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bridge abutment design
i am installing a truss bridge in montana and i am trying to find a design for an abutment that doesn't have piling, just a spread footer.
the bridge is 75' long and spans the high water mark by 12' on either side so i shouldn't have any scouring.
the soil is rocky but not bedrock.
i need to be able to drive concrete trucks across the bridge.
how deep and wide does the footer need to be?
do i need any other structural concrete in the ground if i don't plan on having any wing walls?
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montanabridge,
i assume that you have designed the bridge so you have the dead load of the bridge. if you put the live load of the concrete truck over one abutment you have the total "p" to the footing (plus the dl of the footing.
the problem come with the lateral load. the wind comes from asce7-05, figure 6-23 and probably won't govern the design.
to find v & e (seismic) use eq(12.8-1) you need to calculuate cs, which is equal to sds/r/i. i am not sure what to use for an "r" value for a bridge abutment. once you get e then you can do p/a + or - mc/i (with m being e times the height above the base of the footing)to size the footing and design the reinforcing.
i have a rr flat car bridge project am am dealing with this issue right now. i am interested is listening to what other folks think about what to use for "r" for a bridge abutment.
wind loads (and other loads for bridges) are outlined in the aashto specifications. look there. to decide whether you need wingwalls, you need to look at the grading around the abutment. you will likely need some sort of wingwall, even if they're short.
the aashto specifications will also cover oldpapermaker's question on the appropriate r value to use for seismic loads.

crossframe,
i was looking in asce 7-05, chapter 15, seismic design requirements for nonbuilding structures for some direction but i didn't find anything that seemed to fit.
i don't have the aashto specifications and this is a private bridge on a driveway in the wilds of the state of washington.
i may just use the r value in table 15.4-2 for "all other self-supporting structures" which is 1.25.
how does that compare with the aashto specs?
6 years abck i used a programm (abut 5) from penndot. it was dos based and only 500$. it worked good i guess. you might check it out.
use this link; look for my post it has a link to abutment design per aashto lrfd specs.
i've designed a few private railcar bridges using sofa shaped abutments - the sofa back being level with the top of pavement and the lower seat area is where the bridge bearing plates are located. so you have a really thick retaining wall with the toe towards a slope. due to the mass of the footing, i've easily met the sf for sliding and overturning. but you need to ensure the bottom of the footing is sufficiently deep to protected against scour. the geotech should include in his report recommendation on depth and slope protection. i would not penalize myself with an r=1.25. i think you could make make an argument for using r=3 - "all other steel and reinforced concrete structures distributed mass cantilever. . . "
alanna2,
thanks for the "heads-up". i missed that category. i was leaning toward using section 15.4.2 "rigid nonbuilding structures" which directs: v=0.30sdswi. this just happens to come real close to using r=3 (1/3=0.33), as you suggested.
thanks for the suggestion.
even if you are bearing the bridge girders directly on your footing, you will need some endwalls and short wingwalls to protect the ends of the girders that are probably in the order of two to three feet deep.
for a project of this size and type, i would get a geotech report to establish the allowable bearing - somewhere in the range of 2500 to 10000 psf. big help, right? you may aldo need a recommended coefficient of friction, active and passive pressures to use too, as you may need a keyway to resist lateral sliding from wind and seismic action.
mike mccann
mmc engineering
did you design the truss or did you order it from one of the prefab truss manufacturers? i believe msquared has given you the best advice to get started ... get a geotech to do a boring for each abutment location. second, you do not have conditions to justify avoiding scour investigation. the last time i spoke to someone about this, the federal guidelines stated that unless you have a footing that is founded on rock with an rqd>50, you have scour. while this is a ludicrous number, anything not founded on rock needs a closer look. you may still not have scour, but i would need more information to determine that.
you will need a backwall, a seat and a footing. the backwall should be extended enough to accommodate the grading as mentioned above.
the concrete truck, and a fire pumper truck, will have large loads. you should be refering to the aashto codes for this not building codes. i believe you may need to contact your insurance provider to learn what load the abutment should accommodate. or, you could ask your bridge supplier what the truss was designed for and then design the abutment for that level of loading. if it isn't very high, i recommend you post the bridge. you might be able to contact your county to find someone to speak to about that. i don't think you will have to do seismic loads for a private bridge, unless you are in a seismically active region.
finally, after you have the loading, the trick is to keep the resultant load in the middle third of the footing. to do this, you want the abutment to be as short as possible. also, the heavier the abutment, the easier it is to keep the load 'balanced'. you will need to analyze at least two load cases: one with only the dead load of the bridge and abutment with 'at rest' earth pressure, and a second case with full live load.
good luck.

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