Addition of Interior Bent

[mann]

I need to increase the capacity of an existing structure by adding an interior bent. The bridge is small, only a 27' span over a creek. How would the contractor add an interior bent without affecting the superstructure? Is it even possible? What design details would help with construction? Any thoughts appreciated.

 

[Qshake]

Mann,

Many solutions exist for the problem you've posed.

However, before jumping right in to the solutions let me first mention that many creeks fall under the Department of Natural Resources and Army Corp of Engineers regulations. In short, you, or the agency you represent, could be responsible to the abovementioned agencies for a "No-Rise" condition. This condition stipulates that an obstruction placed in the waterway will not cause more than 12 inch rise in the water level upstream of the bridge for a 100 yr event.

Now, on to the alternatives for construction. Many assumptions have to be made such as whether this is a girder bridge or a slab bridge. Many concrete slab bridges were designed in this span range as were non-composite steel girders and slab bridges. Alternate One. Since the stucture is short (about 27 feet)it is possible to remove the deck (it's probably old and deteriorated anyway)and drive piling, place pile cap and substructure concrete and then place a new deck on the structure. Traffic during the construction phase is rerouted adjacent to the bridge. Perhaps on some large CMP and gravel, which of course all needs to come out at the end of the project. Alternate Two. If the slab absolutely must stay on, smaller piles (like pin piles or micro-piles can be driven with little headroom and the remaining substructure constructed afterward. The slab can be lifted (oh, so slightly) and set to bear snug on the newly constructed substructure. Naturally, this assumes that the deck has adequate top reinforcing to sustain tension from the negative moment introduced at the support. If not, then we're back to replacing the deck (see alternate one). Alternate Three. Since the structure very likely is 27 feet long and has vertical abutment walls at or about the 27 feet range (open span) then why not proposed a triple box culvert (triple cell culvert) these are used by many states as a standard in this situation. With 3-10' cells you've got 30+ feet of span. Alternative Four. Replace it with a precast CONSPAN culvert or equivalent. The precast alternatives are usually constructed in days rather than weeks.

In short, its going to cost a lot to perform the necessary foundation work within those constraints. More than perhaps some other alternatives. Unless rock is close to the surface, you're going to need a deep foundation to prevent undermining of the interior bent foundation. Driving pile can be detrimental to the existing abutment foundation. Whether or not they are founded on pile. Explore your alternatives!

 

[mann]

Thank you Qshake, you are right the structure is a noncomposite W-section superstructure which is in good shape. It was replaced in 1990. So you are saying that it will be necessary to remove the superstructure to place the interior bent? The opening is large enough, hydraulically to allow the placement of the bent. I am trying to determine the most cost effective way to increase the load capacity of this structure, and I am looking at a double 6x12 culvert. Any other advise.

 

[Qshake]

Mann,

No, it's not necessary to remove the superstructure entirely. Since the superstructure involved girders you can remove a portion of the concrete slab and drive pile between the girders. This is done with railroad bridges quite often as traffic can not be easily rerouted. The pile are then cut off and a steel plate is placed over the gap to accomondate traffic (reduced speed, of course). So, for a few hours the road is closed and then reopened. Work can then begin down below without interuption. And the substructure brought to the necessary grade and incorporated in the new slab section. You can even bulk up the reinforcing in the top layer right over the pier for the negative moment.

A more cost effective solution might be making the superstructure composite. This will increase your load capacity the most for the least amount of work. But requires the removal of the slab. Probably one or two weeks of work to replace the slab with new welded studs.

Another option used by many agencies is a bottom cover plate. This has limited results but can work nicely with a lane reduction to increase load capacity. With this option a steel plate is welded onto the bottom flange of the existing girders (the plate should be oversized to facilitate an overhand weld). The superstructure will have to be jacked up slightly to remove dead load deflection and released as the all of the plates are welded. The benefit can be investigated as best for interior only or exterior and interior girders.

Good to see you're looking into a double box culvert. Unless the agency has problems with drift in that area (woody acreage on site) that would be the way to go. If there are problems with drift piling up on the center wall(s) then try an open span precast culvert.
To summarize: From cheapest, cover plates, composite deck, new culvert, adding interior bent. This is a somewhat subjective look at amoritized cost as well. For example, just adding a new bent seems cheap now but does nothing to add life to the superstructure. Still, I don't think you can add a new bent without replacing the deck with new reinforcing.

Good Luck!

 

[tps]

Don't weld to the bottom plate. This will cause fatigue problems and some governing bodies won't allow it. See Fischer et al.

An alternative for strengthening (beyond making the section composite) is to add dwidag bars to the beams which are connected to anchorages at the abutments (connected by bolting the anchorages to the webs so as to reduce fatigue problems), and stress the bars. The post-tensioning will increase the moment capacity but will have little effect on shear capacity. Inclining the bars will help with shear if necessary. Stiffners comprised of angles bolted to the web are generally used to increase shear capacity when required in rehabilitation projects.

You may also want to consider making the bridge integral or semi-integral with the abutment. The thermal movement on a 27' span will not be much.

More recent developments include application of FRP's to the bottom flange but this method of strengthening is not yet in wide use and only a few contractors are experienced.