Bridge Design Manual

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Bridge Design Manual


Background

From the beginning of Texas Highway Department history, there have been reinforced
concrete bridge decks. They were used for the entire superstructure as slab spans and
culverts, placed monolithically with concrete girder spans, and constructed on top of timber,
steel, and precast concrete beams.
Design Specifications
Design specifications have undergone an evolution from none in the beginning through
several empirical formulas for distribution of wheel loads to the completely empirical design
method specified in the Ontario Highway Bridge Design Code. Beginning in 1980 the Texas
Department of Transportation (TxDOT) sponsored an extensive research project
investigating bridge decks designed according to the Ontario empirical method. A few
experimental bridges using the method were constructed. The Federal Highway
Administration (FHWA) accepted this method for phasing into general use by 1991.
However, after a few tentative trials, construction of this type of deck was abandoned. The
potential savings in reinforcing steel were not significant when concrete panels were used,
and the details were just more comfortable with the standard method.
Beginning with Supplement No. 1 to 1944 American Association of State Highway Officials
(AASHO) Design Specifications for Texas Bridges,1 issued in 1945, the design load for
concrete deck slabs was one 24,000-pound axle or two 16,000-pound axles, 4 ft. apart. This
was changed after the 1957 AASHO Specification to one 32,000-pound axle.


Design Stress
Allowable stresses in the concrete have changed from 800 psi for live load in 1931 to 0.4 f 'c
today. AASHTO-required concrete strengths have increased from 3,000 psi to 4,500 psi and
back to 4,000 psi in the 1988 American Association of State Highway and Transportation
Officials (AASHTO) Interim Specification. The required 28-day concrete strength for
concrete used in bridge decks in Texas was 3,000 psi until 1974, when Class S concrete was
introduced, which has six sacks of cement per cubic yard, low water/cement ratio, entrained
air, and a required strength of 3,600 psi. Later, in response to the 1988 AASHTO
provisions, Class S concrete was required to have six and one-half sacks of cement and a
strength of 4,000 psi. Although the allowable design stress in the concrete rose to 1,800 psi,
most states limited the calculated service load stress, using the empirical wheel load
distribution, to 1,200 psi. This limit kept deck slabs from getting thinner as the concrete
strength increased



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