Anchor bolts in wood sill plates

[bnickeson]

Here is a strange "What was the Code thinking?" question...

In the NDS spec, section 11.5.1 deals with minimum bolt edge/end distances and spacings. 11.5.1.3 and Table 11.5.1.C states that the minimum edge distance needs to be 4D from the center of a bolt. So this begs the question, what do you do with 2x4 wall sill plates? Using a standard 5/8" anchor bolt centered in the wall, you don't meet the minimum 4D requirement, nor do you using the minimum 1/2" anchor bolt required by the IBC. So, whether intentional or not, the code is basically implying you cannot use 2x4 walls with anchor bolts that are subject to out-of-plane loads.

I'm just designing a little multi-stall garage, but it is taking a decent amount of out-of-plane load (100+ mph winds) especially at the king studs. Has anyone dealt with this code issue before or am relegated to using a 2x6 wall? I certainly wouldn't mind it but the architect might...

Thanks.

 

[msquared48]

Well, there is the "nominal" size of the sill plate logic.

And there is the direction and intensity of the load to the anchor bolt you are considering, affecting the edge or end you are considering...

I will have to check my NDS tomorrow.

Mike McCann, PE, SE (WA)

 

[XR250]

Use more 3/8"Ø bolts...

 

[woodman88]

Per the attached link a report was done on this. Funny thing is that I can not find the report.


Wood Sill Plate Anchorage to Concrete Report

With assistance from AWC, a report on anchor bolts connecting wood sill plates to concrete with edge distances typically found in wood frame construction is complete. The report findings will be used to support a change to the International Building Code to relax restrictive provisions for anchoring of wood bottom plates to concrete for resistance to seismic forces.

Anchor strength provisions in Appendix D of American Concrete Institute (ACI) Building Code Requirements for Structural Concrete, ACI 318, establish “non-ductile” anchor design capacities that are approximately 1/3 of that historically used for 2x4 and 3x4 wood sill plates loaded parallel to the edge of the concrete. Test results show that ductile yielding in accordance with the National Design Specification® (NDS®) for Wood Construction Mode IIIs or Mode IV (see Figure A) is consistently achieved prior to concrete failure. While ductile connections are assigned increased design capacities in ACI 318, the bending yield behavior of dowels in wood connections is not specifically recognized. An example hysteresis and envelope curve for a single test of a 2x4 wood sill plate with 5/8” anchor bolt exhibiting ductile behavior is shown in Figure B.

Bolt Mode IIIs (wood bearing in side member and dowel bending in concrete)

Mode IV (wood bearing in side member and dowel bending in wood member and concrete)

Figure A. NDS yield modes IIIs and IV.

The study was sponsored by the Structural Engineers Association of Northern California (SEAoNC) Special Projects Initiative. Other contributors to the project and development of the report include members of the SEAoC/SEAoNC Light-Frame Subcommittees, SEAoC Seismology, Simpson Strong Tie, and the American Wood Council. Download the report.

For more information, contact Phil Line at 202-463-2767 or Philip_line@afandpa.org.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[CELinOttawa]

Here you go Woodman: See attached.

 

[manstrom]

More good reading material. Thanks for the pdf.

1/2" anchor bolts as sill plates is probably a 100 year old concept. I think this is a case where the NDS doesn't conform to typical details rather than the other way around. I am not a fan of anchor bolts for sill plates for a number of reasons, but I don't think it's a dangerous option.

If you are concerned, use Simpson MASA straps. Stronger and easier to install.

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

 

[XR250]

MASA straps only work on cast-in-place concrete foundation walls - a rareity in many parts.

 

[JAE]

I love this conclusion of the report that CELinOttawa posted:

The test data for 2x4 and 3x4 plates indicate that the average peak strengths were:  
 more than [bold]6 times[/bold] higher than ductile design strengths obtained from ACI 318‐05 (and ‐08)
Appendix D, and  
 more than [bold]4 times[/bold] higher than the allowable capacity obtained from IBC‐2006 (NDS‐05)  

After using App D for a while it is apparent that its values are very very conservative.

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[JAE]

And another quote from it:

 Many practicing engineers and
building officials are currently mystified by the low anchor bolt capacities obtained from the application
of Appendix D equations for wood framed construction in seismic design categories D, E and F.

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[JAE]

But that document is for load parallel to the wall - not perpendicular as per bnickeson's question.



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[Archie264]

I am not a fan of anchor bolts for sill plates for a number of reasons...

Manstrom, care to elaborate on that? I'd be interested in hearing it. Thanks.

 

[KootK]

For me:

1) Pins seem easier to install.
2) Bolts installed with limited edge distances are terrible for out of plane wall loads. That cross grain tension business.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bnickeson]

Yes, pins (assuming you mean power driven fasteners) are easier to install, but they have essentially zero uplift capacity due to the wood sill plate and how the uplift load is applied. They also have very little lateral capacity (not much more than a nail of the same diameter). I don't believe the IBC permits them in exterior walls, and even if they did it's not a good idea to use them for various reasons.

 

[KootK]

I used to use the round head version of these: Link.

Although it seems that the little guys work too for some applications: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.