FIGURE 36. Transverse section of T-beam bridge.- Verify that the slab size is adequate and design
the reinforcement
The AASHTO Specification does not present moment coefficients for the design of con-
tinuous members. The positive and negative reinforcement will be made identical, using
straight bars for both. Apply a coefficient of YIO in computing the dead-load moment. The
Specification provides that the span length S of a slab continuous over more than two sup-
ports be taken as the clear distance between supports.
In computing the effective depth, disregard the wearing surface, assume the use of No.
6 bars, and allow 1 in (25.4 mm) for insulation, as required by AASHTO. Then, d = 6.5 -
0.75 - 1.0 - 0.38 = 4.37 in (110.998 mm); WDL = (6.5/12)(150) + 15 = 96 Ib/lin ft (1401
N/m); MDL = (^1 Ao)W 01 S^2 = (^1 /io)(96)(4.17)^2 = 167 ft-lb (226 N-m); M1x= 0.8(5 + 2)P 20 /32,
by AASHTO, or MLL = 0.8(6.17)( 16,000)732 = 2467 ft-lb (3345 N-m). Also by AASHTO,
IF = 0.30; Mtotal = 12(167 +1.3Ox 2467) = 40,500 in-lb (4.6 kN-m). The moment corre-
sponding to balanced design is Mb = K^d^2 = 197(12)(4.37)^2 = 45,100 in-lb (5.1 kN-m).
The concrete section is therefore excessive, but a 6-in (152.4-mm) slab would be inade-
quate. The steel is stressed to capacity at design load. Or, As = 40,500/(20,0OO x 0.875 x
4.37) = 0.53 in^2 (3.4 cm^2 ). Use No. 6 bars 10 in (254 mm) on centers, top and bottom.
The transverse reinforcement resists the tension caused by thermal effects and by load
distribution. By AASHTO, At = 0.67(0.53) = 0.36 in^2 (2.3 cm^2 ). Use five No. 5 bars in
each panel, for which,4,= 1.55/4.17 = 0.37 in^2 (2.4 cm^2 ).
5. Calculate the maximum live-load bending moment
in the interior girder caused by the moving-load group
The method of positioning the loads to evaluate this moment is described in an earlier cal-
culation procedure in this handbook. The resultant, Fig. 37, has this location: d = [16(14)
- 4(28)]/(16 + 16 + 4) = 9.33 ft (2.85 m). Place the loads in the position shown in Fig.
38a. The maximum live-load bending moment occurs under the center load.
The AASHTO prescribes a distribution factor of S/6 in the present instance, where S
denotes the spacing of girders. However, a factor of S/5 will be applied here. Then DF =
5.33/5 = 1.066; 16 x 1.066 = 17.06 kips (75.9 kN); 4 x 1.066 = 4.26 kips (18.9 kN); P =
2(17.06) + 4.26 = 38.38 kips (170.7 kN); RL = 38.38(29.33)/54 = 20.85 kips (92.7 kN).
The maximum live-load moment is M1x = 20.85(29.34) - 17.06(14) = 372.8 ft-kips (505
kN-m).