Free standing cantilever wall resisting wind (LRFD)

[precast78]

Ladies and Gents,

I am designing a cantilevered wall with exposed footing (toe and heel doesnt have soil on top. Wall just sits on the ground). I am trying to design it with LRFD and ASCE 7-10. I understand that ASCE 7-10 wind load has been factored already (higher speed). So when I calculate the overturning moment from the wind, I use 1.0 load factor (eq 16-6 IBC) and used 0.9 for my resisting moment on the concrete DL. My questions are:

1. For overturning, do I just have to make sure the resisting moment is larger? It is LRFD design method so I just have to make sure resisting is equal or larger than the overturning correct? I do not need to use 1.5 factor of safety?

2. For checking sliding. Do I just use the pressure I have (1.0 load factor since ASCE 7-10 is already factored) and make sure the normal force (0.9 load factor) multiplied by the friction coef between soil and concrete is larger than the wind force? Is there some kind of reduction for the resisting friction force?

 

[TehMightyEngineer]

Off-topic but watch out for seismic, cantilever walls alone use an R = 1.5 if I recall correctly and even low seismic loads can control.

Maine EIT, Civil/Structural.

 

[precast78]

The only thing I can find regarding overturning moment and LRFD design method is from AASHTO LRFD but this isnt a highway job. It limits the eccentricity to be in middle 2/3 of the footing center of gravity. Is it the same for IBC?

 

[LHiggins]

There is a safety factor of 1.5 required for retaining walls in section 1807.2.3 of the IBC. This is not required by your wall since you are not retaining earth. In general, you wall has a 1.666 factor built into the wind and you have to take a .9 dead load factor to determine overturning. It's best if you try and keep the center of gravity in the middle 1/3 of the footing but that's sometimes difficult to do, ie: zero lot line footings. Being in a high seismic area, I agree with the post on checking seismic (actually should be checking all load cases).

 

[Hokie93]

You do not need to apply a 1.5 factor of safety on top of the LRFD load combinations. This is true for both overturning and sliding. There is an inherent 1.67 factor of safety built into the 0.9D + 1.0W load combination, where W is based on ultimate wind speeds as in ASCE 7-10.