Page 532 - Forensic Structural Engineering Handbook

14.22 MATERIAL-SPECIFIC FORENSIC ANALYSES

should be investigated. Of course, the other trusses should be inspected, particularly in the
areas of high stress. You may notice visible damage from the overstress but that only occurs
if the damage is severe and close to failure. Accumulated fiber damage may be there with-
out being visible and very difficult (or impossible) to ascertain even with testing methods
such as stress wave timing. This potential fiber damage accumulation was determined by
analysis for the failed truss and would also apply to those that haven’t failed. These trusses
have not yet failed but have accumulated fiber damage based on a day of previous over-
stress plus 2 weeks of recent overstress. Let’s say that these other trusses were not over dry-
ers. In this case, using the 27,000-lb assumed tension force, we have potential fiber damage
accumulation but short of the expected failure point.
,
27 000 lb
= 19
.
)]
−
( [ 3 625 11 5) ( 8 51 2 575)
)(
.
.
.
)(
(
as compared with
216
.
= 188
.
8 1 438)
() ( .
where (3.625)(11.5) – (8.51)(2) = net section and (575) = F .
t
The percentage overstress (beyond the baseline ultimate) is 101 percent which is suit-
able for about 10 years in accordance with the Madison curve. The 30 psf snow would pro-
duce about
+
⎛ 30 15 ⎞ (. 244
19 =)
.
⎝ 20 15⎠
+
creating stress at 2.44 times the allowable stress.
244
.
= 130 percent
.
188
which is suitable for about 2 days in accordance with the Madison curve.
The accumulated amount of damage is
2 weeks 1 day
+ = 0 503 or just over 50 ppercent
.
520 weeks 2 days
Less than 50 percent of the bottom chord capacity for overload remains. The recent snow
did not theoretically cause significant fiber damage.
However it could be and should have been that the timber around the critical net section
areas was free of defects and could be considered select structural which would provide
1000-psi allowable tension. You would want to be careful in verifying this but if so the
stress ratio would be
,
27 000
= 109
.
.
( 24 67 1000 1 0 1 0)
)(
)(
.
.
)(
where (24.67) represents the net section, (1000) is F , (1.0) is C , and (1.0) is C
t t f.
The 1.09 value with select structural material is less than the current allowable load
duration of 1.15 and using the lower chord tension with the snow load of 30 psf for a day