v = Vlb'jd = 66,2807(14 x 0.875 x 32.62) = 166 < 225 lb/in^2 (1144.6 < 1551.4 kPa). This
is acceptable.
Compute the moment capacity of the girder at balanced design. Since the concrete is
poured monolithically, the girder and slab function as a T beam. Refer to Fig. 16, Section
2 and its calculation procedure.
Thus, kbd = 0.375(32.62) = 12.23 in (310.642 mm); 12.23 - 5.75 = 6.48 in (164.592
mm). At balanced design,/cl = 1200(6.48/12.23) = 636 lb/in^2 (4835.2 kPa). The effective
flange width of the T beam as governed by AASHTO is 64 in (1625.6 mm); and Cb —
5.75(64X^1 X 2 )(I^OO + 0.636) = 338 kips (1503 kN);yW = 32.62 - (5.75/3)(1200 + 2 x
636)/(1200 + 636) = 30.04 in (763.016 mm); Mb = 338(30.04) = 10,150 in-kips (1146
kN-m). The concrete section is therefore slightly excessive, and the steel is stressed to ca-
pacity, orAs = 10,100/20(30.04) = 16.8 in^2 (108.4 cm^2 ). Use 11 no. 11 bars, arranged in
three rows.
AASHTO requires that the girders be tied together by diaphragms to obtain lateral
rigidity of the structure.
COMPOSITE STEEL-AND-CONCRETE BRIDGE
The bridge shown in cross section in Fig. 39 is to carry an HS20-44 loading on an effec-
tive span of 74 ft 6 in (22.7 m). The structure will be unshored during construction. The
concrete strength is 3000 lb/in^2 (20,685 kPa), and the entire slab is considered structurally
effective; the allowable bending stress in the steel is 18,000 lb/in^2 (124.1 MPa). The dead
load carried by the composite section is 250 Ib/lin ft (3648 N/m). Preliminary design cal-
culations indicate that the interior girder is to consist of W36 x 150 and a cover plate 1Ox
1/2 in (254 x 38.1 mm) welded to the bottom flange. Determine whether the trial section
is adequate and complete the design.
Calculation Procedure:- Record the relevant properties of the W36 x 150
The design of a composite bridge consisting of a concrete slab and steel girders is gov-
erned by specific articles in the AASHTO Specification.
Composite behavior of the steel and concrete is achieved by adequately bonding the
materials to function as a flexural unit. Loads that are present before the concrete has
hardened are supported by the steel member alone; loads that are applied after hardening
are supported by the composite member. Thus, the steel alone supports the concrete slab,
and the steel and concrete jointly support the wearing surface.
Plastic flow of the concrete under sustained load generates a transfer of compressive
stress from the concrete to the steel. Consequently, the stresses in the composite member
caused by dead load are analyzed by using a modular ratio three times the value that ap-
plies for transient loads.
If a wide-flange shape is used without a cover plate, the neutral axis of the composite
section is substantially above the center of the steel, and the stress in the top steel fiber is
therefore far below that in the bottom fiber. Use of a cover plate depresses the neutral
axis, reduces the disparity between these stresses, and thereby results in a more economi-
cal section. Let>>' = distance from neutral axis of member to given point, in absolute Val-
ue; y = distance from centroidal axis of WF shape to neutral axis of member. The sub-
scripts b, ts, and tc refer to the bottom of member, top of steel, and top of concrete,