CMU Shear/Bearing Wall with C joists

[fixxxer0]

In a 2 story building - say 40' L x 20' W with the longer walls being reinforced CMU and the 20' side being spanned by cold formed steel C joists, could the C joists into the CMU wall be counted on at all for lateral force resistance - like a moment frame?

The intent is to have an open floor space, so that rules out any shear walls running perpendicular to the reinforced CMUs. Only one end of the building is going to have a rear wall (would look U shaped in plan view). I don't think that single perpendicular wall in the rear could count as a shear wall for the entire 40' length of the building.

Any ideas on how to take care of the lateral forces in the 20' direction?

 

[hokie66]

The joist won't help. You need a moment frame across the opening on that end, or at least near that end. Then you need a horizontal truss in the roof plane to distribute the load between the shear wall and moment frame on opposite ends.

 

[fixxxer0]

Thanks for confirming what I suspected.

The job has become a build first, engineer later type of mess, and I have to work within the abilities/limitations of the current contractor and budget. This was not my job from the start.

I was considering using a reinforced concrete frame, rather than steel, to easily bond into the long CMU walls. The problem is this method (whether RC or steel) is that it won't be tied into the floor system that was scheduled to be used. I can't see how I could still use the C-joist floor system. If I went with steel id have to worry about tying or isolating the steel from the CMU wall.

I'm not following you exactly with the horizontal truss at the roof level. I was planning on counting on the diaphragm action of the C joists / decking to distribute load between the CMU shear wall and whatever I use on the other end.

Any suggestions on the best way to proceed? Cost is becoming a major issue at this point.

 

[hokie66]

Yes, you could use a metal deck diaphragm. Not an expert on those, as I am in Australia, and we don't use that type deck here. Others should be able to help.

Do the C joists occur at the floor as well as at the roof? Maybe a few details on how it is intended to be built would help. I agree that building first and designing later is doomed to be a 'mess'.

I don't like the idea of a concrete rigid frame. Detailing the reinforcement to make the joints efficient is very difficult, and is difficult to build.

 

[RFreund]

Can you put in a rear wall as a masonry shear wall? Then, it sounds like you have a 3-sided box situation, Correct? (shear walls on the 3 sides?). You could then possibly design this as a cantilevered diaphragm (long walls take the torsion) however I'm not sure on what the stiffness requirements of the diaphragm would be.

EIT

http://www.HowToEngineer.com

 

[tony1851]

If your roof can act as a horizontal truss, with suitable shear connection to the rear wall, consider a simple steel moment-frame at the open end, as sketch attached