Page 405 - Forensic Structural Engineering Handbook

11.36 MATERIAL-SPECIFIC FORENSIC ANALYSES

be repaired by reinforcing or heat straightening. Failures in other modes, such as local or global
buckling of a member, may require temporary shoring and bracing to prevent further damage
or collapse, pending a final repair or removal of the member or structure.
When providing emergency and temporary repairs, care needs to be taken so that use-
ful information from the damaged element is not destroyed. Should a crack develop, it is
highly desirable to avoid welding directly to the crack surface that destroys the surface and
may prevent an assessment of the failure origin.
When cracks are found in an element as a result of fatigue, the rest of the web or flange
may still be carrying load. Often it is desirable to drill holes at the crack tips in order to min-
imize further crack extension and to avoid the undesirable fracture of the section. In some
cases, such as distortion cracking in girder webs where diaphragms are provided for stabil-
ity during construction, the drilled holes may prevent further crack extension and may serve
as a permanent repair. Figure 11.15 shows two holes drilled in a girder web after repeated
distortion has caused web gap fatigue cracking. Centering holes at the crack tips will arrest
the crack and may prevent reinitiation.

FIGURE 11.15 Crack arrest holes in girder web at diaphragm connection plate web gap.

After holes are drilled, it is desirable to check the hole to ensure that the crack tip has
been removed. Otherwise, crack growth will continue and may even accelerate as the struc-
ture is subjected to cyclic loading. Holes are often a temporary retrofit pending develop-
ment of a permanent repair. As is apparent in Fig. 11.15, angles have been bolted into place
between the transverse connection plate and the girder flange to minimize the continued
out-of-plane movement and to prevent reinitiation of the crack beyond the retrofit holes.
A temporary repair was achieved after first making a detailed inspection and identify-
ing cracked copes, and then grinding out these cracks and the flame-cut copes. The removal
of the embedded cracks provided assurance that the connection would not prematurely fail
prior to the installation of bolted splices to ensure structural integrity. Permanent repairs
were achieved by installing bolted web and flange shear splices, as illustrated in Fig. 11.16.
The use of high-strength steel plate and A490 high-strength bolts provided a permanent
repair of all groove welded heavy W-section tension splices in the roof truss system.