Single Story Vertical Distribution of Seismic Forces (ASCE7-05)

[P1ENG]

I have a single story Manufactured container with dimensions of ~60x15. There are no interior shear walls. Seismic controls the design. The container needs fire rating so there is plenty of heavy gypsum on the roof and walls (shear wall is gypsum on cold-formed framing). However, the floor has the majority of the seismic weight because 25% of the floor 500 psf live load is assumed as storage (12.7.2(1)), and an assumed 15% of total container dead load for permanent equipment load.

Questions:

1.) Do I need to distributed the loads vertically per 12.8.3 since this is a single story building on grade? I normally do (perhaps conservatively) by assuming (3) levels: midpoint of floor, midpoint of walls, and midpoint of roof. I have reviewed other engineers' work of single story buildings on grade where the distribution was accomplished by multiplying the weight of each "level" by Cs. Per the P.E. in our office, the distribution of section 12.8.3 is supposed to be done even for single story buildings.

2.) If the building were put on concrete piers, would this change the answer above?

The large mass of the floor is making my shear wall forces unmanageable if vertical distribution is done per section 12.8.3.

Juston Fluckey, E.I.
Engineering Consultant

 

[msquared48]

Normally, the floor of a single story structure does not come into play in the seismic force distribution, only the roof structure weight, and weight of the walls below the roof level.

As to if the placement of the structure on piers would affect this, if the piers were short, 2 feet or less in height, I would ignore the force from the floor unless it was extremely heavy in relation to the roof and wall loads, but any higher, I would include the floor force too, however, I would not redistribute to the roof. I think that would be an unnecessary overkill.

What are your relative weights?

Mike McCann
MMC Engineering

 

[P1ENG]

The seismic weights are:

Roof: 14.93 kip @ 8.97'
Sidewalls: 21.58 kip @ 4.73'
Endwalls: 5.02 kip @4.73'
Floor: 138.32 kip @ 0.25'

So, this is only for vertical distribution? You would still use the floor mass in the base shear calculation? I would set the weight of the floor to zero to determine the Cvx values at each level. Then multiple the base shear (minus floor weight). So my base shear would be Cs*(Wroof + Wwalls + Wfloor) and my vertical distribution would be Cvx*Cs*(Wroof + Wwalls)?

Juston Fluckey, E.I.
Engineering Consultant

 

[JAE]

I have never heard of taking slab-on-grade loads and throwing them up to the roof for seismic design. That is crazy.

The wall weights - yes - they get included in the seismic mass. But not the slab-on-grade nor the materials resting on that slab.

The mass of the floor level perhaps does get involved in the base shear of the entire system at the ground level (i.e. sliding and footing designs for lateral forces) the load path does not go from the slab up the walls to the roof in my view.

 

[P1ENG]

JAE, I agree. It never made sense to me. If that were the case, why not include the weight of the slab too? I talked to my P.E. today and he said it was o.k. to leave the floor mass out so I must have misunderstood him.

Removing the floor mass in the vertical distribution has made my shear wall forces manageable. I have also kept the floor mass in the base shear for determining anchor reactions.

I will leave this thread unsolved for a few days so someone else can chime in if they want.

Juston Fluckey, E.I.
Engineering Consultant