ENCE 717 - Bridge Engineering
Dr. C. C. Fu (1/18/06)
Homework 1
(by Spreadsheet; due: 1/31/06)
For a 100' two-span continuous steel bridges with 50' equal span length, 8'-girder
spacing, 8"-thick slab and 2"-haunch depth (shown below) with 3' overhang, find both
interior and exterior effective widths and girder section properties Ix, Stop (top of
steel girder), Sbottom (bottom of steel girder) of N (section modulus) = , n, 3n, based on
both AASHTO LRFD Specifications (LRFD) and AASHTO Standard Specifications
(Standard) and then compare the results.
Assume: slab fc' = 4000 psi; fy = 50 ksi; n = 8
Steel girder bridge
T slab = 8"
T haunch = 2", W haunch = 16"
Top Flange 12" x 1"
Web 48" x 1/2"
Bottom Flange 18" x 2"
Overhang: 3'
Request:
(1) Interior, LRFD
(2) Interior, Standard
(3) Exterior, LRFD
(4) Exterior, Standard
(5) Compare
Note: For LRFD, assume Leffective = 0.9L for exterior span and = 0.8L for interior span
LRFD (for general material)
4.6.2.6 EFFECTIVE FLANGE WIDTH
4.6.2.6.1
General
The effective span length used in calculating effective flange width may be taken as the actual
span for simply supported spans and the distance between points of permanent load inflection for
continuous spans, as appropriate for either positive or negative moments.
For interior beams, the effective flange width may be taken as the least of:



One-quarter of the effective span length;
12.0 times the average thickness of the slab, plus the greater of web thickness or one-half
the width of the top flange of the girder; or
The average spacing of adjacent beams.
For exterior beams, the effective flange width may be taken as one-half the effective width may be
taken as one-half the effective width of the adjacent interior beam, plus the least of:


One-eighth of the effective span length;
6.0 times the average thickness of the slab, plus the greater of half the web thickness or
one-quarter of the width of the top flange of the basic girder; or

The width of the overhang.
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STANDARD (for precast/prestressed concrete)
9.8.3
Precast/Prestressed Concrete Beams with Wide Top Flanges
9.8.3.1 For composite prestressed concrete where slabs or flanges are assumed to act integrally with the
precast beam, the effective web width of the precast beam shall be the lesser of (1) six times the
maximum thickness of the flange (excluding fillets) on either side of the web plus the web and
fillets, and (2) the total width of the top flange.
9.8.3.2 The effective flange width of the composite section shall be the lesser of (1) one-fourth of the span
length of the girder, (2) six (6) times the thickness of the slab on each side of the effective web
width as determined by Article 9.8.3.1 plus the effective web width, and (3) one-half the clear
distance on each side of the effective web width plus the effective web width.
STANDARD (for steel)
10.38.3
Effective Flange Width
10.38.3.1
In composite girder construction the assumed effective width of the slab as a T-beam
flange shall not exceed the following:
(1)
(2)
(3)
10.38.3.2
One-fourth of the span length of the girder.
The distance center to center of girders.
Twelve times the least thickness of the slab.
For girders have a flange on one side only, the effective flange width shall not exceed
one-twelfth of the span length of the girder, or six times the thickness of the slab, or onehalf the distance center to center of the next girder.