Bridge Components Technical Terms Stringer vs Girder 1

[EngDYIGuy]

Can someone explain to me the difference between stringers and girders? I've got a project involving a concrete bent railroad bridge with multiple steel stringers/girders for the super structure framing. It appears the terms "Girder" and "Stringer" in the bridge industry are used intermittently to describe the same framing component. Appears "Girder" is more universally used term, please explain any defined differences in the engineering/construction industry.

 

[KootK]

In general, a girder will be further down the load path chain than a stringer. A girder is usually the last trabeate member in the chain before a vertical framing element is reached.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[EngDYIGuy]

Please observe the bridge section attached. In technical report would you use the term girders or stringers for the members spanning between bent caps?

 

[Lomarandil]

In that configuration, either term would be appropriate.

I'd probably lean toward calling it a stringer if less than ~30" deep, or a girder if it's a larger member.

 

[TehMightyEngineer]

My understanding of the applicable terminology follows KootK's. If there is only one predominant beam in the structure before you get to the vertical gravity supports then I just default to the term "girders". I suppose you could make an argument that the bent cap is the "girder" and the primary bridge beams are "stringers", but I would disagree with this.

While we're talking about bridge terms, can I go on record to say that the term "bent" has always sounded like a misnomer to me. There's nothing "bent" about them (yes, I know the etymology is probably from historic usage or non-English languages). Maybe it's just me but I've always tried to refer to bridge supports are "piers" rather than "bents".

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[EngDYIGuy]

I would use the term beam for cross members between the stringers. I would use the term "Bent Cap Beam"; and agree with you that the bent cap should not be labeled a girder.

I've heard of the vertical bents being referred to as piers also.

It appears this bridge could be labeled as having either girders or stringers. The member size is W36x150 and there are six (6) of them.

 

[TehMightyEngineer]

Lomarandil makes a good point; if I see a member which is too small to be a typical size for a "girder" I would probably not call it such unless it had other members framing into it and truly was a "girder".

All said, I think you could call it a girder or a stringer and the intent would be clear, as long as you were consistent.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[bridgebuster]

In the NYC area, a rolled bridge beam is a stringer & anything fabricated is a girder. A floorbeam - can be rolled or fabricated - is perpendicular to a girder and supports stringers.
W36 would be a stringer; beams between stringers or girders are diaphragms.

 

[SlideRuleEra]

For W36 these days, I agree with Lomarandil. Whether the beams are called "stringers" or "girders" has more to do with the general size of the members. Consistent with bridgebuster's comments, the difference was more significant in the early 20th century when large beams were not rolled by steel mills. For example, in 1917 the largest rolled size was a light weight W27 (90 lb/ft). Anything larger was a riveted "girder".

A bridge "bent" usually has been used to refer to one "line" of supports for the superstructure. This could be one large pier, perhaps two, three, or more smaller piers that share a common foundation, or a row of piling, as in a "pile bent".

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[EngDYIGuy]

I have seen timber bridges with timber pile piers, and timber framing members spanning the piers are tipically referred to as stingers.

I am unsure of the differences in the terms "Pier" and "Bent", both are used intermittently. However, I'm under the impression that a pier is a continuous member that is both the deep foundation and structural vertical framing member. While a bent, may not be. I am unsure of this though.

 

[bridgebuster]

Is it a pier or a bent? Again, in my area, a pier is more robust than a bent. A bent is usually piles without a footing; the piles are extended up to the pier cap or cap beam level. Some call concrete substructures piers and call steel substructures bents.

If you're in the US, look at your state DOT's bridge inspection manual for the local terminology.

 

[EngDYIGuy]

So your interpretation of pier vs bent sounds similar to mine.

For example, a drilled shaft/caisson foundation that extends as a continuous member above grade as the structural vertical framing member, would be labeled a pier.

Similarly, a driven pile or group of piles in a row that extends continuously above grade with a cap beam, would be labeled a pier.

 

[bridgebuster]

Yes, although I think you meant to say bent for the second example.

 

[IDS]

I don't know the origin of the word "bent", but outside America, as far as I know, the common English language name for a bent is a pier.

The link below doesn't give an origin either, but comes up with some suggestions:

http://www.madsci.org/posts/archives/2002-05/1020354456.Eg.r.html

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/