Page 531 - Forensic Structural Engineering Handbook

TIMBER STRUCTURES 14.21

an approximate solution that deserves more research and/or sophisticated computer mod-
eling, as does the applicability of the current C connector group factor. Research has
g
shown that the first and last connector in a group of bolts receives even a larger percentage
of load in the elastic range than reflected in the C factor. The author has witnessed failure
g
of tearing out a hunk of timber in heavily loaded groups of connectors. This hypothetical
truss failure investigation would warrant closer study of the fracture area considering that
the face of the fractured end would not be visible to the lumber grader and his determina-
tion would typically not be adjusted in the critical connection areas.
The percentage stress at the critical net sections with current snow load coincides with
a duration of load demand: 2.115 ÷ 1.87 = 113 percent effective duration demand which is
suitable in accordance with the Madison curve for about 5 weeks.
Using our example of 27,000-lb tension force, the 30-psf snow (in a previous occur-
rence) would produce about

( 30 15 ) 27 000 lb = 34 714 lb
+
,
,
+
20 15
creating stress at 2.72 times allowable (without the duration of load factor) as compared
with the 1.87 or 145 percent which is suitable for about a half-day in accordance with the
Madison curve.
Why didn’t these hypothetical trusses collapse during the day of 30 psf snow? First of
all, we can’t call it that close and the members in question may have been better than the 5
percent exclusion rate, but it most likely accumulated fiber damage in that event, con-
tributing to the collapse at lower overstress in the recent event.
Once you have determined the critical failure mode(s) of the fractured bottom chord of
the truss, the owner and/or insurance company will probably want you to come up with a
repair or replacement as quickly as possible to keep the mill running. Note that the fracture
may have led to other potential overstresses as the truss deformed to the support of the
dryer. Repair options may include a heavy bearing plate against the end of the top chord
and steel side plates extending beyond the fracture of the bottom chord and connected to it
with timber rivets or with proprietary / 4'' high-strength screws. These have the benefit of
1
not reducing the net section as the original connectors. Although the NDS only lists glu-
lams as appropriate for timber rivet connections, the research of timber rivets in sawn tim-
ber indicates about 90 percent of the values for glulam. Timber rivets through steel plates
are basically heavy-duty metal connection plates which are given a C adjustment factor of
m
0.8 when the initial moisture value is greater than 19 percent (green lumber). The concern
for application in sawn timber is the potential splitting from the high restraint to shrinkage.
In most interior situations you investigate, the existing sawn timber members are going to
be well below 19 percent moisture content. However, the values for group action of the riv-
ets should be interpolated for the appropriate size from the NDS Table 13.2.1a, b, or c, and
then proportional for the lesser of the ratio of the allowable timber tension stress to 24f V4
Douglas fir glulam allowable tension stress or the ratio of allowable shear stresses. This is
conservative. For more accurate analysis, the Canadian Building Code has a methodology
to specifically determine capacities of “glulam rivets” by checking tension or shear capac-
ity separately. The old reduced (pre-1995) shear values should be used on these formulas.
The author also uses the timber rivet wood capacity design criteria to check to effects of
proprietary screws as they are a similar high capacity, closely spaced, timber connection
system that has the potential of timber failure around the connection group prior to failure
of all of the connectors.
The other concern you should have (whether or not the owner or insurance company
shares this view) is the capacity of the remaining trusses in the building. One truss failed
and even if the other trusses did not, they have potential accumulation of fiber damage that