Page 526 - Forensic Structural Engineering Handbook

14.16 MATERIAL-SPECIFIC FORENSIC ANALYSES

timber and other structures. Discussions regarding wind and seismic loading with respect
to performance more specific to timber structures is included.
Wind Load. Wind is a capricious force whose maximum intensity may last for only a few
seconds yet most building codes did not provide for application of an appropriate duration
of load factor for timber. A 1.33 factor was allowed for an allowable stress increase due to
probability of load and applicable to all materials. Although the NDS specified that proba-
bility and duration of load increases can be combined, the local building departments sel-
dom allowed that interpretation. Even so, the previous use of a 1.33 factor for load duration
for wind or earthquake was very conservative. Even the current 1.6 factor for 10 minutes
at the maximum wind speed may be conservative. Wind force is not distributed uniformly
over a given area. There are small areas of a building subject to very high local pressures
such as roof corners, addressed in the ASCE 7 and later versions of the Uniform Building
10
Code as components and cladding. These higher pressures (as compared to system load-
2
ing) are appropriate for members and member uplift reactions up to 1000 ft . These local
high pressures were not even considered in most codes 25 years ago. Even the conservative
nonuse of the duration of load factor doesn’t overcome these loads. Although a properly
designed primary lateral force resisting system is typically conservative with the older code
criteria, a roof diaphragm that loses sheathing near the ends of the building or at other dis-
continuities does not perform as intended. It is a common occurrence, following a major
windstorm, to find a wind-damaged structure standing adjacent to one which is intact.
Investigation typically reveals that those left standing had a viable lateral force resisting
system. Many older timber buildings were not appropriately designed for wind force resis-
tance. The roof structure was often sent to a timber construction company for engineering
and the lateral force resisting system was just built. That is also true today with many light-
timber-framed structures such as residences. Common problems with light-framed con-
struction with respect to wind load resistance are
1. Use of a double plate at the ceiling line of a gable end wall where a 2 × 4 flat gable end
“truss” is placed rather than extending studs to the roof diaphragm.
2. Discontinuities of the roof diaphragm due to intersecting roofs where inadequate inte-
rior shear walls exist.
3. Roof truss companies design for wind uplift based on system forces rather than the
much higher component forces that vary based on location.

An initial step in a forensic investigation of wind load damage should be wind gauge
records from the National Climatic Data Center or other sources as published estimates are
usually very high.
Seismic Load. Timber structures have performed comparatively well in seismic events.
They are light-weight and, as for wind, were not given appropriate credit for utilization of
a load duration factor, as explained in the wind load discussion. The ability of timber to
resist short-term overloads helps in those situations where the applied seismic acceleration
far exceeds the design value. Consequently, timber structures appropriately designed for
seismic forces have performed well. Nailed structural panel shear walls and diaphragms are
also very ductile and perform well in events where seismic acceleration exceeds the design
values. However, more research is needed for other timber systems such as concentric
braced frames to verify adequate detailing to ensure ductility in severe seismic events.
Utilizing energy absorption with the buckling of connector plates prior to brace failure as
with accepted steel systems could certainly be developed so timber braced frames and
arches could again be a code-accepted method of seismic lateral load resistance. Utilizing
the ductility of timber rivet connections may also prove appropriate for such systems.