Page 315 - Forensic Structural Engineering Handbook

10.4 CAUSES OF FAILURES

UNDERSTANDING THE DESIGN-CONSTRUCTION
PROCESS AS IT RELATES TO TEMPORARY
STRUCTURES

Safety of temporary structures is the concern of the designer, the contractor, the building
official, and the insurer, as well as the workers on the job and the general public. Yet, this
most important component of the construction process is still not a “field” of practice but
a neglected stepchild, at times claimed, at other times disclaimed by both designers and
contractors, and almost totally neglected by researchers and educators. Although increasing
in recent years, still surprisingly little guidance exists in the civil engineering profession for
their design, erection, maintenance, and removal. A tacit attitude seems to prevail in the
design-construction industry: “These things” are temporary and generally less important,
therefore greater risks are acceptable than in permanent structures. Certainly, less care and
control is exercised with temporary than with permanent structures. Possibly as a result, far
more failures and loss of life occur during construction than in completed projects. The liter-
ature, although increasing in recent years, is still scant: mostly manufacturers’ promotional
pamphlets; a few public authority, government, and military manuals; isolated specifications;
and a handful of journal papers. The OSHA (US Department of Labor, Office of Safety and
Health Administration) regulations are just that regulations—they do not provide technical
information. The first and, so many years after its publication, still the only comprehensive
book believed to exist on the subject is the Handbook of Temporary Structures in
Construction, Robert T. Ratay, ed., published by McGraw-Hill first in 1984, and the second
edition in 1996.
Design Philosophies and Practices. It is an axiom among some designers that for
temporary works during construction, lower safety factors may be applied than for com-
pleted permanent structures. In light of the uncertainties with temporary structures
and their history of failures, this is an inappropriate and even dangerous attitude if
applied indiscriminately. If we put aside the notion that “these things” are temporary
and generally less important (i.e., if we do not allow greater risk to life and property at
a construction site), then the design loads should perhaps be even more severe, the
allowables even lower, and the calculated safety factors even higher than those for per-
manent structures.
On the other hand, a school of thought persists that short-term construction loads are
more closely predictable and can be more effectively controlled than the live loads during
the decades of use—and who knows what alternate uses—of the permanent structure.
Therefore, indeed, lower design safety factors may be acceptable on temporary structures
as well as on permanent structures during their construction.
Several factors are inherently involved in the establishment of design loads, allowable
stresses, and safety factors that are used for the design of any structure. These include the
following:

• Intended function of the structure
• Nature of loads
• Predictability of occurrence of loads
• Certainty in the magnitudes of loads
• Possibility of simultaneous occurrence of loads
• Strength and deformation characteristics of the material
• Reliability of the rated strength of the material (or member)
• Possible secondary stresses, redundancy, and instability