Approximate and refined analysis methods - Computational Analysis and Design of Bridge Structures - page 27

Each hatched area equals to half of shaded area

W e W e

σ max

Half width Half width

Figure 2.10 Shear lag effect on stress distribution and equivalent effective width.

increase/decrease of the deck longitudinal flexural stress near the intersec-
tion to the beam web is called shear lag effect (Figure 2.10). This effect
can be taken into account during the stress calculation by assuming only a
portion of flange working to resist bending moment, or so-called effective
flange width. In AASHTO, the effective flange width is assumed constant
along the bridge, although some may assume otherwise. The prior-to-2008
AASHTO LRFD Specifications for interior beam's effective flange width
take the least of

• One-fourth of the effective span length (the effective span length
may be taken as the actual span for simply supported spans and the
distance between points of dead load contraflexure for continuous
spans).
• 12 times the average slab thickness, plus the greater of web thick-
nesses, or one-half the width of the top flange of the girder.
• Average spacing of adjacent beams.

For exterior beam, the effective flange width may be taken as 1/2 the effec-
tive width of the adjacent interior beam, plus the least of

• One-eighth of the effective span length.
• 6 times the average slab thickness, plus the greater of one-half the web
thickness, or 1/4 the width of the top flange of the girder.
• Width of the overhang.

The current AASHTO LRFD Specifications (2013) are using the full tribu-
tary areas of the girder, which is the third criterion shown previously.