F.S. for Overturning and sliding 2

[casasf]

Can someone refer to the code section for using the factor of safety against overturning and sliding for a footing carrying a simpson wood strong wall? I found the 1.5 factor for retaining walls but could not locate for shallow foundation. I can use 1.5 for both. 40 years ago there used to be a factor of 1.25 unless my memory is deceiving me! Your recommendation is appreciated.

 

[r13]

IBC does not address safety factors for shallow foundations, however I would follow the rules for retaining walls. One exception is overturning due to earthquake, which IBC (1808.3.1) directs to ASCE7 12.13.4 for proportioning the footing. I've no recollection on 1.25 factor.

 

[JoshPlumSE]

ASCE has the basic load combinations along the lines of:

0.6DL +/- 0.7EL
0.6DL +/- 0.6WL

Where:
DL = Dead Load
EL = Earthquake Load (strength level)
WL = Wind Load (Strength level)

These equations could be re-written in terms of SERVICE level wind and seismic. Where 0.7EL = service level seismic and 0.6WL = service level wind.

0.6DL +/- (EL or WL)_service

Compare this to the older code which had a 1.5 safety factor with the following load combination:
0.9DL +/- (EL or WL)_service

I'd write this as:
0.9*Stabilizing Moment Due to Dead Load > 1.5 * over turning moment due to (EL or WL)_service
Dividing both sides by 1.5 brings:
0.6*Stabilizing Moment Due to Dead Load > 1.0 * overturning moment due to (EL or WL)_Service
Which is what ASCE's new equations give.


Ergo, the implied safety factor in the code is 1.5. That was true in the 1997 UBC and it is true now. Even if the code doesn't explicitly state it.

 

[canwesteng]

You might want to go with 1.5 if the overturning is due to live (such as kick from a portal frame, or impact), since the ASD load combinations don't really provide a built in FOS for overturning caused by live/dead loads, hence the 1.5 FOS for retaining walls. Otherwise it is built in per Josh's answer.

 

[casasf]

Thank you for your input. I will use the criteria for the retaining walls. Great responses from all.