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Tier 1 Seismic Retrofit Building Type

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MillR

Structural
Jan 15, 2007
67
Using AISC 41, I am conducting a Tier 1 seismic analysis of an existing hospital building that has previously undergone a seismic retrofit. The original building (1947) used steel moment frames. The retrofit (1981) used concentric braced frames. I am trying to pick the building type for the current analysis. S1 Steel Moment Frames can't be right, it's undergone a retrofit. S2 Braced Frames could be correct, although I don't know if the retrofit was intended to replace the moment frames or augment them, and it seems as though it would be conservative to neglect the moment frames. However, the description of S4 Dual Systems with Backup Moment Frames is very close to the building I have. I know this is ultimately a judgment call based on the best guess for how the building will behave, but as this is my first interaction with AISC 41 any thoughts would be helpful. Thanks in advance.

 
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Per the ASCE 41 commentary:

asce41_eiir7i.png


If the building was built in 1947, I'd guess that the design lateral loads were fairly light, and the retrofitted braces are substantially stiffer than original moment frames.
 
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