Bridge

1. Data and Specification Clear Span = Clear width = Live loading = Wearing Surface = Concrete Strength, f'c = Grade 40 reinforcement, fy =

6.25 8 HS20 1.44 21 276

m m kPa MPa MPa

By AASHTO Specifications, an allowable concrete stress of fc = 0.4 f'c =

8.400

MPa

and the allowable steel stress is fs = 0.5 fy = 138.000 MPa 2. Slab Design Consider a 1 meter strip of slab parallel to traffic:

0.6

8

m

m

0.6

m

1 m strip Cross section of slab Bridge

1

Effective span = 6.55 m Plan  

m strip

23.5

Unit Wt of Concrete =

Effective span of slab, S = Minimum Slab Thickness = Trial Slab Thickness, t = Weight of slab = Weight of wearing surface = Total deadload/m of Span = Deadload Moment is =

6.55 m 1.259 ft 0.4 m 9.400 kN/m 1.440 kN/m 10.840 kN/m 58.133 kN-m 1.613 m Width of slab over which load is distributed= E = 4 + 0.06 S where E and S in feet E = 1.22 + 0.06 S where E and S in m

0.384

kN/m3

m

The load on each rear wheel is (16.00 lb) 71.168 kN and the load on a width of slab is therefore Rear wheel = 71.17 kN PRear = 44.122 kN Position of live load for maximum moment: and that in the front wheel is PFront = 16.435

Rear wheel =

R= 16.435 kN

26.51

88.243

kN

44.122 kN 4.27 m

kN

kN

44.122 kN

1.07 1.07

2.13 m

3.28 m

3.28 m

29.747 kN

58.496 kN CASE 1

and the liveload moment is (with one wheel on the load span) ML =

72.249

kN-m

The impact coefficient is:

50 I= ≤0.30 L+125 I= The impact moment is therefore: MI =

L( infeet 0.341 21.675

) >

0.3

use I =

0.3

kN-m

The total moment due to dead, live and impact effects is MT = 152.057 kN-m Consistent with the AASHTO specification, the elastic design equations will be used. EC = 21538.106 MPa n= 9.286 say 9 k= 0.354

j= R= dreq'd =

0.882 1.311

340.548 mm

dfurn =

362.500 mm

MPa 400

mm

Comment: OK The actual effect depth is sufficiently close to the computed requirement and a 300 mm thick slab will

be used. The required main reinforcement is As = 3446.184 mm2 using 25 mm Ø bars Ab = 490.874 mm2 Required Spacing, s = 142 mm Therfore use

25

mm Ø bars

@

120

mm

o.c main bars.

The amount of transverse reinforcement required for proper distribution of concentrated loads (also provide for shrinkage) is < 50% 100/√S = 21.575 Atr = 743.500 mm2 (controls) Ast = 800.000 mm2 using 16 mm Ø bars Ab = 201.062 mm2 Required Spacing, s = 251 mm Therfore use

16

mm Ø bars

@

250

Use A = 800.000 mm2

mm o.c

transverse bars.

To facilitate screeding off the slab, the required curbs will not be monolithic. Required edge beams will

consist of an edge slab section additionally reinforced. Thickness of the curb = Width of the curb = Dead load carried by edge beam =

0.25 0.6 9.17

m m kN/m

49.150

kN-m

46.615

kN-m

13.985

kN-m

The dead load moment is MD = The specified live-load moment is

ML = 0.10 P20 S = The impact load moment is MI =

The total moment due to dead, live and impact effects is MT =

109.750 kN-m

The resisting moment of the given section is

Comment: MR = Rbd = 2

103.364 kN-m

NOT OK

Comment: Since the curb is connected to the concrete slab by dowel bars, this will create a composite action between the slab and the curb. (It is left for the student to verify the adequacy of the trial design taking advantage of composite action).

The steel required is As = 2487.349 mm2 using

Using

25 mm Ø bars Ab = 490.874 mm2 n = 5.067 say 6

-

25

mm Ø bars

6

Case 1

Case 2

3.2

Two Wheels Right

One Wheel

4.27

One Wheel

One Wheel

Left

16.435 kN

44.122 kN

44.122

4.27 m

4.27 m

3.28 m

3.28 m

CASE 2

From Case 1: MMax =

65.681

From Case 2: kN-m

MMax =

72.249

kN-m

kN

say

1 m strip

Therfore use

#REF!

#REF!

#REF!

@

#REF!

250

mm o.c

transverse bars.