Large Scale Central

Howe Truss Bridge Engineering?

Modeling
#1

Okay the next question is how does one engineer a Howe. The two known as the span which is 35" on the rail. That is what is there now with the appropriate gaps at each end. the other known is I would like it to be 12" top of rail to bottom of the upper cross beams. I am not too concerned about width because it will be on the wider side as I want scale walks on each side because this will be used as a “man/wagon” bridge so the entire deck will be planked. So width will be what ever looks good and gives clearance.

My design questions are how do you determine the number of diagonal members and their angle so that you get the height and length right? It also determines the angle of the blocks that the diagonals rest on (the defining characteristic of the Howe design).

#2

Dan’s bridge is just about exactly what I am looking to build. Great example of the Howe design

#3

I used plans from Garden Texture to build mine.

#4

Their website is still up but whether Bob is still selling plans is another story.

#5

Ken Brunt said:

Their website is still up but whether Bob is still selling plans is another story.

Ken,

Are the plans customizable to my specific length? I would love to follow a plan but to get it so that it fits the space is the key. I understand the design concept. Its just the actual angles that I am unsure of how to figure out. And the angles will change with length as the bridge lengthens or compresses. At least thats how I see it in my mind. It would have been a way better idea to build the bridge from a plan and then build the walkway to accommodate the bridge. But I didn’t do that and unless the plans are close, +/- a couple inches, then I am a still stuck. I could make some modifications at this point to the walkway to support the bridge but not a lot. Everything is pretty well fixed in its location.

#6

Devon

I know I have sent you this before on my bridges. Quadrangular Warren Through Truss Bridge Pair On The Mainline

Scroll down to that article.

I bring it up only because I highly recommend the inclusion of steel pipe in any long wooden bridge span (regardless of the design). Mine are 72" and have two lengths of 3/4" steel pipe hidden in the deck. You never know what will show up on your rails. A couple years ago I had a guest bring a 75 lbs live steam K-28 to a “Run What You Brung” day. Apprehensive at the long spans he was very relieved to find out about the pipes.

As always I tend to build for durability and not always for Prototypical accuracy.

#7

http://kormsen.info/bridges/

scroll down about two thirds, where the trussbridge begins.

the beam triangles, that make the criss-cross have the same length on all three sides.

#8

Boomer,

I like the way you think! The hidden steel pipes is a stroke of genius. (https://largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-smile.gif)

#9

Thanks to both Boomer and Korm for those links. I am just about ready to give up on the idea of a prototypical Howe truss and just build a nice look stout workable truss bridge. Using the info from both links I can make a nice looking truss bridge and only I know that it is not a prototypical Howe truss as was used on the CR&N which I am not copying to a T anyways. I do like the pipe idea or at the very least some angle aluminum. Even thought you won’t be able to have more that one or two cars and a small loco on it it still will be nice. This is not a through track but instead is on the tip of my WYE which serves as an industry siding that only holds one car. So weight isn’t going to be to big a problem; over kill is always good.

#10

Devon Sinsley said:

Ken Brunt said:

Their website is still up but whether Bob is still selling plans is another story.

Ken,

Are the plans customizable to my specific length? I would love to follow a plan but to get it so that it fits the space is the key. I understand the design concept. Its just the actual angles that I am unsure of how to figure out. And the angles will change with length as the bridge lengthens or compresses. At least thats how I see it in my mind. It would have been a way better idea to build the bridge from a plan and then build the walkway to accommodate the bridge. But I didn’t do that and unless the plans are close, +/- a couple inches, then I am a still stuck. I could make some modifications at this point to the walkway to support the bridge but not a lot. Everything is pretty well fixed in its location.

The plans are for a 42" long bridge. Both my bridges are different sizes then what the plans called for. One is longer and wider than the other. It was built originally for use on a curve. I just used the plans as a guide to how it was built.

Garden Railways also had a set of plans for a Truss Rod bridge. It was 18" long. Now whether those plans are still available I have no idea.

#11

Judging by the spans this one is 36".

#12

Well, the cross members should be close to 45 degrees from the horizontal. They can be steeper then that, but seldom lass then that. So you know the height, and you know the length of what you want. See if you can divide the length by the height and get an even number of bays. If not, then add one more bay to it and that would be how many bats it should have to get the diagonals close to 45 degrees.

Rereading your post, you want 35 long and 12 inches high. 12 inches sound a bit tall, but running with that, you could build it 36 inches long, 12 inches high and have 3 bays. Or, if you go for steeper then 45 degrees, you could have 4 bays.

#13

Okay the next question is how does one engineer a Howe.

Devon, some years ago that question was posed over on MLS, and they found a website for a military college that had the whole works on it. How to design and build all sorts of bridges - including a Howe Truss. Does anyone else remember, and know where it is?

#14

If you are confident in cutting the blocks to the correct angle, no reason you can’t use something like this for guidance: https://www.usbr.gov/lc/region/programs/bridgebuilding/Presentation.pdf

Or this: http://www.girr.org/girr/tips/tips2/howe_truss.html

I would also recommend threading your own rods and using that for compression - you could build it without glue, though I wouldn’t recommend doing that!

#15

If you use steel, you can simplify your build;

Width rarely has any bearing on a bridge’s strength, up to the point where they add extra weight and reduce the load. The wider the stretch, the taller and thicker the plate. Maybe you should plank the ties and charge tolls between trains.

#16

John Caughey said:

If you use steel, you can simplify your build;

Blank post

Gotta watch my post count

#17

in the track and trestle section about 2 or 3 pages back there is a long post about truss bridges and lots of pictures of several examples. Ok it was about 9 pages back and you posted it!

Topic: Howe Truss design?

#18

Yes Pete I do remember posting that. And inam still going on that information. I know about enough to be dangwrpus. The problem I ran into was taking that information and actually putting it to use on my given span. I thought it would be easy until I tried to figure it out. I know what a Howe is supposed to be I just don’t really know how to engineer it.

But with all the great info on this thread I am pretty sure I can come up with a usable design. The only reason. It had to.be a “Howe” truss and not some other truss design is because the CR&N (my prototype) used Howe bridges. It is clearly called put for in the track book I have and I have photo evidence of one of the bridges. So I was hoping to keep.with the theme. I’ll figure it out I am sure. At the very least I knownincan make some form of a truss design.

#19

Geez sorry for the horrible typing. Between my fat fingers and horrible eyesight I can see what I am typing on my pbone.

#20

You are excused my friend. (https://largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-wink.gif)