Memorandum



September 6, 2005


TO:                  All Design Section Staff

FROM:            Jugesh Kapur
SUBJECT:       Use of Deck Members for Bridges


The following revisions are made to the design and construction of precast decked girders, and precast slab superstructures:

    The use of decked bulb-Tee, Double-Tee, and Ribbed Girders shall be limited to state routes with the Average Daily Traffic (ADT) of 30,000 or less. An asphalt overlay with membrane over girders shall be specified for all girders of these types.

    Asphalt overlay is no longer allowed for precast solid or voided slabs bridge superstructures. Instead, a minimum of 5 Cast-In-Place (CIP) concrete topping shall be used for these types of bridges. The CIP concrete topping shall be class 4000D concrete with one layer of #4 epoxy coated reinforcement in both transverse and longitudinal direction spaced at 1-0 maximum. The requirement for 5 CIP concrete topping does not eliminate the need for welded ties and grouted keys. This type of cross-section may be considered composite for structural design calculations.

    Air-entrained concrete is no longer required for decked girders and precast slabs. WSDOT Standard Specifications, and Bridge Design Manual (BDM) Standard Drawings are being revised for these revisions.

PGSuper program is being upgraded to incorporate the design of decked members and precast slab superstructures.


Background:

The recent increase in use of decked members and precast slabs for state highway bridges urged WSDOT to revisit the current design and construction practice of this type of superstructures. The above revisions are to improve the long-term durability and structural performance and not encourage the over use of these types of superstructures. The precast I-girders or Tub girders with CIP deck slab remain the preferred type of superstructure for WSDOT bridges.

The limitation on ADT for decked members is to reduce the potential for longitudinal overlay cracking, and to improve the durability of joints between girders. Decked members with ADT of 30,000 or less have successfully been used in Washington State for many years.

The requirement for CIP concrete topping for precast slabs improves the long-term durability, and the structural performance of this type of superstructures.

Welded ties are still required for the erection of precast slabs, and grouted keys are necessary to prevent deck cracking due to abrupt change in stiffness between precast slabs.

The air-entrained concrete for top surface of precast decked members and precast slabs covered with asphalt overlay or CIP concrete topping is no longer required.


If you have any questions regarding these issues, please contact Dewayne Wilson at 705-7214, Rick Brice at 705-7174, or Bijan Khaleghi at 705-7181.