Page 105 - Bridge and Highway Structure Rehabilitation and Repair

80 SECTION 1 ADMINISTRATIVE ISSUES

this vital information. Immediate issues that a structural forensic engineer may be faced with
when arriving at a collapse site and in theensuing days include issues such as:
• Safety
• Preserving perishableevidence
• Reserving samples
• Documentation
• Interviews
• Document gathering
• Preliminary evaluation of causes of failure.
3.2.4 Formal in-Depth Inquiry into the Defects Contributing to Failure

The condition of a failed bridge needs to be thoroughly examined. The following are some

of the deficiencies a forensic engineer will typically encounter in his investigations:

1. Inability to define loads accurately, such as magnitude and unpredictable level of stress
distribution from settlement.
2. Limited redundancy in the structural system.
3. Inability to fully include plastic behavior or composite action between the concrete deck slab
and repeated beams, arching action, creep and shrinkage strain distribution in the deck slab.
4. Lack of information on fracture mechanics in general and lack of understanding of fracture
of new materials, in particular.
5. Inelastic behavior of connections and joints, splices, gusset plates, bolts, and welds.

6. Complex behavior as a unified assembly of uncombined multiple components of mixed
(old and new) materials and structural systems, resulting from rehabilitation or widening
methods.
7. Delamination and reduction in strength of the concrete deck due to deicing salts (as observed
from chain drag test).
8. Malfunction and locking of old bearing assemblies due to lack of maintenance, freezing of
expansion bearings, large thermal forces causing compression, and local buckling of truss
members and fl anges.
9. Inability to prevent scour at pile top.
10. Inability to fully incorporate different types of soil interaction at abutments.
11. Lack of drainage behind abutments and pressure build-up behind abutments.
12. Inadequacy of Rankine, Coulomb, or Mononobe-Okabe theories for non-homogenous soil
conditions for wing walls and stub abutments resulting in unstable design.

3.2.5 Forensic Engineering as a Diagnostic Tool
An area where forensic science is required is failure from blast loads. Accidentalexplosions
have resulted from numerous sources, including high explosives. Theseimpulsive events call for
specialized forensic analysis methodology. The damaging loads exist for only short durations,
making conventional static analysis inappropriate for backing out possible rootcauses. Examin-
ing structural load path transferduring a blast is required in order to provide additional support
toportions of the structure under attack.
An important part of an explosion forensic analysis is to use damage indicators from the
surroundings to determine thestrength of the blast wave. The damage indicators are:
1. Deformed structural members.
2. Deflections in metal panels.

3. Debris throw.