Page 543 - Forensic Structural Engineering Handbook

TIMBER STRUCTURES 14.33

split the timber members. Collapse was prevented primarily by bracing to adjacent
trusses prior to the emergency installation of shoring towers. As indicated previously, the
load duration effects on cross-grain tension are even more significant than for other tim-
ber stresses. The cross-grain tension induced by the connectors eventually split the tim-
ber under significantly less load than previously resisted, indicating the accumulation of
fiber separation damage. This study by Al Bassett, P.E., S.E. pointed out that there were
no group connector effects in the original timber design criteria. These bowstring trusses
also had the same excessively high allowable stress criteria typical of that era that lead to
failures today.

Case Study 5. Dept. of Social and Health Services Building, Port Angeles,
Washington
This case study illustrates bowstring trusses with eccentric joints and previous fire damage.
Early in the 1996–97 Pacific Northwest snow storm, this building had a major collapse
of the roof structure. Careful study of the geometry and other clues from the collapsed
structure by the author indicated the location of, and which, truss initiated the collapse.
Further observation showed a tension failure in the bottom chord near the heel joint that
appeared brittle and did not pull fibers from the adjoining fractured ends. This fracture was
near the heel joint at a knot and there was a / 4'' char layer from a previous fire under white
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lacquer paint. Truss analysis did show overstress with design snow loads originally at
20 psf, then later, 25 psf. The actual measured snow load was between these values. In addi-
tion to direct tension, moments were transferred into the chord members from joint eccen-
tricity. Final analysis indicated that the previous fire had a very significant effect on the
failure. Besides the obvious elimination of the char layer in resisting stress, this burned
layer leaves little notches in the adjacent heat damaged area of the tension member.
Additional tension is added from the eccentric moments. The other significant effect of fire
on a tension member is that the heat damaged area near the edges has a greater reduction
of strength than reduction of stiffness so a higher proportion of stress is attracted to weaker
wood. The truss which was determined to be the initiator of the roof collapse also had the
most fire damage.

Case Study 6. Rosemont Horizon Arena, Rosemont, Illinois
This case study illustrates lateral buckling of unrestrained glulam arches.
The sports and general-purpose arena was constructed in 1979 adjacent to Chicago’s
O’Hare Field. A total of 14 glulam barrel arches, spaced at 24-ft centers and spanning approx-
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imately 288 ft, frame the sports arena structure. Arches are 10 / 4 in × 73 / 2 in 24F glulam sup-
3
plied in three segments, connected by two moment connections. Framing between arches
consisted of glulam purlins at 4-ft centers with wood decking/diaphragm over.
On August 13, 1979, at a point when 12 of the 14 arches were in place, collapse occurred
rapidly, killing 5 workers and injuring 19 others. Initially, several collapse theories were
discussed in the extensive media coverage including vibration and wingtip vortex from
low-flying aircraft. Placement of purlins and decking/diaphragm was following well
behind erection of the arches. Causes were attributed to lack of workers available, lack of
coordination between the two operations, and schedule pressure. Every sixth purlin at 24-ft
centers was of a larger size and designed as a tie beam bolted in place between arches.
Investigation by Don Neal, P.E. S.E. revealed that about one-third of these bolts were in
place, and no bolts secured the last arch set.