Converting Non Composite Box Girder Topping to Composite Deck

[minorchord2000]

We are evaluating a 5 span concrete box girder bridge with an existing non-composite asphalt topping. After removal of the existing asphalt topping, can the proposed new 5" concrete deck be made composite with the existing concrete box girders. The bridge is 9 girders wide and the thickness of the top slab of the box is 5 inches. The bridge was constructed circa 1958 and each span is a simple span with no continuity between spans. The ideal solution would be to make the bridge continuous for HL-93 live load and place negative steel reinforcing within the new concrete slab at the intermediate piers. Does anyone have experience with this and if so, how to install slab ties connecting the slab to the top of the existing box girders. Can we achieve composite action by scabbling the tops of the existing box girders? Thanks for your consideration.

 

[BridgeSmith]

"Can we achieve composite action by scabbling the tops of the existing box girders?"

You have to provide at least the minimum amount of shear reinforcement across the joint. In the AASHTO LRFD Bridge Design spec, the requirements are in Article 5.8.4. If you're designing to AASHTO, you'll need to provide fully developed shear reinforcement of some kind across the joint. I can't imagine any other design codes being substantially different.

You'll either need numerous small anchors you can develop with 5" embedment, or you'll have to assess whether the webs are wide enough to accommodate the drilled anchors you'll need. Most likely, an adhesive anchorage system, such as Hilti's HIT RE500, would be the way to go for that. Another possibly is a system that can be anchored on the underside of the top flange, but may be hard to do if you can't access the inside of the box girders. Otherwise, you'd have to chip off the top of the box girders to cast a new top flange around the existing vertical steel from the webs, which is even more labor-intensive than the other options.

There's also the matter of the continuity of the bottom flange between sections for the compression force to be transferred.

In similar situations, if the load rating was unacceptable, we found the most cost-effective solution for us was to replace the entire superstructure or the whole bridge.

 

[damorim]

As HotRod10 mentioned, calculations would most certainly show that interface friction due to scabbing of the deck alone is not sufficient to develop full composite action... some level of interface shear reinforcement will likely be required. I like the suggestion to go into the webs if suitable. If there is adequate web thickness and minimal risk to damaging rebar, coring (not drilling!) anchorages into the webs could allow you to develop sufficient shear reinforcement. Do you have any record drawings with details on the girder sections and of the elevation of the girder ends at the piers? How much of a gap between them? I'm assuming there are expansion joints of some kind over each of these substructures. Do they share common bearings or separate bearings? Bearing type? etc. Number of details to work out but it could definitely be done.

I'm assuming you load-rated the girders and have no concerns there of course.

 

[BridgeSmith]

"...calculations would most certainly show that interface friction due to scabbing of the deck alone is not sufficient to develop full composite action..."

Actually, what I was attempting to convey is that, per the AASHTO bridge design spec, no interface shear friction capacity can be utilized without at least the minimum prescribed interface reinforcement.

Why no drilling for anchors in the webs?

 

[damorim]

Sorry HotRod10, I understood that but wasn't clear in my post. That being said, if your Client is fairly progressive and IF the concrete friction by itself is more than adequate without rebar, depending on your level of comfort, the Client may allow you a design exception and not require shear rebar - better be satisfied its sufficient though. As I noted however, I doubt the interface shear friction alone is sufficient... but you never know without some calculations! As for drilling, I just find that if you are worried about tolerances and causing damage, coring is the safer bet - cleaner hole with less chance for spalling depending on the web thickness. Coring would likely be more expensive however.

 

[BridgeSmith]

"...IF the concrete friction by itself is more than adequate without rebar..."

What I'm saying (and the code is also saying) is that in order to have friction there must be reinforcement crossing the interface or substantial normal force to hold the faces together. There may be some adhesive bond, but not friction. For the situation under consideration, the friction force is near zero because the normal force is near zero, regardless of how high the coefficient of friction may be.

I suspected your reasons for coring rather than drilling, but I wondered if there were other reasons as well. Thanks.