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.
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.
