Alan Williams Seismic Design Review 7th Edition Problem 1.48

[bassplayer45]

I was able to determine the design base shear in problem 1.47, not a problem. In 1.48 they want to know the load in the diaphragm. Based on ASCE 7, this should be done by first determining the load at the first floor using the distribution of seismic forces, Fx = Cvx * V (12.8-11), then we would apply Fpx = (Fi / wi ) wpx (12.10-1)

The solution in the book has it determining a tributary load based off of the floor weight, and 2, 12 foot wall sections.

First, Why arent they considering the total seismic wall weight for that floor (using 4 walls and distributing the base shear respectively to the floors)? We used the total structure weight for base shear determination. Second, are they assuming 6 foot above and 6 foot below are supported by floor 1? How are they arriving at the weight tributary to floor 1? it appears they are assuming the total base shear is applied to floor 1.

Appreciate the help.

 

[kevo_55]

I don't happen to have the problem in front of me but the reason why only two walls are counted in diaphragm forces and not all four is because the end walls are at the supports of the floor itself.

Remember, you are basically looking at a simply supported beam so only the weight acting on the span is what is doing the work.

I hope this helps.

 

[McEngr]

Perhaps to expound on kevo's comments, think of the walls parallel with the direction of force as having its own inertia and thereby not inducing (or needing) help from the diaphragm. It's essentially a stiffness problem, not a tributary problem.

 

[bassplayer45]

Yeah, i have been going over the problem today and i remember encountering some problems similar where the shear in the direction of the seismic force stays in the walls parallel to the force, thus only walls normal to the load need a diaphragm to resist. I always get mixed up when we use the total weight to get base shear, since normally these types of problems just have you use the walls normal to the direction of the seismic load to get the base shear in that direction (this is correct, no?). Since the problem before had you calc the total weight using all 4 walls, i immediately think you need to use the total weight to get the diaphragm load. If it would have said calc the base shear in the east direction, then i think i would have been able to zero in on it more.

 

[bassplayer45]

To expand on this further. Essentially, to get the seismic base shear in the north south direction, we only include the weight of the walls on the north and south side of the building because the east and west walls act as supports and dont contribute to the seismic weight in that direction? I ask because I have seen some problems that use all the wall weight, but I , remember being taught to use only the walls that contribute.

 

[bassplayer45]

To expand on this further. Essentially, to get the seismic base shear in the north south direction, we only include the weight of the walls on the north and south side of the building because the east and west walls act as supports and dont contribute to the seismic weight in that direction? I ask because I have seen some problems that use all the wall weight, but I , remember being taught to use only the walls that contribute.