Page 112 - Forensic Structural Engineering Handbook

DESIGN CODES AND STANDARDS 2.17

loads and load combinations, dead and live loads, construction loads, lateral earth pres-
sures, and environmental loads.
The construction loads, load combinations, and load factors were developed to account
for the relatively short duration of load, variability of loading, variation in material
strength, and the recognition that many elements of the completed structure that are relied
upon implicitly to provide strength, stiffness, stability, or continuity are not present during
construction. The load factors are based on a combination of probabilistic analysis and
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expert opinion. The concept of using maximum and arbitrary point-in-time (APT) loads
and corresponding load factors is adopted to be consistent with ASCE 7.
The basic reference for the computation of environmental loads is also ASCE 7.
However, modification factors have been adopted to account for reduced exposure periods.
Furthermore, certain loads may be disregarded due to the relatively short reference period
associated with typical construction projects, and certain loads in combinations may effec-
tively be ignored because of the practice of shutting down job sites during these events, e.g.,
excessive snow and wind.

FHWA/AASHTO Guide Design Specification for Bridge Temporary Works. Following
the collapse of the Route 198 Bridge over the Baltimore Washington Parkway in 1989,
FHWA determined that there was a need to reassess, on a national level, the specifications
used to design, construct, and inspect falsework for highway bridge structures. Toward that
end, FHWA sponsored a study to identify the existing information on this subject and to
develop a guide specification for use by state agencies to update their existing standard
specifications for falsework, formwork, and related temporary construction.
As part of the FHWA study, a questionnaire was sent to the fifty U.S. highway depart-
ments. Information relating to design and administrative policies for falsework and formwork
construction and the bridge construction activity for each state was requested. Virtually every
state was found to have general requirements and guidelines for the construction and removal
of falsework and formwork. However, only approximately one-half of the states specified
design criteria. Similarly, only twenty-two states had accompanying design or construction
manuals that included specific design information. States that are more active in constructing
cast-in-place concrete highway bridges generally were found to have more comprehensive
specifications and guidelines.
Besides identifying the content of state specifications, the survey provided some insight
into each state’s administrative policies concerning falsework and formwork. Approximately
two-thirds of the states require the submittal of plans and calculations, sealed by a registered
professional engineer, for any significant falsework construction. The survey showed that
most states also conduct their own reviews and inspections, subject to availability of staff,
complexity of design, and so forth.
In 1993, the Federal Highway Administration produced a new guide specification for
the design and construction of falsework, formwork, and temporary retaining structures
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used to construct highway bridge structures. The new guide specification was developed
for use by state agencies to update their existing standard specifications. This specification
was subsequently adopted by AASHTO in 1995. 50

REFERENCES

1. The BOCA National Building Code/1996, Building Officials and Code Administrators
International, Inc., Country Clubs Hills, IL, 1996.
2. Uniform Building Code, 1997 ed., International Conference of Building Officials, Whittier, CA,
1997.
3. Standard Building Code, Southern Building Code Congress International, Inc., Birmingham, AL,
1997.