Anchor Bolts in Reinforced Concrete Piers 2

[jike]

I have recently waded thru all the formulae in ACI 318-2002 Appendix D for computing anchor bolt capacities! IBC 2000 Section 1913 is similar.

It seems to me that everything is based upon capacities in unreinforced concrete.

When anchor bolts, subjected to shear, are embedded in a reinforced concrete pier, it would seem that the ties will work like stirrups to resist the shear if they are spaced at d/2. The d/2 requirement is closer tie spacing than normally required for piers. The compressive force in the pier (if there is any)will obviously increase the shear capacity. To prove that the anchor bolts are OK in shear, it seems logical to check the allowable shear of the reinforced pier instead of the breakout strength of the anchors in the unreinforced concrete. Steel strength and pryout strength of the anchors also need to be checked.

When anchor bolts, subjected to tension, are embedded in a reinforced concrete pier, it would seem that the overlap of the anchor bolts and the reinforcing steel has to be long enough to transfer the tensile forces. To prove that the anchor bolts are OK in tension, it seems logical to check the tension that can be transfered to the reinforcing instead of the breakout strength of the anchors in the unreinforced concrete. Of course, the steel strength of the anchors also need to be checked.

I would then use the minimum values of the capcities in the combined stress equations.

For anchors in footings, I use the formulae in the code for anchors in unreinforced concrete.

Is this how others are currently doing it or are you relying on old tables like I used to? You need a major spreadsheet to handle these new calculations. Pretty involved process.

Any comments?

 

[JAE]

Most of the research into Appendix D is based on unreinforced concrete to some extent as there is such a wide variability in how an anchor bolt and pier reinforcing are related geometrically.

In many many cases, pier reinforcing is comprised of vertical corner or side bars with square ties around the perimeter. Sometimes cross ties are included, but not always.

Anchor bolts are usually placed in the central region of the plan of the pier. In this area, there are usually no reinforcing bars that cross the cone of failure of an anchor bolt.

Even if there were provisions for reinforcing, you would have to ensure that the bars not only crossed the shear cone, but were also developed beyond the shear cone. This just isn't usually dealt with by engineers. And even if you did, the location of the rebar relative to the base or point of the cone would greatly affect the capacity. Very difficult to estimate capacity.

 

[mrengineer]

Appendix D looks like a pain in the neck to me. For years now I have used the simple interaction charts in the A.I.S.C. Engineering Journal Fourth Quarter 1992 article "Design Aid: Anchor Bolt Interaction of Shear and Tension Loads". If I have a pier situation and am feeling ambitious, I consult the Engineering Journal Second Quarter 1983 article "Design of Headed Anchor Bolts", which discusses the interaction between the anchor rods and the pier reinforcing.

 

[jike]

The '92 Engineering Journal Article looks useful. I am familiar with the '83 article.

Why do cast in anchor bolts have lower factors of safety, say 1.7 then do drilled in anchors or even bolts for structural connections? Don't structural bolts even have a factor of safety of 4? I can see why drilled in anchors have higher factors of safety but cast in bolts can have similar problems with cutting oil and grease (one of my pet peeves) not being cleaned off before they are cast in the concrete.

 

[pmkPE]

I have casually reviewed the IBC 2000 Section 1913. Basically what they are checking for is the resultant shear capacity of the cone(s) that are developed beginning from the head of the AB to the nearest surface and/or other adjacent cones. This is assuming that the concrete shear will take all of the load. Therefore reinforcement is not required to be considered. If you were to consider the reinforcement in a new design your construction documents would need to be explicit regarding how your ABs are connected to the reinforcement such as welding, special connectors or even lapping under the reinforcement. This way you could potentially neglect the contributions of the concrete shear capacity acting alone for the AB.

The important consideration in any AB design is how are the forces transferred from the AB to the surrounding concrete? If you use development length of the AB you also need to have adequate length on the reinforcement in the pier or footing to first develop then transfer the forces into the footing. If you use the shear cone method minding all of the potential reducing effects of edge distance, overlapping cones and such you can evaluate the capacity withour regards to the reinforcement. The problem with coming up with a formula for using the combined capacity of the concrete's shear cones and the reinforcements is it is not practical to make assumptions for the type of reinforcement, grade, configuration, etc. As was stated above it would need to be on a case-by-case basis.