A36 Anchor Rod Embed length 1

[MotorCity]

I have (4) 1" dia. A36 anchor rods, each carrying 22k tension. I am trying to determine their embedment length into a 3' dia concrete pier. I cannot use the ACI development length equations because they are smooth, not deformed, bars. I referred to the AISC LRFD manual table 8-26 (p. 8-90 of the connections manual)and it requires a minimum of 12d, or 12" which I cannot believe is adequate (just my engineering judgement). Others have suggested to calculate the embedment length based on a bond stress of 100 to 120 psi. Using 120 psi, this gives a length of about 58" which seems excessive. I cannot use headed anchors because the failure cones interfere since the concrete pier is only 3' dia.

Suggestions?

 

[dig1]

MotorCity,

Try using vertical rebar and determine the embedment length of the rebar, which will be the embedment depth of the bolt (plus cover and shear cone from center of bolt to rebar, see ref below). The rebar has to be extended/detailed below the bolt enough to develop the rebar's strength.

Use Fisher's AISC manual for Industrial Buildings.

Just used this for bigger bolts (3" to 6") in Chile and it really saved depth and width of foundation.

If you need more info just post again.

 

[mrengineer]

My understanding is that you you can either develop shear cone or provide adequate pier reinforcement. There is an article in AISC Engineering Journal Fourth Quarter 1992 by Mario Scacco which uses 12d for A36 on either basis. Article by Shipp in Second Quarter 1983 goes into pier reinforcement in great detail, and also uses 12d. I am not up to speeed yet on ACI 318 design requirements.

 

[bjb]

I do it the way dig1 does it. See AISC Design Guide 7, "Industrial Buildings" for more info. You can download it from the AISC website, but it will cost you unless your an AISC member

 

[JStephen]

I don't see why you can't use headed anchors. Even if the failure cones interfere, that doesn't mean you have zero strength- just reduced strength.

 

[DaveAtkins]

In a pier, I always lap the anchor bolts with the vertical bars in the pier suffiently to develop the required strength in the pier reinforcing. Otherwise, the top of the pier would pull off of the reinforcing.
However, if the area of your pier is sufficiently large, the concrete alone will be strong enough to resist the tension.
But the fact that you are using smooth, unheaded rods is something that may control, and I don't know what the bond to a smooth rod would be. Can't you use "all-thread", where the entire rod is threaded?

DaveAtkins

 

[MotorCity]

Sounds like there are a few good suggestions:

Unfortunately, I'm not a member of AISC so I don't have access to any of their member resources.

I did consider the reduction in strength from overlap of the failure cones, but as I spaced the bolts further apart to minimize the overlap, the size of the baseplate became excessive.

I also thought of using threaded rods, however I'm not sure if you could consider these to be "deformed bars" and follow the ACI development length calculation. (Although they are "more deformed" than smooth rods.)

I am leaning toward lapping the anchor rods with the vertical steel as DaveAtkins suggested. Right now I only have a steel cage around the pier (minimum steel). As a rule, I always neglect any tension resistance offered by the concrete. If I understand the suggestion correctly, I would add one rebar per anchor rod and lap them.

 

[UcfSE]

I would use all-thread rod and a nut and thick washer at the end to make it "headed". Then use the ACI 318-02 Appendix D to calculate the pullout capacity of the rods. You have to consider steel strength, concrete break-out strength, hook or head strength, and blow-out of the side face. All of these are included in the ACI. Concrete break-out strength will be influenced by the group interaction and the edge distances of the members as well as the embedment and concrete strength. I recommend you use a headed anchor instead of a hooked one because the hooked anchors are so weak. It doesn't take very much embedment for the hook strength to control the design.

You should check the design equations for development length also. It seems like there was a factor in there to take smooth bars into account. I would not rely on the threads to necessarily act the same way as a deformed bar. You're making an unnecessary guess. Bar deformations are larger and stronger than are the threads, they aren't likely to behave the same way.

 

[aggman]

I am going to give a response, but I first want to make a general observation. In terms of allowable stress design if you have (4) 1" dia A36 anchors then your allowable tension is 19.1 ksi per Table 1-B ASD. With 22k per bolt your stress is 28 ksi. So you may want to take another look at your numbers even if you are using an upset rod, unless these are LRFD numbers. To design the anchorage I would use headed anchors and I would use the ACI-02 approach which is most current. You will get a failure plane along the perimeter of the column based on a 35 degree projection cone. The ACI theory is based on unreinforced concrete. Use the development of from the top of your perimeter bars above the failure plane to determine the extra capacity you get from the rebar. If it is not enough, extend the anchors deeper until you can develop the strength and or hook the tops of the bars and use the hooked development length. If you don't use the vertical rebar to help hold the cone then you will not generate enough capacity.

 

[MikeDB]

For A36 Anchor Bolts you need to use a headed bolt, nut or hook on the end of the rod to develop pullout strength. You can not rely on any bond development to a smooth rod. ACI 318 Appendix D is the source required for most building codes in the US. Where there is no applicable building code I have used 25 bolt diameters as a general guide when used inside a rebar cage with a minimum of 6" from bolt to bolt or bolt to edge of concrete.