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Reinforcing Existing Steel Deck Diaphragm

Reinforcing Existing Steel Deck Diaphragm

Reinforcing Existing Steel Deck Diaphragm

(OP)
1) I am designing major modifications to an existing one-storey structural steel building. I am not able to verify the horizontal diaphgram capacity of the steel deck to resist the higher seismic loads based on the latest code. I will need to add new horizontal bracing. Existing roof structure consists of OWSJ with back to back angle top chord. Curious where people usually place new horizontal bracing (height wise). If I were to place it flush with the bottom of the OWSJ top chord, I am going to interfere with some existing electrical conduits, but this seems like the most sensible thing as it allows the new horizontal bracing to be continuous. If I were to place the new horizontal bracing flush with the top of the OWSJ top chord, this would avoid the interferences, but I am not sure how the load would be transferred through the gap between the OWSJ top chord back to back angles. Where are horizontal bracings usually installed (height wise) in an existing building and how are they usually detailed?

2) The building has moment frames in one direction and horizontal bracing in the other. I have to add a new staircase with a CMU or cast-in-place enclosure. This shaft is within the footprint of the structure and near one end of the building. I am thinking it would introduce torsional sensitivity to my structure - and given that this is mostly an existing structure, it may not be a good idea. My question is: Do I need to provide a physical expansion joint between the two structures? Or can I ignore this shaft, not provide an expansion joint, and not take it into account in horizontal analysis as long as I do not detail the shaft as shear walls and not provide a doweled/mechanical connection between the new shaft and the structure.

Thanks for your input.

RE: Reinforcing Existing Steel Deck Diaphragm


I am not sure that i visualize the situation you're describing.
My questions ;
- Why you don't assume flexible or or semirigid Diaphragm Condition (12.3.1.1) ?
- What is the function of staircase for single storey bldg ?
- Can you model as flexible diaphragm so the structure will not be torsional sensitive ? or can the staircase be detached from the structure ?

You may consider to post some pictures and more details to get better responds.
...

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

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