Loading to Box Culverts

7/29/2019 Loading to Box Culverts

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LOADING TO BOX-CULVERTS

University of Dar es SalaamBy Dr-Ing. JK Makunza


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General AspectsBox culverts are drainage structures which

consist of two horizontal slabs and two or morevertical walls. The slabs and walls are built

monolithically, and are ideally installed for a road

or a railway bridge crossing with highembankments crossing a stream with a limited

flow. Reinforced concrete rigid frame box

culverts with square or rectangular openings areused up to spans of4.0 m. The height of the

vent (h) with respect to Figure 1, generally does

not exceed 3.0 m.


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l

L

h Ht

t s

w

f

f

standard fillet

f = 150 mm

Figure 1: Single cell box culvert.


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Box culverts are economical due to their

rigidity and monolithic action and separate

foundations are not required since the bottom

slab resting directly on the soil, serves as raft

foundation. For small discharges, single celled

box culvert is used and for large discharges,

multi-celled box culverts can be employed. The

barrel of the box culvert should be of sufficient

length to accommodate the carriage way and

the kerbs.


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5Figure 2: Double cell box culvert


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Figure 3: Triple cell box culvert


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Analysis Assumptions

Frame

The box culvert shall be analyzed, as a rigid frame

with all corner connections considered rigid.

Sidesway

Sidesway is not considered in the analysis

Section Properties

The centerlines of slab, walls and floor are used for

computing section properties and for dimensionalanalysis. Standard fillets which are not required formoment or shear or both shall not be considered incomputing section properties.


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Minimum Thickness

The following minimum thickness shall be used

Top slab: ts = 200 mm, but taken as 80-

100mm per 1.00m length

of the span

Floor slab: tf= 250 mm

Wall: tw = 25 mm per 300 mm of wall

height but not less than 230 mm.


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Design LoadsThe structural design of a reinforced concrete box culvert

comprises the detailed analysis of rigid frame for moments,

shear forces and thrusts due to various types of loading

conditions outlined below:

1. Concentrated Loads2. Uniform Distributed Loads

3. Weight of Side Walls

4. Water Pressure Inside Culvert

5. Earth Pressure on Vertical Side Walls6. Uniform Lateral Load on Side Walls


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1. Concentrated Loads

In cases where the top slab forms the deck of the bridge,

concentrated loads due to the wheel loads of the BS 5400HB type loading have to be considered.

If P = wheel load due to HB loading which include the

impact factor of.25%, the dispersal length = 1.75D, and D

= depth of soil fill, then the load intensity on the culvertslab,

W = (P/(1.75D) kN/m (1)

The soil reaction of the bottom slab is assumed to beuniform. The notations used for the box culvert and the

type of loadings to be considered are shown in Figure 4


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Concentrated Loads

Case 1(b)

P P1.80 m

Case 1(a)

P P1.80 m1.75 D

D

Figure 4: Point load due to vehicles


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2. Uniform Distributed LoadsThe weight of embankment, wearing coat and, deck slab and the track

load are considered to be uniformly distributed loads on the top slab

with the uniform soil reaction on the bottom slab. Minimum D = 300 mm

w/m 2

w/m 2

Case 2

s.DD

Filldepth

HA - Udl

HA - KEL

kN/m

BS 5400 HA Loading

Figure 5: Uniform distributed loads


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3. Weight of Side WallsThe self weights of two side walls

acting as concentrated loads are

assumed to produce uniform soilreaction on the bottom slab.

Ww= is the weight of one

wall, and is given by:

Ww= twHJc kN/m transversal

Where

tw= wall thickness

H= height of wall, and

Jc= density of concrete = 24kN/m3.

Case 3

Ww

Figure 6: Load from walls


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4. Water Pressure Inside Culvert

When the culvert is full

with water, the pressure

distribution on side walls is

assumed to be triangular

with a maximum pressure

intensity ofp = Jwh at thebase

where Jw= density of

water andh

is the depth offlow.

p/mp/m2 2

Case 4

h

Intensity of water pressurep = Jwh

Figure 7: Water pressure


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5. Earth Pressure on Vertical Side WallsThe earth

pressure on the

vertical side walls

of the box culvert

is computed

according to the

Coloumbs Theory.The distribution of

soil pressure on

the side wall is

shown in Figure 8. Case 5p/m2 p/m

2

D

h

I

IJ

sin1

sin1hp sSoil pressure,

Figure 8: Soil pressure


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6.Uniform Lateral Load on Side Walls

Case 6

p/m2 p/m2

Uniform lateral pressure on

vertical side walls has to be

considered due to the effect

of live load surcharge. Also

trapezoidal pressure

distribution on side walls due

to embankment loading can

be obtained by combining thecases (5) and (6).

Uniform lateral pressure due to the effect of surcharge loads is obtained

from:

I

I

sin1

sin1LoadseargSurchp

Figure 9: Lateral load due to surcharge loads


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Design Moments, Shears and Thrusts

A box culvert is analyzed for moments, shear

forces and axial thrusts developed due to thevarious loading conditions by any of the classical

methods such as moment distribution, slope

deflection or column analogy procedures.

Alternatively coefficients for moments, shearsand trusts from various structural analysis books

are very useful in the computation of the various

force components for the different loading

conditions.


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Table 1a: Some standard formulae for analyzing box culverts

A B

Mi Mk

EI = Constant

i k

l

A B Mi Mk

q

2ql

2

ql

12ql2

12ql2

q

ql35.0 ql15.0

20ql2

30ql2

q

ql15.0 ql35.0

30

ql2 20

ql2

q qi k

lq15.0q35.0 ki lq35.0q15.0 ki 2ki l30

qq5.1 2ki l

30

q5.1q9


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Table 1b: Some standard formulae for analyzing box culverts

A B

Mk

EI = Constant

i k

l

A B Mk

q

8ql3

8

ql5

8ql2

q ql

40

11 ql

40

9 120

ql7 2

q

10

ql

5

ql2

15

ql2

q qi k

l

40

q4q11 ki

l40

q16q9 ki 2ki l120

q8q7


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Conclusion:Design Of Critical Sections

The maximum design moments resulting from the combination of

the various loading cases are determined. The moments at the

centre of span of top and bottom slabs and the support sections

and at the centre of the vertical walls are determined by suitably

combining, the different loading patterns. The maximum moments

generally develop for the following loading conditions:

1. When the slab supports the dead and live lads and the culvert isempty.

2. When the top slab supports the dead and live lads and the

culvert is running full.

3. When the sided of the culvert do not carry the live load and theculvert is running full.

The slab of the box culvert is reinforced on both faces with fillets

at the inside corners.

Documents

Loading to Box Culverts