Risk-Informed Approaches to Safety Regulation - Structural Integrity of Offshore Wind Turbines - page 138

in the connection; because the cost of connection design is determined
primarily by fabrication rather than materials, providing the additional
conservatism has little economic impact). Similar benchmarks have been
adopted for most other building construction materials.
Bridges
The American Association of State Highway and Transportation Offi-
cials (AASHTO) LRFD Bridge Design Specifications dates from 1994, with
the 2007 edition being the latest. The probabilistic design methodology
adopted there is essentially the same as that used for building structures.
The supporting study (Nowak 1995) focused on the strength of individ-
ual bridge girders, with truck loads applied to the individual girders
through empirically derived girder distribution factors for moment and
shear. AASHTO uses essentially the same LRFD format as is used for
ordinary buildings and other structures. The load and resistance factors
in the LRFD Bridge Design Specifications (AASHTO 2007) were devel-
oped in such a way that bridge girders achieve a reliability index, β, equal
to 3.5 at the inventory or design level for a service period of 75 years. No
distinction is made between steel, reinforced concrete, and prestressed
concrete girders in terms of their target reliabilities, nor is the target reli-
ability index dependent on the girder span or on whether the girder is
simply supported or continuous over internal supports.
Offshore Platforms
Formal design guidance for offshore structures originated in 1967 with
the release of American Petroleum Institute (API) RP 2A (API 1967).
This standard used a working stress approach, consistent with the pre-
vailing steel design practice for land structures. In 1979, work began on
development of an LRFD version of API RP 2A. The format was parallel
to that developed by Galambos et al. (1982). The calibration strategy
focused on developing partial factors for identified components that
would yield a platform design having members and connections equiv-
alent to those resulting from use of the existing working stress code. This
approach was summarized by Moses and Larrabee (1988):
The traditional one-third allowable stress increase for environmental loading
found in working stress design (WSD) has been replaced in the Draft RP2A-