Wood Blocking in Stud Walls 1

[KarlT]

For wood framed walls I have seen the following general notes:
-Provide wood blocking at 4'-0" max o/c - typ. (seems like overkill to me!)
-Provide wood blocking at mid-height of all load bearing stud walls (works fine for short walls, but I would be less comfortable with that for tall walls say 20 ft and above)
-Provide wood blocking at 8'-0" max o/c - typ. (seems more reasonable, since most walls 8'-0" high that are sheathed and drywalled don't need additional blocking) Is the blocking intended more for temporary conditions, where the wall may get loaded before it gets sheathed on one side? What max. blocking spacings does everyone else typ. use in your general notes?

 

[JAE]

Similar questions were raised in this thread:

thread173-95297

 

[dicksewerrat]

Some times the blocking is a fire stop.

 

[KarlT]

Thanks JAE, I feel the same as boo1......you are my hero too! I wish I would have discovered this site a few years ago, it would have saved me a lot of stress and worry when I didn't have much help as a junior engineer.

I was thinking a bit more about this and also reading up on the wood design manual and I think the 4' blocking dimension is suited for a wood shearwall application in which the plywood sheets are placed in a horizontal, overlapping pattern. This effectively ties the edges of the plywood to the wall and gives you the full "blocked" shearwall design values. If the blocking is not placed at all plywood edges, then a reduction factor is applied to the shearwall strengths to account for it being an "unblocked shearwall"

 

[JAE]

KarlT - yes, I agree on the value of this site. In fact, I once asked an "older" engineer mentor of mine about his preference for staying involved heavily with ASCE, NSPE, etc. and he answered something to the effect that if you ever get in a bind, or have a serious issue to resolve, it is very helpful to have other engineers that you KNOW that you can call on to get advice, provide objective observations, etc.

While local engineering societies provide that personal relationship tie, Eng-Tips sort of does this as well. After spending some time on the site, you get to "know" some of the handles and get a flavor for their expertise and preferences.

 

[mark1234]

KarlT,

I know this thread is old but can you site the literature which provides a reduction factor for an unblocked plywood shearwall? What is it?

Thank you.

 

[bjb]

Are you even allowed by the IBC to have an unblocked wood shear wall? In 2306.4 it specifically requires it, and also in footnote b of table 2306.4.1. The code also allows calculations for shear walls to be based on the principles of mechanics, so maybe APA has something that allows you to come up with the capacity for an unblocked shear wall.

 

[archeng59]

all edges of an APA rated sheathing panel used for a shear wall needs to be supported. it's possible to eliminate the blocking if the sill plate to cap plate dimension is 8 ft and the sheathing is installed with the long side (8 ft) of oriented vertically, which is typical for most wood framed buildings I design. However, I have designed a few buildings where the sill plate to cap plate dimension is 10 ft. in those cases, I used blocking along the top of the 8' panel. Another instance where I used blocking was when the contractor requested approval to orient the long side of the 8 ft panel horizontally. I told him to use blocking, which ticked him off but he did it anyway.

The NDS chapters 3 and 4 provide guidance on unbraced lengths of compression and flexural (bending) members. sometimes blocking is needed to satisfy the l/d and d/b criteria.

 

[DaveAtkins]

Per current codes, there is no such thing as an unblocked shear wall. Unblocked diaphragm, yes, but not an unblocked shear wall. I always have this note on my drawings: PROVIDE BLOCKING BETWEEN ALL STUDS AT HORIZONTAL JOINTS IN OSB.

DaveAtkins

 

[boo1]

If you use the unblocked diaphragms shear values, then the blocking may omited for shear walls.

Building tip: When we use the 10' high walls we use a two foot rip of PT plywood (w SST nails) at the bottom, fully blocked.

 

[DaveAtkins]

boo1,

Where in the Code does it say that you can use unblocked horizontal diaphragm values for a vertical shear wall?

DaveAtkins

 

[bjb]

It seems to me that the 2000 IBC requires blocking at panel joints in wood shearwalls in 2306.4.1.

The APA has a document "Report 154, Wood Structural Panel Shear Walls" where they talk about unblocked shear walls and give some ultimate loads. They are sensitive to wall stud spacing. Their use doesn't seem to be supported by the IBC, so I wouldn't design one. Maybe this info. could be of use if you had to come up with the capacity of a shearwall that had been built without blocking, maybe in an older building. Since NY only recently went with a model building code, I wouldn't be surprised to find some older buildings having unblocked wood shear walls. You can download it from the APA website.

 

[boo1]

In essence, the shear wall table is really just a “special case” diaphragm table. Shear walls are little more than cantilevered diaphragms. Can you use the unblocked diaphragm table for vertical shear wall values?

APA discussed shear walls in:
Design and construction recommendations for engineered diaphragm systems in floor, shear wall, and roof systems. Updated December 2004. (Form L350 - 32 pages)

I use Florida building code and I could not find a requirement for blocking in my code book. I see unblocked homes all the time in my area. I agree though, anything I design is blocked.

 

[boo1]

see nice report at:

http://www.icc-es.org/reports/pdf_files/ICC-ES/1539.pdf

 

[boo1]

What is the requirement for the attachment of the top of shear walls? Must they terminated to the roof diaphragm?

 

[archeng59]

not certain I understand your question completely, boo1. but will answer that the shear wall provides lateral stability for the building and the roof diaphragm acts to transfer lateral loads to the shear walls. they need to be tied together adequately to transfer the diaphragm shears to the shear walls. there are clips available from Simpson and other manufacturers that can be used.

 

[bjb]

The 2000 IBC in 2305.1.4 requires that a positive connection is required between the shear panel and the attached components. In the IBC structural provisions handbook on page 320, they have some figures that illustrate what they are trying to say and they all show blocking. They don't come out and explicitly say it, but it seems to me like the IBC is expecting full depth blocking. As long as you can provide a reliable load path to transfer the shears, you should be ok. I just don't see how you can do it without blocking.

If you have something like trusses with deep energy heels, I question if metal clips can transfer roof diaphragm shears into the wall. In the case of wind, there might be an uplift force concurrent with the lateral force, and you have to check the clips with an interaction equation. I have seen test results for shear transfer between a sloped roof diaphragm and walls without blocking, and the values increase when clips are used. However, these tests were based on trusses with heels that were about 6" deep. With something deep like an energy heel or I joist I think there is an increased tendency for the truss or I joist to overturn. For these situations, I think you need full depth blocking. The APA Engineered Wood Handbook goes into this, I found it helpful. The transfer of roof diaphragm shears into the walls is an area where it seems like there is not a lot of agreement between engineers. Prior to adopting the IBC, I never saw full depth blocking in my area.