Weird roof truss

Aug 02, 2006 2 Replies
A

At my new job we're working in an old railroad repair building, from about 1880 (heavily sandblasted!).



Rigth above our heads are some kinda unusual looking trusses that hold up the barrel roof.



The trusses span about 120 feet, are about 12 feet high, and consist of a straight bottom beam, about 8x14, zig-zag braces, and a curved upper beam also about 8x14.



Running from the top to bottom beams are a number, about 8 steel rods, with plates and nuts on the outer ends.



What confuses me is the rods are of wildly different diameters-- the ones near the middle are about 3/4 inch in diameter. The ones near the walls are about 2.5 inches in diameter!



I can't quite picture how there could be more stress near the walls than at the center, and by that large a factor.



Any ideas?



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D

... best guess is that the vertical steel rods were intended to hold the roof onto the truss, as in the instance high winds or other forces of nature. However, as you proceed from the wall to the center of the truss, you incur a bending moment at the point where the truss is attached to the wall. ... Consequently, the design allowed for the placement of the heaviest rods closest to the wall, with the lighter rods further out, towards the truss center. ... If the top of the rods was not attached to the actual roof structure, I'll have to come up with a different explanation.

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G

The rods (in tension) and the diagonal braces (in compression) are taking the shear load of the truss. Shear load is maximum at the supports (walls) and zero in the center, while bending load is maximum at the center and zero at the walls, and is taken primarily as tension in the lower beam and compression in the upper.

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