Shear walls 1

[braves25]

I have an existing building (single story wood truss roof framing) with CMU walls on all four sides. They are wanting to expand the building and remove on of the CMU walls and expand out the side. The addition will be constructed of wood stud walls. Does the IBC allow the mixing of CMU and wood shear walls? Thanks!

 

[msquared48]

if you have four individual lines of shear walls, all external, and one entire line is being replaced with wood, I do not see why not. Torsion could be induced though due to different lateral deflections.

If you mixed wood/CMU types in the same line of shear walls, that is where I would have a problem.

In the past I have mixed steel frames and concrete shear walls in the same line for a hospital expansion, but you have to do an iterative rigidity analysis to properly assess the relative loads taken.

Mike McCann, PE, SE (WA)

 

[TehMightyEngineer]

What seismic design category is the building?

Maine EIT, Civil/Structural.

 

[TLHS]

I have no idea about actual prohibitions in mixing systems. From a practical standpoint I would suspect it depends on your diaphragm and how your load transfers. If you've got a stiff diaphragm your load transfer is going to be related to the stiffness of the shear walls. As such, your wood is going to take significantly less load than the existing CMU wall did and all your loads will shift.

If you have a flexible diaphragm you're going to have to think about it and decide if it's actually flexible *enough* to make up for the differential flexibility of the CMU and wood shear walls. I would suspect it's in the middle zone where the diaphragm and wall stiffnesses are comparable and you really should be modelling the stiffness of the whole system or you should be checking it as a flexible and stiff diaphragm and then enveloping the solutions.

 

[Iasonasx]

I do not see how IBC would prohibit the mixing of the two types of materials, but in terms of mechanics and behavior, you will have something more complicated to analyze and design, and it may not be worth the effort. Without knowing the geometric formation of the structure, my opinion would be to keep the same materials and keep the design clear.

 

[manstrom]

You can mix the two. Stick with a flexible diaphragm analysis. CMU shear walls are something like 100 times more rigid that wood walls.

 

[XR250]

The change in materials will put a discontinuity in your diaphragm chord that needs to be addressed

 

[steellion]

I'm assuming if you have wood trusses, you probably have a plywood diaphragm. That would qualify as a flexible diaphragm, but I would consider conservatively checking your system as a flexible and a rigid diaphragm and designing for an envelope case of the two. Since the masonry walls are so much more rigid than a wood shear wall, they will end up getting more of the load and taking some torsional force.