Civil Engineering Reference
In-Depth Information
17.5.1.1 Bond
The cover requirements for bond are given in
Table 17.4
.
17.5.1.3 Nominal cover
The nominal cover, which should be specified on the draw-
ings, is the minimum cover (the maximum of the bond and
durability requirements) plus an allowance in design for devi-
ation (Δ
c
dev
). The allowance in design for deviation is effect-
ively a construction tolerance and should be taken as 10 mm,
unless measures are taken to ensure that a smaller tolerance
will be met.
17.5.1.2 Durability
The minimum cover for durability is denoted
c
min,dur
and can be
determined from tables 4.1, 4.3, 4.4 and 4.5 of BS EN1992-
1-1. However, in the UK, tables 4.3 to 4.5 should not used and
BS 8500-1 (BSI, 2006) should be used instead.
Floor type
Span range: m
Practical minimum
thickness: mm
Span to depth ratio
Single span
End bay
Multi-span
Cantilever
Flat slab
6-9
200
23-28
Post-tensioned flat slab
7-12
200
26-40
Flat slab with drops
6-12
200
23-30
Flat slab with flared
column heads
6-12
200
21-31
Waffle slab
6-12
250
14-23
Biaxial voided slab
7-12
150
20-35
One-way slab
4-11
150
23-27
27-32
7-10
Beams
-
-
15-20
17-26
6-10
Slab supported by band
beams
4-11
150
27-40
33-40
Band beams
6-12
250
13-24
14-24
Ribbed slab
6-12
250
17-24
17-27
Lattice girder slab
4-9
150
20-30
23-35
Hollowcore slab
4-16
150
24-45
Precast double tee units
8-16
300
17-26
Two-way slab
4-12
150
25-34
29-39
-
Notes:
1. The span-to-depth values given encompasses a range of loads from 2.5 kN/m
2
to 10 kN/m
2
; a lower value should be used for the higher loads.
2. The span-to-depth values given covers a range of spans; as the span increases a lower value should be used. The combination of high load and long span may be outside the
range of span-to-depth values given and a more detailed check should be carried out.
3. For flat slabs, punching shear must also be checked; for small columns in particular, the slab depth may need to be increased to be economic.
4. Hollowcore units are often supplied to fixed depths, which can vary depending on supplier.
5. The use of composite toppings on hollowcore and double units could reduce depth of the units required.
Table 17.2 Preliminary span-to-depth ratios for various floor elements
Percentage of
reinforcement
Ultimate axial load: kN
1000
1500
2000
3000
4000
5000
6000
8000
10000
1.0
250
300
350
400
475
525
575
650
725
2.0
225
275
325
375
425
475
525
600
675
3.0
225
250
300
350
400
450
500
550
625
4.0
225
250
275
325
375
425
450
525
600
Notes:
1. Suitable for concrete class C30/37 (higher strength will reduce the section sizes).
2. Suitable for internal columns, the following factors can be used to increase the vertical loads:
• Edge column - 1.5
• Corner column - 2.0
3. Columns are not slender and are braced (i.e. not suitable for moment frames)
Table 17.3
Preliminary column sizes (mm) (data taken from Goodchild
et al
., 2009)
