ASCE Standard 7, ''Minimum Design Loads for Buildings and Other Structures,'' has contained provisions for load combinations and load factors suitable for load and resistance factor design since its 1982 edition. Research in wind engineering in the intervening years has raised questions regarding the wind load factor 1.3 and load combinations in which the wind load appears in ASCE 7-95. This paper presents revised statistical models of wind load parameters based on more recent research and a Delphi, and reassesses the wind load combinations in ASCE Standard 7 probabilistically. The current approach to specifying wind loads in ASCE 7 does not lead to uniform reliability in inland and hurricane-prone regions of the country. It is recommended that the factor accounting for wind directionality effects should be separated from the load factor and presented in a separate table in the wind load section, that the wind load factor should be increased from 1.3 to approximately 1.5 or 1.6 to achieve reliability consistent with designs governed by gravity load combinations, and that the exposure classification procedure in ASCE Standard 7 should be revised to reduce the current high error rate in assigning exposures.
SUMMARYMany concrete gravity dams have been in service for over 50 years, and over this period important advances in the methodologies for evaluation of natural phenomena hazards have caused the designbasis events for these dams to be revised upwards. Older existing dams may fail to meet revised safety criteria and structural rehabilitation to meet such criteria may be costly and di cult. Fragility assessment provides a tool for rational safety evaluation of existing facilities and decision-making by using a probabilistic framework to model sources of uncertainty that may impact dam performance. This paper presents a methodology for developing fragilities of concrete gravity dams to assess their performance against seismic hazards. The methodology is illustrated using the Bluestone Dam on the New River in West Virginia, which was designed in the late 1930s. The seismic fragility assessment indicated that sliding along the dam-foundation interface is likely if the dam were to be subjected to an earthquake with a magnitude of the maximum credible earthquake (MCE) speciÿed by the U.S. Army Corps of Engineers. Moreover, there will likely be tensile cracking at the neck of the dam at this level of seismic excitation. However, loss of control of the reservoir is unlikely.
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