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effective flange width for concrete box girder

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Can anyone help me with the following?

I am seeking information regarding the allowable "effective" width of a flange
for a concrete box girder under seismic loading.  "Effective" flange width for
concrete tee beams is outlined in Section 8.10 of ACI 318.  I am reviewing a
case where the flange on the box girder is about twice as wide as the ACI 318
effective width.  The designer argues that the girder only needs to figure using
the ACI effective width for static load combinations and that under seismic
loading the entire width of the flange can be used in compression and the rebar
in the entire flange width can be used in tension.  I have questioned whether or
not the girder can perform in accordance with this assumption.  (I am concerned
about buckling of the compression flange and the ability of a rebar to be
developed in tension a long distance from the web.)  The response from the
designer has been that the effective flange width is only a crack control
provision and that that the entire flanges will be effective in both tension and
compression under seismic loading.

Any input will be greatly appreciated.

Eric Lindquist