Page 521 - Forensic Structural Engineering Handbook

14.12 MATERIAL-SPECIFIC FORENSIC ANALYSES

discussion on field installation of laminations near the end of the chapter.) Some moment
connections (not usually critical or beam stretcher connections) require stiffness to behave
as intended. Although these types of connections are difficult to successfully achieve in
timber due to secondary concerns such as changes in moisture content and splitting poten-
tial, they need to be constructed with nonslip low deformation (with elastic load) connec-
tions. This would typically rule out bolts and screws or nails through steel plates or straps
as they have holes larger than the connector. Nails themselves provide a relatively flexible
connection. Snug-fit connections such as split rings (where the bolt is primarily providing
clamping force) are suitable but the best solution is usually timber rivets that deform the
steel plate they are driven through for a very tight fit and rely primarily on friction parallel
to grain (rather than crushing of the soft cambium layer) for lateral resistance in the wood.

Adjustment Factors Applicability

A common error in design or evaluation of structural timber is the use of tabulated design
values without adjustment of those values for specific use conditions. The list of adjustment
factors for design values has grown over time, and use of the adjustment factors is often
perceived as a confusing complication in timber design. All have a rational basis in design
refinement. In practice, most designs use only one to three adjustment factors. Several fac-
tors are used only in specialized applications. The “Applicability of Adjustment Factors”
table from the NDS, furnished here as Table 14.1, provides a quick review of the adjustment
factors required for a specific design. A similar table is provided in NDS for applicability of
adjustment factors required for connection design in timber. The adjustment factors for tem-
perature in service is shown in Table 14.2. When the temperature drops below 100°F, the
adjustment factor returns to 1.0 but if the temperature exceeds 150°F, damage is permanent
even with future lower temperatures. A discussion of the load duration factor C is included
D
here due to its importance for proper understanding in forensic engineering of timber.
Load Duration Factor C . This factor addresses the time-dependent property of timber
D
to sustain a given loading (see Tables 14.3 and 14.4). Factor C is applied to all tabulated
D
design values except MOE, which is not time-dependent, and compression perpendicular
to grain which is based upon a deformation limit. However, the shear modulus appears to
be affected by duration of load. The load duration factor is used in virtually all design and
evaluation calculations.
A unique property of structural wood is its ability to sustain higher loads for a short time
than under extended or permanent loading. Tabulated allowable design stresses are for nor-
mal loading duration defined as a 10-year period. Design stress adjustments must be
applied for other loading durations, as shown in Tables 14.3 for ASD and 14.4 for LRFD.
LRFD indicates the λ factors to be used with load combinations rather than time of load.
These factors are based on the Madison curve shown in Fig. 14.7 which was based on clear
specimens of wood and is fairly accurate for higher grades of material. However, from full
size member testing, the effects of longer-term loads appear more significant and the curve
for lower grade material is a straighter line with the curve tending downward at longer dura-
tions. If the forensic engineer can get a good estimate of load history, it is usually easier to
consider the allowable stress factors of duration of high load (overload) that could affect
the life of timber rather than the LRFD factors that consider duration of a load combina-
tion, more applicable to design or forensic situations where the load history is not known.
There are some advantages to the LRFD approach for forensic engineers. Appendix N
3
of the 2005 NDS lists the adjustments to allowable stresses for LRFD. Also, the LRFD
3
Manual Section 1.1.4, Time Effect Factors in the previous National Design Specification
specifically refers to “cumulative damage” more than once. “Cumulative damage” is a very