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Re: Lag screws in withdrawal from end gr

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Charles Greenlaw wrote:

. > Another mystifying addition to the 1991 NDS etc., is the provision 
. > allowing built-up lumber columns by nail-laminating a stack of two-inch
. > lumber, faces to faces. Bowstring truss top chords have been done this 
. > way, where through bolting only occurred at the web connections. I have
. > pictures of top chords of not-overloaded 100-ft span trusses made this 
. > way in a warehouse, and the chords buckled like the edge of a leaf of 
. > lasagna-- one panel curved up, the next down, and so on. The nails 
. > completely failed to make the stack of 2x6's monolithic for l/d purposes. 
. > Resulting truss sag caused the solid 6x8 bottom chords to split open 
. > along the tension splice bolt holes by the prying action of their long, 
. > rigid steel side plate splice straps, that could not follow the 
. > sag-caused joint rotation. What saved these tension splices (and the 
. > building from collapse) was a couple of half-inch vertical "stitch" bolts 
. > at each joint, with ordinary cut washers, that stopped the split 6x8's 
. > from separating more than a half inch along a long line of horizontal 
. > one-inch bolts. (The stitch bolts were loose by nearly that same 
. > half-inch, due to seasoning shrinkage on the 8-inch dimension, at all the
. > unsplit joints.) 
. > 

I, too, have seen what is described, however, I have attributed splitting at 
the bottom chord splices a result of shrinkage of the wood where unyielding 
steel shear plates have been installed rather than prying of the splice 
plates.  (I also have major concerns about the effect of elevated attic 
temperatures on wood properties and don't believe that the effects of 
temperatures under 150 deg. are "essentially" reversible as stated in the 
codes.)  In addition, many of the 60' - 100' span trusses have (2) 2 X's or 3 
X's as the bottom chord and were designed long before it was known that the 
allowable tension stresses permitted by the code were extremely 
non-conservative.  In one instance, 9 of 13 100'+ bowstring trusses with (2) 
3 X 12's for the bottom chord had failed, several of them with a vertical 
break thru the member away from a joint.  Installing stitch bolts in 2 X and 
3 X chord members would seriously reduce the strength of an already 
inadequate member.  Incidently, the roof in this case did not collapse, which 
I attributed to the roof sheathing boards being installed diagonally, making 
the sheathing a self-supporting shell, rather than a truss supported roof.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona