4: Reinforced pad base The axially loaded pad
base in Design Example 2 is to be redesigned as a reinforced base,
founded in the weathered sandstone. Assuming settlements have been
judged to be...
Fig. 11.20 Reinforced concrete strip design conditions.
Rectangular and tee-beam Continuous
Strips.IntroductionRectangular beam strips are briey discussed
previously and the inverted T-beam strip in section 9.3.7 where it
is mentioned that the main difference in the two beam foundations
relates to the relationship between the width of beam required to
resist bending moments and shear forces and that required to
achieve the allowable bearing pressures.
If the two widths are similar then the rectangular beam tends to
be economic. However, on relatively poor-quality sub-strata the
beam width required to achieve the allowable bearing pressures
often far exceeds that required for bending and shear resistance.
In the latter case it tends to prove economic to reduce the beam
width and spread the load through a ange slab on the soft of the
beam.
1 Design decisions The economic design of continuous beam strips
can be greatly affected by the choice of curtailment of the lengths
of beams. They are generally used where longitudinal...
2 Sizing of the design The sizing of the rectangular beam is
similar to the sizing of the up stand beam of the inverted T, i.e.
based mainly upon bending moments and shear forces. However, the
beam width... Design Decisions: Continuous Beam Strips.The economic
design of continuous beam strips can be greatly affected by the
choice of curtailment of thelengths of beams.They are generally
used where longitudinal bending moments are a major problem for the
foundation design, i.e. in variable ground, soft sub-strata, or
where loading is variable in the length of the beam. They are also
used in some areas of mining activity etc., where bending from
differential subsidence movement is critical but where tensile and
compressive ground strains in the foundation can be controlled.The
decision to use a continuous beam strip usually follows the need
to(1) Reduce differential settlements below framework columns.(2)
Combine foundations which would otherwise tend to overlap.(3) Ease
construction by the use of continuous strips rather than separate
pads when they are becoming closely spaced.The decision to use an
inverted T rather than a simple rectangular beam would result from
bearing pressure criteria demanding excessive beam widths for
bearing when compared to widths required to resist bending and
shear.
Trench Fill StripsA brief description of trench ll strips is
given previously. The design of such strips is relatively simple,
and it is true to say that there is more design involved in making
the decision to adopt such a foundation than in analyzing and
sizing the appropriate trench ll.Trench ll is often used in an
attempt to:
(1) Reduce the foundation width where brickwork below ground
would need a wider footing to suit working space,
(2) Reduce the labour content of construction, and
(3) Speed up the construction of the footing, for example, in
conditions where trench supports are not necessary for short
periods but would be required if the trench were left open for a
signicant time.
The saving in excavation, labour, time and/or temporary works
can in some situations be quite considerable. However, in loose
ground the quantity of concrete used can become both difcult to
predict and/or considerable in quantity particularly if trenches
meet or cross at right angles.
Strips excavated through poor ground to reach suitable bearing
strata can prove troublesome due to instability of the trench
sides, particularly at changes in direction of the strip (see Fig.
11.1). This can be overcome by using suitable trench supports.
However, the problem can often be more economically assisted by
good design.
Fig. 11.1 Trench instability at change in direction.
For example, Fig. 11.2 shows two alternative designs for the
same house foundations: in (A) the trenches would fail under much
less critical conditions than the trenches in (B) since this scheme
avoids trench direction changes and hence avoids the corner failure
conditions of the trench sides.
Fig. 11.2 Trench ll alternatives.
A disadvantage in some situations is the tendency of the trench
strips to pick up, via passive resistance, any longitudinal or
lateral ground strains which may occur in the strata around the
foundation. This can prove to be a major problem in active mining
areas and in sub-strata sensitive to moisture changes such as
shrinkable clays. In some situations this problem can be overcome
by the insertion of a compressible batt against the trench faces
(see Fig. 11.3), but this must be considered for all directions and
for conicting requirements since passive resistance is often
exploited in the superstructure and foundation design.
Fig. 11.3 Trench ll with compressible side formers.
In addition the high level of the concrete can create problems
for drainage and services entering the building if these are not
pre-planned and catered for. The top surface should be low enough
so as not to interfere with landscaping and planting. In some
situations concrete trench ll can create undesirable hard spots,
and stone trench ll should be considered.
Stone trench ll used under the strip loads to transfer the loads
to the lower sub-strata is more yielding than concrete trench ll
which may produce excessive differential movement between the main
strip load area and the general slab (see Fig. 11.4).
Fig. 11.4 Stone versus concrete trench ll.
In soft wet conditions, the soft materials at the surface of the
trench bottom can be absorbed into the voids of rst layer of no nes
stones blinded by a second layer of well graded stone. The second
layer prevents the soft materials from oozing up through the
hardcore. This can prove to be a clear advantage for difcult sites
where the material is sensitive and wet and where good clean trench
bottoms are difcult or impractical to achieve. By this method a
stable trench ll can quickly and easily be achieved in relatively
poor ground (see Fig. 11.5).
Fig. 11.5 Trench ll in poor ground.
Compaction difficulties can be experienced in narrow trenches
cut in dry or relatively stiff sub-strata where compaction of the
ll at the edges is partly restricted by the frictional resistance
of the trench sides. This tends to show itself in the concave
surface of the compacted layer (see Fig. 11.6). However, this can
be overcome by using suitably graded stone in relatively thin
layers and by extra compaction at the edges of the trench.
Fig. 11.6 Concave compacted surface.
Selection of suitably graded and shaped stone is particularly
important, for example, single sized rounded stone will tend to
compact automatically during lling in a similar way to say lling a
trench with marbles. The marbles immediately fall into contact on
more or less the maximum compaction due to the standard radius
involved. However, in some locations it is important to avoid
forming a eld drain within the ll which may attract moving water;
therefore well graded material is essential in these
situations.