Alcan Structural Engineering

Strap footings

Strap footings, as shown in figure 10.11, are used where the base for an exterior column must not project beyond the property line. A strap beam is constructed between the exterior footing and the adjacent interior footing - the purpose of the strap is to restrain the overturning force due to the eccentric load on the exterior footing.

The base areas of the footings are proportioned so that the bearing pressures are uniform and equal under both bases. Thus it is necessary that the resultant of the loads on the two footings should pass through the centroid of the areas of the two bases. The strap beam between the footings should not bear against the soil, hence the ground directly under the beam should be loosened and left uncompacted. As well as the loadings indicated in figure 10.11 EC2 recommends that, where the action of compaction machinery could affect the tie beam, the beam should be designed for a minimum downward load of 10 kN/m.

figure 10.11

Strap footing with shearing force and bending moments for the strap beam

Centroid of bases to coincide with -I resultant of N, _1 and N2

p., = net upward pressure at the ultimate limit state

Sending Moments

To achieve suitable sizes for the footings several trial designs may be necessary. With reference to figure 10.11 the principal steps in the design are as follows.

1. Choose a trial width D for the rectangular outer footing and assume weights W\ and W2 for the footings and U\ for the strap beam.

2. Take moments about the centre line of the inner column in order to determine the reaction R\ under the outer footing. The loadings should be those required for the serviceability limit state. Thus and solve for R\. The width B of the outer fooling is then given by b = r-i pD

where p is the safe bearing pressure. 3. F.quate the vertical loads and reactions to determine the reaction R2 under the inner footing. Thus

4. Check that the resultant of all the loads on the footings passes through the centroid of the areas of the two bases. If the resultant is too far away from the centroid then steps (1) to (4) must be repealed until there is adequate agreement.

5. Apply the loading associated wilh the ultimate limit slate. Accordingly, revise equations 10.5 and 10.6 to determine the new values for A'| and R2. Hence calculate the bearing pressure pu for this limit state. It may be assumed that the bearing pressures for this case are also equal and uniform, provided the ratios of dead load to imposed load are similar for both columns.

6. Design the inner footing as a square base with bending in both directions.

7. Design the outer footing as a base with bending in one direction and supported by the strap beam.

8. Design the strap beam. The maximum bending moment on the beam occurs at the point of zero shear as shown in figure 10.11. The shear on the beam is virtually constant, (he slight decrease being caused by the beam's self-weight. The stirrups should be placed at a constant spacing but they should extend into the footings over the supports so as to give a monolithic foundation. The main tension steel is required at the top of the beam but reinforcement should also be provided in ihe bottom of the beam so as to cater for any differential settlement or downward loads on the beam.

tf, + R2 - (Ni + N2 + W{ + W2 + W,) = 0 and solve for R2. The size S of the square inner footing is then given by

+1 -1

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