Page 220 - Forensic Structural Engineering Handbook

7.4 CAUSES OF FAILURES

SELF-STRAINING LOADS 7.38
Nature and Consequences of Self-Straining Loads 7.38
Loads Due to Temperature Changes 7.39
Loads Due to Moisture Changes 7.40
SETTLEMENT 7.41
Nature and Consequences of Settlement 7.41
Determination of Settlement Loads 7.42
REFERENCES 7.43

INTRODUCTION

In a highly simplified manner, designing a structure for strength and stability may be
looked at as satisfying an equation, and every engineer knows that there are “two sides
to the equation.” One side represents the resistance characteristics of a structure, and the
other side represents the applied loads. In the evaluation of a nonperformance or failure,
a thorough investigation must reveal the facts concerning both sides so that the investi-
gator can weigh the merits of competing failure theories. Ultimately, the investigator
must be sufficiently convinced to rule out improbable causes and to develop defensible
opinions about the true reasons for a failure. Then the investigator must be able to con-
vince a body of skeptics under aggressive challenge by parties who feel harmed by those
opinions. This effort necessarily involves critical analyses of the loading history for a
failed structure.
The investigator should start with loads on the structure at the time of the failure.
These loads, which can be generated by gravity, wind, seismic events, and several other
causes, are the most obvious potential contributors to a failure. However, often an inves-
tigator must assess whether loads other than those on the structure at the time of failure
are relevant to the investigation. Sometimes a structure’s prior load history contains
events or circumstances that affect the ability of the structure to survive a subsequent
extreme event. Cyclic loads can cause fatigue damage. A single overload can weaken a
connection or cause other inconspicuous damage without leading to obvious or immedi-
ate failure. When such damage occurs, it might lead to a later failure that otherwise would
not have occurred.
Often it is a challenge to determine the loads on a structure at the moment of failure.
Building components and contents can be in disarray in a debris pile. Wind storms and
earthquakes obviously have passed by the time investigators arrive at a site. Even snow
loads often are hard to assess, since they can vary widely even over relatively small roofs.
Furthermore, snow and ice often melt and rain dissipates before investigators can collect
important data.
When the loads on a structure cannot be measured directly, the investigator often needs
to conduct substantial research, interrogation, and analyses to estimate—sometimes from
circumstantial evidence—the loads that most probably affected the structure at the critical
times. In addition to being theoreticians, investigators are detectives: They must be obser-
vant and resourceful as they analyze the debris for clues about the failed structure’s loading
history.
The purpose of this chapter is to present some issues that an investigator should con-
sider when evaluating loads on structures that have failed. Some guidance is given to
assist the investigator during calculations of loads. For detailed analytical procedures, the