SUBJECT: WSDOT Policy for use of Transformed Section Properties
This design memorandum describes WSDOT’s policy in using transformed section properties for design of prestressed girders. The early findings of the NCHRP 12-83 “Reliability-based service and fatigue limit-states calibration” indicates that the live load reduction factor at service III limit state as indicated in the AASHTO LRFD Table 3.4.1-1 “Load Combinations and Load Factors” may not be applicable when transformed section properties are used. As a result, the following recommendation on Service III load combination shall be observed:
1. Transformed section properties shall not be used for design of prestressed girders. Use of gross-
section properties remains WSDOT’s standard methodology for design of prestressed girders
-
including prestress losses, camber and the flexural capacity.
2. In special cases transformed sections properties may be used for the design of prestressed girders
-
with the approval of the State Bridge Design Engineer. In these cases the live load reduction
-
factor at service III limit state load combinations shall be as follows:
γLL
= 0.8 when gross section properties are used
γLL
= 1.0 when transformed section properties are used
Background:
The early findings of the NCHRP 12-83 “Reliability-based service and fatigue limit-states calibration” indicates that the transformed section properties may not be used in conjunction with the 0.8 live load factor at service III limit state. The current live-load load factor of 0.80 at service III limit state is based upon “rough” calibration following trial designs to yield comparable member proportions as the provisions of the Standard Specifications.
Reliability-based service and fatigue limit-states calibration (NCHRP 12-83 by M&M) is underway with results available within the year. In the meantime, the use of transformed sections is discouraged and the live load factor at service III limit state is raised to 1.0.
If you have any questions regarding these issues, please contact Bijan Khaleghi at 705-7181.
cc: Mark Gaines, Bridge Construction - 47354
F. Posner, Bridge and Structures – 47340
BDM Revisions:
Chapter 5 Concrete Structures
5.6.2 Design Criteria
5.6.2 - B4
Transformed section properties shall not be used for design of prestressed girders. Use of gross section properties remains WSDOT’s standard methodology for design of prestressed girders including prestress losses, camber and the flexural capacity.
In special cases transformed sections properties may be used for the design of prestressed girders with the approval of the State Bridge Design Engineer. In these cases the live load reduction factor at service III limit state load combinations shall be as follows:
γLL
= 0.8 when gross section properties are used
γLL
= 1.0 when transformed section properties are used
Note: Click here for a PDF of this design memo.