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Re: Split Ring Trusses and Epoxy Repair

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At 12:48 PM 9/21/99 -0400, you wrote:
>hello,
>
>I have a project where we inspected some built up split ring trusses.  In
>most of the web members the ends are split, some as much as 1/4" wide.
>First, where can I get information on split rings?  Second, are the split
>rings still effective  even with the splits through the end of the member?
>Last, has anyone heard of using an adhesive to repair this kind of problem
>and where could I get some information on it?
>
>Thanks,
>
>Mike zaitz
------------------

Mike, in the 70's I surveyed about 6 miles of circa 1942-vintage split-ring
connected low-slope warehouse roof trusses at McClellan AFB near here.
Chords ranged up to 3x12; webs were 3x and 2x lumber. Splits were chronic,
but failure to carry load was infrequent. There is no snow here, only
reroofing. 

My take on the splits is that they were due to restraint of initial
cross-grain shrinkage, and restraint of later cross-grain
expansion/contraction cycles with seasonal humidity changes. The split-ring
connectors were in patterns of two or three in two or three rows at each
major connection. Horizontal grain of the outboard twin chords restrained
cross-grain movement of the twin diagonally oriented webs, which were next
inboard of the chords, and the center, single vertical web also fought
against diag web cross-grain movement. Chords also had splits, typically
between connectors at the joints.

Conventional truss analysis for the loads indicated a competent design.

Split rings are a shear transferring device located at the faying surface
between adjoining members. The rings fit into circular grooves routed into
the faces of contacting pieces. The routing tool pilots in predrilled bolt
holes, and the bolts both clamp the stack of lumber together and add some
shear resistance in the event of slippage of the split rings. See the NDS
for illustrations. Old ones have them.  

It would seem that a split ring bears on the edge of the routed grooves, and
that those edges stay put only by virtue of the lumber's localized shear
strength parallel and oblique to grain. The rings in part bear on the disc
of wood within the ring.

Endurance of these wood properties in the face of fluctuating stresses
induced by cross-grain lumber movements and by truss-action "secondary
stresses" (non-pin joint moments) is the problematic condition, in my opinion.

In the trusses I dealt with, we specified replacement of some members with
new kiln-dried lumber or modern LVL lumber, but mostly we added mechanical
restraints against existing splits growing wider and freeing the connectors.

My lingering impression is that lumber trusses have a size effect that
compromises the endurance of their lapping joints. The departure from
easy-twisting pin conditions as the connector pattern gets big seems to
matter a great deal.  

I would worry about epoxy adding to the problem of internal struggles within
a lumber truss joint assembly.

Charles O. Greenlaw  SE   Sacramento CA