The National Academies

NCHRP 20-07/Task 325 [Final]

Updating the AASHTO LRFD Wind Loads Provisions
[ NCHRP 20-07 (Research for AASHTO Standing Committee on Highways) ]

  Project Data
Funds: $160,000
Research Agency: Modjeski & Masters
Principal Investigator: Wagdy G. Wassef
Effective Date: 11/12/2012
Completion Date: 8/31/2014

The AASHTO LRFD Bridge Design Specifications (LRFD) was written in the early nineties with wind load provisions that were derived from the American Society of Civil Engineering (ASCE) Minimum Design Loads for Buildings and Other Structures/ASCE 7-88 and adapted for bridge design. These provisions were based on the fastest-mile measure of wind speed which was used by the national weather service. However, since then, the national weather service has changed to the 3-second-gust measure of wind speed and ASCE 7-88 has been updated. The revisions were based on 17 years of ongoing research of wind loads on structures and included new wind speed maps based on the 3-second-gust.

The LRFD uses base wind speed and base wind pressure. The base wind speed is 100 mph and the base wind pressure is 40 or 50 psf depending on the structural element. The specifications require adjusting the wind speed for the design elevation and for the exposure or the upstream surface conditions. This is conservative for the majority of the country where the 50 year recurrence wind is 70-80 mph. But, this is non-conservative for hurricane-prone regions in which ASCE 7-88 wind speed maps do not properly account for the higher wind speeds associated with hurricanes.

The current method in the LRFD does not provide for consistent reliability across different regions and locations. For example, a bridge designed according to these wind loads will have lower reliability if located along the coasts than if located inland. At the same time, the design wind speed will likely be exceeded during the life of a bridge along the coasts while it is very unlikely the design wind speed would be exceeded for inland locations.

Based on the above, there is a need to update the LRFD wind load provisions to provide more uniform reliability and to take into consideration the technological changes and the research advances since these specifications were written. This is supported by NCHRP Report 489: Design of Highway Bridges for Extreme Events, which recommends that future research in wind engineering develop new wind design maps that would provide more uniform safety levels for different regions of the US.

The objective of this research was to propose revisions to the wind load provisions in Section 3—Loads and Load Factors and other related sections of the AASHTO LRFD Bridge Design Specifications.

Products Availability: Agency final report was submitted to AASHTO SCOBS, Technical Committee on Loads Distribution (T-5). (NCHRP Staff: W. Dekelbab)

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