Combining Steel Moment Frame & CMU Shear Walls
Combining Steel Moment Frame & CMU Shear Walls
(OP)
I have a one story Seismic Design Category D building with a steel beam/metal deck roof. I assume this is a flexible diaphram. The building is almost rectangular, but it has a couple of jogs in one corner.
For loads perpendicular to the long dimension wall, there are CMU shear/bearing walls on each end and one near the middle of the building. I will load these shear walls with the tributary widths for the wind and seismic loads.
For the loads perpendicular to the short dimension walls, there is a long CMU shear wall on one side. The other side is adjacent to an existing building that I don't want to make a structural attachment. So, we have a steel moment frame about 15' away from the existing bldg. The short dimension of the building is about 48' wide.
1. How should I go about distributing the wind and seismic loads between the moment frame and the shear walls?
2. Should 1/2 of weight of the CMU partition walls (4" to 8") contribute to the seismic load at the roof level?
For loads perpendicular to the long dimension wall, there are CMU shear/bearing walls on each end and one near the middle of the building. I will load these shear walls with the tributary widths for the wind and seismic loads.
For the loads perpendicular to the short dimension walls, there is a long CMU shear wall on one side. The other side is adjacent to an existing building that I don't want to make a structural attachment. So, we have a steel moment frame about 15' away from the existing bldg. The short dimension of the building is about 48' wide.
1. How should I go about distributing the wind and seismic loads between the moment frame and the shear walls?
2. Should 1/2 of weight of the CMU partition walls (4" to 8") contribute to the seismic load at the roof level?






RE: Combining Steel Moment Frame & CMU Shear Walls
If your diaphragm is flexible, then distribute the lateral forces by trib area. If it is a rigid diaphragm then distribute lateral forces by stiffness. You should be doing the same thing in the long direction. Don't just use trib area if it is a rigid diaphragm.
2.
Yes
RE: Combining Steel Moment Frame & CMU Shear Walls
Regarding #1, due to the 15' cantiltver of the roof diaphragm, that implies that the diaphragm forces perpendicular to the short face direction need to be analyzed as a rigid diaphragm to make it work. Otherwise you have a pinned connection over the mnoment frame and that will not work. I would look at the reactions to a uniformly loaded cantilevered beam - AISC beam diagram #24 - for starters, and play with the results from there.
Mike McCann
McCann Engineering
RE: Combining Steel Moment Frame & CMU Shear Walls
The other alternative would be to take all of the shear on the 3 exterior walls of the addition and leave out the moment connections. The open side is against the existing building. However, the diaphram is metal roof deck and should be considered flexible.
RE: Combining Steel Moment Frame & CMU Shear Walls
You could run the solution as I described with the frame, then, considering the deflections, determine the relative rigidities of the frame and CMU walls, and re-analyze using a rigid diaphragm analysis. This should lower the force to the frame. The allowable deflection or story drift will control the size of the frame anyway, not bending stress.
Mike McCann
McCann Engineering
RE: Combining Steel Moment Frame & CMU Shear Walls
I revised my simple beam analysis and the frame load is about 2.5 times the shear wall load. Oops!
What is the simplest way to evaluate and compare the rigidity of the 2 different systems?
RE: Combining Steel Moment Frame & CMU Shear Walls
RE: Combining Steel Moment Frame & CMU Shear Walls
Had too do this for a multi-story hospital expansion in Anchorage years ago with steel moment frames and concrete shear walls. Method worked well. No earthquakes there yet!
Mike McCann
McCann Engineering
RE: Combining Steel Moment Frame & CMU Shear Walls
RE: Combining Steel Moment Frame & CMU Shear Walls
maybe you could use the 0.5 factor for getting the max load going to the moment frame, but I would not use any factor to find the load going to the shearwall.
This will be a little conservative, but will give you a good envelope. After all, those EI coefficients are just approximations and it is very difficult to predict how it will actually behave. Also, I don't know how different masonry is from concrete, but 0.5 sounds reasonable.
RE: Combining Steel Moment Frame & CMU Shear Walls
I have 2 walls (offset by 8'-10") parallel to the moment frame. One is 31' long and one is 75'-8" long. The walls are 17'-4" tall from the footing to the top.
RE: Combining Steel Moment Frame & CMU Shear Walls
RE: Combining Steel Moment Frame & CMU Shear Walls
If you have Amrhein's masonry textbook, there are tables for the rigidity of CMU shear walls, based on height and length.
But when all is said and done, I would just put all the lateral load into the shear wall. I doubt any moment frame will even be close to the stiffness of these long shear walls. Yes, I know that a steel deck is supposed to be considered a flexible diaphragm. But engineering judgment seems to indicate it will behave like a rigid diaphragm in this situation.
DaveAtkins
RE: Combining Steel Moment Frame & CMU Shear Walls
RE: Combining Steel Moment Frame & CMU Shear Walls
RE: Combining Steel Moment Frame & CMU Shear Walls
I would also expect such a lopside center of rigidity that the perpendicular shear walls would be needed to handle the torsion on the building (if modeling as a rigid diaphragm)