Gravity load distribution

[LOKSTR]

I have a single storey steel building with metal deck. The building plan is shown below. I have a braced frame on one side(grid-A) of building and moment resisting frame on the opposite side(grid-D).

|-----------------|---grid-A
force---> |-----------------|
|-----------------|
|-----------------|---grid-D

My questions are;

1. when I check the stability of both frames, what gravity load distribution(UDL on each frame) will be on each frame? Will it be half of building area on each frame (considering flexible diaphragm)?

2. For designing column of the frame, What axial load I should consider? Will it be whatever axial force is in the columns based on gravity load UDL or whatever axial load is due to tributary area of individual column?

 

[AggieYank]

1) No. The fact that your grids "A" and "D" are lateral force resisting doesn't increase the amount of gravity load on them. The other columns will still carry only their own gravity load based on their own tributary area.

2) The load on each column is from Live Load, Dead Load, Snow?, etc, based on the tributary area for each column. The load combinations can be found in the IBC 2003, or whatever code you are designing from. It seems as if you're asking the same question twice. The dead load / live load / etc is based on the tributary area of the member, which in this case is the column.

 

[JAE]

1) - Well, for lateral stability of the frames - the two intermediate frames carry dead load and their P-delta forces would translate to the adjoining frames A and D in proportion to their location and their individual lateral drifts, which are different because A is more stiff than D.

You have a lateral deflection at A and one at D. From this you can estimate the lateral deflection at B and C.
The P-delta forces from the dead loads on each of frames A through D would be applied laterally at each corresponding frame and a simple lateral beam analogy would distribute the loads - some to A and the rest to D....assuming you have a flexible diaphragm.

2) Not sure of your question here. For column design - each column has an axial load based on the supported elements and that is what you'd use for axial design.

 

[AggieYank]

JAE, would you really do a P-Delta analysis for the structure he has described?

I could understand doing a P-Delta analysis when you have a heavy load on the roof, or multiple floors, where the extra load from a P-Delta analysis will be meaningful, but in this case, I don't see it as necessary. In fact, it is rare when I go through the process of a P-Delta analysis, even though with programs like RISA-3D, and others, its fairly easy.

 

[JAE]

Aggie...I guess I didn't mean to present that I would do it...just responding to the question of how to do it if you do it.

But ya...you're right - if the frame is light - then Pdelta probably isn't a big issue. But the info above doesn't really give you a weight, height, stiffness, etc.

 

[LOKSTR]

Thanks for the reply, but I think I could not explain my question properly.

When we apply lateral force to a frame( x-braced or moment frame) , we get additional axial forces in the columns of the frame which we add to axial loads due to individual column axial force (by multiplying tributary area & load(psf)).

My question is, if we have to do a p-delta analysis, we apply gravity load( depening upon the area which this frame has to stabilize) on the frame and do the analysis and get the forces in frame members. If we compare the frame column axial loads after analysis , with the loads calculated as described above, those are quite high. So which loads are to be used for column design?

 

[JAE]

If you have to do a Pdelta analysis per the code, then you simply perform a Pdelta analysis on each required load combination (D, D+L, D+L+W, etc). You don't do a Pdelta for just the lateral loads and then add them to gravity loads separately. The gravity loads, in fact, are an essential part of the Pdelta effect.

The axial loads in a single column due to non-Pdelta and Pdelta combinations shouldn't change all that much.

 

[TPNY]

Wouldn't you use the load combination factors? If the lateral force is due to wind, then the compression load on the column from wind would get that respective factor, and the gravity loads would have the dead load factor, etc...

 

[JAE]

Yes, I agree.