The effective width provisions have nothing to do with crack control.
Instead they are related to the distribution of compressive stresses in the
slab (or tensile stresses in the reinforcement). There is a short
discussion of this consideration in "Reinforced Concrete Structures" by Park
and Paulay (p. 99). For seismic design, the same issue is addressed in more
detail on pages 189-193 and 285-288 of "Seismic Design of Reinforced
Concrete and Masonry Buildings" by Paulay and Priestley.
In each of the cases noted above the discussion focuses on T-sections (not
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Michael Valley, P.E., S.E. E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc. Tel:(206)292-1200
1301 Fifth Ave, #3200, Seattle WA 98101-2699 Fax: -1201
> -----Original Message-----
> From: Eric Lindquist [mailto:ESL(--nospam--at)eqe.com]
> Sent: Tuesday, March 07, 2000 2:49 PM
> To: seaint(--nospam--at)seaint.org
> Subject: effective flange width for concrete box girder
> 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