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Wood truss bridge

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Wood truss bridge
Posted by UNCLEBUTCH on Monday, January 08, 2018 9:13 AM

I need/want to build a wood truss bridge. I've found a ton of pics.and drawings,photos of kit and scratch built.

I can not find anything that will tell me the size of lumber/timbers to use for the different parts. I assume it depends on span.

Anyone point me in the right direction ?  thanks

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Posted by RR_Mel on Monday, January 08, 2018 9:45 AM

I built mine about 28 years ago from a centerfold in an MR magazine.  As best as I can remember the plans either had dimension or I sized the timber to match the size of the timbers on the centerfold.
 
My LHS stocked Northeastern Scale Lumber and he had all the correct sizes.  I built my trestle the same way.
 
 
 
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Posted by 7j43k on Monday, January 08, 2018 9:57 AM

In Paul Mallery's "Bridge & Trestle Handbook", there's a chapter on wood bridges.  There are drawings with the sizes of most members noted.  It is an excellent book.  

If you have a particular bridge in a photo in mind, the above will likely give you very good hints on sizing.

It looks to me that, if you had a nice drawing for a bridge that spanned 100', and you wanted one for 200', you wouldn't just scale the plans up.  They tend to add panels, also.  You'll note that the number of panels varies.

Anyway, I recommend checking Mallery's book out.  I have a 1976 copy, and I think I bought it not long after that date.  I've read it cover to cover a couple of times since.  And I pull it out once in awhile, for discussions like this one.

 

 

Ed

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Posted by VOLKER LANDWEHR on Monday, January 08, 2018 10:06 AM

Kalmbach's Model Railroad Bridges & Trestles contains a chapter about wooden bridges with dimension and details.

Amazon offers a number of used books: https://www.amazon.com/Model-Railroad-Bridges-Trestles-Designing/dp/0890241287

Regards, Volker

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Posted by mbinsewi on Monday, January 08, 2018 10:10 AM

The Dec., 1988 MRR has some info on timbers sizes, and designs.  Pretty intensive article.

Mike.

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Posted by RR_Mel on Monday, January 08, 2018 10:52 AM

Ed jogged my memory.  I got the info from Bob Hayden’s Model Railroad Bridges & Trestles.

Page 39 
 
 
 
Mel
 
Modeling the early to mid 1950s SP in HO scale since 1951
  
 
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Posted by selector on Monday, January 08, 2018 11:28 AM

Spans much longer than about 30' would require either metal girders, or a metal truss of kind, or a Howe truss if wooden.  You would never see a wooden span of 100' because the typically greatest length of timber in trestles and the like, the tallest bents, would be 30'.

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Posted by mbinsewi on Monday, January 08, 2018 11:52 AM

I just read through the article by Harold Russel that I referred to in my last post, in the Dec. 1988 MRR.

It was actually part 6 of a series that Harold did, starting in July of 1988.

A lot of the same illustrations that are in Bob Hayden's book.

Mike.

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Posted by rrebell on Monday, January 08, 2018 1:58 PM

selector

Spans much longer than about 30' would require either metal girders, or a metal truss of kind, or a Howe truss if wooden.  You would never see a wooden span of 100' because the typically greatest length of timber in trestles and the like, the tallest bents, would be 30'.

 

Generally speaking yes but it has been done.

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Posted by selector on Monday, January 08, 2018 2:32 PM

I don't mean to put you to the test, but would you be so kind as to provide an example of a 100' wooden span that is unsupported with at least one central bent, pylon, pier, crib, or cables of some kind?  I'm not talking about an arched structure, such as a road or ox-cart bridge over Ye Olde River, I'm talking about one with a supporting deck, at grade, that would withstand the mass of even a 60 ton locomotive midway along its span.

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Posted by VOLKER LANDWEHR on Monday, January 08, 2018 5:02 PM

Here is an example: Spokane International/UP Kootenai River bridge near Bonner's Ferry: http://art.nelson-atkins.org/objects/63624/bridge-number-1069-kootenai-river-bonners-ferry-idaho-fi

Five spans of 125'. And the same bridge: http://www.moosecreek.ch/bilder/history-prototype-rallroads.html/history-howe-truss-bridge.pic/pic1.jpg

The book Model Railroad Bridges & Trestles contains drawings of a Northern Pacific 150-foot Howe truss bridge.

If you google you can find a lot of Howe truss bridges of more than 100 ft span.
Regards, Volker

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Posted by selector on Tuesday, January 09, 2018 11:50 AM

Thanks for those examples, Volker, which support my contention earlier...no simple spans, but spans with a Howe Truss or some other girder/truss arrangement.  Even then, none of the timbers involved exceeds 9m in length, or if they do, by perhaps one more meter.  

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Posted by 7j43k on Tuesday, January 09, 2018 12:04 PM

Selector is no doubt referring to "beam bridges", the simplest of which is a log felled across a chasm.  It is a single element or multiple parallel elements.  Each beam is not fabricated or assembled.

He implied that when he mentioned soon after the term "a Howe Truss if wooden".

This is a railroad beam bridge:

 

 

They are generally so small they aren't even noticed.  Until you put a bunch in a row and call them a trestle.

Girder bridges are a form of beam bridge (they are, of course, assembled.  In their way).  But I recall no wooden girder bridges.

We "all" know there are wood framed trusses longer than 100'.

Finding a 100' wood beam bridge that will carry trains might just be more of a challenge. 

 

Ed

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Posted by VOLKER LANDWEHR on Tuesday, January 09, 2018 1:21 PM

7j43k
He implied that when he mentioned soon after the term "a Howe Truss if wooden".

I just overlooked Selector's first post and therefore misunderstood his next one.

7j43k
Finding a 100' wood beam bridge that will carry trains might just be more of a challenge. 

As a civil engineer (structural design) I wouldn't even look for one.

7j43k
We "all" know there are wood framed trusses longer than 100'.

Having seen lots of model railroads with the wrongly selected types of railroad bridges I thought I show an example (after misunderstanding the question).

It doesn't happen on American layouts alone it is even worse in my country (Germany). But in the USA you have the luxury of a number of good books on the subject that we don't have. Two were already mentioned here.
Regards, Volker

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Posted by selector on Wednesday, January 10, 2018 12:57 AM

I could have worked harder to be more clear.

I did mean that you would never see a simple beam span of timber over about 30' which was intended to support a 70 ton locomotive and several logging cars or ore hoppers.  This is because trees used for timbers in trestles and other bridges don't often go much further than 30 feet from nearest the 'wide' end until they start turning into cambrium and bark.  So, when I began to research timber trestle construction years ago, it seemed that from site-to-site, and railroad-to-railroad, the longest timbers 12" x 12" on a side were about 30'.  You can't join such timbers to cover a 100' span unless you use a truss of some kind.  Most trestles that were long and had to span a river between bent frames employed a box truss, often of the Howe design.  I have stood under such a truss that was at least 50 feet long, that being on the Kinsol Trestle on Vancouver Island.  It is complicated and massive with many metal rods under tension.  My guess is the Howe truss, by itself, would top out near 200 tons.

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Posted by VOLKER LANDWEHR on Wednesday, January 10, 2018 4:56 AM

selector
I could have worked harder to be more clear.

I should simply have read your first post.

In your first post you state: You would never see a wooden span of 100' because the typically greatest length of timber in trestles and the like, the tallest bents, would be 30'

Perhaps I misunderstand you again. If you mean that higher trestle bends are made of shorter posts of approximately 30' on top of each other you are right. That is possible as compressive forces can be transferred over a butt joint.
http://www.highestbridges.com/wiki/images/2/2f/CedarRiverTrestle3.jpg

The link shows the Cedar River Logging trestle in Washington State is 203' feet high.

The story height differed from railroad to railroad. RGS had typically 16' with a maximun of 20'.

GN Rwy bents were 27' to 33' high:  http://3.bp.blogspot.com/-IOtJjna4m-Q/UYkR6hkYZrI/AAAAAAAADmw/KPZv7IhhUjA/s1600/4.jpg

I think post length differed with availability of fitting timber.
Regards, Volker

 

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Posted by j. c. on Wednesday, January 10, 2018 5:57 AM

check out the Dungeness river bridge a 150 ft wooden howe truss.

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Posted by VOLKER LANDWEHR on Wednesday, January 10, 2018 9:16 AM

You misunderstood as did I. The question was: are there examples of timber beam bridges with spans of 100'.

The answer is no, it doesn't work.
Regards, Volker

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Posted by selector on Wednesday, January 10, 2018 12:19 PM

j. c.

check out the Dungeness river bridge a 150 ft wooden howe truss.

 

We have all agreed that many rail Howe Truss bridges are wooden, and that other types of wooden trusses are suited for, and used for, railroads.  In many cases their spans run much more than 50 feet.  What we are attempting to settle is whether or not a BEAM bridge can have a span beyond 30-40 feet for railroad tonnages and still be made of timber.  The answer is an unequivocal....NO.  The span would sag too much in the middle and deviate from grade.  That's why we invented trusses....they are many times stronger, and can maintain their shape closely, even on long spans.

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Posted by j. c. on Wednesday, January 10, 2018 1:15 PM

you missed the piont that the op was asking as he stated a truss bridge  the reply i made was to him not you.

selector

 

 
j. c.

check out the Dungeness river bridge a 150 ft wooden howe truss.

 

 

 

We have all agreed that many rail Howe Truss bridges are wooden, and that other types of wooden trusses are suited for, and used for, railroads.  In many cases their spans run much more than 50 feet.  What we are attempting to settle is whether or not a BEAM bridge can have a span beyond 30-40 feet for railroad tonnages and still be made of timber.  The answer is an unequivocal....NO.  The span would sag too much in the middle and deviate from grade.  That's why we invented trusses....they are many times stronger, and can maintain their shape closely, even on long spans.

 

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Posted by jjdamnit on Wednesday, January 10, 2018 1:44 PM

Hello all,

I too recommend the Kalmbach Publishing Model Railroad Bridges & trestles; Bob Hayden, Ed., Seventh Printing 2006, for the information you are looking for. 

This might be out of print but is still available on Amazon.

Check out this link:
https://www.amazon.com/Model-Railroad-Bridges-Trestles-Hayden/dp/B001NOA2OG/ref=sr_1_1?ie=UTF8&qid=1515612574&sr=8-1&keywords=model+railroad+bridges+trestles

On page 3 there are charts on bridge load classification and bridge construction; listing the types of bridges, building material and span in feet related to time of construction; 19th- and 20th-century and modern (21st-century).

Page 49 has a chart of strip wood sized (actual) that translates to different scale size lumber from N to O scale.

I believe this is the information you are asking about.

Hope this helps.

"Uhh...I didn’t know it was 'impossible' I just made it work...sorry"

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Posted by selector on Wednesday, January 10, 2018 2:42 PM

j.c., it was not clear from the way you posted your reply in the order in which it came during discussions of types of spans.  I understand your aim now...thanks.

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Posted by j. c. on Wednesday, January 10, 2018 5:39 PM

selector

j.c., it was not clear from the way you posted your reply in the order in which it came during discussions of types of spans.  I understand your aim now...thanks.

 

 

the reason it was so late as i had to find out how to spell dungeness on my puter i listed it as listed as drb.

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