Page 529 - Forensic Structural Engineering Handbook

TIMBER STRUCTURES 14.19

Experience with cantilever beam systems suggests that the economic benefits often van-
ish when analyzed for unbalanced span loading. Deflection should be checked as the
reduced section, based upon bending criteria alone, may result in a beam too flexible and
subject to ponding. The portion of beam past the inflection point where compression is at
the lower portion of beam should be considered in regard to buckling stability. One prob-
lem with cantilevered glulam beams is that the deflected shape is complex, and it may be
preferable to use a straight beam, rather than attempting to camber.
A recent development in glulam technology utilizes high-strength composite fibers
bonded into the tension zone of a glulam beam. The fiber panels are fiberglass, Kevlar, or
similar-type material. The composite panel is bonded to the beam tension face with a wood
cover board for cosmetic purposes. Strength of the tension zone is increased with the goal
of reducing member size. Addition of the composite reinforcing may alter the member siz-
ing from strength-controlling to stiffness-controlling. Availability of composite-reinforced
glulam at this point is through proprietary suppliers.

Accumulation of Fiber Damage. As indicated previously, the duration of load or time
effect factors are related to a phenomenon known as damage cumulation in timber. The
term damage cumulation could describe damage from other factors such as decay, chemi-
cals, erosion, fire, insects, or physical impact. Therefore the author prefers to use the term
“accumulation of fiber damage” to be more specific as damage to the wood due to previ-
ous loading, perhaps more accurately considered previous overloading. Significant studies
have been done at the Technical Institute of Denmark and the University of British
Columbia regarding this effect. Timber can be accurately modeled as a cracked viscoelas-
12
tic material according to Dr. Lange Fuglsang Nielson to best predict accumulation of fiber
damage.
Damage cumulation also occurs at an even greater degree in cross-grain tension, often
induced directly or indirectly at timber connections. The author uses the term “accumula-
tion of fiber separation damage” for this situation.
A forensic engineer usually becomes involved when there is damage to one or more ele-
ments in a structure. In severe situations, that may lead to total building collapse. When the
damage is caused by loading, it is probable that members or connections were overstressed.
This may have been caused by inadequate design, excessive load, construction error and/or
factors affecting strength beyond the original design criteria (temperature, decay, moisture,
chemicals, etc.). Whatever the cause, the timber members or connections have probably
been overstressed, so we are looking at conditions near the ultimate capacity rather than at
the allowable stress levels. It is in the high stress range that fiber damage may accumulate
and reduce the capacity of timber member or connection to resist loads in the future or pre-
vious high stresses may have reduced the capacity of the timber to resist the recent load that
caused failure. In fact, many failures in timber have occurred under lesser loads than the
timber structure had carried in the past.
So how do we apply this concept to forensic engineering? Let’s look at a hypothetical
situation that includes facets of previous forensic investigations by the author. Let’s say
you have been called to investigate the failure of a timber roof truss in a paper mill. The
bottom 4 × 12 chord failed at the interior split ring connector but collapse was prevented
by the paper dryer structure a few inches below the truss. You measure 20 psf of snow on
the roof that has been there for 2 weeks and the original plans indicate a 25 psf snow capac-
ity. You analyze the truss and find that with the original allowable design values in 1949
for specified construction grade lumber the truss was capable of resisting 25 psf of balanced
snow with 10 psf of dead loads. You have a certified lumber grader determine that the bot-
tom chord member is Douglas fir #2 by current standards. You determine 15 psf of current
dead load and also that the roof had 30 psf of snow for a day 10 years ago. Your truss analy-
sis indicates 27,000 lb of tension at the net section which is overstressed slightly more