Epoxy For HDU Holdown Anchor Bolts Residential Shallow Footing 1

[jdgengineer]

I wanted to see what you all are doing to defend the use of epoxy for HDU holdowns in a typical residential footing. If we assume a shallow footing (say 12" wide) and the outside face of the studwall is at the outside face of concrete (2x4 wall) the edge distance from the center of the bolt to the edge of the concrete may only be 1 3/4".

Following Appendix D procedures, I calculate a capacity of that bolt to resist seismic forces (taking into account overstrength magnifications) to be ~1,000 lbs. See attached calculations from Simpson Strong-Tie software.

This would essentially mean, that epoxy could only develop roughly 1/3 the capacity of an HDU2 holdown where the edge distance is so close.

As a result, our office has, for the past few years, not allowed epoxy in these conditions and require that the contractor drill down through the footing and place a nut and washer on the underside of the existing footing. We obviously get a lot of flack from contractors for this approach. The footing would still need to be checked for bending / shear as a result of the uplift. This is often difficult to defend in existing conditions that are minimally reinforced.

What do you all do?

 

[jdgengineer]

I have heard, but don't know the specifics, that AC308 was revised to allow epoxied rebar to be designed following Appendix D requirements OR Chapter 12 development / lap splice requirements. Part of me is tempted to start threading rebar for attachment to holdowns (instead of all-thread). Obviously, that is a bit "gimmicky"

 

[structSU10]

shouldn't it be 0.7*2.5? that would give 1893#.

Generally I try to get as much edge distance as I can to avoid the problem. If I need more room, I make a fatter wall, or move the end of my shear wall in if I have to.

Just curious, How do you justify the drill through method? this is a footing cast against the earth, such that they have to undermine the footing to install? is the nut protected after the fact? is flow fill used to fill the void?

 

[jdgengineer]

I don't believe it's 0.7 * 2.5.

To convert strength to ASD we multiply by 0.7 (Assuming only seismic force is relevant)
To account for increasing the forces by Omega, I've reduced the capacity by omega (moved omega to other side of equation)

They would have to "undermine" the footing for the nut. But we are talking only a 18" wide hole or so. We have assumed the footing can span across that small opening. Yes, the nut is protected by grout / concrete with 3" cover on all sides

From a calculation point of view we say the drill through option is now a bearing problem on the bottom of the concrete and therefore outside of the scope of Appendix D. It's also a bit "gimmicky", as you still have to check the footing for bending / shear which will often govern.

 

[jdgengineer]

This would be an existing condition where the studwall is already framed on the footing. Therefore, without bending the bar and drilling the hole at an angle I believe we are stuck with the 1 3/4" edge distance. We have also added "haunches" in the past or "underpinning footings", but both of these seems a bit much for small holdowns considering that the existing footing outside of the haunch or underpinning is likely the weak link regardless.

 

[structSU10]

The load combination has 0.7*Ω*E, so ignoring other loads, R = 0.7*Ω*E so E = R/(0.7*Ω)

 

[jdgengineer]

I think we are talking the same thing about load combinations, but just handling the definition of E a little bit different. We typically convert to ASD for wood framing to be more compatible with Simpson Catalog and shearwall values and then back convert for concrete as necessary. A bit confusing, sorry.

Simpson Catalog = ASD Level = 0.7 * E,strength = "E,ASD"
E, strength = E,ASD / 0.7

R = Omega * E,strength = Omega * E,ASD / 0.7.

Therefore, if you were trying to develop an HDU2 holdown w/3,100# capacity (ASD loading), I believe you would need 3,100 / 0.7 * 2.5 = 11,071# Appendix D strength capacity.

Regardless, about load combinations, how are you defending the HDUs with epoxy? Even if I misinterpreted omega value, you are still only ~0.5 of HDU2 capacity with common residential edge distances.

 

[perland]

Attached is a detail that I use for conditions where HD's with epoxy don't work. The loads used for this detail are uplift x 2.5. Hope this helps.

 

[jdgengineer]

Hi perland, thanks for the detail. We've done similar details in the past. Only issue is the strap would be visible from the outside unless there is some type of finish over the exterior concrete (which sometimes there is).

Do you use epoxy for holdowns? If so, how do you defend? Or do you typically go to this detail?

We run across drawings from other engineers from time to time and I still see HDU8 holdowns epoxied with very close edge distances. These are done by respectable engineering firms. I'm assuming they are just using old typical details without rerunning the calc for Appendix D, but we have these details thrown to us often when contractors don't like that we don't allow epoxy.

In talking with Simpson Strong-Tie, they seem to be aware of the issue. They've mentioned other engineers commonly neglect the overstrength factor and make some argument that the holdown device is ductile. Simpson I don't believe agrees with this argument and even so you would need to design for at least 3x HDU capacities to arrive at expected strength vs allowable strength (assuming testing governed connection strength).

 

[XR250]

Never have had to do a retrofit, but as other have mentioned, I typically fatten up the wall and install the HDU flush with the interior face.

 

[perland]

Hi, dgengineer

I have also had multiple conversations with Simpson Strong-Tie about this issue. Basically it is very hard to make an epoxy anchor that close to the edge work for a "good" uplift load. For existing footings, I use the above attached detail. If a contractor complains about the visibility of the strap then I give them an option of drilling thru the footing with a new concrete pad below.

" ... These are done by respectable engineering firms. ..."
Not knowing the details of a specific design, all I can say is that probably they are making a mistake. If so, the two main reasons are 1) Loads provided by Simpson's software are in LRFD and need to be reduced for ASD and and 2) The calculated overturning loads need to be multiplied by Omega that probably it is not being done so.


The requirements for ACI Chapter D are (at least to me) very confusing and counter intuitive. For example if you increase the top edge distance in my detail from 6" to 7" inches, the connection will fail because per chapter D the 7" will make all shear to be taken by the top bolt rather than two bolts when it is 6"!


Jason Oakley was the engineer that gave me a ton of information about this issue.

http://seblog.strongtie.com/author/joakley/

Victor

 

[jdgengineer]

Thanks guys for the input. Sounds like our findings are more or less in line with everyone elses. Thanks for all the input.

perland -- Appendix D calcs sure don't always make sense. Similar to the issue you described, if you make the anchor embedment deeper with a shallow edge distance the anchor often will be weaker due to the full projected area of breakout growing at a much faster rate than the truncated cone and additional embedment. I have a hard time believing a deeper anchor is weaker than a shallower one, but what do I know.

I personally think Appendix D is overly complicated. I wrote a fairly intense spreadsheet before Simpson / Hilti came out with their software. My calculations matched up perfectly with their software calculations once released, but it took a lot of iterations in the spreadsheet to dial in all the factors. I am a firm believer that anyone attempting to do a hand calc for Appendix D will make a mistake somewhere. To essentially 100% have to rely on software for something as simple as an anchor to concrete seems a bit odd to me.