Summary of design requirements

Grid C flexure

End supports:

radius to uout from face of column

Column strip: 10 no. H20 U-bars (max. 200 mm from column) in pairs

(where 200 x 200 hole use 8 H20 T1 in U-bars in pairs)

Middle strip: H12 @ 200 T1

Column strip and middle strip: H20 @ 200 B

Central support:

Column strip centre: for 750 mm either side of support: H20 @ 100 T1

Column strip outer: H20 @ 250 T1 Middle strip: H16 @ 200 T1

Grid 1 (and 3) flexure: Spans:

Column strip: H16 @ 200 B2* Middle strip: H12 @ 300 B2

Interior support:

Column strip centre: 6 no. H20 @ 175 T2 Column strip outer: H12 @ 175 T2 Middle strip: H12 @ 300 T2

Grid 2 flexure: Spans:

Column strip: H16 @ 250 B2* Middle strip: H10 @ 200 B2

Interior support:

Column strip centre: H20 @ 200 T2* H16 Column strip outer: @ 250 T2 Middle strip: H12 @ 300 T2

Punching shear

Use H10 legs of links in perimeters at max. 175 mm centres. Max. tangential spacing of legs of links, st max. = 270 mm Last perimeter, from column face, min. 767 mm Edge (e.g. at C1, C3 assuming no holes):

Use H10 legs of links in perimeters at max. 175 mm centres. Max. tangential spacing of legs of links, st max. = 175 mm Last perimeter, from column face, min. 940 mm

Edge (e.g. at D1, D3 assuming 200 x 200 hole on face of column):

Use H10 legs of links in perimeters at max. 175 mm centres. Max. tangential spacing of legs of links, st max. = 175 mm Last perimeter, from column face, min. 1123 mm

Note

* rationalise centre of bars in column strips T2 and B2 to 175 mm centres to suit punching shear links.

Shear Links Per 8110

Figure 3.25

Reinforcement details bay C-D, 1-2

Note

* Spacing rationalised to suit punching shear links.

Figure 3.25

Reinforcement details bay C-D, 1-2

Note

* Spacing rationalised to suit punching shear links.

Eurocode Punching Shear Link Arrangement

Figure 3.26

Punching shear links at column C2 (102 no links) (column D2 similar)

Figure 3.26

Punching shear links at column C2 (102 no links) (column D2 similar)

Details For Punching Shear Links

Figure 3.27

Punching shear links at column D1 (and D3) (penultimate support without hole similar)

Figure 3.27

Punching shear links at column D1 (and D3) (penultimate support without hole similar)

3.5.14 Commentary on design

Method of analysis

The use of coefficients in the analysis would not usually be advocated in the design of such a slab. Nonetheless, coefficients may be used and unsurprisingly, their use leads to higher design moments and shears, as shown below.

Method

Moment in 9.6 m span per 6 m bay (kNm)

Centre support moment per 6 m bay (kNm)

Centre support reaction VEd (kN)

Coefficients

842.7

952.8

1205

Continuous beam

747.0

885.6

1103

Plane frame columns below

664.8

834.0

1060

Plane frame columns above and below

616.8

798

1031

These higher moments and shears result in rather more reinforcement than when using other more refined methods. For instance, the finite element analysis used in Guide to the design and construction of reinforced concrete flat slabs[29] for this bay, leads to:

■ H16 @ 200 B1 in spans 1-2 (cf. H20 @ 200 B1 using coefficients),

■ H20 @ 125 T1 at support 2 (cf. H20 @ 100 T1 using coefficients) and

■ 3 perimeters of shear links at C2 for VEd = 1065 kN (cf. 5 perimeters using coefficients)

■ 2 perimeters of shear links at C3 (cf. 7 perimeters using coefficients)

Effective spans and face of support

In the analysis using coefficients, advantage was taken of using effective spans to calculate design moments. This had the effect of reducing span moments.

At supports, one may base the design on the moment at the face of support. This is borne out by Guide to the design and construction of reinforced concrete flat slabs[29] that states that hogging moments greater than those at a distance hc / 3 may be ignored (where hc is the effective diameter of a column or column head). This is in line with BS 8110[30] and could have been used to reduce support moments.

Shear reinforcement

H10 punching shear links are required for columns D1 and D3. Whilst the other columns were found to require only H8s, H10s have been adopted throughout to avoid confusion in detailing or on site. The cost differential would have been marginal.

With added area, the numbers of links could have been reduced on the other columns. A rectangular arrangement (300 x 175 grid) of H10 links would have been possible. However, as the grid would need to change orientation around the columns and as the reinforcement in B2 and T2 is essentially at 175 centres, it is considered better to leave the regular square grid arrangement.

Use of shear reinforcement in a radial arrangement, e.g. using stud rails, would have simplified the shear reinforcement requirements.

Curtailment of reinforcement

In this design, the reinforcement would be curtailed and this would be done either in line with previous examples or more practically in line with other guidance [20' 21].

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