Concrete Edge Breakout Strength for Pier Anchor Bolts

[OSUCivlEng]

I am checking the concrete edge breakout strength of some anchor bolts for an exterior girder in a pier cap. The bridge is in Seismic Zone 1, so the only shear forces are due to the seismic forces. Of course the edge breakout strength fails miserably. Increasing the edge distance and cap depth helps some, but not nearly enough. The only other option is to add anchor reinforcing. This got me to thinking if I should even worry about the concrete strength, since the top reinforcing bars will be #9 bars with 180 degree hooks on the end. This seems to be similar to what is shown in Fig. RD.6.2.9(b) of Appendix D of ACI 318-08. However, the commentary for section D6.2.9 says that anchor reinforcing larger than #6 is not recommended. I am curious as to what others have done in this situation. Thanks.

 

[bridgebuster]

Here's the typical NYSDOT pedestal reinforcement detail. We use this for CIP or post-installed anchor bolts. FYI - the downstate region of NY is considered to be a moderate seismic zone.

 

[OSUCivlEng]

Thanks. That would work for tension in the anchor bolts, but I don't see how it would do anything for edge breakout for shear.

 

[bridgebuster]

I guess you guys do things much differently there. We provide 8" of cover on the anchor bolts.

 

[OSUCivlEng]

I have 15" of cover on the anchor bolts and it is still failing. The transverse loading from EE I is 160 kips per bearing. Some of the problem (the way I see it anyways) is that Appendix D doesn't really account for any of the reinforcing that is already there.

 

[KootK]

You can always resort to a strut and tie model. Bolt lateral bearing stresses become a strut that makes its way to the pier reinforcement. So long as the cap reinforcement is sufficiently anchored to the strut(s), you should be good to go.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[graybeach]

Can you put #4 or #5 hair pins around the anchor bolts with the ends toward the middle of the pier?

 

[umrce]

I'd recommend reconsidering your failure mechanism if possible. We assume where I design for seismic zone 1 loads that the transverse loads are distributed to all the bearing anchor bolts equally within a given span segment. In the longitudinal direction we assume the fixed anchor bolts will fail since it would be so expensive to allow for them to take all the load. We then design the remaining expansion bearing within the span segment for the load in that direction.