Force transfer around openings
Force transfer around openings
(OP)
Per IBC 2305.3.7.1 allows for alteration of shearwall design by a "rational analysis". (Actual engineering!) I would consider the shear transfer mechanisms to be the wall top plates, to the top window sill with blocking full length across the segmented wall providing nailing for a coil strap or something similar, to the bottom window plate fully blocked across the segmented wall and strapped, to the sill plate/anchor bolts. Uplift to occur at the ends of each segmented wall, hold downs specified there if req'd. The first firm I worked for commonly specified a detail like this when we ran into trouble with the 3.5:1 ratio and needed the wall length.
The firm I am currently at has some difficulty justifying the use of this due to the relative stiffness of the other walls in the same wall line. Ergo, the wall segment with the highest stiffness will absorb more of the load than the rest. Does anyone else currently specify something similar (I know someone does I saw it in a church on lopez island under construction) and how do you contend with the stiffness issues?
Thanks all,
Will
The firm I am currently at has some difficulty justifying the use of this due to the relative stiffness of the other walls in the same wall line. Ergo, the wall segment with the highest stiffness will absorb more of the load than the rest. Does anyone else currently specify something similar (I know someone does I saw it in a church on lopez island under construction) and how do you contend with the stiffness issues?
Thanks all,
Will






RE: Force transfer around openings
I saw a thread similar to this previously and I don't think it was ever resolved...
RE: Force transfer around openings
RE: Force transfer around openings
Thanks!
RE: Force transfer around openings
1. each segments of the shear wall you count for shear capacity still need to meet the required 3.5:1 ratio.
2. the whole idea of perforated shear wall is to reduce the hold down anchors. these hold downs could we huge and expensive.
3.the actual shear wall capacity will be reduced due to reduction factors. what you gain is the reduction of anchors only.
Correcte me if i am wrong!
RE: Force transfer around openings
The general practice in wood design is to distribute the shear load in the shear walls on a linear basis not a stiffness basis. Donald Breyer's book "Design of Wood Structures" has an excellent discussion of the topic and helps one to understand this type of approach.
RE: Force transfer around openings
If a blocked wall is in the same wall line, with the same edge nailing (aside from the straps) could you consider that part of the wall line? Does this book address that specifically?
RE: Force transfer around openings
In his book Donald Breyer explains the basic design assumptions made in using a linear design approach.
I'm sure there are times when an engineers stucture doesn't meet those assumptions and the enginner has to take a different approach.
In explaining the linear method Breyer compares a wood shear wall system with a masonry system. If you don't have Breyer's book I would highly reccomend it. Another good book is "Wood Engineering & Construction Handbook" edited by Keith Faherty & Thomas Williamson.
RE: Force transfer around openings
I have done some research to figure out how to actually design the conection around the openings in the shearwalls. There is no straightforward guide that i have found to be useful. Teh Wood Engineering and Construction Handbook by Faherty has an example but is limited by lack of discussion and many, many errors (typos). I am in agreement with you that the stiffer walls will resist more loads but that when using a flexible diaphram analysis you dont really consider that.
If you have since found other good reference materials that show practicle examples of shearwall anaylsis with openeings let me know.
RE: Force transfer around openings
Force Transfer method: forces are transfered around the opening using blocking and straping. Each wall panel on each side of the shear wall still requires holdowns but the 3.5 ratio is avoided as mentioned.
Perforated Shear Wall: the IBC03 code limits these to 350#/ft. The number of holdowns are reduced to only the ends. I believe a force transfer analysis must be done.
RE: Force transfer around openings
Also for performated shearwall design you do not have to detail force transfer around the openings because the IBC03 reduces the capicty rating of your shearwall. This is how the ICC account for the unknown factors of force transfer around openings.
This discussion is all good, but there is still the issue of actually figuring out (engineering) the transfer around openings. What is an acceptable method for this analysis?
RE: Force transfer around openings
From what I've seen part of the analysis is looking at the connection b/t the cripple under the header and determining if a strap is req'd...from that I would assume if you are strapping there then you would have to locate a hold down below it...
I would say the main benefit is being able to cut simpsons walls out of the construction costs...
I started a spreadsheet but I've been pretty busy lately...
RE: Force transfer around openings
You need to look at the WOOD ENGINEERING AND CONSTRUCTION HANDBOOK by Faherty. His analysis shows how you can design with force transfer around openings (although it does have many typos and is hard to follow his assumptions) and there are only compression and tension forces at the ends of the wall. Meaning that holdowns are required at the ends of the entire wall not each wall segment.
Again, there is no reason to do the design of transfer around openings if you still have holdowns at the ends of each segment.