Shear capacity increase for wood header or plates 2

[cg3375]

Is there an allowance or duration increase allowed in shear capacity when designing a wood moment frame in place of a shear wall (with wind only) for either the top plates or header such as the 1.4 allowed in panels per IBC section 2306.3.2?

Example, SYP #2 has a value of 90 psi. Can this be increased for a wind only collector and if so how much (1.4, 1.33, 1.6)?

Any opinions or thoughts are appreciated.

Thanks !

 

[RARWOOD]

Load duration factors may be applied to the allowable shear. If you are designing your moment frame using the NDS, you would use a load duration factor of 1.6 for wind.
The local building code may be more restrictive than the NDS.

You should contact APA-EWS for more information. I believe that they publish information on the design of wood portal frames.

 

[MiketheEngineer]

A wood moment frame is pretty hard to design and build. The connections just do not make it.

You might want to use a "Portal Frame" as outline by APAWOOD.

 

[JAE]

I agree that per NDS the Cd factor does apply as stated for short time (wind) loads.

I also agree with Mike that a wood moment frame is pretty tough to detail - the bolts tend to gouge the holes a lot and leave you with a much more flexible frame than you might think.

 

[cg3375]

All of the input is appreciated.

If I stay with SYP #2, I'll have 725 psi Ft, 1660 Fc, and 175 Fv all adjusted to Cd 1.6 to work with.

My problem with the APA portal frame is if I read them right, I am maxed out at 10 feet tall. This wall is 18'-6. Also, for wind, I am limited to minimum 3.5:1 ratio with a wood wall panel, and nowhere in this 24 foot long wall segment do I have a clear 5.3 foot wide section to bring the shear down. That's why I was looking at using one or a combination of several wood moment frames (PSL columns on the cords anchored to the footing, with headers or top plate collecting between). The moment connection at the top of the frame was gong to determine if I needed multiple frames or not. I really don't want to start in the direction of the Simpson SSW walls and felt I could engineer this low V (total 1500 lbs along 24 feet..63 plf) into a wood moment frame.

Any other thoughts/suggestions?

 

[RARWOOD]

A typical rule of thumb in wood design, is to avoid moment connections where every possible. Unfortunately to do so is not always possible. As both MiketheEngineer and JAE have indicated above the problem is with the conncetions. Tolerance in the fastner holes and deformation in the wood create member deflection before the joint is working effectively. Sometimes that deflection is acceptable and some times it is not.

If I was trying to do what you are describing, I would put in some small steel tube frames. Another option might be to put in some diagonal wood bracing.

 

[msquared48]

If you specify "no splits" which also applies to knots and wanes, you can double the allowable shear stress, outside of any load duration factors.

Mike McCann
MMC Engineering