Model Railroading > Howe truss question

I'm most of the way through a large wood Howe truss kit. While the kit instructions indicate using a lap joint, or 2 pieces for the cross bracing between the trusses, I'm curious if the prototype would go through that trouble, or if it would be full size pieces (in this case 6x8 and 8x8) staggered vertically so they cross at the midpoint. Obviously in HO scale using full size pieces will be significantly easier than cutting a lap joint. Thoughts?

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Darren Boes
Lethbridge, AB
Southern Alberta Railfan

I can't really answer your question but my great-great grandfather built barns in Iowa many, many years ago. He had special tools and from what I understand He would make some interesting joints, etc. (also using pegs?) for barns that would last a long time. I don't think the Amish build them like Josephus did.

Victor A. Baird
​http://www.erstwhilepublications.com

 



Edited 1 time(s). Last edit at 03/27/17 10:24 by wabash2800.

When i was helping rebuild the snowshed at Cumbres Pass 20 years ago, the joints in the verticals and the main horizontals where rabbet jointed with 3 foot pins of iron joining them. They where not fun to pound into the vertical. I would say construction technics of a hundred years ago, plus the work to this day by the Amish with traditional barn/structure building using every wood joint possible is totally increadable in this day and age. Slightly off setting a peg joint to help draw in a timber is a classic way of tight joint construction.

Blows me a way looking at some of the stuff that is still around....

I've been reviewing some drawings of wooden Howe trusses in my files.

As pointed out in anothers' post, wooden post and beam structures for barn and home construction were usually constructed using mortise and tenon joints secured by wooden pegs.  Bridgebuilders avoided this.  The philosophy of structure builders was that it didn't matter if you removed wood (a mortise) from a beam, as long as you replaced it with a tenon.  Not so with bridgebuilders.

Also, in building wooden bridges, wood is NEVER used as a tension member.  Use wood ONLY in compression.  Metal rods are used for tension members.  In the  set of drawings below, "C" is a compression member, "T" is a tension member.

Now, back to your Howe truss.  You're asking about the interlocking of crossmembers between the trusses.  There may be one to three types of these, but I'm not which you are referring to..  (1) All trusses will be tied together below the floor (which supports the rails).  On a pony truss, these are the only cross ties.  (2) On a Through Truss, the side trusses will be cross tied above the train, and on deck truss will be cross tied at the bottom of the structure, well below the floor. Both types one and two lie in a horizonal plane.  (3) On a deck truss there will be cross ties in a vertical plane,

It is not likely, as noted above, that on a bridge these would be mortise and tenoned.  The side truss diagonals will invariably be "X" braces with the diagonals leaning left being in a different plane than those leaning right.  The "X"s of the type (1), (2), and (3) above may lie in the same plane, but are not likely to be notched to lap into each other.  Type (3) braces may lie in separate adjacent planes.  I have drawings of Great Northern bridges in which type (2) cross braces are notched to fit into each other, but are not lapped.

Hope this helps.

-John



 

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Thanks for that...my gut was right about a wood member being notched (weakened) seeming out of place. These braces go in a horizontal plane between the upper and lower chords of the deck truss, easier to draw and install than vertical cross bracing (it is a mass produced kit after all). I should have pictures to share on this forum in the next couple months.

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Darren Boes
Lethbridge, AB
Southern Alberta Railfan