metalchair
Structural
- Oct 25, 2022
- 13
Hello All. I have an odd situation here and am seeking advice. Any help would be greatly appreciated.
I have recently been tasked with designing lateral braces for an existing industrial building that did not include bracing in the original design. From an analysis standpoint the building is currently unstable in certain areas, so bracing must be added. This building is in a relatively low seismic activity zone (SDC C) and the wind loads are relatively low (Vult = 115mph). I have considered the following three scenarios already:
1) Installing a second beam directly below the existing rafter with a bolted flange plate moment connection. -> This solution results in story drift that exceeds the limit set by ASCE 7.
2) Installing a second beam directly below the existing rafter with pinned connections and create a truss between new member and bottom of existing rafter. -> Again, this results in story drift that exceeds the limit set by ASCE 7.
3) Installing bracing members in a manner so the frame is treated as an ordinary concentrically braced frame per AISC 341. -> The customer has indicated having braces extend all the way to the middle of the span will create clearance issues with already ordered process equipment.
I want to design an eccentrically braced frame with the link beam being a large center segment of the existing rafter. The issue with this is that I am working with existing frame sizes, and AISC states that link beams and columns within an eccentrically braced frame system must be highly ductile. The sections here to be used as the link beam and column are not highly ductile. The purpose of high ductility is to ensure that the members are able to achieve their full plastic stress before failing.
My question for you all: If my analysis including these eccentrically braced frames (and appropriate overstrength factors, etc.) indicate that the rafter links and braced columns are no where near even yielding, let alone their plastic limit, is it truly necessary to insure highly ductile sections are used? Or do you feel it would be fair to disregard AISC 341's ductility requirements considering the circumstances? In my opinion, with how low the loads are on the structure, to eliminate entire bracing schemes due to the sections used seems overly conservative.
I have recently been tasked with designing lateral braces for an existing industrial building that did not include bracing in the original design. From an analysis standpoint the building is currently unstable in certain areas, so bracing must be added. This building is in a relatively low seismic activity zone (SDC C) and the wind loads are relatively low (Vult = 115mph). I have considered the following three scenarios already:
1) Installing a second beam directly below the existing rafter with a bolted flange plate moment connection. -> This solution results in story drift that exceeds the limit set by ASCE 7.
2) Installing a second beam directly below the existing rafter with pinned connections and create a truss between new member and bottom of existing rafter. -> Again, this results in story drift that exceeds the limit set by ASCE 7.
3) Installing bracing members in a manner so the frame is treated as an ordinary concentrically braced frame per AISC 341. -> The customer has indicated having braces extend all the way to the middle of the span will create clearance issues with already ordered process equipment.
I want to design an eccentrically braced frame with the link beam being a large center segment of the existing rafter. The issue with this is that I am working with existing frame sizes, and AISC states that link beams and columns within an eccentrically braced frame system must be highly ductile. The sections here to be used as the link beam and column are not highly ductile. The purpose of high ductility is to ensure that the members are able to achieve their full plastic stress before failing.
My question for you all: If my analysis including these eccentrically braced frames (and appropriate overstrength factors, etc.) indicate that the rafter links and braced columns are no where near even yielding, let alone their plastic limit, is it truly necessary to insure highly ductile sections are used? Or do you feel it would be fair to disregard AISC 341's ductility requirements considering the circumstances? In my opinion, with how low the loads are on the structure, to eliminate entire bracing schemes due to the sections used seems overly conservative.
