Seismic Base Shear 1

[VTHokie91]

I am working on my first precast big box. As big boxes go, it's not that large, at 120' by 160'. I am getting significant pushback from the wall supplier, in that the base shear quantity that I have provided is "an order of magnitude too high".

I have attempted to follow ASCE 07 in determining seismic characteristics, and determined the base shear by eqn 12.8-1 where

V = Cs * W​

I believe that I have Cs under control. W is the area where we're having the intense discussion.

Section 12.7.2 defines "W" to include the dead load, with some additions for partitions, snow, equipment, etc. Section 3.1 defines "dead load" to include walls, with no exceptions. As such, I have included full dead load of all four walls.

However, this decision is being questioned, and I am told that industry standards range from a lower end of using only the upper half of each orthogonal wall, to using the full height of each orthogonal wall, to using the upper half of all four walls. I am told that using the full dead weight of all four walls exceeds typical standards of care.

But I do not see anything in ASCE 07 permitting me to use a subset of the wall weight.

I appreciate your comments.

 

[TLHS]

First off, none of the things they're talking about would end up with results being an order of magnitude too high.

Anyway, for base shear you should definitely be using everything in my mind. It'll all end up at the bottom somehow.

For the design of your shear walls or other lateral force system, you don't necessarily have to use your full base shear. I'm not horribly familiar with ASCE 07, but your code might have rules for load distribution across different floors. If it doesn't, consider how your loads actually get to the bottom. What they're talking about sounds a lot like how you might decide to design a diaphragm or your bracing system. Part of your wall weight goes up into the diaphragm and gets carried down by the lateral force system, part of your wall weight goes down into the floor/foundation. That's still all part of the base shear though.

It sounds like there may be some confusion about the overall structure base shear and its relation to the required shear wall strength.

All your weight has an associated seismic force. Sit down and sketch out what load path it's all likely to take and you'll probably be in a better situation.

 

[jittles]

Follow the load paths to determine what you need to design for.

When you get lateral loads, with walls spanning vertically, does the full load get thrown up to the roof level, or does the bottom half get transferred directly to the foundation? This lower half of the wall weight might still contribute to the overall base shear, but you have to track what elements are actually supporting that load.

For diaphragm design in a certain direction, how do the orthogonal walls load the diaphragm? Do the parallel walls load the diaphragm at all?

For shear wall design, what loads do they directly support?

If you are getting into a situation where you want to dial in your calculations, you could end up with different answers for diaphragm vs shear wall design in both directions (unless you had a perfectly square box).

As a rule of thumb, in my office we typically lump everything together for a light-framed wood structure, but when you get to concrete walls it can be of great benefit to break it down to a higher degree.

 

[JoshPlumSE]

Some thoughts on this:
1) Dead Load: If there is any dead load that is supported by the slab on grade directly (as opposed to wall supported) then this could be left out. Same thing goes for equipment load.

2) Partition Weight: It depends on whether there really will be partitions. Though there probably is a reasonable proability that there will be partitions. Even so, it depends whether the partitions will end up being supported by the walls. Again, if you have ground floor partitions that are really supported directly by the slab, then I don't think they'd need to be considered.

3) Snow load is not my area of expertise. Plus, this can vary greatly based on the jurisdiction. But, in general, you don't generally need to include the FULL snow load.

4) Weight of walls: In multi-story buildings where you determine the floor weight for each individual floor, the lower half of the walls are not tributary to any floor level. Instead it is tributary to the foundation level. Do you include this weight in the value used to calculate the TOTAL base shear or not? I've seen it done both ways.

Agree with TLHS that none of these things should result in an "order of magnitude" difference.

 

[TehMightyEngineer]

4) Weight of the lower wall should be included in the weight for base shear. However, when you come up with a story force you should calculate that using the half-walls (vertical force distribution and all that).

Maine EIT, Civil/Structural.

 

[VTHokie91]

Thanks to each of you for the thoughtful and prompt replies. I'm trying to work this out today, as our office is closed tomorrow for Good Friday.

First, I should clarify that I think "order of magnitude" was simply hyperbole. Although I once swapped inches and feet, to my extraordinary embarrassment.

I agree with all points about disregarding loads that bear directly on the slab. I did not include them in the original calculation. Also, my roof snow loads are small enough that 12.7.2.4 eliminates them. This is really an issue about the wall weight.

I am going to rerun the analysis splitting the base shear into two orthogonal directions. Since the building is rectangular, this should provide the additional detail that will keep the project moving forward.

Thanks again.

 

[DST148]

@VTHokie91>> I have older version of IBC Seismic Design Manual. I have attached two relevant
pages from the manual.
For a single story structure with masonry / precast walls(heavy) and wood / metal roof(light),
the assumptions made in calculating the seismic weight of the walls can make a sizeable difference in the base shear.
For nonloadbearing partition walls supported on slab on grade and laterally supported by the floor above, half the weight of the partitions should be assigned to the floor above.