Small railroad bridge

Discuss park gauge trains and large scale miniature railways having track gauges from 8" to 24" gauge and designed at scales of 2" to the foot or greater - whether modeled for personal use, or purpose built for amusement park operation or private railroading.

Moderators: Glenn Brooks, Harold_V

Forum rules
Topics may include: antique park gauge train restoration, preservation, and history; building new grand scale equipment from scratch; large scale miniature railway construction, maintenance, and safe operation; fallen flags; track, gauge, and equipment standards; grand scale vendor offerings; and, compiling an on-line motive power roster.
Post Reply
Springhill-RR
Posts: 26
Joined: Sun May 30, 2021 3:17 pm

Small railroad bridge

Post by Springhill-RR »

We are building a small railroad bridge for a 2 ft. gauge track. Total length is 10 ft. We have concrete supports on both ends and middle. Our total load is less than 1000 lbs. Should we use "H" steel, 3" square tubing, or 3" channel iron? We have galvanized 3" angle that we could attach treated 4x4's to for attaching 4x6 crossties.
User avatar
Bill Shields
Posts: 10442
Joined: Fri Dec 21, 2007 4:57 am
Location: 39.367, -75.765
Contact:

Re: Small railroad bridge

Post by Bill Shields »

H or I beam

You have to consider 1000# in the center of the bridge and distributed weight of track and ties plus riding cars and passengers...and assume that at some time the future the center support may settle a bit

Plus the possibility of a derail in the center of the bridge with maybe 6 men to get it back on the track.

All of a sudden your 1000# load looks very light.

And what happens when someone brings a 3000# loco to run?...ask him to weigh it..giggle

You can never use too much steel

Find some used structural beams
Too many things going on to bother listing them.
Glenn Brooks
Posts: 2929
Joined: Mon Nov 10, 2014 1:39 pm
Location: Woodinville, Washington

Re: Small railroad bridge

Post by Glenn Brooks »

Personally I’d be inclined to build it with 4x6 wood stringers, with one bent in the middle. Traditional RR design. Easy to build, using 10’ timber’s. Carrying capacity up around 4/5 tons+. Plus you have an authentic trestle when your finished.

Glenn
Attachments
BF282B23-3465-4957-8DED-3A90057D608A.jpeg
002D90EF-B9F2-407D-8993-81D4C8B735BA.jpeg
043E88D4-24D6-46CF-89AE-39593572A49A.jpeg
AC494D19-9951-4753-9A93-5453DB5D2352.jpeg
EAB8750C-DB4F-4C44-AF5A-72FF4AC4E67A.jpeg
636CCFD5-6D6D-4673-980A-86BEAE6778E1.jpeg
8D36E52E-FDF2-41E6-AAAD-115F7A9D17FF.jpeg
44D04190-115F-4897-8E17-2E35FF1EB711.jpeg
A5CE0788-AC29-4204-BE84-6EBC82B4DA32.jpeg
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....
rkcarguy
Posts: 1730
Joined: Tue Aug 22, 2017 10:33 am
Location: Wa State

Re: Small railroad bridge

Post by rkcarguy »

Glenn's trestle is really nice, I'd agree to use your 4x6's for stringers, the 4x4's for cross ties, and incorporate the 3x3 angle for brackets and braces (you don't say how much you have). For a 10' span I'd probably want 2 bents, but I'm all for overkill so it's up to you.
FYI, treated lumber contains copper and other chemicals dissolved in an acidic compound. Especially in wet environments, the addition of galvanized steel parts essentially creates a battery, which will quickly corrode and consume the galvanized zinc (anode) brackets and fasteners. I highly recommend either sealing the treated lumber (including the holes you've drilled) with an outdoor water proofer, or at least bed the fasteners in wax, sealant, or caulking. You can use tarpaper soaked in water proofer as an isolator between any angle brackets and the lumber. I've taken apart deck structures that are less than 5 years old and had the heads pop off the galvanized fasteners used (that looked good from the outside) because the rest was literally corroded away.
rkcarguy
Posts: 1730
Joined: Tue Aug 22, 2017 10:33 am
Location: Wa State

Re: Small railroad bridge

Post by rkcarguy »

FYI, this is from our engineer we work with where I work. I had proposed just using two steel beams with 4x4 ties carraige bolted to the top chord. Unofficially, of course, you know how it is.
"If I were building a 5 to 6 foot wide bridge with two steel stringers spanning 8 feet, I would use any of the following for the steel stringers: W4x13, W5x16, W6x12 or W6x15. I like these better than a W12 because the shorter shapes are less likely to roll or buckle. I used 100psf live load or 5000# in the center of the span for my check."
If you wanted to go free span, two W6x15's should do it with proper beam seats. Add a couple steel cross-braces and your loading goes up exponentially. I have access to lots of scrap so I will probably be going this route when I start laying my track.
Glenn Brooks
Posts: 2929
Joined: Mon Nov 10, 2014 1:39 pm
Location: Woodinville, Washington

Re: Small railroad bridge

Post by Glenn Brooks »

+1 for what Ryan said. Not sure what to do about treated wood these days. Use rebar for spikes maybe. In lieu of galvanized spikes.

Forgot to mention, my bents are on 4’ centers, and the ends bed a couple of feet behind the revetments- for stability.

The uprights are 3x5” recycled, old growth, clear grain fir - seasoned in salt water at a cannery for 50’ or 75 years. ( I have a secret stash I use for special projects )So fully saturated with salt and fish oil that bugs and dry rot don’t stand a chance. They don’t even weather when stored outside in the rain. Nothing affects them! I treated them with hardware store cuprinol just to darken the wood a bit.

Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....
rkcarguy
Posts: 1730
Joined: Tue Aug 22, 2017 10:33 am
Location: Wa State

Re: Small railroad bridge

Post by rkcarguy »

The problem is the steel in general. You've got the copper, the acid, and the steel/zinc which end up being the anode (eaten) metals. Essentially add moisture and you have a battery, electrolysis will consume the lessor metals.
Coating or bedding galvanized fasteners (try to get hot dipped stuff, not zinc plated) in something will buy you more time. I was examining an older wooden bridge and it looked like they had used hot dip galvanized threaded rods bedded in tar where they passed through the piles and creosoted lumber. Or, you can spring for stainless. FWIW, deals can be found on stainless threaded rod if you can get it wholesale so it may not be that much more than hot dip galvanized.
Post Reply