ASCE-5 combinations used at soil-structure interface

[nour75]

I have a discussion with my coworker about footing design subjected to vertical forces due to D, L, and E loads. The soil report permits a 1/3 increase in allowable soil stress for transient load conditions.

I say the following points,
1. To get the max expected soil compression stress and the required footing horizontal dimensions, it is better to use the ASD combination in ASCE-05, (D+0.75x0.7E +0.75L), where E=Eh+Ev (E is reduced by 25% according to ASCE-05 item 12.13.4).

2. To get the max expected soil tension stress, it is better to use the ASD combination in ASCE-05, (0.6D+0.7E), where E=Eh (E is reduced by 25% according to ASCE-05 item 12.13.4 and Ev=0 according to ASCE-05 item 12.4.2.2 exception 2).

3. To design the footing (depth & reinforcement) it is better to use strength design combination in ASCE-05, (1.2D+0.5L+E), where E=Eh+Ev (No reduction for E by 25% according to ASCE-05 item 12.13.4).

4. To check the soil bearing capacity we should follow the soil report in permitting a 1/3 increase in allowable soil stress for transient load conditions

My coworker says,
• In point No.1 we should not use E=Eh+Ev but we should use E=Eh because ASCE-05 item 12.4.2.2 exception 2 tells in determining demands on the soil-structure interface of foundations Ev should be zero.

• In point No.4 no increase in allowable soil stress for transient load conditions according to IBC-2006 item 1605.3.1.1.



Any one can give professional comments about my three assumptions as well as my friend point of view?

 

[Lion06]

I agree with your co-worker's second bullet item, unless you use the alternate load combinations which ends up being pretty close to a wash.

As for your assumptions, I suppose they seem reasonable, but I would say that you need to check every load combination, not just the one you think will govern.

 

[kslee1000]

1. No comment on point 1.
2. For point 4, suggest to check it with your geotechical engineer to see if his recommedation is in accompliance with the governing code. He might have took that (reduction) into consideration already.