Designing the foundation is by no means the most boring task a design engineer will undergo. Nor does it mean that it is the least prestigious. You see dude, the mightiest skyscraper needs to rest on a mightier foundation. Otherwise, its glory will be mythical, one that would remain on the design drawings and will never be constructed to reality.
Foundation design requires a lot of common sense. A lot of it. In fact, foundation design is all about making sense of the super substructure that will support the superstructure.
For a few guidelines, I’ve listed the essential ones below. If I missed anything or if you might find anything missing or you want to augment my discussions, please hit reply and key in your thoughts. I, including the technical community of structural engineers will greatly appreciate your thoughts about it.
- Make sure that all loads above the foundation are transferred correctly. Check base reactions. Check applied vertical point loads for gravity loads such as dead and live loads and see that they match the overall base reaction from the structural 3D model. Check for horizontal base reactions for wind and seismic loadings.
- Know thy geometry. This is an exciting part for me. If it’s just a pad foundation or a typical 3-pile or 4-pile-group pile cap, it can really get a little boring. But if the foundation system is supported directly on soil or a piled raft with a playful rise and fall of elevations and mind-bending underside crankings then it’s game on. This also implies the designer to be mindful of possible MEP trenches or pits. It is important to model the geometry as near to the actual situation as possible.
- Model the correct soil support. Consult the geotechnical reports for pile spring values and soil springs for rafts usually defined as the subgrade modulus. Take note that the subgrade modulus is defined as the allowable load (kN, kips) per square meter (or square feet) per millimeter (or inch) settlement. Usually, the unit of the subgrade modulus in SI is in kN/cu.m There were also instances when we modelled the pile, 75 mm into the cap soffit to account for the strut and tie effects.
- Apply hydrostatic uplifts where required (emanating from ground water table). Aside from gravity floor loads, the hydrostatic uplift can be critical for piles with high tensile loads and bearing pressure for rafts. More of that on the proceeding item. The knowledge on the underside of raft topography is critically at play here. Remember that the deeper the pits and depressions are, the greater the uplift loads are.
- There should be no soil tensile stresses because obviously, only compressive stresses (bearing pressure) are resisted by the soil. Only piles can carry tension loads due to lateral loads and hydrostatic uplift. If there are positive tensile stresses, it should be investigated. And if there really is tension to be resisted, it might be required to run a non-linear analysis to capture the effects of uplift or to alter the existing geometry of the foundation in order to eliminate the uplift.
- Never underestimate the power of manual load take downs. This is the most powerful tool for foundation sensibility checks especially for piled foundations.
- Same layout and loading, same answer. Be it column loads or pile reactions, it should be almost the same. The “fishy” one will stand out with this check.
- Check for settlements. Differential settlements should be kept below the allowable levels because it will cause additional stresses to the whole building. How? Imagine a portal frame and one column settles by a certain amount of vertical displacement. The beam connecting the columns will incur an additional bending moment which might be very significant. So the whole system is not really “at rest” because of the initial stresses brought about by excessive differential settlement. Another visual effect of which is cracking of brittle partitions such as glass façade, block works, and cracks on corners of doors and windows.
- Check the flotation. Your structure is not Noah’s ark! Large hydrostatic uplift can cause a building or structure to float. Don’t ignore uplift forces, it’s a must to consider this in the design process because it’s a basic serviceability requirement.
- Extra serviceability requirements. Aside from flotation, one should check that the structure is stable by checking the sliding and overturning.
- Avoid punching the raft. Other than one way shear and bending, one must ensure that columns or pile caps especially single-pile pile caps do not punch through the supporting raft. What we do is we do a handcalc of the actual stress from the column load to the critical perimeter and compare it to the allowable stress.
- Minimize the crack. In some parts of the world just like in the Middle East, it is but common to check crack widths emanating from service bending moments to mitigate the resulting crack and its probable effect on the corrosion of reinforcement. This aims to provide a compact and robust section with optimum reinforcement. We usually delimit our cracks to 0.20mm for raft/foundation undersides and 0.3mm on top (except where there is a water tank on top of the raft, and that we take 0.20mm as a limit, or it also depends on the nature of the fluid retained where a more stringent requirement of 0.10mm crack or lower is required).