Page 542 - Forensic Structural Engineering Handbook

14.32 MATERIAL-SPECIFIC FORENSIC ANALYSES

In 1985, one outer diagonal web separated during a snow loading condition due to the
moment couple induced by truss joint rotation. At the truss heel the distance from intersection
of top chord and outer bolt of web connection is 28 in. The truss was shored and the split
web replaced, including stitch bolts to resist perpendicular to grain forces.
The web failure was caused by a combination of a partial joint fixity and moment from
heel eccentricity causing cross-grain tension in web members. This is an example of an ade-
quate design for primary forces, but failure of a truss member caused by secondary effects
of joint rotation as the truss deflected under loading. The moment couple induced in truss
members was high due to a relatively flexible truss and a long moment arm caused by the
rigid steel plate connection.

Case Study 3. Tinora High School, Defiance, Ohio
In January of 2004, a glulam roof beam flexural failure occurred in the gymnasium of the
building. Gray Engineering and Design, LLC of London, Kentucky, was contacted by the
owner and requested to perform a structural investigation and analysis of the failed beam
and to make subsequent recommendations of possible repair scenarios. The gymnasium
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had been constructed in the early 1960s and consisted of 8 / 4 × 48 glulam girder beams
spanned 90' and were curved downward by approximately 3' at center span. The failure
had initiated at a knot near mid-span at the bottom lamination and the crack had propa-
gated upward and outward approximately 12'' in a flat “V” pattern at the time of the struc-
tural investigation. The addition of ballast weight nearly 20 years earlier had resulted in
an approximate 40 percent increase in dead load only bending stresses. Using current
design criteria the beams used in the gymnasium were overstressed approximately 24 per-
cent under permanent dead loads and approximately 62 percent under full design load
requirement of DL + SL.
Based on the extent of damage sustained by Beam #5 and the calculated overstress
condition of the other glulam beams, Gray Engineering and Design recommended a
repair and reinforcement method which included shear dowel reinforcing and the addi-
1
tion of post-tensioning cables installed 8 / 4'' below the beams to be utilized for both the
“failed” beam and the remaining beams. Western Wood Structures, Inc. of Tualatin
Oregon was selected to perform the design and installation of the repair-reinforcing
work. Shear dowels consisted of steel-reinforcing bars in epoxy-filled holes drilled ver-
tically on the member centerline. One-inch rebar was inserted in 1 / 8'' holes. These shear
3
dowels served three purposes. In any cracked beam, the shear capacity is compromised.
These shear dowels were developed to restore the horizontal shear capacity of a damaged
beam in place. The shear dowels can also resist radial tension which could be induced in
an inverted curved glulam depending on the curve, dead load, post-tensioning force and
eccentricity of the cables. In a bending failure, the shear dowels are also used to stitch the
beam back together.
More detailed information regarding this investigation and repair can be found in an arti-
cle by Gary W. Gray, P.E. and Paul C. Gilham, P.E. in the September 2006 STRUCTURE
Magazine and on line at www.westernwoodstructures.com. 13

Case Study 4. University of Puget Sound Field House, Tacoma, Washington

This case study illustrates state-of-the-art criteria changes, cross-grain tension from con-
nectors, and accumulation of fiber-separation damage.
In October of 2008, prior to winter snow loads that followed, the multiconnector heel
joint of a bowstring truss supporting the roof and intermittent snow loads for 60 years