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Transfer of tensile forces in anchor reinforcement

Transfer of tensile forces in anchor reinforcement

Transfer of tensile forces in anchor reinforcement

I have a concrete pedestal 3’x3’x 4’-6” high supporting a 14”dia x 40’ high stack. It has 12 nos. of anchor bolts in a bolt circle diameter of 18-3/4” and is closely spaced at approx. 4.8”. The failure that I would like to focus on is concrete breakout in tension.
ACI 318-11 Sec. D5.2.9 indicates that “Where anchor reinforcement is developed in accordance with Chapter 12 on both sides of the breakout surface, the design strength of the anchor reinforcement shall be permitted to be used instead of the concrete breakout strength in determining ΦNn. A strength reduction factor of 0.75 shall be used in the design of the anchor reinforcement”
In my previous projects using ACI 318-05 when I encounter this type of failure, what I have understood is to design the pedestal with sufficient embedment length and area of rebar in tension. Lap an anchor bolt according to a a 35 degree tensile breakout cone.
1. Have the anchor with sufficient edge distance of 6do then place my rebar ≤ 0.5Hef from the anchor centerline.
2. Get the required embedment length Hef. Calculate development length as per ACI 318-11 Sec 12.2.3 Eq 12-1 plus say assume 2” (as clear distance of vertical bars to top of concrete) plus X/1.5 where X is the actual distance of the center of the anchor to center of the reinforcement.
3. Calculate the max tension load T per anchor. Get the required rebar area per anchor.
That is As = T / (0.75Fy). Then divide this area to the area of single rebar that you will use.
If you are designing the reinforcement as per Chapter 12, the statement in ACI 318-11 Section D.5.2.3 where Hef is the maximum of ca,max/1.5 and s/3 can be neglected right? Since you can dictate your new Hef depends on the procedure above. With this assumption, you can set your Hef > 25 to satisfy Chapter 12 development length requirements.
If you will draw projection of 0.5Hef in each anchor bolt, you can see how many rebar are in the 0.5Hef circle (projection). How do you actually account the number of rebar within the circle especially if it overlaps with another 0.5Hef circle (projection) since it is closely spaced as above?
Given the same projection, what is your view if the 0.5Hef circle (projection) covers another anchor? Say 10kips per anchor and the circle covers 3 anchors within the 0.5Hef circle (projection), are you supposedly be adding 10k+10k+10k as your max tension load to get the required rebar area per anchor?
The proposed detail in Fig RD-5.2.9 Anchor reinforcement in tension where you can see top bars and an inverted U bar on both sides of concrete breakout. Is this still required if you have already detailed your reinforcement as per Chapter 12 above?


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