Page 540 - Forensic Structural Engineering Handbook

14.30 MATERIAL-SPECIFIC FORENSIC ANALYSES

carefully mixed and applied. Resorcinol glue used to be clamped, potentially by a closely
spaced nail pattern. (See Case Study 11, William Shore Memorial Pool.)
Although nail laminated beams were produced, those with the laminations flat cannot
handle much moment or shear. One can make a paperback book as stiff as a board if one
glues all the pages together, but shooting in some nails does not develop that kind of stiff-
ness (see Shear Reinforcement section). It doesn’t take much nail slip for a nail lam beam
to behave more like a stack of individual laminations than a cohesive beam. There are a few
nail lam arches still standing. They have much less shear transfer required between lami-
nations (with balanced loads) and the friction to the adjacent lamination due to curvature
helps. This is not a value we want to rely on and it will relax over time.

EXAMPLES AND CASE STUDIES

Several case studies are included to illustrate examples of distress, forensic evaluation, and
repairs discussed in the body of this chapter.

Case Study 1. Hood River Valley High School Theater, Hood River, Oregon
This case study illustrates secondary effects from fixed and eccentric truss joints not fully
accounted for in the original design, cross-grain tension at web-chord truss connections,
and accumulation of fiber separation damage.
The theater structure was designed and built in 1977 with walls of tilt-up concrete and roof
framing of light timber trusses. The trusses were about 65' long, 45'' deep at 32'' on center.
The wood chord wood web trusses were of a unique design where web-to-chord and web-to-
web connections consisted of glued finger joints. The truss manufacturer had many of their
finger-jointed connections tested from which design values were developed. The trusses were
designed as pinned joints with “fudge” factors applied to handle the fixity effects.
Roof collapse occurred on January 12, 1979 under snow loading of about 8 psf, well
below the design snow load of 40 psf. A significantly higher snow load had occurred the
previous year without a noticeable problem with the roof. Initial forensic investigation by
others centered on the strength of adhesive bond at web-to-chord finger joints. Neal
Engineering Associates were retained for forensic evaluation by the school district.
Analysis by the author working with Mr. Neal considering fixed web-to-chord joints indi-
cated definite moment transfer at these locations, particularly at the slightly eccentric top
chord-to-tension web intersection adjacent to supports. The combination of direct web ten-
sion and moment produced high cross grain tension forces at the top chord. This moment
exceeded the effects allowed by the “fudge factors.” Several undamaged trusses were
removed from the structure and tested to destruction on a test rack.
Failure of the three trusses tested occurred well below design load with an average of
dead load plus 56 percent of design snow load. The failure mode could be observed in the
test rack. Failure occurred not in the adhesive but as cross-grain tension in the wood at web-
to-chord finger joints as was highlighted in our analysis. Nodal rotation of the truss con-
nection caused a triangular-type tension distribution across the finger joint connection. The
outer web connection failed in the top chord, followed by inner web connections in rapid
succession. The more significant load duration effects for cross-grain tension played a role
in the accumulation of fiber-separation damage.
Case Study 2. Rainier Beach Library, Seattle, Washington
This case study by Neal Engineering Associates illustrates nodal rotation of fixed truss con-
nections and secondary effects.