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.