Sliding Resistance

[dherna45]

I am designing a grade beam foundation, and wanted to check its sliding resistance. I know that I have to considered friction and lateral earth pressure, but I am not sure to what surfaces of the grade beam I need to apply this lateral earth pressure to. Thanks

 

[MotorCity]

Typically grade beams are not checked for sliding because they are assumed to span between supports (usually piles or other grade beams). In other words, they are assumed to not make contact with the soil since they are spanning as beams. As for lateral earth pressure, usually grade beams are surrounded on soil by all sides, so lateral earth pressure cancels out on both sides of the grade beam. If one side of your grade beam is exposed, then simply apply lateral soil load to the vertical face of the beam and design accordingly.

 

[dherna45]

Basically This is what I am designing a soft story retrofit on a open wall line using a moment frame. I analyze the moment frame just due to seismic forces. So the sliding force is the base shear.
If I ignore lateral pressure, then the sliding resistance will just be based on weight of foundation and friction which is small compared to base shear. I dont know what to do.

 

[MotorCity]

What is the purpose of the grade beam? It appears as though there is no vertical load on it other than self weight. Any friction resistance to sliding will occur at the underside of the pad footings. You can also consider the passive resistance of the soil based on the vertical surface of the pad footing and the vertical face of the end of the grade beam. If you have a concrete slab, I would be tempted to engage it in diaphragm action for lateral resistance if possible. Not sure how wide your grade beam is, but you may have created a large discontinuity by running the column through it. Side note....I would use at least f'c = 3 ksi for the footing.

 

[dherna45]

Well, the purpose of the grade beam is to make the steel columns fixed and tie them together. They receive the moment at the end of the column and create more weight to prevent overturning. I dont know if grade beam is the correct term used, but thats what we call them.