Page 545 - Forensic Structural Engineering Handbook

TIMBER STRUCTURES 14.35

Case Study 8. First Nazarene Church, Portland, Oregon
This case study by Donald Neal, P.E., S.E. illustrates notching and tapering of the tension
face of the beam.
The roof structure of a large church was designed using glulam roof framing. Two
1
1
beams 14 / 4 in × 88 / 2 in spanning 118 ft supported the entire roof system with glulam beam
and light truss subframing. To satisfy architectural constraints the tension faces of beams
were tapered 30 ft at each end. To facilitate the upper column seat, a notch was located
within the upper taper cut. See the beam elevation sketches in Fig. 14.13.
The church had been occupied approximately for 1 year when total collapse of the roof
occurred on September 9, 1981. Approximately 20 min elapsed between the time failure
was recognized and the time when total collapse occurred. Sticking doors and other build-
ing deformations had been occurring for a period of weeks or months preceding collapse,
but were not initially recognized as a failure in progress.
The collapse occurred under mild weather conditions with no snow or other roof live
loads present. The collapse was triggered by a separation at the notch located in the upper
taper cut near the beam mid-depth. The separation progressed toward the mid-span of the
beam, separating the beam into two segments. Forensic evaluation determined that failure
was solely due to effects of the notch and taper cut. Fiber separation damage accumulation
was obviously occurring from the time of construction. The beam was otherwise sized in
accordance with code requirements.

Case Study 9. Wilson High School Auditorium, Tacoma, Washington

This case study illustrates added dead load beyond the original design assumptions, state-
of-the-art criteria revisions, and accumulation of fiber damage.
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3
The auditorium was constructed in 1969 utilizing an 11 / 8 × 63 / 8 glulam beam as a main
girder spanning over 100 ft. The ceiling had two layers of gypsum board for fire resistance;
however, the engineer had used the lesser weight for a suspended acoustical ceiling. A
gallery of lights and projection equipment on a floor with two 20-ft-tall sheetrock-covered
walls were added later.

Failure
12 line
1 / 9 16

1
88 / "
1
14 / " Glulam 2
4
5
10'-0 / "
8
5
30'-0" 68'-0 / " 30'-0"
8
128'-0 5 / "
8
118'-0" LL c Columns
c -
FIGURE 14.13 Roof beam elevation. First Nazarene Church, Porland, Oregon.