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effective flange width for concrete box

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Eric,

The effective flange width is a fictitious width over which, for all 
practical purposes, the stress can be considered uniform, whereas in reality 
the stresses are greatest where the web connects to the flange and diminishes 
as you get progressively further away from the web (shear lag).  (It is 
fictitious just like the Whitney stress block is fictitious.  The ultimate 
compressive stress distribution in a concrete beam is certainly not 
rectangular, but the Whitney stress block gives the "right answers."  
Similarly, the stress distribution in a flange does extend beyond the 
"effective width," but using a uniform distribution over the "effective 
width" gives the "right answer.")

If you can't find proper guidance for determining the "effective flange" 
width and reinforcing distribution for box girders in the ACI code, I would 
look to AASHTO's code for guidance.  IIRC, AASHTO has a distribution of 
reinforcing with a certain amount within the effective flange width and some 
more distributed beyond the effective flange width.

Hope this helps.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Eric Lindquist wrote:

>>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.<<