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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.