View Q&A

1.  Can ASTM A706 rebar be substituted for all the A615 rebar in FTB's, and if so, can the A706 rebars be tack welded together without compromising the crashworthiness of the design?

I see no reason that you should not be able to substitute the A706 for the A615 as long as long as the grade of steel stays the same or is better and there are no changes in bar sizes or lengths. The A706 should have better ductility and should not pose any other issues.

A fabricator down here wants to tack weld pre-tied rebar cages together to make them more rigid and easier to handle when they are placed into the forms.  It appears he wants to fabricate his own homemade "welded wire fabric".  I am concerned about doing this in a precast yard and not a factory environment.  If we mandate that the welders be certified and that AWS welding procedures be used, I think we would also need to require A706 rebar.  Currently we require A706 rebar for the connector loops only as it is a more ductile steel and can better accommodate the tight bend radius.

With regard to tack welding the rebar, I don't see a huge problem with it either as long as you hold the contractor to similar quality controls as those observed by welded wire fabric manufacturers. It appears from you email that you plan to do that. As long as the tack welds are not compromising the capacity of the rebar or causing stress concentrations, it should not be an issue.

2.  On another project, the contractor has proposed using the proprietary 1" diameter high strength (A449) anchor bolts shown on page 11 of the following attachment in lieu of the 1.25" diameter F1554 Grade 36 anchor bolts specified on our standard:

What do you think of using a smaller diameter, higher strength bolt than that used in the crash tests?  The contractor is also proposing to use an adhesive that we have not classified as a "high strength" adhesive.  We only allow "high strength" adhesives for this application due to the shallow embedment depth of the anchor bolts necessitated by our 7" to 8" deck thicknesses.

I would like to allow this type of proprietary bolt, maybe the same 1.25" diameter as was used in the crash tests though, because the bolts can be easily removed from the bridge deck when the barrier is removed or relocated.

For our design and testing of the bolt through tie-down, we specified a 1.25" dia. A307 threaded rod. The rod was embedded approximately 12" into the concrete with a high strength epoxy. The epoxy and embedment depth were chosen such that the full strength of the threaded rod was developed. In this case the threaded rod ultimate strength was 60 ksi and the threaded area was 0.969 in^2 which yields a maximum load of approximately 58 kips.

The Kelligrout specs you sent suggest that for the A449 1" dia. bolt proposed you need 9.5" of embedment to develop the strength of the bolt. The bolt capacity for the A449 1" dia. bolt corresponds to a ultimate strength of 120 ksi and a threaded area of 0.606. This would correspond to a capacity of 72.72 kips. Therefore the tensile capacity of the bolts is not an issue.

We are concerned with the bending capacity of the anchor rods. The bending section of the 1" diameter rod is approximately 50% less than that of the 1.25" dia. rod. So while the strength of the rods is different, the ultimate bending capacity of the 449 anchor is actually slightly lower than the A307 rod. We did get significant bending of the anchors in the full-scale testing and thus are leery of using the smaller diameter anchor. In addition, the A449 anchor is made of high strength steel that has lower ductility than the A307. This translates to lower energy absorption during the impact and an increased potential for bolt failure. The smaller diameter of the 1" anchor also increases the bending load because of the increased clearances in the holes of the barrier. Because of these concerns, we would not recommend the 1" dia. A449 anchor at this time.

The grout itself seems acceptable. I looked at the specs for the grout on their web page and it appears that they can achieve the full strength of the 1.25" dia. A307 rod with 12" of embedment.

As an alternative, we did develop a bolt through option that bolts through the bridge deck with washers and nuts underneath. This system is easier to remove than the epoxy system.