I'm Moment Frame Happy!

[ars001]

I have a unique situation where I have a 1 story steel framed building with bar joists and metal deck roof. The building is appox. 70'x180'. The problem lies in the fact that the building elevation including the metal deck steps up 2'-0" at 3rd points along the 180' dimension. So that basically means I need a braced or moment frame at those locations. So for lateral resistance to wind loads perp. to the 180' dimension of the building I will need frames at the outer walls and a frame at each of the jogs in the roof diaphragm (total of 4 frames). Do I have any other ways around this? Seems like alot of frames for a building 180' wide. Thanks

 

[JAE]

Can't you develop vertical mini-braces to take the upper diaphragm forces down to the lower diaphragms? Or perhaps the columns can simply cantilever up to transfer the forces from one level to another.

 

[ars001]

Yes, I have thought of that. But what happens when I take and put the reaction into my adjacent roof diaphragms. Then I'm basically treating my outer bays as 3 sided diaphragm or an open c shape(assuming I will have frames perp to the frames we are talking about). Is this acceptable with a metal roof deck?

 

[lkjh345]

Depending on how high the building is, you could try cantilever columns. If the building is fairly tall, this idea isn't the greatest idea.

Just throwing out ideas, though this may be no better than moment frames.

 

[JAE]

Well, through the vertical jog in the diaphragm you would still want to create a tension/compression chord at the level of the lower roofs, that would extend through/under the clerestory.

 

[ars001]

JAE, I think I see what you are doing... Essentially you are looking at the whole building as a simple span beam and transfering the shear load by diagonal braces or by cantilevering the tops of the columns at the roof jogs. Also creating a continuous member chord member as you just explained to take out my chord forces. The problem I have is the building jogs up 2' and then at the next third point jogs up another 2' not back down.

 

[JAE]

Ah...that is different.

But if your lowest chord can be developed all the way through, and you transfer down to a perimeter chord of beams at the high end, maybe that could work.

 

[ars001]

I think we are getting somewhere!! Do you think its logical to transfer my chord force (from higher to lower roof) thru a cantilevered column or say a couple as I did to transfer the shear load to the adjacent roof?

 

[JAE]

The cantilevered column can work...you just need to check strength and stiffness response.

This sort of feels like a case where I'd be tempted to model the whole thing in RISA 3D or similar program to get the full results of the various load paths, stiffnesses, etc.

 

[ars001]

Yeah, I agree. I probably should model it in Risa. I was trying to simplify it as much is possible and try doing it by hand. Obviously its not that simple. I agree there is alot of strenght and stiffness stuff going on with cantilevered columns and so forth. I want to make sure everything is accounted for since I have a fair amount of brick veneer on the building. Thanks for all of your help!