0.6 D load cases with friction

[campbelltc]

On a PEMB with some fairly light lateral loads, I am trying to resist wind loads partially with friction on the bottom of a spread concrete footing. With the 0.6D load with wind uplift, the vertical component of the load is essentially zero, so if you use the 0.6 factor on the dead load prior to calculating the frictional force, there would be no friction on the bottom of the footing.

However, I know that there actually IS a downward force on the vertical component that would allow for friction to be used as a part of the lateral resistance. It is my understanding that the 0.6 factor is actually 0.9 divided by a 1.5 factor of safety. So can you argue that the 0.6 factor should be taken times the frictional force of an unfactored vertical force and use the friction that is known to be there? It would be similar to using the older way of comparing lateral loads with a 1.5 factor of safety for sliding.

 

[ishvaaag]

Just by comparison with Spain's current code CTE you must be right, see attachment. When favourable, factors on own weight stand at 0.8 for strength and 0.9 for stability. So even if coeffs. for variable actions stay at 1.5 in the check it is as comparing 1 to 0.6, service level.

CTE should be a close approach to that in Eurocodes (since some years already in convergence). I have not checked this.

 

[JoshPlumSE]

I would think you could use a 0.9 DL case.... as long as the sliding resistance for that case is 1.5 times what is required.

Doesn't the IBC still have the 0.9 Load Combinations as an "alternate" set of load combinations. ASCE-7 may not have them, but I thought they were still in the IBC.