Has anyone used "pin" at bottom of cmu wall in metal building?

[jfudo]

NCMA perscribes flashing to break the bond at the bottom of a cmu wall in a pre-manufactured building to in effect create a pin connection. This is required to account for the H/100 drift that the metal building engineers design too. We would like to use this detail, but we need supporting evidence that this is functional.

More specifically, 23' eave height, the masonry is 14' high with a girt fastened to the top of the cmu wall. The total building sway at the top of the masonry is around 1". Doing a cantilevered wall that doesn't move with the building is nearly impossible

Does anyone have any other recommendations on integrating the cmu with the metal building?

 

[mrengineer]

We did structural design on high school in 1997 with metal building frame and cmu/brick veneer exterior wall. We considered using NCMA hinge detail but decided it was too picky and unconventional. We specified maximum metal building drift of eave height/150 and have had no reported problems. Interior cmu walls were cantilevered from floor slab and terminated just above the ceiling. We also showed control joints at intersection of interior and interior masonry walls. The only other special thing we did was to go ahead and show the exact wide flange girt that we wanted at eave for lateral support of the masonry wall.

 

[jfudo]

Thanks for the response. Our situation is a little backwards, this is a design build project and we already have the metal building designed. We are stuck with H/100. I have one question on your example, were the exterior walls anchored metal building so they moved with it?

 

[mrengineer]

The exterior wall was attached to the metal building frame. We showed an eave strut on top of the metal building frame and specified that strut should be designed to transfer wind into the cmu shear wall (for overall building stability parallel to the cmu wall), then showed a W8x31 girt a couple of feet below the eave strut to which we attached the cmu for lateral support of the wall. The fact is that using a metal building frame did not make a lot of sense structurally, but politically we were requested to do so since there were a couple of metal building manufacturers in this small town.

 

[jfudo]

I didn't want to ask the question of why a school will be built that way. I thought what we were using it for, a tank maintenance facility for the national guard, was inappropriate.

Thanks again.

 

[DaveAtkins]

I have notes from a presentation given by Jim Fisher on Structural Design of Industrial Buildings. Fisher says that allowable drift can be determined in the situation you describe by assuming a 1/16" crack opens up in the bottom joint of the masonry veneer. By similar triangles, this results in H/58 drift for 3-5/8" veneer, and H/122 drift for 7-5/8" veneer.

DaveAtkins