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K Design Graphs and Charts for RaftFoundations Spanning Local Depressions

Table K.2 and multiply each by the appropriate moment

factor Km which is also obtained from Table K.2.

(4) Sum the results from (3) above to give (TuKm) and

use Fig. K.2 for slabs with top reinforcement only and

Fig. K.3 for slabs with top and bottom reinforcement to

determine the area of reinforcement required for the

selected effective depth. Note that the area of reinforce-

ment is required in both directions (i.e. a square mesh is

needed).

(5) If heavy point or line loads are present, a shear capacitycheck should also be undertaken.

(6) A similar design process is adopted for designing raft

beams, using Table K.3 and Figs K.4 and K.5.

The reader is advised to read the text in Chapter 13 before

using these charts. The charts and figures are repeated here

for quick reference and the following is an aide-mmoire for

the experienced user of the procedure.

(1) Select the required design span of the depression

based on the engineers experience and using Table K.1

and/or Fig. K.1 as a guide.

(2) If there is no bottom reinforcement in the slab, use the

chart on the right of Fig. K.1 to select the minimumeffective depth required.

(3) For each loading type on the area under consideration,

calculate the total factored load on the design span from

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350 Appendices

4.0

3.0

2.0

1.0

0

4.0

3.0

2.0

1.0

0

slabs with topreinforcement only

minimum allowableeffective depth (mm)

225

200

175

150

125

100

75

soilclassification

assumeddiameterofdepression(m)

designspanofdepressionL(m)

D

C

B

A

0 0.5

depth of hardcore/compacted material hfill (m)

1.0

Fig. K.1 Design span for local depression. For soil classifications see below.

Table K.1 Design diameter for local depression

Soil classification

A

Consistent firm

sub-soil

B

Consistent soil

type but variable

density, i.e. loose

to firm

C

Variable soil typebut firm

D

Variable soil

type and variable

density

Soil type

One only of: clay,

sand, gravel, sandy

clay, clayey sand

One only of: clay,

sand, gravel, sandy

clay, clayey sand

Two or more of: clay,

sand, gravel, sandyclay, clayey sand

Two or more of: clay,

sand, gravel, sandy

clay, clayey sand

Assumed diameter

of depression (m)

1.01.5

1.52.0

2.02.5

2.53.5

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Appendices 351

Uniformlydistributed loadfS (kN/m

2)

Top reinforcement only Top and bottomreinforcement

Parallel lineload P(kN/m)

Lateral lineload P(kN/m)

2 way lineload P(kN/m)

Point load P(kN) 2.0

1.5

1.5

1.5

1.0

Km

T1 TB1

T2 TB2

T3 TB3

T4 TB4

T5 TB5

FS=fS(L2/4) FS=fS(L

2/4)

P=PL

P=PL

P= 2PL P= 2PL

P=P

P=PL

P=PL

P=P

Table K.2 Load types and moment factors for raft slabs spanning over a depression of diameter L

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352 Appendices

500

450

400

350

300

250

200

150

100

50

0

d= 200

(K

mT

u)

0 50 100 150 200

AREA OF REINFORCEMENT EACH WAY =AS (mm2/m)

CONCRETE GRADE 35REINFORCEMENT GRADE 460

250 300 350 400

d= 225

d= 175

d= 150

d= 125

d= 100

d= 75

Fig. K.2 Design chart for slabs with top reinforcement only.

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Appendices 353

1000

900

800

700

600

500

400

300

200

100

0

d= 200

d= 175

d= 150

d= 125

d= 100

d= 75

0 50 100 150 200

AREA OF REINFORCEMENT EACH WAY PER FACE =AS (mm2/m)

CONCRETE GRADE 35REINFORCEMENT GRADE 460

250 300 350 400

d= 225

(K

mT

u)

Fig. K.3 Design chart for slabs with top and bottom reinforcement.

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354 Appendices

Uniformlydistributed loadfS (kN/m

2)

Internal beam Edge beam Corner beam

Parallel lineload P(kN/m)

Lateral lineload P(kN/m)

2 way lineload P(kN/m)

Point load P(kN) 2.0

1.0

1.0

1.0

0.5

Km

I1

I2

C1E1

E2 C2

2

L

I3 E3 C3

I4 E4 C4

I5 E5 C5

FS=fS(L2/4)

P= 2PL

P=PL

P=PL

P=P

FS=fS(L2/8)

P= 3PL/2

P=PL/2

P=PL

P=P

FS=fS(0.64L2)

P=PL/ 2

P=P

P=PL/ 2

P= 2PL/ 2

2

L

Table K.3 Load types and moment factors for raft beams

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Appendices 355

5000

4500

4000

3500

3000

2500

2000

1500

1000

500

0

d= 550

d= 500

d= 450

d= 400

d= 350

d= 300(K

mT

u)L/b(kN)

0 400 800 1200 1600 2000

AREA OF REINFORCEMENT PER FACE/m WIDTH =As/b(mm2/m)

CONCRETE GRADE 35REINFORCEMENT GRADE 460

Fig. K.4 Design chart for internal and edge beams.

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356 Appendices

2500

2250

2000

1750

1500

1250

1000

750

500

250

0

d= 550

d= 500

d= 450

d= 400

d= 350

d= 300

(Km

Tu

)L/b(kN)

0 400 800 1200 1600 2000

AREA OF REINFORCEMENT IN TOP FACE/m WIDTH =As/b(mm2/m)

CONCRETE GRADE 35REINFORCEMENT GRADE 460

Fig. K.5 Design chart for corner beams.