Wind Load / Load Path

[ChrisKn]

I've been working on a big-box retail store with 3-sides CMU walls and a front wall that is conctructed of light gauge materials with a variety fo eifs covers/etc. The roof of the building consists of bar joists/joist girders. Their are two lines of joist girders. One line is supported by interior HSS columns and masonry piers in the side walls. The second line of joist girders are suported on HSS columns and are tight/parralel to the front wall. The joist girders obviously span the two bays (in the front half of the building between both lines of joist girders, and in the rear half of the building between the center line of joist girders and the rear wall. The light gauge framing that forms the front of the building is hanging from the joist girder along the front wall.

The wind loads on the side wall of the building would transfer 1/2 to the foundation & 1/2 to the roof structure. Normally the loading in the roof diagram would transfer to the front and rear walls of the building which would act as shear walls. However, teh front wall of the this particular bldg. is non-existant as a shear wall since the open constraints of teh wall do not allow x-bracing between the columns.

So does the entire wind loading actually only transfer loading to the rear masonry shear wall?? Also does teh attachment of angles between the top & bottom of the josit girder and the front light gauge wall (used to hang the light gauge system) constitute any problems??

Hopefully I'm looking at this laod path situation correctly?? Any help is appreciated.

Chris

 

[Lion06]

Well, you have a 3-sided torsion box. All of the shear for that direction will go into the rear wall. You have to consider the torsion on the building due to the center of rigidity in this direction not coinciding with the location of the applied shear. This will actually end up reducing the shear on the rear wall (but I wouldn't do that to be conservative) and put a shear equal to the torsional moment divided by the distance between the two sidewalls into those two side walls simultaneously with the shear on the rear wall. This is assuming a rigid diaphragm, I'm not sure how to handle this if it is a flexible diaphragm.
Just as a side not, if there is a parapet be sure not to take 1/2 of the ceiling hieght plus the parapet - I don't know why, but I have seen a lot of people do this.

 

[Lion06]

How is the light gauge front wall that is being hung from the joist girder getting supported laterally? Is it being kicked back to the joists framing in to the joist girder? Has this load been accounted for in the design of the joists?
I am not too clear on your very last question about the angles from the top and bottom of the joist girders, can you be a little more clear?

 

[msquared48]

R$EMINDER: Also, according to 2006 IBC section 12.3.1, the diaphragm you describe is flexible by definition. You will need to prove through deflection calculations that the untopped steel deck will qualify as a rigid diaphragm qualify to transmit the torsion - see section 12.3.1.3.

Is wind going to control here, not seismic? Where are you located. For seismic, with no lateral resistance at the storefront, it will throw you out of the realm of the simplified method. You will also be penalized under section 12.3.3.4. So, thinking here, why not add an ordinary steel moment frame(s) at the front window wall - steel tubes. This makes many problems go away and the tube frames are generally deflection controlled.

Mike McCann
McCann Engineering

 

[Lion06]

Mike-
I am not sure that will make the torsion problem go away.
The masonry shearwall is MANY, MANY times stiffer than a moment frame. As such, the center of rigidity in that direction will not move very much.
I guess if this is a flexible diaphragm then the moment frame gets the shear based on trib area instead of stiffness, so that will definitely be beneficial.

ChrisKn-
Is this a flexible or rigid diaphragm?

 

[msquared48]

Your right. I was thinking of using a flexible diaqphragm - trying to eliminate the inherent problems of a rigid one by adding the frame(s). Thanks for clarifying my answer.

Mike McCann
McCann Engineering