Interior Shearwalls for Wood Framed Structures 2

[medeek]

I'm still not sure that I am entirely satisfied with my treatment of interior shearwalls for residential work lately however I have come a long way in the way I analyze each situation a little more carefully than before. Thanks to some good texts and some excellent advice and feedback from engineers on this forum I"ve managed to cobble together an acceptable interior shearwall detail drawing. I have posted it to my CAD library here for all who are interested:

http://design.medeek.com/cad/cadlibrary.html

My reason for posting this structural detail drawing is to elicit further comments or recommendations which would hopefully improve this structural detail even further, not only for my own use but for other engineers who face similar challenges with residential structural engineering.

I am sure there are other situations or configurations which this detail(s) does not adequately cover and I would be interested in exploring those as well.

A confused student is a good student.

 

[manstrom]

A few comments:

The contractors that I work with almost never get an anchor bolt in the right place for a hold down. Most of the time these are drill and epoxy. It's ok to prescribe an embedded bolt, but have an option for a post installed anchor

You may want to specify shear nailing for the shear wall to the elements above / below. Not the sheathing nailing, but the shear connection between the wall and the blocking, etc.

Nails come in many flavors. Most gun nails are "sinker" dimensions or so. "Common" nails aren't common. Check out ESR 1539 for equivalent nailings to code minimum attachments.

http://www.icc-es.org/Reports/pdf_files/ESR-1539.pdf

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[medeek]

Updated the drawing to include some notes about nailing the SWL bottom plate to the rim joist/blocking below as well as another note for the connection between the top plate and shear panels.

I haven't decided on what is the best option or way of calling out an alternative to the cast in place bolts, still thinking about that.

I see 16d nails being called out in many structural details I've studied but maybe they are implying a box nail (0.135" DIA) in these call outs.

A confused student is a good student.

 

[jayrod12]

It is a quite common detail to have nails installed partially into the bottom of you nailer board and cast into the concrete. I personally don't use it, but I've seen it more times than I can count.

 

[medeek]

You'll notice that this detail page also includes a typical eave detail which shows sheathing nailing at the overhang. I haven't really given too much thought to this nailing other than to reference the diagram of a similar overhang situation I found in the the WFCM 2012. However, upon further investigation I noticed the WFCM has some really nice prescriptive tables that appear to be quite useful in this regard.

Most of my residential projects are in a high wind region (155 mph ultimate -> 120 mph ASD) so some consideration for nailing the roof diaphragm for out of plane forces is in order. I'm wondering what other engineers are typically doing for this sort of thing. So far I haven't gone so far as to call out special nailing requirements for perimeter nailing versus interior nailing but if I were to follow the WFCM prescriptive tables the nailing specification for the roof sheathing could become quite complicated. Would this be too much to expect from your average residential contractor/carpenter?


A confused student is a good student.

 

[jayrod12]

An average carpenter should know the standard nail spacing, 6" perimeter 12" field. If you need more than that then say so, but put it clearly in bold letters right on the detail or else you likely won't get it the first time.

 

[medeek]

Just when I think I've got shear walls more or less figured out... something comes along and significantly humbles me.

Looking at the interior shearwall example in the drawing above you will note that I've shown a double 2x bottom plate, which it turns out is completely wrong. Somewhere I had learned that when the ASD unit shear of a shearwall exceeded 350 plf that it should have a 3x or (2) 2x sill plate. My first mistake here was not differentiating between a sill plate and a bottom plate (sole plate). They can be one and the same but with a TJI floor they are obviously not the same thing.

If I was to use a 3x sill plate under some of my higher loaded shearwalls then all of the sill plates would need to be 3x or double 2x otherwise the TJI floor would be all wonky (at different levels). Using a 3x bottom plate really doesn't make sense either since there are no adjoining panel edges at the bottom of the shearwall, interior or exterior.

Digging a little deeper I find that the 2006 IBC 2305.1.11 required a 3x sill plate for an ASD shearwall load over 350 plf. This was omitted from
the 2009 / 2012 IBC due to plate washer requirements (see SDPWS 4.3.6.4.3) and it appears that most of the shearwall code in the IBC was removed and it now references the SDPWS. The only reference I find pertaining to a 3x member in both the SDPWS-2008 and the SDPWS-2015 is for adjoining / abutting panel edges. Is there any current code requirement for a 3x sole / sill plate at upper floors or even at foundation levels?

It appears I've been operating with outdated information.

A confused student is a good student.

 

[medeek]

The only rational reason I can see for using a double or even a triple 2x sill plate is when the shear force is so large that the anchor bolts will crush the wood fibers in their holes and there is not enough length in the shearwall to increase the number of anchor bolts. Hence, you will see portal frames with only enough room for 1 or 2 anchor bolts which need more capacity from the wood and a triple 2x sill plate is utilized. Other than this type of situation the number of anchor bolts and their diameters should be able to be increased such that only a single 2x sill plate is ever required. Correct me if my logic is wrong here, I might be missing something.

With regards to a shearwall framed on top of a TJI floor. If bolt diameter is maximized and shearwall length is too short to realistically add more bolts one could always add flat blocking between the TJI/lumber joists similar to what is shown in the portal frame details. In this way the TJI floor remains at the same level as the other floors framed under non-shearwalls.

A confused student is a good student.

 

[medeek]

Or one could turn this around and offer the option to use a 3x or double 2x sill plate so as to reduce the number of anchor bolts or their diameters. If I understand it correctly typically the critical interface with sill plate anchor bolts is the wood fibers failing in bearing or splitting and not the concrete, embedment or the bolts themselves.


A confused student is a good student.

 

[medeek]

I just found this other thread on 3x sill plates:

http://www.eng-tips.com/viewthread.cfm?qid=273357

Looks like I'm not too far off.

A confused student is a good student.

 

[medeek]

As well as this paper which explains the reason for the elimination of the 3x sill plate:

http://www.strongtie.com/ftp/articles/2010-Building-Code-Update.pdf

I guess this is old news for pretty much everyone but I'm surprised nobody called me on it with regards to the detail drawing above.

A confused student is a good student.

 

[medeek]

I've updated the shearwall calculator so that it no longer requires a 3x sill plate when the unit shear exceeds 350 plf. However, I have not eliminated the option for it either, in case a more substantial sill plate is required due to anchor bolt requirements and space limitations.

I wonder how many practicing engineers and building officials are still using the 2006 code and may not even be aware of the changes that removed the 3x sill plate requirement. I recently specified 3x sill plates for a local residential job and the building dept. did not call me on it. In fact, their inspector noticed the builder had not installed the 3x sill plates and had subsequently required a letter from me for a fix. Next time I stop in I'll bring it up to see where they stand and hopefully eliminate any future confusion as well as apologize for my outdated methods.

A confused student is a good student.

 

[CELinOttawa]

Medeek - This is a very impressive effort. Your truss calculator as well. Kudos!

 

[medeek]

Thank-you CELinOttawa.

The truss calculator is probably the largest project I've ever programmed but is still mostly incomplete as it currently stands. It is primarily an academic exercise on analyzing MPC trusses per the TPI 1. In order to make is a usable design tool there is a to-do list a mile long that I need to complete. Unfortunately, lack of a way to fund the project severely limits my progress at the moment. I have had a number of solar panel companies interested in its progression contact me with questions and suggestions so it may have some real world potential.

A confused student is a good student.

 

[CELinOttawa]

If you were interested, I'd actually be quite keen to help with the development....

 

[medeek]

Shoot me an email with what you propose.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com