RC Raft Foundation (BS8110) - Calculation guidance

Introduction

Tedds a RC Raft Foundation (BS8110:Part1:1997) calculation. This article looks in depth at this calculation in order to provide greater understanding and guidance on its use. The notes included with the calculation provide detailed accounts of how bearing pressures and bending moments are calculated. These are not reproduced here, instead an overview is presented with particular attention being paid to areas of the calculation where questions have been asked recently by Tedds users.

Theory

The approach adopted by the calculation is best described as that for a 'flexible' raft. It is most appropriate for relatively low-rise structures of regular shape and loading on relatively poor ground. The suitability for each particular design case should be verified by the Engineer, further guidance on applicability is given below.

The calculation breaks the raft into four discreet locations, and with the exception of adding certain areas of reinforcement obtained from one area to those obtained from another, considers each area in isolation. The four areas are:

• Slab internally
• Edge beam thickening/slab edge away from a corner
• Edge beam thickening/slab edge at a corner
• Internal beam thickening (if applicable)

In order to explain how bearing pressures are calculated it is helpful to clarify how it does not calculate the bearing pressures. The calculation does NOT add up all loads on the raft, work out their centroid, divide the total load by the area of the raft and adjust the resulting bearing pressure for the resultant moments. The bearing pressure at each of the four locations is determined in isolation so for example, an increase in the longitudinal line load to the perimeter of the raft will not register an increase in the bearing pressure for the case of the slab internally. The actual mechanics of calculating the bearing pressures at each of the four locations are explained in detail in the calculation notes and are therefore not reproduced here.

Because the raft is not considered holistically, the Engineer must ensure that the results obtained are valid for the raft being designed. This is best illustrated by way of examples.

Example 1

Consider a raft of 10m x 3m with a high line load to the full perimeter. At the edge away from a corner the calculation will determine the width of raft required, i.e. perpendicular to the edge of the raft, to distribute the load without exceeding the allowable bearing pressure (it will also check that there is sufficient slab top steel to enable this width to be mobilised). Now consider if the width required is calculated as 1.75m, this applies to both sides of the raft therefore the total width required is 3.5m. The minimum overall width available is only 3.0m and therefore the design should be rejected by the Engineer or the raft modified, for example by increasing the projection of the raft outwards from the edge loads.

Example 2

Consider the same raft of 10m x 3m with a high line load but now applied to the two short sides and only one of the long sides. The calculated width along the sides to satisfy the bearing pressure requirement would remain at 1.75m. An eccentricity will exist between the applied edge load and centroid of the ground reaction resulting in an overturning moment. The calculation will check the adequacy of the slab top steel for this moment but it is not able to check the overall stability of the raft. Because there is no longitudinal line load to the opposite edge there is no force to counteract the generated overturning moment and again the design should be rejected by the Engineer.

It is not expected that these examples would arise for a relatively low-rise regularly shaped and loaded structure.

For each location the calculation assesses the raft's ability to distribute the applied loads without exceeding the allowable bearing pressure, in addition to its ability to span over a theoretical circular depression, which is assumed to occur in the ground beneath the raft. For the bearing pressure check, it is assumed that the ground below the raft is intact. The diameter of the depression is assumed to include an allowance for local overstressing of the soil around the perimeter of the depression.

Raft Loading Input

For any given raft the Engineer should assess which is the most highly loaded area of internal slab, edge beam/slab edge away from a corner, edge beam/slab edge at a corner and if applicable internal beam thickening, bearing in mind the diameter of the depression used in the calculation. For EACH of the four locations ALL loads (excluding raft self weight) applied to the raft over the theoretical circular depression MUST be input. This may mean that dead and live udl's have to be input more than once, i.e. at each location. Similarly longitudinal line loads to the raft perimeter have to be input at both the edge location and corner location. The calculation is written in this way to enable different loads to be input at different locations. Future enhancements to the calculation may be considered so that the udl loads only have to be input once.

It is strongly recommended that the sketches are accessed when entering the load information, to view the sketches click on the sketch icon on the left hand edge of the interface page. There is a separate sketch for each location which describes the various loading types and dimensions required.

Loads which fall outside the depression should not be entered. For example, if the diameter is 2.0m and there are point loads applied to the raft edge at 3.0m centres then only one point load should be entered. Conversely, if the point loads are at 1.0m centres then two should be entered as two can occur over a depression. For both the bearing pressure and depression span check the calculation will assume that both point loads are coincidental. Therefore at the Engineer's discretion, the value of each could possibly be reduced to give a more realistic design, or alternatively the loads could be approximated to line loads. If not the calculation will be conservative.

Conclusion

The design of a raft foundation is inherently approximate. Even with sophisticated finite element software, the ground/structure interaction can never be modelled 100% accurately as the ground stiffness will vary across the site and future settlement patterns cannot be accurately predicted. Engineers should appreciate that this Tedds calculation provides an approximate design and that the results obtained should be interrogated to ensure they are valid for the raft being considered. Notwithstanding this, for a relatively low-rise regularly shaped and loaded raft, the calculation will provide a safe and economic design.

Extract taken from Tedds Newsletter - Issue 4 2006