a few bridge questions

[nkp_174]

Here is a basic analysis of your bridge...
compressive strength of each pier...appx 250tons.
shear strength of deck (neglecting the strength of ties and lack of stringers)...90tons (a 4-axle diesel is around 80t)

Be advised that buckling and bending are probably where it would fail. I would consider such a bridge to be believable for small locomotives (porters, small shays, small heiselers...) but it is unsatisfactory for any modern mainline power. Weight does not scale linearly, and that is why it holds the model f-unit.

[Spitfire]

I'm no structural engineer, although it sure is great to hear from an expert! I just use common sense and observation when mixing materials, whether it's a bridge or a building.

You will see stone foundations supporting wood structures. Wood supporting stone - never. Wood supporting steel - that doesn't work either. Curved bridges are not really curved, but a series of angled straight sections. Think of a brick arch. It is a curve, but the bricks that make it up are still straight, they are just angled (except to the keystone of course which is angled on both sides).

There may be exceptions to this, but I think it's always more believeable to go with standard practice rather than hunting for some unusual prototype to justify "breaking" the rules.

As for mixing truss bridges with timeber trestles, there were many instances of truss style bridges being built of timbers. There are even kits of wooden truss bridges. <!-- m --><a class="postlink" href="http://www.walthers.com/exec/productinfo/472-3053">http://www.walthers.com/exec/productinfo/472-3053</a><!-- m -->

cheers
Val

[LynnB]

Some great edumacational info from Micheal. Micheal if you happen to fall upon my layout progress please ignore all my bridges

[nkp_174]

Some great edumacational info from Micheal. Micheal if you happen to fall upon my layout progress please ignore all my bridges

Thanks.

Don't worry, none of my locomotives are prototypically accurate...they have little electric motors in their fireboxes and boilers stuffed with lead. Also, all of my cars are equipped with "friction" brakes instead of automatic air brakes So, I can't really complain about other people's bridges (mine are fair game!).

[Bigsteel]

the F-unit was just for a size comparison,and i wanted to see how strong it was .but itll be holding small shays and log cars anyways so it dudnt really matter .nice to hear from an expert though,i thought it would be able i to hold more in full scale though,seeing as the timbers are approx 2 ft diameter square.but anyways,ill post the other bridge soon,its copied from a real bridge so nit picking this time NKP --josh

[nkp_174]

from a real bridge so nit picking this time NKP --josh

Ok! I'll have my finger ready...

I am not an expert...I am a civil engineer...but not a structural engineer. I have been trained for structural analysis...but not the fancier stuff like finite element analysis and such. I'm more of a dam engineer than a bridge engineer...(I had a chance to use that line again! )

If your bridges are made from rarer woods and factors of safety are relaxed...they could hold much more. You ought to see some of the bridges in George Hilton's American RR Narrow Gauge...

Michael

[jim currie]

As i said i would in first post here is a photo of the SP bridge over the Gila river near Roll AZ it has a timber approach on the north end . built in 1923.
Jim

[Bigsteel]

thanks for the post guys.nkp,im going to add more pilings and beams lengthwise and even more bracing.it should be fine after then.it will atleast give an impression of a rickety logging bridge ,and great pic jim , its pretty interesting how they did that,althoug nit uses concrete to support the metal truss itself.-josh

[jim currie]

if you want rickety how bout this.

[Bigsteel]

jim,thats a little bit more rickety than i had planned for ,i did reinforce the whol structure and turned it into somewhat of a truss bridge.i cant post pics on this computer,so if someone can link them for me im chessie1992 on photobucket,if not,itll just wait till monday.--josh

[LynnB]

I like the rickety bridge look.
Here's the links also posted in the main layout topic

[sgtcarl1]

Big Steel:
Way cool trestle/bridge work. I learned how to build trestles like that way back in 1960! But I dis-remember, now!!

[Bigsteel]

thanks lynn ,i couldnt have posted them pics without you ,and for anyone wondering,the first pic is of my new bridges larger more complicated brother on the same canyon i was thinking of staining some of the timber to look old but leaving the others look new like part of it burned down.id also leave a junk car or two below the bridge and some burned timbers.--josh

[Bigsteel]

ok,here is the finished reenforced bridge,itll need some more staining as its kinda blotchy right now.but you get the idea ,bye the way,doe anyone recognize the reefer? thanks wayne --josh





and hers a beter pic of my other bridge,you can see the truss rods and internal bracing that was a pain to build,but it looks sooo cool

[iis612]

Josh,

That is a sweet looking bridge!! It kinda reminds me of one on the former CSX line coming south from the Saginaw yard, over the Cass River. That one didn't stand nearly as tall, but the supporting structure was very similar. It, too, was all wood construction. I remember coming south towards Flint after some serious rainfall with the river water level near the flood plain level. It would actually touch the ties. That bridge was built in the 1930's, I think.

Matt