Wood Shear Wall Capacity, 40% Increase for WInd 1

[wannabeSE]

I am looking an article or documentation that explains why the IBC allows a 40% increase in shear wall and diaphragm capacity for wind loads, while the 97 UBC did not allow the increase. In a 2008 thread (

http://www.eng-tips.com/viewthread.cfm?qid=223905)

UcfSE posted a link to and article that explained the change, but the link does not work.

 

[johnbridge231]

The current shear wall and diaphragm tables are based on a 2.8 minimum safety factor and it was agreeg that a 2.0 safety factor is adequate, thus a 40% increase in tabulated values.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[wannabeSE]

John,
Thank you for responding. However, I already now that the shear wall and diaphragm allowable capacity (safety factor) has changed for wind loads. I am wondering why.

 

[LorenAWC]

The change actually happened first around the 95 SBC and ASCE 7 was the motivating factor. Around that time the wind loads jumped up and designers were understanding about roof problems, but they questioned why suddenly their shearwall and diaphragm designs no longer worked despite a lack of failures.

Meanwhile PS-2 was using a factor of safety of 2.8 to get design values for wind, and people focused their questioning on this. Why use a factor of safety of 2.8 for design values when the other connections surrounding that diaphragm or shearwall only have a factor of safety of around 2.0? That is where the 2.0 comes from.

While the factor of safety wasn't changed in response or calibrated to the increase in loads, it was a motivating factor. Designers realized that their factor of safety was high and modified it. Now we have the 1.4 increase or the two tables as shown in the Special Design Provisions for Wind and Seismic.

 

[wannabeSE]

Loren,

Thank you very much. It is not the answer that I expected. I would have guessed that additional research showed that the shear walls performed better with wind loads. I am going to need to look at a renovation project a little closer. It would not be prudent to shorten a shear wall designed under the 1997 UBC and justify this by assuming a 40% increased capacity (and checking the hold downs and sill bolts for the increased load).