Column Anchorage into Concrete--Number of Bars

[sjhgeotech]

I have a steel column baseplate that is connected to a concrete foundation with anchor bolts. The anchor bolt nuts have corroded off and there is no way to get new nuts on the connection. The column is one leg of a four legged radio tower and is in complete tension of about 55 kips, max load. The best solution is to cap the existing baseplate with a concrete block that is connected to the existing foundtion using grouted rebar. I am assuming that I will use Section 12.2.2 of ACI-318 to determine the embedment length of the bars in the existing foundation. How many bars should be used if I am using #8 bars. A single 60 ksi bar would give me 60 kips of capacity but I think four bars would give me the symmetry I need. Is there a pull-out cone for each bar that I need to assume that subtracts from the full tension capacity?

Thanks.

 

[civilperson]

Drilled and epoxy set rebars will take less length than development length. One #8 (area of 0.79 sq. in.) with a phi of 0.75 and 125 ksi tensile strength will suffice for 74 kips of factored load. Over lap of pull-out cones will deduct from the strength of the embedment. See Appendix D of ACI 318.

 

[DaveAtkins]

Also, if you are dealing with a pier and not a massive footing, the top of the pier can pull off of the vertical bars in the pier--so your new bars would have to extend far enough down into the pier to develop the existing bars in the pier.

DaveAtkins