Memorandum

In prestressed girder bridge widenings with one or two lines of new girders, the end and intermediate diaphragms shall be placed prior to the slab casting to ensure the stability of the girders during construction.  The closure shall be specified for deck slab but shall not be required for diaphragms.  The designer shall investigate the adequacy of the existing girder adjacent to the widening for the additional load due to the weight of wet slab transferred through the diaphragms, taking into account the loss of removed overhang and barrier.  The diaphragms must be made continuous with existing diaphragms.

A similar procedure shall be used for steel I girder widenings.  Normally, crossframes should be installed between existing and new girders prior to slab placement that allow vertical displacement but no twisting or rotation of the new girders.  The common method is to install one bolt (not fully tensioned) per crossframe connection in the bay between new and existing girders.  The remaining bolts for these connections are installed after slab and closure placement.   Bolt holes should be drilled in the field to eliminate the need for reaming.  Bottom laterals for straight girders are not needed when this method is used.  Crossframes between new and existing girders should always be used for straight bridges being widened by 1 or 2 girders.  Straight bridges, which are to be widened by 3 or more girders, may or may not require the installation of crossframes between new and existing girders.

 Background:

The WDSOT Bridge Construction Office has raised concerns about the stability of prestressed concrete and steel I girder bridge widenings with one or two lines of girders.  For additional information and a simplified method for determining construction forces in the crossframes, refer to the Transportation Research Record 1712, Paper No. 00-1268 “Methods for Controlling Stresses and Distortions in Stage-Constructed Steel Bridges” by Swett, Stanton & Dunston.

 If you have any questions regarding these issues, please contact Nathan Brown at
705-7219, Geoff Swett at 705-7157 or Bijan Khaleghi at 705-7181.

 JK:dsb
 BK

 cc:     J. Weigel, Bridge and Structures - 47340
          F. Higgins, Bridge and Structures -  47340
          Z. Y. Fahoum, Bridge and Structures - 47340