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Bridge Abutments

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  • Member since
    April 2003
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Bridge Abutments
Posted by Anonymous on Saturday, July 12, 2003 6:07 PM
There have been a lot of articles, over the past 30 years, on bridge construction but nothing on their supports. How are the heights and widths determined. In order to maintain level track how do you shim them so the shims don't show.
( HINT how about an article on this [:D])
  • Member since
    April 2003
  • 302,278 posts
Bridge Abutments
Posted by Anonymous on Saturday, July 12, 2003 6:07 PM
There have been a lot of articles, over the past 30 years, on bridge construction but nothing on their supports. How are the heights and widths determined. In order to maintain level track how do you shim them so the shims don't show.
( HINT how about an article on this [:D])
  • Member since
    April 2003
  • 302,278 posts
Posted by Anonymous on Saturday, July 12, 2003 9:32 PM
pennGG1 -

For those who may not be aware, bridges are supported by abutments at the ends and piers, if necessary, for the interior supports. One reason abutments are different from piers is that they have to retain at least a small amount of soil used to support the track beyond the bridge.

As for setting up my abutments, I build them deeper than the scene requires and then set them to support the bridge (and track) at the desired elevation. Then I run the scenery up to them and cover the very bottom where any shimming may have been needed.

The visible portions of the abutment are the wings, the seat and the backwall. The height of the backwall above the seat is the depth of the girder plus the depth of the bearing. This is not too difficult to understand when thinking of a beam or girder bridge, but can become complicated when working with a truss with a floor system used in conjunction with the main truss supports and their bearings.

I am working on some bridge projects but it will be some time before you ever see any of my work in any magazines. Good Luck - Ed
  • Member since
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Posted by Anonymous on Saturday, July 12, 2003 9:32 PM
pennGG1 -

For those who may not be aware, bridges are supported by abutments at the ends and piers, if necessary, for the interior supports. One reason abutments are different from piers is that they have to retain at least a small amount of soil used to support the track beyond the bridge.

As for setting up my abutments, I build them deeper than the scene requires and then set them to support the bridge (and track) at the desired elevation. Then I run the scenery up to them and cover the very bottom where any shimming may have been needed.

The visible portions of the abutment are the wings, the seat and the backwall. The height of the backwall above the seat is the depth of the girder plus the depth of the bearing. This is not too difficult to understand when thinking of a beam or girder bridge, but can become complicated when working with a truss with a floor system used in conjunction with the main truss supports and their bearings.

I am working on some bridge projects but it will be some time before you ever see any of my work in any magazines. Good Luck - Ed
  • Member since
    April 2003
  • 302,278 posts
Posted by Anonymous on Monday, July 14, 2003 7:17 PM
As an architect, I can tell you that an abutment is essentially the same thing as a foundation, such as the footings under your house. The main function is to distribute the weight of the bridge and its traversers into the ground, as well as to hold back the loose soil either side of the span. The size is determined by a variety of factors, such as the soil bearing capacity, depth of bedrock, depth of the span, slopes, etc.

In general, the concrete must be poured down below grade to at least below the frost line, which is how deep the ground freezes during winter. In modeling terms, it would be simplest to approximate this by providing extra abutment and then "hiding" it underground, as you bring the scenery up to it. This is much easier than trying to fashion the abutment to fit the profile of the scenery, and also ends up looking better.

As for width, this mainly depends on what the abutment is holding back. If the approach to the bridge is over a fill, then it will be loose soil that mounds up at a certain angle-of-repose (think how sand in an hourglass piles up). The abutment and its wings must be wide enough to retain all of this fill, which makes it pretty broad at the base. At the top, of course, it must be wide enough for your roadbed. Ties that are wider than the rail guage and the ballast beneath them that spreads even wider are serving the same function of distributing weight, so this determines the soil width at the top of the fill; off the top of my head, I'd guess that 20 feet of soil fill is sufficient for a single track. If the angle of repose is essentially a 1:2 ratio (rise:run), and your abutment stands 10 feet tall, then at the base it will have widened to 60 feet (10 ft rise = 20 ft run each side).

1:2 is a very optimistic angle of repose, but selective compression may require us to go even steeper than that. As for wings that bend around, I think that is just a trick to minimize width without compromising the angle, as well as a way to prevent stream flow from cutting behind the abutment. I might be wrong, though.
  • Member since
    April 2003
  • 302,278 posts
Posted by Anonymous on Monday, July 14, 2003 7:17 PM
As an architect, I can tell you that an abutment is essentially the same thing as a foundation, such as the footings under your house. The main function is to distribute the weight of the bridge and its traversers into the ground, as well as to hold back the loose soil either side of the span. The size is determined by a variety of factors, such as the soil bearing capacity, depth of bedrock, depth of the span, slopes, etc.

In general, the concrete must be poured down below grade to at least below the frost line, which is how deep the ground freezes during winter. In modeling terms, it would be simplest to approximate this by providing extra abutment and then "hiding" it underground, as you bring the scenery up to it. This is much easier than trying to fashion the abutment to fit the profile of the scenery, and also ends up looking better.

As for width, this mainly depends on what the abutment is holding back. If the approach to the bridge is over a fill, then it will be loose soil that mounds up at a certain angle-of-repose (think how sand in an hourglass piles up). The abutment and its wings must be wide enough to retain all of this fill, which makes it pretty broad at the base. At the top, of course, it must be wide enough for your roadbed. Ties that are wider than the rail guage and the ballast beneath them that spreads even wider are serving the same function of distributing weight, so this determines the soil width at the top of the fill; off the top of my head, I'd guess that 20 feet of soil fill is sufficient for a single track. If the angle of repose is essentially a 1:2 ratio (rise:run), and your abutment stands 10 feet tall, then at the base it will have widened to 60 feet (10 ft rise = 20 ft run each side).

1:2 is a very optimistic angle of repose, but selective compression may require us to go even steeper than that. As for wings that bend around, I think that is just a trick to minimize width without compromising the angle, as well as a way to prevent stream flow from cutting behind the abutment. I might be wrong, though.
  • Member since
    April 2003
  • 302,278 posts
Posted by Anonymous on Tuesday, July 15, 2003 9:19 PM
I built my piers and abutments with two helps: Bridge and Trestle Handbook by Paul Mallory and studying numerous bridge photos from protoype magazines. If you have a heavy looking bridge, then a heavy foundation is required. Light for light, medium for medium.

I kind of used a rule of thumb that the top of a pier should be at least 50%, up to 100% wider than the bridge pedestal, and for an abutment, at least 25%, up to 50% wider than the pedestal. The pedestal should be at least 1' from the side edge. My piers are sloped, I used a 15:1 ratio, for example, if the pier is 10' wide at the top, 15 feet below the top, the pier is 1 foot wider on each side (12' wide total). Visually, to me anyway, this creates an obvious slope, but it doesn't eat up real estate.

My abutments are not sloped, and I too, bring scenery up to cover the shims. If I chose to model a sloped abutment, I would build a non-sloped track support, and then build a slop to the support. If you model a sloped abutment, the slope should be the same angle of the soil or rock that the right of way has just gone through - gentle for soil, gentle or steep for rock.

You could also just let the soil slope naturally around a straight abutment if turbulent water isn't near by, or put large riprap around the base of an abutment to protect it.

There are a lot of options, because man's imagination has always tried to outdo nature.

Happy modeling.
  • Member since
    April 2003
  • 302,278 posts
Posted by Anonymous on Tuesday, July 15, 2003 9:19 PM
I built my piers and abutments with two helps: Bridge and Trestle Handbook by Paul Mallory and studying numerous bridge photos from protoype magazines. If you have a heavy looking bridge, then a heavy foundation is required. Light for light, medium for medium.

I kind of used a rule of thumb that the top of a pier should be at least 50%, up to 100% wider than the bridge pedestal, and for an abutment, at least 25%, up to 50% wider than the pedestal. The pedestal should be at least 1' from the side edge. My piers are sloped, I used a 15:1 ratio, for example, if the pier is 10' wide at the top, 15 feet below the top, the pier is 1 foot wider on each side (12' wide total). Visually, to me anyway, this creates an obvious slope, but it doesn't eat up real estate.

My abutments are not sloped, and I too, bring scenery up to cover the shims. If I chose to model a sloped abutment, I would build a non-sloped track support, and then build a slop to the support. If you model a sloped abutment, the slope should be the same angle of the soil or rock that the right of way has just gone through - gentle for soil, gentle or steep for rock.

You could also just let the soil slope naturally around a straight abutment if turbulent water isn't near by, or put large riprap around the base of an abutment to protect it.

There are a lot of options, because man's imagination has always tried to outdo nature.

Happy modeling.

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