ACI 318-05 - Dragstrut connection to concrete 1

[PostFrameSE]

I am needing to transfer 11kips laterally out of a roof diaphragm into an 8" x 24" concrete counterfort as another engineer called it. It is a well-reinforced member with the highest stirrup within 2" of top of concrete. As I look at sizing anchor bolts at the top to transfer my 11k lateral load into the wall, I'm getting really confused by Section D.6.2.2. Is "h sub a" in Figure 6.2.1(b) that height above the highest stirrup where concrete is constrained? The definition says it's the thickness of the member in which an anchor is located. Well.....I don't think they're wanting me to input 10 feet!! Can somebody help me out here? Thanks.

 

[civilperson]

Take the distance from the near anchor to the face as c sub a one. If the height of your counterfort is greater than the distance 1.5x(c sub a one), then use figure RD.6.2.1(a) which has breakout dimension of 1.5x(c sub a one) in the vertical. The stirrups add to the concrete breakout strength by the area of steel crossing the pyramid plane times the yield strength.

 

[PostFrameSE]

Thanks civilperson,

Would it be too conservative, since my stirrup is capable of resisting all of that 11k load (despite being held down 2" or so from the top of the concrete) that my anchor bolt could be sized as a bending member....as though the top 2" of concrete wasn't even there and my bolt was loaded laterally with a ~2" moment arm?

I have a group of anchors (3) in a line spaced 6" apart. The one is 4" from the loaded edge, and the other two of course are 10" and 16" respectively from the loaded edge. According to one of the drawings in fig. RD.6.2.1(b) it shows my scenario with a two bolt set up. If I have three bolts can my "c sub a one" be 16"? It appears if I use that figure, that I'm not allowed to use the depth of 1.5ca1 but rather just "h sub a."

I think I'm making this more complicated than I should.

Thanks for your time

I think that makes sense. I have a group of anchors in a line. My nearest face of my anchor is 4" from the en

 

[civilperson]

Connect the three anchor bolts by using a plate with holes to apply the lateral load. Share the load to the three bolts, each receiving one-third, and analyze just as you described as a 2" cantilever beam. Use the stirrup strength to resist the horizontal shear if it is sufficient, (quicker than calculating concrete side blowout strength). Anchor bolts need stirrups in the top 5" of the concrete, two No. 4 or three No. 3.

 

[PostFrameSE]

That is what I was planning on doing with the plate. One last question........."anchor bolts needing stirrups in the top 5" of concrete...." What am I missing? I'm not finding that provision in the ACI. Can you help me out please?

Thanks so much for your professional advice!!

 

[civilperson]

ACI 318-08, 7.10.5.6

 

[PostFrameSE]

Thanks so much. I must have glanced right over that!!!

 

[PostFrameSE]

Civilperson,

I know why I glanced over it.......my concrete is not a compression member as it really is only transferring shear forces. I think I read the note under 7.10.5 ties referring to compression members and skipped over the rest. I don't do a whole lot of concrete design. Having those ties up top can't hurt.

Thanks again.