Page 178 - Forensic Structural Engineering Handbook

THE FIRST STEPS AFTER A FAILURE 5.11

High-quality field notes, photographs, and possibly video are invaluable for document-
ing the collapse configuration and the removal process.
Aerial photographs can provide a valuable overall view of the site.
Consideration should be given to both vertical and oblique photographs. Vertical pho-
tographs most clearly show the position of features and components in plan. The intended
use of the photographs should be considered in selecting the person or firm who will take
the photographs. If the photographs will be used for general information purposes, then
high-quality hand-held photographs may suffice. If photogrammetric techniques will be
used to precisely determine the location of features, or if the photographs need to be viewed
stereoscopically, then specialized aerial photography equipment will be necessary. Oblique
photographs are useful for capturing the relative heights of elements, and provide a more
natural three-dimensional view. Oblique photographs should preferably be taken from all
four sides. (See Fig. 5.8.)

FIGURE 5.8 Aerial photography, combined with land surveying techniques, is often used to
document large scale collapses, such as that of the Schoharie Bridge (1987).
For large sites, it may be desirable that professional surveyors lay out baselines or grid
lines for reference. The position of collapsed elements can then be measured and recorded
in relation to these reference lines.

Steel Fracture Surfaces
Features of fractured surfaces, if preserved, can provide crucial information regarding the
failure of a steel structure. Examination of failure surfaces can assist in determining
whether fatigue played a role, the order of magnitude of the cycles experienced, the rela-
tive magnitude and suddenness of the applied load, the ductility of the failure, whether the
fracture originated at a preexisting crack, and other useful information. Some of these tell-
tale features exist at the microscopic level, and thus are fragile. They can be obscured—or
even destroyed—by light corrosion. Corrosion of freshly exposed steel can occur rapidly,
especially in humid or seaside environments.
Fracture surfaces can be readily protected against corrosion by spraying with acrylic
paint. Use of a pigmented coating will help provide visual confirmation that a surface has
not been overlooked. The coating can be easily removed in the laboratory with a solvent.
(See Fig. 5.9.)