|TO:||All Design Section Staff|
|FROM:||J. Kapur - 47340|
|DATE:||September 18, 2001|
|SUBJECT:||Prestressed Concrete and Steel I Girder
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.
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
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 &
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.
J. Weigel, Bridge and Structures - 47340
F. Higgins, Bridge and Structures - 47340
Z. Y. Fahoum, Bridge and Structures - 47340