PS Girder Bridge: AASHTO Requirements Pertaining To Stirrup Extension Into Deck? (Interface Share)

[Future_SE]

I am currently designing a concrete bridge that has typical AASHTO girders with a concrete deck. I understand that some state design manuals sometimes add a requirement for the shear stirrups in the beam to be extended into the deck at a certain depth but I cannot seem to find a specific depth requirement in AASHTO or ACI.

I was able to note that as per AASHTO 5.7.4.1 (Interface Shear Transfer)- "All reinforcement present where interface shear transfer is to be considered shall be fully developed on both sides of the interface". However, I have only really seen shear stirrups that extend approximately 5"-7" above the top flange (even with typical details required by States) so surely this is not enough embedment for full development. These typical details also do not seem to incorporate proposed haunches into their details so excessive haunches (though usually supplied with additional reinforcement) can also be a concern.

Does anyone have any insight on any AASHTO requirements regarding standard looped stirrup depth into the deck?

 

[BridgeSmith]

The only thing I could find in that regard is this:

5.10.8.2.6—Shear Reinforcement
5.10.8.2.6a—General...
Shear reinforcement shall be located as close to the
surfaces of members as cover requirements and
proximity of other reinforcement permit.

So basically, the reinforcing needs to be closed loops that extend as far into the deck as they can without violating the top cover or displacing/interrupting the top reinforcing. You probably see variations in the depth because different states use different cover depths, different deck thicknesses, and different configurations for the top mat of deck reinforcing.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Enginerdad]

This is a really good topic, because you're absolutely right. For interface reinforcement to perform as expected it needs to be fully developed on both sides of the interface. But, as an example, a state I work in a lot calls for the stirrups to be 3"-4" below the top of slab, which is only 3.5" into the slab, plus a 2" haunch. In fact, that same DOT has a note on their standard details that says the following:

"The embedment depth shown does not produce full development."

I would approach this as any other situation with insufficient embedment depth; scale the available strength of the rebar by the ratio of As provided/As required, and provide additional reinforcement to make up the difference. In other words, if your embedment depth is half of the required depth for full development, provide twice as much reinforcement to compensate. This is covered by the excess reinforcement factor in 5.10.8.2.4b.

Also make sure you're accounting for the dead load of the deck as a permanent force normal to the interface plane when you calculate the required interface reinforcement. It can be a non-negligible help in keeping your reinforcement reasonable.

 

[BridgeSmith]

But, as an example, a state I work in a lot calls for the stirrups to be 3"-4" below the top of slab, which is only 3.5" into the slab, plus a 2" haunch. In fact, that same DOT has a note on their standard details that says the following:

"The embedment depth shown does not produce full development."

Are the stirrups they show open, or are they closed loops? It's hard for me to see how a loop into the deck cannot be fully developed, at least on the deck side. I can easily see how an open end of a stirrup, even with hooks may not have full development. Is the embedment in the beam shallow?

Rod Smith, P.E., The artist formerly known as HotRod10