top of water above low beam elevation
top of water above low beam elevation
(OP)
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
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





RE: top of water above low beam elevation
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.
RE: top of water above low beam elevation
RE: top of water above low beam elevation
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 ?
RE: top of water above low beam elevation
RE: top of water above low beam elevation
RE: top of water above low beam elevation
RE: top of water above low beam elevation
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.
RE: top of water above low beam elevation
RE: top of water above low beam elevation
RE: top of water above low beam elevation
RE: top of water above low beam elevation
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.