How wide should a bent be?

[Glenn Brooks]

So I am getting ready to build bents to support a 22' long RR bridge deck that spans my driveway in front of my shop, also another 30' bridge over a small gulley in the back of the property.

The width of my existing bridge structure is 12" on centers to accommodate 12" ga. track. it is made up as a weldment consisting of 8" x3" light weight steel I-beams, with angle iron cross bracing. It doesn't stand very far above the driveway -only 5" or so to the bottom of the I beams. However I need to support the center of the span with a box style bent.

My big question is: what is the ideal ratio for the width of a bridge bent? In other words, if The gauge is 12" wide, how wide should be the upper beam of the bent? Most all bents I've seen extend out beyound the width of the decking a certain amount for stability. - but I've never seen a formula or engineering specification that specifies the optimum width, compared to the width of the bridge decking.

I am thinking the upper beam of the bent should be perhaps 3 times as wide as the track gauge + some magical extra amount for side planking? Anybody know the answer to this?

Thanks
Glenn

[makinsmoke]

Glenn,
If you have the bent support structure figured out and are simply asking about the width of the
bridge and therefore the horizontal top of the bent, then decide if all you want is to have room on top for a scale bridge width, or room for feet
if one has to step off the equipment.

I think that's all you need to decide.

[Cary Stewart]

It almost sound like you are going to build vertical bents without a wider base. Yes there is a ratio that some use - I don't know what it is. RR engineering books may offer some help. Even the steel bents that we built for the LALS west end truss bridge (concrete piers) and the through deck bridge sections which are wider than the top support.
Cary

[Glenn Brooks]

Hi Cary, thanks. I do intend to build a wider base than the top cross beams of the bents - although with my driveway bridge sections, I've pretty well settled on a box structure-similar to cribbing- to support the two center spans. The vertical height is 13" overall, only 5" between bottom of the bridge frame and the driveway. Not much, but enuf of a drop to be a serious safety hazard if the bridge structure isn't stable. So in a flash of repressed genius I decided to make rectangular solid abutments out of 4"x6" timbers.

The forthcoming bridge over the gulley -might reach 40' LOA - will need the traditional bent treatment, with wide bases anchored to footings.

Glenn

[makinsmoke]

Hello Glenn,
Here is a very good treatise on wooden trestles specifically the ones on the
Rio Grande Southern.

http://riograndesouthern.com/RGSTechPages/rgstec1.htm

Scaling the dimensions down will provide a very good and tried bridge design that will also look very good.

One can also fudge the scale and increase the material size. Done across the board the effect is not noticeable. If you only increase the dimensions or size of a couple of parts it makes the bridge as a whole look strange.

I built a low trestle in Oklahoma using these plans, but used common available lumber. 2x4's for the bents and top and bottom timbers, and 2x6's for the stringers. I bolted the stringers together using all thread, in paired assemblies of three, with spacer blocks between the three. So you have six 2x6's longtitudinally under the track. I cheated and just used regular track panels with 2x2's for ties, just longer.

Interesting note. The RGS in places used guard rails on some of their trestles outside the running rails to allow flanging the bridges.

Hope this helps.
Brian

[Glenn Brooks]

Brian, this page is perfect. Thanks! I found a bunch of 100year 3x5" clear grain fir cribbing last year for the bents. Really nice looking, strong, full dimensional lumber, so I think it will serve well for 3" scale construction. (The track is 12" ga.) The dimensions and slope of the uprights mentioned in the article are exactly what I've been looking for. Hope to start building mid June.

Glenn

[Patio]

What is a bent?

[makinsmoke]

Hi Patio,
Open the link above and all questions are answered.

Take care,
Brian

[Patio]

Thank you Brian.