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# some questions about special concentric braced frame

## some questions about special concentric braced frame

(OP)
I was asked to design a special concentric braced frame (special Concentrically Braced Frame in seismic design category D, with R=6). it's a one story X-brace with HSS squares as diagonal braces. the height of column is about 14' and span of beam is about 29'. Beam and columns are typical W-shape.
I have some questions
1. about the un-braced length of HSS tube. Can I calculate the unbraced length as sqrt(14^2+29^2)/2=32/2=16 ft? So the HSS in tension acts as a lateral brace for the HSS in compression.

2. About the brace connection to the beam and column. I thought the tension design load for the connection shall be the lesser of expected yield strength of HSS square or the maximum tension force the system can transfer to the X-brace (load combination with over-strength factor). But I checked AISC Seismic provision F2.6, it seems the over-strength factor load combination is not mentioned. in my case I think the tension force from the load combo with overstrength factor give less design load than the expected yield strength of HSS.

3. anchorage of column and diagonal brace base plate to the concrete spread footing. the anchor bolts and shear key/lug should be designed with the same load as I mentioned above (question 2)?

Any thoughts or comments? Thank you.

### RE: some questions about special concentric braced frame

For #2, I believe you can only use that exception of using only load with overstrength, if you disregard all compression braces in the structure. Many programs have this option as a "click" but don't actually remove the compression braces from taking load so beware how the program, if you are using one, is transferring diaphragm load.

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