Bridge construction and Grade (real life) question

[Rail4Pete]

Hi, On a route I am building(this is a DEM map).Because I can't go (to see with my eyes ) at every places I want to reproduce, I must make some deductions myself and sometimes, I must use only my imagination....even with a DEM map.Here I go: I got an elevation of 600 feet on a 8 miles curve segment of track. So the grade is steep. At some point I come to a bridge (over a stream or river). My question is: Would they have build a ''track'' bridge with the continuation of the 2.5% grade in real life ? (I mean would the bridge have been constructed with a lower end and a higher end to continue the constant grade) Or a bridge is always 0% grade (same height at both end)?

Thanks for your help !
Rail4Pete

P.S. Hope my question is understandable...english is not my

 

[schweitzerdude]

I have faced the same question using Transdem terrain

And if I am building track on a grade and need a bridge, I have no hesitation to continue the grade on the bridge. I am not a bridge engineer, but I can't see any reason a bridge has to be level. In fact, on a 2% grade, a level bridge would look out of place.

Let's see what people who build railway bridges for a living (or have more experience in railway bridges then we do) have to say but I believe I'm correct.

 

[llebrez]

There are several different types of bridges. Some are "objects" and are at 0% both ends. Others are splines and you can lay your track over them at your desired inclination, and there are the ones considered as tracks, and as you know, they can also be "tilted". It al depends on how nice you want them to be.

 

[leeferr]

In real life, bridges will usually follow the grade of the approaches. In actuality, the bridge is built first and then the approaches are constructed, but in the end, the approach and the bridge are usually at the same grade. It can create a dangerous situation for grades to abruptly change at the transition between the approach and the bridge. You will find very few bridges that are level and even the ones that are will fool you, they're not really level. Bridge beams are generally built with what is called a camber in them. A camber, simply explained means that there is actually a curve in the beam with the center of the beam being higher than the two ends when it is fabricated. In theory, if the anticipated loads are calculated correctly, the camber will flatten out when a load (vehicles) is applied to the beam. It gets much more complicated, but that's the easiest way for me to explain it.

Bottom line, every bridge is designed and built for each particular situation and none are really level or flat. Regarding railway bridges, I don't believe that there's much camber in a railway bridge beam. They are generally sufficiently overdesigned so that very little camber is required. They just don't flex a lot under the applied loads.

Mike

 

[JCitron]

Mike,

The camber is really obvious when you see a flatbed trailer. The trailer actually curves quite noticeably and is completely gone when the load is on it.

This being a truck and not a bridge, I suppose is more flexible.

John

 

[Rail4Pete]

So up to know....my railway bridge with a 2% grade would be ok ?

@ llebrez: I'm not sure you understood my question....I'm not questionning how to make it in surveyor, I'm talking about ''real life''. There is a real railway bridge on the grade I'm reproducing, but I can't go there to see if it has been build at level or with a 2% grade....but thanks for trying
Rail4Pete

 

[pdwood]

The answer is yes. Bridges generally keep the grade of the track nearby. They would not last long in Norway if not!

pdw

 

[Rail4Pete]

The answer is yes. Bridges generally keep the grade of the track nearby. They would not last long in Norway if not!

pdw

Just curious, why they would not last long if not in grage ?
Rail4Pete

 

[sniper297]

I was a partner in a small construction company, we primarily subcontracted for assorted home remodelers. One job we got about 20 years ago was building an attached 3 car garage on the front of a house. Nothing unusual about that, except the house was on a lot with an angled line. Since the local ordinance dictated at least 10 feet from the property line, the architect drew this thing coming out 6 feet, then it made a 22 1/2 degree left turn for 24 feet. Looks good on paper, but building it was one screwball problem after another since all the framing, concrete anchors, drywall, siding, roof sheeting and shingles all had to be adapted to adjust for that 22 1/2 degree angle.

My guess would be that unless there's an overwhelming reason not to, most structural engineers would prefer to build bridges level simply because it's simpler to do. If there's enough real estate to allow it, my guess is they would level out the grade, build the bridge level, then start the upgrade or downgrade further on. If the distance wouldn't allow for it, then they would be stuck with building the bridge on a grade with all the problems associated with non level construction.

 

[bendorsey]

I gotta agree with all the guys. Bridges usually continue the grade. I made one for the DLS that has a 4% grade.

Ben

 

[leeferr]

I was a partner in a small construction company, we primarily subcontracted for assorted home remodelers. One job we got about 20 years ago was building an attached 3 car garage on the front of a house. Nothing unusual about that, except the house was on a lot with an angled line. Since the local ordinance dictated at least 10 feet from the property line, the architect drew this thing coming out 6 feet, then it made a 22 1/2 degree left turn for 24 feet. Looks good on paper, but building it was one screwball problem after another since all the framing, concrete anchors, drywall, siding, roof sheeting and shingles all had to be adapted to adjust for that 22 1/2 degree angle.

My guess would be that unless there's an overwhelming reason not to, most structural engineers would prefer to build bridges level simply because it's simpler to do. If there's enough real estate to allow it, my guess is they would level out the grade, build the bridge level, then start the upgrade or downgrade further on. If the distance wouldn't allow for it, then they would be stuck with building the bridge on a grade with all the problems associated with non level construction.

Level just wouldn't work in most cases for highway bridges except maybe the Sahara. In addition to maintaining the grade of the approaches, being level would not provide enough slope to get water off to the sides. In any roadway or railway bed design, one of the most important things is to get water away as quidkly as possible. Water is one of the most destructive forces found in nature. Railway bridges don't normally have to worry about water run-off though since ususally, there is not ballast to contend with as the ties are resting on sleepers. There's always exceptions to the rule though.

Mike

 

[leeferr]

Mike,

The camber is really obvious when you see a flatbed trailer. The trailer actually curves quite noticeably and is completely gone when the load is on it.

This being a truck and not a bridge, I suppose is more flexible.

John

Yes it is. If anyone has taken notice of a flatbed trailer, the camber can be as much as a foot in the center. Their framework is designed to be very flexible since they're moving and twisting all the time.

Mike

 

[JCitron]

Thanks, Mike for the explanation.

This is a great thread, folks, one where we can actually learn something new!

John