Tensile strength of concrete

[keyPitsimplE]

Reference thread507-111215

I have an old house remodel where we need to attach hold down straps to an existing concrete basement wall. The wall is 6" thick and appears to be in good condition, but is likely unreinforced or minimally so. Based on past local experience I am certain the concrete would test at much higher than 3000 psi for compressive strength. I realize it is not ideal to rely on the tensile strength of concrete at all, but when remodeling old houses we do not always have the luxury of doing everything perfectly. Pouring a new basement foundation wall is not practical. Drilling and epoxying threaded rods into the top of a 6" wall not a good option because I know it would not get me the strength I need (about 10,000 lb seismic uplift force). I am considering using a 1/4" x 3" steel strap with 7/8" bolts epoxied 4" deep into the side of the wall and (24) 1/4" x 2-1/2" SDS screws into the 4x6 DF2 hold down studs. (2) A307 7/8" bolts have an allowable shear capacity of 6 kips each for the steel. Ignoring cracked concrete for a moment, the 2012 catalog for Simpson SET Epoxy gives me an allowable shear load of 7085 lb per bolt with a 12" critical edge distance & 4" embed with 2000 psi concrete. If I set the (2) bolts 12" & 18" respectively below the top of wall, and figure a failure plane at 45 degrees up each side from the upper most bolt to the top of wall (ignoring the lower bolt to be conservative), using 100 psi allowable tensile strength and the resulting 204 sq in of "failure area", I get a 20,400 lb rough check. I have had the discussion with the owner about what current code requires with respect to cracked concrete, etc. vs. what the older codes allowed, so he is aware and willing to accept the risk. Much of the house will remain old, so the remodeled part will certainly be stronger. I realize this is my own professional judgement call, but does anyone think this is an irresponsible route to take? If so, what would you do instead?

I sincerely appreciate any thoughtful input I can get.

 

[KootK]

My thoughts:

1) I'm fine with using concrete in tension this way. Most of ACI 318 appendix D relys on concrete tensile strength.

2) It seems as though you've come up with your own method to approximate appendix D. I'd just use appendix D outright.

3) I don't believe that it is appropriate for the owner to be defining his own level of risk in this situation.

4) Tensile stresses on your 204 in square failure area would be acting perpendicular to the failure plane and thus at 45 degrees to the applied load. As such, your capacity would be multiplied by 0.707.

5) Doing it your way (not app D), I would argue that it would be best two use the plain concrete shear provisions on each side of the breakout cone rather than making direct use of concrete tension. Perhaps it's the same thing at the end of the day.



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.

 

[Ron]

Agree with KootK that your failure mode is shear-tension, not direct tension. Your method is plausible as long at the numbers work. If your capacity is insufficient, engage more of the wall, provided you don't overlap the shear planes.