Page 536 - Forensic Structural Engineering Handbook

14.26 MATERIAL-SPECIFIC FORENSIC ANALYSES

The remainder must be determined on site or from documents. This may involve inclement
weather, unpleasant site conditions, and inconvenient time frames. Avoid undocumented
second-hand information. Structures do not become distressed or fail without a reason. A
forensic engineer must identify and document in a report to his or her client the reasons for
a structural problem.
Knowledge of the subject matter, in particular engineering properties of wood and tim-
ber characteristics, is necessary to evaluate structural timber members. Access to older, out-
of-date codes and standards is of great value as evaluation often involves older structures.
All possible in situ plus laboratory testing within budget constraints should be performed
and guesswork minimized. Nondestructive tests such as wood moisture content readings
and stress wave testing are possible on most structures. Where specimens are available,
bending, strip tension, compression, and MOE tests from a laboratory are valuable. Test
data enhance a forensic report by reducing speculation in the conclusions but keep in mind
timber variability (particularly sawn timber) even in the same members.
The tenacity and inquiring mind of a detective are great assets for a forensic engineer.
Keep an open mind and be prepared to learn something from every commission. Expect the
unexpected.
Full-Structure In Situ Load Testing. Full-structure load testing is often proposed fol-
lowing evaluation and/or repair of a distressed structure. It is easily visualized, and the
structure either passes or fails. Full-size load testing of timber bridges and buildings, if
done at all, should be used with caution because of the problems of isolating the variables
and possible damage to members or nonstructural elements. It is difficult to know what per-
centage of the design load may be applied without damage to some of the members or con-
nections. One does not want the testing to cause the structure to accumulate fiber damage
that would reduce its future capacity.
Caution should be employed in use of full structure load testing for the following reasons:

1. Costs may be substantial.
2. How much load should be applied and for what duration? The load duration factor C D
is based upon the assumption that the reserve capacity of the member can withstand a
specified load in excess of the design allowable for a specified duration during the life
of the structure. It is often difficult to determine what percentage of overload to use in
a performance test without risking yield or damage to structural elements.
3. It may be difficult to isolate the structure being tested from nonstructural elements fas-
tened to the structure. May elastic deflection be isolated from nonelastic deflection?
What if the structural frame tests are satisfactory, but nonstructural walls, gypboard,
windows, etc., are damaged—who pays for the damage?
4. When testing is done, be cautious about agreeing to an unrealistic criterion of accep-
tance. Load testing was initially agreed upon after construction of a timber domed struc-
ture for a university. The specification required a load test representing full dead plus
unbalanced live load, and the criterion of acceptance was that deflection under full load
is measured and 80 percent deflection recovery be achieved upon removal of the test
load. The structure performed very well under test load. Deflection under full-design
loading was less than calculated, but 80 percent recovery was not achieved, as some of
the initial deflection was nonelastic due to small amounts of crushing as the connectors
seated. As such, the structure did not pass the acceptance criterion, and some negotia-
tion was required prior to acceptance.
5. Occasionally, removing and testing a portion of the structure to failure can be helpful
but not necessarily conclusive. (See Case Study 1, Hood River Valley High School
Theater.)