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# What is the depth of the average bridge?

## What is the depth of the average bridge?

(OP)
My question is basically defined in the subject heading. For a work project I need to know the depth of the average bridge (in the UK, if there is a difference globally). I have searched many bridge web sites and can only find data on bridge width and span. I'd be very grateful if anyone has any relevant information. Thankyou.

### RE: What is the depth of the average bridge?

A little more of an explanation is needed.  What do you mean by average?  By average do you mean typical highway bridges or the average of larger, longer span bridges?

The depth of a bridge is directly proportional to the load and span.  Inherent in the load previously mentioned is the bridge width.  A 100' bridge carrying vehicular traffic will be shallower than the same span which carries rail traffic.  So, you must let us know what the nature of traffic on the span.

Also, there are many ways to span a specific distance.  For instance, an 80' span could be accomplished with either wide flange beam or prestressed I-girders.  If the span were a little bit longer the bridge may be plate girder or if not too much longer a deeper Prestressed girder.  Selection of the superstructure is not always a function of span, but of many factors including maintenance and aestetics.

In general, typical highway bridges could be broken down this way:

Here in the States, a short (30 feet) prestressed deck girder bridge could be as shallow as 30 inches, while longer span bridges (100-120 feet)could be as shallow as 7 feet.  Values do not include parapet.  Longer span (200) could be 10 feet plate gider or maybe a deck truss.  Note:  All of the above values assume slab and girder or deck beam.

Many other types of superstructure exist: post-tension concrete box girders, truss bridges ( a 600 foot Warren truss might be 90 feet tall), cable stay (with box grider), suspension bridges(could be double deck) and tied arch bridges.

If you have some specific information, it would help immensely to pinpoint an answer to your question.

### RE: What is the depth of the average bridge?

(OP)
Thankyou for your reply. It has given me a few ideas to work from.  You may have a point that I need to analyse typical highway bridges and longer span bridges individually.  It is any form of road bridge that I am investigating.  Could you suggest the typical depth of these for me.  I understand that there are many factors involved but an average value would be of great use to my calculations.

### RE: What is the depth of the average bridge?

If you can provide me with an idea of what typical road bridges in your area are constructed of, I will be happy to provide you with the data.  Again, here are some ideas.

Slab on girder bridges

Continuous Wide Flange Sections.  Generally span to 80' and will be 36 inches deep.  With an 8.5 inch deck and say 1.5 inches for the fillet, this results in a depth of 3 feet 10 inches.  Our wide flange bridges are generally limited to 21 inches as a minimum so this results in a depth of 2 feet  7 inches.

Prestressed I-girder bridges.  A 2 feet 8" girder will span up to approx. 50+ feet while a 6' girder will span to about 110 feet.  Again, using a 8.5 inch deck and 1.5 inch fillet, this results in a depth of 3.5 feet or 6 feet 10 inches.

Plate Girders can be used to span various distances.  The smallest we've used are 42 inch girders (4feet 4 inch depth) and the largest I've seen is 10 feet (10 feet 10 inch depth).  Plate girders are fabricated piece by piece so they can fabricated to match many constraints.  Often you will see plate girders that fishbelly (are deeper) over the supports and transition to smaller sections near midspan.

There are other types of superstructures as well but slab on girder is presently the most commonly used.  If your interested, I can post numbers for these but please understand that the list is fairly long.

Good Luck!

### RE: What is the depth of the average bridge?

(OP)
Thankyou for your help.  I seem to have enough informaion now to continue with my investigation.  Your help was much appreciated.

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