Renovating Box Beam Non Composite Deck to Composite Deck
Renovating Box Beam Non Composite Deck to Composite Deck
(OP)
Gentlemen:
I am analyzing an old 3 span non-continuous, pre-cast box girder bridge, with simple spans of 49.25 feet each. The superstructure is 27 feet wide and consists of 9 voided box sections 3 feet wide and 27" deep. Currently the driving surface is only about 2" of asphalt. Our scope of work is to see if we can make the 3 spans continuous for live load be removing the asphalt and replacing with a 5" concrete deck and reinforcing the deck at the supports to resist the tension due to negative bending. I am using the AASHTO LRFD code. My issue is how to get composite action between the proposed deck and the top of the existing precast girder. The top flange of the box is only 5 inches thick. Is there another way to get composite action without the use of rebar dowels drilled and epoxied into the top flange of the girder which is only 5" thick? I do not think that this is feasible since I cannot develop the bars in that short distance. The bridge was built in 1958. I know I can roughen the top flange of the girder by scabbling, but I do not think that will be enough to get true composite action. Does anyone think that epoxy adhesive applied to the top of the deck will help? Any ideas will be greatly appreciated. Thank you.
I am analyzing an old 3 span non-continuous, pre-cast box girder bridge, with simple spans of 49.25 feet each. The superstructure is 27 feet wide and consists of 9 voided box sections 3 feet wide and 27" deep. Currently the driving surface is only about 2" of asphalt. Our scope of work is to see if we can make the 3 spans continuous for live load be removing the asphalt and replacing with a 5" concrete deck and reinforcing the deck at the supports to resist the tension due to negative bending. I am using the AASHTO LRFD code. My issue is how to get composite action between the proposed deck and the top of the existing precast girder. The top flange of the box is only 5 inches thick. Is there another way to get composite action without the use of rebar dowels drilled and epoxied into the top flange of the girder which is only 5" thick? I do not think that this is feasible since I cannot develop the bars in that short distance. The bridge was built in 1958. I know I can roughen the top flange of the girder by scabbling, but I do not think that will be enough to get true composite action. Does anyone think that epoxy adhesive applied to the top of the deck will help? Any ideas will be greatly appreciated. Thank you.
RE: Renovating Box Beam Non Composite Deck to Composite Deck
If I was faced with this situation, I would approach it by providing continuity from the sidewall reinforcing to the new reinforcing in the top of the new deck. This most likely would required chipping away the concrete above the sidewalls, mechanically splicing the vertical steel extending up from the sidewalls to new bars that are lapped with new reinforcing in the top of the deck. Alternately, you could remove the entire top slab and cast a new continuous top slab for the box girders.
The third option, and possibly the most cost-effective, is to replace the superstructure in its entirety. Prestressed girders that have bars or strands extended so that they can be spliced, or continuous steel girders could work for this option.
Rod Smith, P.E.
RE: Renovating Box Beam Non Composite Deck to Composite Deck
As Hotrod10 pointed epoxied dowels are out of question, and so is any epoxy applied in between the girder and an overlay concrete, as the epoxy have a tendency to creep under load.
Short dowels with maximum 4" embedment grouted into top flange may work. The crucial issue will be the condition of the prestressing strands - if good you may squeeze another 40-50 years of service.
You may lift the bridge at the piers prior to making in continuous, and lower after casing of the new overlay, to post tension the zones above the piers.
RE: Renovating Box Beam Non Composite Deck to Composite Deck
Tony G
RE: Renovating Box Beam Non Composite Deck to Composite Deck
Getting new prestressed girders of the same size, with cast-in reinforcing stirrups extended for the cast-in-place continuous slab, could be a good option.
Btw, when we rehab bridges, any design work is done per the spec. for which the bridge was designed. For now, that's typically the AASHTO Standard Specifications, not the LRFD.
Rod Smith, P.E.
RE: Renovating Box Beam Non Composite Deck to Composite Deck
I thought of that. This old bridge uses 250 ksi tendons f'c = 5 ksi for the girders and grade 40 for the mild steel. I will use the oldest AASHTO we have. Thanks again for your timely and thoughtful responses. The existing stirrup layout could be the deal breaker in trying to add a 5" concrete slab to the existing girders even non-compositely.
RE: Renovating Box Beam Non Composite Deck to Composite Deck
If the loading on the substructures is a concern, you may want to consider steel girders to cut down on the weight of the superstructure.
Rod Smith, P.E.
RE: Renovating Box Beam Non Composite Deck to Composite Deck
This option may be used if the spans have enough capacity to accommodate 5" slab, still working as simple supported.