Gravity load distribution
Gravity load distribution
(OP)
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?
|-----------------|---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?






RE: Gravity load distribution
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.
RE: Gravity load distribution
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.
RE: Gravity load distribution
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.
RE: Gravity load distribution
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.
RE: Gravity load distribution
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?
RE: Gravity load distribution
The axial loads in a single column due to non-Pdelta and Pdelta combinations shouldn't change all that much.
RE: Gravity load distribution
RE: Gravity load distribution