Load combinations for Foundation Design as per IS 456 : 2000

Question

IS 456 : 2000 stipulates load combinations for design of collapse and serviceability, but not exactly for foundation design. Is it appropriate to use serviceability load combinations for foundation stability check and collapse load cases for reinforcement design, if so please suggest clauses of code where it mentions such statement, else give your resolution as per code.

DSANJU · Answer

Please refer to Table 18 of IS 456:2000, page 68. partial safety factor for loads for collapse and serviceability are provided. you need to create load combinations with these factors.

DSANJU · Answer

Yes, for foundation design you must have serviceability and collapse/ strength load combinations.

Shuvam Sarkar · Answer

Agreed, but where it is mentioned in the code, would you please refer the clause?

Shuvam Sarkar · Answer

Dear Sanju, 
 Thanks for your prompt response. But try to understand the question first, I want to know where it is clearly mentioned about load combinations for foundation design only. Collapse and serviceability also have different meaning in code. So specific foundation design load combinations code requirements, I need.

DSANJU · Answer

It is the basic requirement that you fix the foundation size from serviceability load cases and design it (reinforcement ) using strength load combinations. As you are following Limit state of design - these are basic requirements. You may refer to any standard text book for this.

Kris Sathia · Answer

Don't know if the IS 456 code has guidelines specifically for foundations. 
 The following is what the ACI code 318-2014 says in section R13.2.6.1 for foundations: 
 The size of the base area of a footing on soil or the number and arrangement of piles are usually established on the basis of these permissible values and unfactored applied (service) loads, such as D, L, W, and E, in whatever combination that governs the design. 
 To proportion a footing or pile cap for strength, it is necessary to calculate the contact soil pressure or pile reaction due to the applied factored load. These calculated soil pressures or pile reactions are used to determine the required strength of the foundation for flexure, shear, and development of reinforcement, as in any other member of the structure. 
 As per the book "Reinforced Concrete Design Third edition by Pillai and Menon", section 14.2.1, soil pressure checks are done for service load combinations because the allowable soil pressure (bearing capacity) is usually provided by the geotechnical engineer for service load conditions. 
 In section 14.3.1, the authors say the following: The area of the footing is fixed on the basis of the allowable bearing pressure and the applied loads and moments under service load conditions with partial load factors applicable for the serviceability limit state. Once the base area of the footing is determined, the subsequent structural design of the footing is done for factored loads, using the partial load factors applicable for ultimate limit state.