Overstrength Factor for Foundation Design

[bridgemaster]

Hello!

Is the seismic overstrength factor applied in the ASD/LRFD load combinations when checking for bearing/lateral/axial resistances? I have searched other threads and codes and found good information on how the overstrength factor applies to a foundation's strength design, but I am struggling to find guidance on if overstrength factors are to be applied to geotechnical designs of foundations.

I am designing a pipe rack with options for a drilled pier or shallow footings. Any help would be appreciated!

 

[Okiryu]

Hi, are you referring to the 1/3 increase of allowable bearing pressures for short-term loading that is typically recommended in geotechnical reports? I think that the above language is stated in Section 1806.1 in the IBC 2012. The allowance for this increment is not always granted, and it will depend on how soils behave under cyclic loading.

But also, see Section 12.13.4 of ASCE 7-10 which allows for reduction in overturning at soil interface for the earthquake loads in the different load combinations.

 

[bridgemaster]

Okiryu,

I am referring to the seismic overstrength factor, Ωo, which is a coefficient for the seismic load effect within the different load combinations (ASCE 7-10 Section 12.4.3). You may choose to include or not include Ωo in the load combinations, depending on the structure you are designing and its intended purpose. I want to know if there is a requirement to include this factor when running the different geotechnical design checks.

I am familiar with the 1/3 increase in allowable bearing as sometimes recommended in geotechnical reports, but I do not think that is related to the overstrength factor.

 

[Okiryu]

Hi bridgemaster, for axial checks, I think that since in most of the times, the geotechnical capacity of the foundations are determined based on settlement considerations (serviceability considerations), unfactored loads (DL+LL) are used.

For lateral analysis of pile foundations, I have been provided to the structural engineers different pile sizes with their respective ultimate lateral loads in order to have maximum deflections of say 25 mm (I use this maximum deflection based on my interpretation of Section 1810.3.3.2 of the IBC 2012). For shallow foundations, I provide passive earth pressures and ultimate friction coefficients to the structural engineer. However, I am not sure how structural engineers use our geotechnical parameters. Perhaps you can expand more on this.

Other geotechnical engineers in this forum may provide more input on this...would also like to see opinions from the structural engineers...

 

[ATSE]

In the past, OSHPD (California hospital plan checkers) has required omega thru the entire seismic frame load path to use omega, down below the anchor bolts and into the foundation element.
The IBC and ASCE 7 are ambiguous on using omega for the foundation, but very few engineers actually use omega below the base plate / column dowels.
The discussion has been previously addressed on previous Eng-Tips posts.

 

[bridgemaster]

I agree that the way ASCE 7 and IBC directs the usage of the Ωo is fairly ambiguous.

No one at my office has any clue, and all the Eng-Tip threads that I have found concerning Ωo with foundations do not address my specific question. The best idea I have is as follows:

Ωo is applied to certain elements because the engineer wants to control which portion of the seismic resisting system yields first and which elements don't. I am pretty sure that in my case, I do not want my foundation or soil to yield, so I should apply Ωo to the foundation elements in a manner that ensure they do not yield. This basically means I will use Ωo in my loading not only when designing the strength of the foundations and its attachment to the supported structure, but also when determining the bearing stress, lateral capacity, etc. of the foundation.

 

[Okiryu]

Interesting. bridgemaster, a question: If you are checking the geotechnical parameters by yourself and if you are using ASD method and considering that the geotechnical parameters are already affected by a FOS in the capacities, are you still considering to use Ωo in your loading? On the other hand, if you have a geotechnical engineer in your team, so he/she will provide soil parameters/capacities, do you compare these parameters/capacities with your factored load cases? Just wondering how structural use the information provided by geotechnical. Thanks !!