Analysis and design of large mat foundations supported on soil using FEM considering soil -structure/foundation interaction (loading and design governed by ASCE/ACI and / or IS codes)

Indu

I agree with what Seth posted. One should analyze the foundation with strength level loads to find out the true strength level demand. Talking in terms of STAAD.Pro, one needs to create REPEAT LOADs to combine the gravity and lateral loads with appropriate load factors and then analyze the foundation to find out the strength level moments/shears. Multiplying the results from service load cases by strength level load factors should be avoided because of the nonlinear nature of the problem as you rightly pointed out in your post.

Indu

I agree with what Seth posted. One should analyze the foundation with strength level loads to find out the true strength level demand. Talking in terms of STAAD.Pro, one needs to create REPEAT LOADs to combine the gravity and lateral loads with appropriate load factors and then analyze the foundation to find out the strength level moments/shears. Multiplying the results from service load cases by strength level load factors should be avoided because of the nonlinear nature of the problem as you rightly pointed out in your post.

Saikat

Great to hear from you!!
Responding to both of you (Seth and you as well).

1. Of course, we do use reference loads / repeat load commands to analyse such nonlinear problems. That is the correct way to handle such iterative non-linear stuff which requires rebuilding the stiffness matrix at each step involving the active springs in compression.
2. As an established practice / principle, foundation sizing (max. bearing pressure, stability ratios and limiting loss of contact) is finalized based on the service level load combinations. The redistribution of bearing pressure due to loss of contact is carried out (by specifying compression only spring) for each service level load combination at each step which yields the final contact area and the maximum bearing pressure for each case.
3. With my limited knowledge / experience, a practising engineer re-analyzes a large mat foundation with strength level load combinations only to obtain the strength level demands (internal forces and moments) to design the concrete thickness and rebars. He / She never looks back into the foundation sizing aspect (max. bearing pressure, stability ratios and limiting loss of contact) again.
4. This is a case of large mat foundation (not a rigid mat) having considerable eccentric loading . So, the bearing pressure distribution and the internal design forces are a function of the deflection pattern which are, in turn, the function of applied loading and spring stiffnesses as well.
5. Consider the case of strength design of beam / columns supporting suspended floors. The strength level forces are obtained by linear multiplication of the primary load cases with strength level load factors and following algebraic sum. This large mat foundation problem is distinct from the strength design of a suspended floor supporting beam / column due to the reason mentioned in point no. 4 above.
6. Moreover, soil is considered to be a naturally existing heterogeneous medium which is not man made. Therefore, as a general practice, strength level design is not applied to soil as it is done for reinforced concrete / structural steel. A large factor of safety of the order of 2.5 ~ 3.0 is applied on the ultimate bearing capacity of soil to limit the net safe bearing capacity used in design.
7. However, as the bearing pressure distribution for each of the service level load combinations is finalized as mentioned in point no. 3, the corresponding strength level demands (internal design forces and moments) should be established with the same bearing pressure distribution with corresponding load factors at strength level.
8. The major problem arises owing to the fact that these days different load factors are assigned to different primary loads in the ACI / IS codes (for example, 0.6, 0.9 or 1.2 with Dead Load, 1.6 with Live / Wind loads, 1.0 / 1.4 with seismic loads etc.). Since the load factors are different, it is quite difficult to determine a single common / average load factor as a multiplier. Earlier, in the Indian codes (IS), there used to be common load factors like 1.5 or 1.2 and it was quite straightforward to handle the strength level demands.
9. Another point of caution in case of such large mat foundation with significant loss of contact - it is not prudent to conclude that by designing the foundation (for thickness and reinforcement) for the internal design forces / moments obtained due to the bearing pressure distribution for larger loading (strength level loads) would yield conservative results consistently when compared to the one which is designed for internal forces obtained from a base pressure distribution for service level loads with appropriate load factors.

Kindly share your views on this.
If you guys / someone in this forum has some reference wherein the foundation design including sizing / stability and strength level design is carried out with an envelope of service level and strength level combinations, kindly share.