Irregular Lateral Force Resisting System - Edge Columns + Masonry

[ahypek]

Structure is 60' x 21' and 45' high.

There's 60' of 8" masonry shear wall in one direction but in the other direction we have excessive openings so there will be cold formed curtain wall with steel framing for lateral at each end of the building. Two columns spaced at 21' is giving me unacceptable drifts and is economically not worth it. I added an additional column and obtained reasonable sections with acceptable drifts. The owner is having a panic attack and demands two columns per side. EoR is claiming that I can consider contribution of lateral stiffness of the 60' masonry wall. My issue is how do I approach this? Realistically I know that the wall will provide some stiffness but do I assume an equivalent column section and analyze my diaphragm as a continuous beam with a spring in the middle to represent the wall? Do I anchor the column to the masonry and consider it a composite section? What would the effective width be?

I appreciate any help.

 

[JLNJ]

Is this a three-sided structure? One 60' side with a masonry shear wall and two 21' sides with steel moment frames? A picture might be worth a thousand words.

If the unreasonable drift is in the 21' moment-frame direction, the wall in the other direction won't help much, regardless of the attachment.

 

[ahypek]

Exactly. North and Southwalls are 60' of 8" CMU and East/West is 21' of curtain wall with a steel moment frame. I agree that it won't help much but I'm stuck dealing with a very stubborn man who without any proof is asking me to prove his concept works. How much width of each wall would be effective to contribute flexural rigidity to the steel? Would I use similar constraints as I would with a composite beam/deck/slab?

 

[JLNJ]

Is he asking you to use the CMU as a flange of sorts which will work with the moment frame in the cross direction?

If it were all masonry I guess you would have an L shaped piece, but I don't see the flange of the "L" helping the stem when the stem is a moment frame.

I suppose you could model it and see what those connecting forces look like. If the structure were not so tall there might be some nominal flexural stiffness to be gained by including the CMU in the cross direction. I can't picture it curing what ails the structure.

Once you figure out if it helps you, then you can consider attachments. Usually CMU attachments have some flexibility in its attachment to the building frame and this might hurt your chances to force them to work compositely.

Sounds like a red herring.

 

[Deker]

Have you considered using more transverse frames throughout the length of the building? How many stories? Wind or seismic? How are you bracing your 45' tall 8" CMU walls?

 

[KootK]

I'd refuse the composite wall/frame option. In my opinion, that would entail so many unknowns that I'd never feel good about it. And I doubt that the composite action, over just the frame columns, would accomplish all that much anyhow.

Is the layout of your end wall penetrations such that you might be able to create a two tier moment frame? You know, looking like a two story moment frame but without a diaphragm behind the lower level.

I agree with Decker about the intermediate frames as well. 60' x 21' is starting to get up there with regard to diaphragm aspect ratio.

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.