or steel girders. Except for the deviations indicated, the Standard Specifications for High-
way Bridges, published by the American Association of State Highway and Transporta-
tion Officials (AASHTO), are applied.
The AASHTO Specification recognizes two forms of truck loading: the //loading, and
the HS loading. Both are illustrated in the Specification. For a bridge of relatively long
span, it is necessary to consider the possibility that several trucks will be present simulta-
neously. To approximate this condition, the AASHTO Specification offers various lane
loadings, and it requires that the bridge be designed for the lane loading if this yields
greater bending moments and shears than does the corresponding truck loading.
In designing the bridge members, it is necessary to modify the wheel loads to allow
for the effects of dynamic loading and the lateral distribution of loads resulting from the
rigidity of the floor slab.
The basic notational system is: DF = factor for lateral distribution of wheel loads; IF =
impact factor; P = resultant of group of concentrated loads.
The term live load as used in the following material refers to the wheel load after cor-
rection for distribution but before correction for impact.
DESIGN OFA T-BEAM BRIDGE
A highway bridge consisting of a concrete slab and concrete girders is to be designed
for these conditions: loading, HS20-44; clear width, 28 ft (8.5 m); effective span, 54 ft
(16.5 m); concrete strength, 3000 lb/in^2 (20,685 kPa); reinforcement, intermediate
grade. The slab and girders will be poured monolithically, and the slab will include a %
in (19.05 mm) wearing surface. In addition, the design is to make an allowance of 15
lb/ft^2 (718 N/m^2 ) for future paving. Design the slab and the cross section of the interior
girders.
Calculation Procedure:
- Record the allowable stresses and modular ratio given
in the AASHTO Specification
Refer to Fig. 36, which shows the spacing of the girders and the dimensions of the mem-
bers. The sizes were obtained by a trial-and-error method. Values from the Specification
are: w = 10 in stress calculations;/,. = 0.4/c' = 1200 lb/in^2 (8274 kPa); for beams with web
reinforcement, vmax = 0.075/c' = 225 lb/in^2 (1551.4 kPa);/, = 20,000 lb/in^2 (137.9 MPa); u
= 0.10/c' = 300 lb/in^2 (2068.5 kPa).
- Compute the design coefficients associated with
balanced design
Thus, k = 1200/(12OO + 2000) = 0.375, using Eq. 21, Section 2. Using Eq. 22, Section 2,
J=I- 0.125 = 0.875. By Eq. 32, Section 29 K =^1 / 2 (1200)(0.375)(0.875) = 197 lb/in^2
(1358.3 kPa).
- Establish the wheel loads and critical spacing associated with
the designated vehicular loading
As shown in the AASHTO Specification, the wheel-load system comprises two loads of
16 kips (71.2 kN) each and one load of 4 kips (17.8 kN). Since the girders are simply sup-
ported, an axle spacing of 14 ft (4.3 m) will induce the maximum shear and bending mo-
ment in these members.
