Page 303 - Forensic Structural Engineering Handbook

9.14 CAUSES OF FAILURES

Service Life
The service life is the period of time after installation during which the properties of a com-
ponent or system actually exceed the minimum acceptable values when properly main-
tained. The service life of a structure depends on human factors, such as the changing needs
of property owners and communities; changing aesthetic values; and changing economic
and political climates. Durability is also affected by exposure to weather, pollution, and the
general wear and tear from use. Consequently, design life and actual service life of com-
ponents vary, and may not be completely defined.
Whether designed and classified to last only a short time or for the entire life of the
building, the “premature” failure of a component occurs when the actual service life is
shorter than the design life. Premature failures are typically caused by defects or improper
maintenance that allows deterioration to progress.
ASTM E2018, Standard Guide for Property Condition Assessments: Baseline Property
13
Condition Assessment Process, defines the term “remaining useful life” (equivalent to
remaining service life used here) as a subjective estimate based upon observations, or aver-
age estimates of similar items, or a combination thereof, of the remaining time that a com-
ponent or system is able to function in accordance with its intended purpose before needing
replacement. Such period of time is affected by the initial quality of the item, the quality of
the initial installation, the quality and amount of preventive maintenance exercised, climatic
conditions, and extent of use, among other factors.

Estimating Durability
Economics play a critical role in the durability of structures. Most owners consider design
life in planning capital improvement programs, which factors into the calculation of life-
cycle costs. On purely economic criteria, the lowest life-cycle cost is usually the best choice
among multiple options. Owners often choose to implement capital projects with short
design lives simply because they lack one important piece of information: they do not have
a basis for evaluating actual product durability.
At present, there is no standardized measure of the actual durability of most construc-
tion products. Therefore, a professional performing a forensic investigation must utilize a
combination of personal experience and published literature to evaluate material failures in
the field.
Information regarding the design life and durability of building products can be obtained
from a variety of sources, including professional and technical society guidelines; manu-
facturers’ technical literature; published test results from recognized laboratories; pub-
lished case histories and journal articles; and experience of the designer and his or her
colleagues.
Standards. Professional and technical societies often provide unbiased industry stan-
dards written by committee and published with the consensus of designers, manufacturers,
and contractors. Such standards improve communication between producers and con-
sumers regarding the suitability and durability of building components. On the other hand,
manufacturers often provide conflicting and unclear information regarding the durability of
the materials and components they market. Using published standards, different claims
made by manufacturers may be evaluated with more confidence.
Research regarding the service life of buildings indicates the need for more standards
regarding the durability of components and systems. Further, the more complex interaction
of several products within a system must also be considered and possibly standardized to
ensure reliable performance and to facilitate identification of deficiencies.