Dual System: Steel Ordinary Moment Frames with Ordinary Reinf Masonry Shear Walls 1

[tgraybs]

I'm designing a 3 story building with a lateral force resisting system consisting of steel moment frames not specifically detailed for seismic resistance (R=3). Building is Seismic Design Category C. Diaphragms are rigid. There are reinforced masonry shear wall stairwells located in a few areas, and I am considering utilizing these walls in combination with the moment frames to resist lateral forces. I'm hoping for some guidance from the seismic experts on here on the implications of combining these two systems, specifically:

1) Are the moment frames still required to resist 25% of the design seismic forces, as outlined in ASCE 7-10 12.2.5.1? Or is that requirement just for intermediate and special moment frames?
2) If the 25% requirement does not apply, are the lateral forces simply distributed based upon stiffness? Wouldn't the much stiffer shear walls suck up all the lateral load, leaving little to no load for the moment frames to be designed for?

Thanks in advance.

 

[wannabeSE]

1) Yes, The configuration conforms to the definition of dual system in chapter 11 (Frame> Structural System> Dual System)

 

[Guastavino]

I believe that your thought process in #2 is why the answer to #1 is yes.

With that said, it's my interpretation that as long as the moment frames CAN take 25% of the design forces, then you are OK even if they DON'T take that much based on stiffness. Any other one agree of disagree with my interpretation?

With that said, in different directions you can have two different systems and not use the combination requirements, or so that's my understanding. IE if you have Shear walls N-S and then MF's E-W, I think that is OK.

 

[MacGruber22]

For SDC C, I believe you also have the option to combine the systems without designating it as a dual system. To me, specifically classified dual systems are meant to limit combining particular systems mainly in SDC D, E, and F.

Without all the particulars of your job, I would say that a general unintended consequence of combining stairwell shear walls and moment frames with a rigid diaphragm *could* be torsion. You rarely have a say where you would like stairwells to exist, so you may be stuck with a large torsional irregularity created by introducing the stairwells as participating elements. Depending on your geometry, it may be worth running your frame as only a moment frame and as the combination to see what ends up being more advantageous.

"It is imperative Cunth doesn't get his hands on those codes."

 

[KootK]

In code:

V_walls = proportion based on stiffness

V_frames = max (25% total shear, proportion based on stiffness)

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[tgraybs]

Thanks for the input, everyone.

MacGruber - I had the same line of thinking as you. As far as I can tell, there is no "dual system" classification in ASCE 7 for Ordinary Moment Frames...only Special and Intermediate Moment Frames are listed (Table 12.2-1 Parts D and E). It seems to me that I have the option of combining the systems without designating it as a dual system - so long as I use the most stringent R-value between the two systems. I'd still probably design the moment frames for at least 25% of the force.

I'll be sure check how adding stairwells effects the torsion on the building's LFRS.