Force transfer around openings

[WBUW]

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

 

[WBUW]

Also if anyone knows where to find an example of an analysis / detailing that'd be great. I am NOT interested in regular preforated wall example as per IBC I already know how to do that, I am looking for a rational analysis including moments, shear and compression etc not a look at this fancy table and multiply by .82.

I saw a thread similar to this previously and I don't think it was ever resolved...

 

[skier1578]

The SEAOC design manual, Vol. II has an example of this. See design example 1, 9a.

 

[WBUW]

I found one example in the past couple days but it was from DCLU not SEA...

Thanks!

 

[sean2000]

My understanding of the perforated shear walls:

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!

 

[RARWOOD]

Contact APA-EWS. In the last couple of years there have been quiet a few code changes relating to shear wall loading and aspect ratios. APA-EWS can provide you with several ways of solving your problem.

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.

 

[WBUW]

Yes, the linear process is what everybody uses... However, the stiffest element in the wall line will take the load, ergo a wall with 2" Edge nailing vs 6" edge nailing cannot be in the same wall line.

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?

 

[RARWOOD]

WBUW

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.

 

[KeySol]

WBUW-

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.

 

[seattlemike]

There are two types of construction being discussed here, and it is important to distinguish between the two (because the code does, and one requires more holdowns).

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.

 

[KeySol]

I think that you are incorrect seattlemike. When designing shearwalls using the force transfer method you are also reducing the number of holdowns in the wall to only at the ends of the entire wall. You do still have to have wall segments at the ends that meet aspect ratio but it is a different hight used to determine this, see IBC03 figure 2305.3.4 This is the reason behind the force transfer method. Otherwise there is no reason to use this method because it would cost more in construction cost then typical pier design.

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?

 

[WBUW]

I think the force transfer method requires hold downs at each segment. I think the 3.5:1 ratio still applies too, only instead of the wall height you get to use the pier height as shown in the IBC...

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...

 

[KeySol]

You are coorect in saying that the aspect ratio still applies, and yes only to the wall pier not full height of wall. And yes the analysis does indicate whether or not you need strapping and blocking around the edges of the opening. But if you have to have holdowns at each wall segment there is no reason to do the analysis of force transfer around the opening. The standard shearwall pier design will suffice. The entire point of doing force transfer around the openings is so you can only have holdowns at the ends of the wall. Not at each segment.

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.