Design Memorandum



TO:                  All Design Section Staff

FROM:            Bijan Khaleghi
DATE:             July 28, 2008
SUBJECT:       Full Depth Intermediate Diaphragms


Prestressed concrete girder bridges are often damaged by over-height loads. The damage may range from spalling and minor cracking of the bottom flange or web of the prestressed concrete girder to loss of a major portion of a girder section.

This design memorandum describes WSDOT policies for use of intermediate diaphragms for I-shape and deck bulb tee prestressed concrete girder bridges. This memorandum incorporates recommendations from the TRAC Report WA-RD 696.1 titled "Effect of Intermediate Diaphragms to Prestressed Concrete Bridge Girders in Over-Height Truck Impacts” completed on April 2008 by the Washington State University.

The use of intermediate diaphragms for I-shape and deck bulb tee prestressed concrete girder bridges shall be as follows:

    1.   Full depth intermediate diaphragms as shown in Fig. 1 shall be used for Interstate 5 bridges, and other bridges
          crossing over roads of ADT > 50000.

    2.   Either full depth or partial depth intermediate diaphragms as shown in Fig. 1 and 2 may be used all bridges not

          included in item 1.

This memorandum is primarily for the new bridges. Bridge widenings should be considered on the case-by-case basis depending on the width of the widening and number of added girders.


Background:

The recent TRAC Report WA-RD 696.1 completed on April 2008 by the Washington state university, studied the Effect of Intermediate Diaphragms to Prestressed Concrete Bridge Girders in Over-Height Truck Impacts utilizing a quasi-static and explicit dynamic numerical finite element analyses of prestressed concrete girder bridges with and without intermediate diaphragms subjected to over height load impacts. The scope of the project was to study the resistance of I-shape prestressed concrete girders to over height load impacts considering the following parameters:

  •  Location of intermediate diaphragms,
  •  Size of the intermediate diaphragms
  •  Prestressed concrete girder types
  •  Spacing of prestressed concrete girders
  •  Length of prestressed concrete girder
  •  Dynamic load types
Among all the above parameters the depth of the intermediate diaphragm deemed to be most important factor in improving the bridge resistance to over height impact loads. The full depth intermediate diaphragm remove the weak point from the girder web to bottom flange while providing a frame action between girder bottom flange and diaphragm to resist the over height impact loads.


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

cc:   Mohammad Sheikhizadeh, Bridge Construction - 47354

       F. Posner, Bridge and Structures – 47340




                                                Fig. 1   Full Depth Intermediate Diaphragm


                                                Fig. 2   Partial Depth Intermediate Diaphragm (Current Practice)




                               Construction of Full Depth Intermediate Diaphragm Form

                                    C7324 Rush Rd to 13th St (Labree Rd Br 5/218)
                                              ...Courtesy of Jesse Beaver