Wood X-bracing added to a wood shearwall residence

[ELR1]

I have a two-story wood sheathed residence located on a hill side. There are two stories of living space above an open area out over the hill side. The two up hill sides (north and east) of the house are carried on continuous footings. The two down hill sides (south and west) of the house are carried on unbraced wood posts. I want to augment the LFRS system by X-bracing some of the posts in each direction on the downhill side. Location is seismically active (Western Washington). So, I believe I have mixed framing systems (wood shearwalls and braced frames) in each direction at the lowest level. All levels above the lowest level are wood shearwalls. What would be the appropriate SFRS (R, Omega, Cd) selection in ASCE 7-05 Table 12.2-1 for the wood X-braced portion of the overall LFRS system in this situation?

 

[EngineeringAdam]

Because you have a light framed building you can use a different R, Omega, Cd per line of resistance.

However, you have to use the worst case R, Omega, Cd for that line.

I don't think there is an exact match for wood braces. Are you planing on using wood braces? or using steel rods to act as tension bracing?

Either way I would use B.4. Ordinary steel concentrically braced frames. R = 3.25, Omega = 2, Cd = 3.25

Some might argue that the wood braces should have a lower R, and I couldn't really disagree, so maybe to be conservative you can use G.7., it's for cantilevered timber frames, but your forces will be higher.

 

[ELR1]

Thank you. I was thinking along the same lines as you. I will likely go with B.4. Yes, I am planning on wood braces designed for tension-compression.

 

[DST148]

Since the proposed LLRS in wood is not included in the Table, I would agree with EngineeringAdam's suggestion in the last paragraph - LLRS type G.7, and R = 1.5.
@ELR1: Based on the description given in OP, it seems you have two-story light-framed wall system partially supported on stilt in the lower story. On two sides of the lower story, the light-framed wall system is discontinued. I would definitely be conservative for such scenarios in high seismic zone, even though the code makes an exception in case of detached one and two family dwellings.

 

[msquared48]

Do a repetitive unit load deflection analysis for the shear wall and X brace wall. That should tell you the relative rigidities to apportion the overwall wall shear.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com