top of water above low beam elevation

[dabbler101]

Hello all,

I am currently working on some preliminary design calculations for a bridge in Lee, MA that spans over the Housatonic River. The design span is 100', and the cross section is composed of (6) 48" deep spread box beams. Unfortunately, due to several site constraints, the bottoms of all six beams are below the ten year design flood elevation.

Sec. 3.18.1.1.2 of the AASHTO Standard Specifications for Highway Bridges states:

"In cases where the corresponding top of water elevation is above the low beam elevation, stream flow loading on the superstructure shall be investigated. The stream flow pressure action on the superstructure may be taken as Pmax with a uniform distribution"

My question is do I solely need to investigate the stream flow loading on the fascia beam, or do I need to look at stream flow loading on all beams below the design flood elevation?

Any help with this would be greatly appreciated.

Regards,
Jim

 

[jimaitken]

Jim

I would be conservative and assume you will have force on all of the beams below the waterline, but I would also assume a reduction of force on the interior beams, since the fascia will be seeing most of the load, but may not prevent the other beams from being loaded.

I would also include in my pier and abutment details a keeper block at the controlling interior beam as well as for the fascia beams. These are detailed in the MassHighway Bridge manual. You may have to provide an extra diaphram to prevent the bottome flange from rolling as well as to ensure the load gets fully transferred to the entire bridge rather than just the fascia beam.

 

[MichSt]

Before you get too carried away with how to distribute the stream flow pressure to the beams, do a quick calc to see how large of a force you have. Can it easily be resisted by the fascia beam only with your standard position dowel and keeper details?? If so, you don’t need to worry about how to distribute the load – just assume it all goes to the fascia beam. If the load is large enough to require some “beefing up” of the standard details (extra position dowels, larger keepers, etc), then you may want to think about distributing the load to the other beams.

 

[jimaitken]

My response was a bit overzealous with regard to redistributing load. MichSt is right. Don't jump in w/ both feet until you have no other alternative.

I have a few additional questions though. How far below the flood elevation is the bottom of the beam? Do you have to worry about bouyancy or are the boxes tied into the endwalls/integral abutments/piers ?

 

[MichSt]

I have a comment, not on your question, but just in general to your situation. In my firm we typically use adjacent box beams for river crossings. With adjacent boxes the bottom of the bridge is flat, with no snag points for debris that could be coming down river during a flood. We try to avoid steel beams or spread boxes, unless adjacent boxes are too expensive. I have seen plenty of spread box beam bridges for river crossings, but adjacent boxes is just the way I typically go.

 

[MichSt]

….you will also most likely be able to use a shallower beam section with adjacent boxes (in your case maybe a 42” or 39” box), which will help provide a larger hydraulic opening to pass your design flood.

 

[cvg]

I'm curious why the bridge is so low? We typically design bridges for the 100-year flood with freeboard. Generally, we are checking hydraulic forces on the low chord for the 500-year storm.

 

[dabbler101]

There is an 8" gravity sewer main that crosses the bridge, the highway department does not want to get into a large sewer main replacement project and they do not want to add pumps.

There are two other utilities on the bridge as well, including gas and water.

Because of the existence of these utilities we felt like spread boxes was the only way to go. It didn't seem feasible to have all three utilities mounted on the down stream side of the bridge.

The highway department has indicated in the scope of work that the bridge should be designed for the ten year flood elevation.

The bottom of the fascia beam is approximately 1'-4" below the ten year flood level at the lowest point along the span.

Thanks again for your thoughts and suggestions.

 

[MichSt]

Based on the info you provided, I agree with the use of spread box beams for your bridge. My first comment on the stream flow loading still stands.

 

[cvg]

not having read your scope of work, I would interpret design for 10-year flood to pass under the bridge. Your bridge sounds like it is designed for something well under the 10-year flood. By the way, most health departments require sanitary sewer manholes be above the 100-year flood level. You may want to check that.

 

[LCruiser]

The thing people forget about in a situation like this is that when that event comes, there will be trees, debris, etc. in the water. You could end up with a dam there, depending on the potential for that type of stuff to get into the waterway.

 

[wiktor]

The bridge should not be designed to be partially submerged during ten year flood, until it's a temporary structure. Typical requirement for a permanent bridge is 50 year flood or a 100 year flood. You have to make it clear to a client, if you have to proceed with the design.
If this partial submerging is acceptable (good hydraulic calculations are needed - you will be restricting the flow, so additional incerase in the water level could be expected) design all the beams (girders) to withstand the water pressure. And provide scour protection.