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seismic design of braced timber frame

seismic design of braced timber frame

I'm interested in input on how to design a braced timber frame with current codes.  Specifically, due to a change of use, I've been asked to analyze a historic barn, and bring it up to current seismic code.  The barn is constructed of heavy timber frames with knee braces and endwalls sheathed with 1x horizontal board siding.  The last versions of the IBC and ASCE7 have eliminated timber frames from the seismic design provisions, except for Cantilevered Column systems.  The last code I can find that mentions braced timber frames is the 1997 UBC.  The 1997 UBC allowed braced timber frames with R values of 2.8 where the bracing also carries gravity loads, and 5.6 when the braced frames are independent of the gravity loads.  So, my dilemma is what, if any, R value to use when analyzing the strength of the existing frames.  I guess we could resheathe all the roofs and walls with plywood and install steel moment frames in between the existing frames and ignore the strength of the timber frames, but that would be quite cost prohibitive.  Anybody have any input here?

RE: seismic design of braced timber frame

You said it is an "historic" barn.  With all this renovation planned to upgrade it, I hope it's not on the National Historic Register.  If it is, you will be very limited as to the structural upgrades you can make.

Mike McCann
McCann Engineering

RE: seismic design of braced timber frame

The bottom line is that wihtout significant structural renovation, this barn cannot be brought up to IBC level seismic code.  I would recommend an alternate approach by using the Internation Existing Buildings Code, and by reference ASCE 31-03 (Seismic Evaluation of Existing Buildings) and ASCE 41-06 (Seismic Rehabilitation of Existing Buildings).  This is a relatively new approach, and I'll warn you that if you go this route, you will be doing a lot of education on the part of the owner, building department and architect.  But, on the plus side, these standards are based on Performance Based Design and allow a significant amount of latitude for engineers to assess, evaluate and renovate existing and historical buildings.

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