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Wood Shear wall headers

Wood Shear wall headers

Wood Shear wall headers

(OP)
Ladies and gents:
Attached to this post are two details of built up wood headers in a wood shear wall.  The structure is a four story wood structure, located in a high seismic zone. These details are for the headers located in the exterior walls where the shear is transferred around the openings.  We had to use this method because of low aspect ratios, there are numerous openings in the exterior wall.  The maximum shear transferred is 700 lb/ft, at the lower floors.  The shape of the structure is in a U, we have found that wind governs in one direction, where seismic governs in the other direction.  The original header was to be (3) 2x12's with spacers. However, the architect is looking for VE issues and is questioning all the exterior headers.

Header A is what the architect wants to use and swears that this is "common in the industry".  

Header B was suggested by a person in our firm as an option.

I have experience in metal stud structures but not wood; but for only single and double story buildings.  Anything over three was steel frame with concrete deck and curtain wall framing.  Hence, Header B makes more sense to me since it resembles a box beam.
My questions are:
1. Has anyone seen Header A?  If yes, then how did you analyze the load path?
2. In a wood shear wall, where does the shear force load path occur? (I was told that all the forces occur on the exterior.  Is this correct?)
3. How does one analysis Header B?  (I was thinking: NDS – beam/column calc with shear converted to axial load, wind force as my out of plane bending, un-braced length of zero due to the top and bottom 2x6, divide the axial in half for each 2x8.)

Any insight/suggestions will be greatly appreciated. Thanks.
 

RE: Wood Shear wall headers

I have seen Header A used sometimes in "non-engineered" houses. Your architect is getting mixed up between what is common practice in houses that are not engineered and what is allowed in engineered commercial buildings. If he was able to structurally design 4 story structures in high seismic zone in your area, he wouldn't need you.

Stick with Header B!

RE: Wood Shear wall headers

I don't like either one.  I typically use (3) 2 X members for a header in a 2 X 6 wall.  That leaves 1" for insulation.

DaveAtkins

RE: Wood Shear wall headers

DaveAtkins detail is most prevalent in my area.

BA

RE: Wood Shear wall headers

I use DaveAtkins detail as well.

Also, how does one get from (3)2x12 to (2)2x8?

RE: Wood Shear wall headers

i assume that with a 2X shearwall above the header, you are dealing with a perforated shearwall system, unless the top of the header is at the bottom of the joists above?

I also agree with Dave Atkins - or use a 4X if appropriate which allows even more for insulation.

Mike McCann
MMC Engineering

RE: Wood Shear wall headers

I have seen all of the above.  In the single family detached header A is becoming more popular to allow for blown in insulation.
Header type B is a box header but using 2x material.  I'm not a fan unless insulation is placed inside.

Rday raises a good question, if the original header is (3)-2x12 then there is nothing that can be done unless engineered wood (LVL, LSL) is used to reduce material and could be cheaper than (3)-2x12.  (3)-1 3/4" x 7 1/4" should work depending on your load.  If the (3)-2x12 are overkill then reduce to (3)-2x10 or (3)-2x8 where possible.   

Jim Houlette PE
Web: www.evstudio.us
Online Magazine: www.evstudio.info
 

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