Page 547 - Forensic Structural Engineering Handbook

TIMBER STRUCTURES 14.37

close inspection of these splits, an alarming observation was made. A fracture extending
halfway across the bottom lamination at about the third point of the span was noticed.
Closer observation revealed this same situation in another glulam beam. It took some per-
suasion to convince city officials to shore the beams. After all, this structure had supported
“heavy” snow loads before. The architect had sized these bottom face tapered glulams by
depth required for flexure at the center and that for shear at the supports, tapering the beam
between. With uniform loads the moment reduces at a much lower rate than the section
modulus from the center of the span. This caused the maximum flexural stress to occur
close to the third points where the fractures were observed. Insulation board was used in
place of rigid insulation increasing the dead load. Although many repair options were
explored, the best solution was the field addition of three to four tension laminations
(some of the beams were at different spacings). After the beams were jacked to remove
dead load stress and supported on the sides to allow the added laminations with resorcinol
glue which restored the original design capacity of 20-psf snow. Later, new membrane
roofing and rigid insulation increased the snow load capacity to the current 25-psf require-
ments. This work was done by city maintenance personnel under the direction of Donald
Neal, P.E., S.E.

CLOSURE

To summarize, the forensic engineer investigating a timber structure needs to be particu-
larly aware of the potential of the “accumulation of fiber damage” and “fiber separation
damage” for both the immediate problem being investigated and for the remaining struc-
ture that may have been affected by a recent or previous event. The significant variability
of timber, particularly sawn timber, presents an additional challenge to the forensic engi-
neer attempting to hone in on the specific cause(s) of the problem. Previous fire damage,
often painted over with a white lacquer, or impact damage filled in with putty, can have
consequences far beyond the direct evidence of damage. This of course is also true with
recent structural timber damage from a fire or impact. Where there is evidence of moisture,
it is important to consider the possibility of decay. A weakness of timber is its very high
variability in resistance to cross-grain tension which can originate from timber connections.
Excessively high allowable stresses that may have been used in timber, especially prior to
the 1970s, and other state-of-the-art design changes, make older timber structures more
vulnerable to failure. Secondary effects from fixed and eccentric truss joints not fully
accounted for in the original design are also common problems.

REFERENCES

1. ASTM D 4442-92 (Reapproved 2003), Standard Test Methods for Direct Moisture Content
Measurement of Wood and Wood-Base Material, American Society for Testing and Materials,
Philadelphia, PA.
2. ASTM D 4444-92 (Reapproved 2003), Standard Test Methods for Use and Calibration of Hand-
Held Moisture Meters, American Society for Testing and Materials, Philadelphia, PA.
3. National Design Specification for Wood Construction (NDS), American Forest & Paper
Association, Washington, DC, ANSI-AF & PA NDS-2005 and previous editions where supple-
ments, such as the LRFD Manual for Engineered Wood Construction, were separate.
4. AITC, Inspection Manual AITC 200-92, 02, for Structural Glued Laminated Timber, American
Institute of Timber Construction, Englewood, CO.