Arrangement of reinforcement

-Par:): Section .3 3.6.4.4 Shear conribution from solid blocks

3.6.4.5 Shear contribution by jointsbetween narrow precast units

13.6.4.6 Maximum design shear stress

3.6.4.7 Area of shear reinforcement in ribbed, hollow block or voided slabs

] given in 3.4.5.8—3.4.5.10.Advantage may be taken of enhanced shear strength of sections close to supports as 13.6.5 Deflection in ribbed, hollow block or voided construction generally

Note that if a slab is designed as simply supported it must be considered as simply -supported when checking deflections.

3.6.5.1 General

If a slab is designed as simply supported according to the rules given in 3.6.2 then it should be treated as simply supported for the purposes of checking the ratio of span to effective depth.

3.6.5.2 Rib width of voided slabs or slabs of box or I-section units

3.6.6 Arrangement of reinforcement

3.6.2 allows that a continuous ribbed slab may be designed as simply supported with a so-called anti-crack steel provided over-the supports. The system of treating continuous slabs as simply supported has arisen in practice because of the difficulty, or even sometimes impossibility, of fixing enough top steel in the ribs over supports to resist the El moments which would arise from treating the slabs as continuous. From the point of view of safety, this is likely to be satisfactory. However, from the point of view of serviceability, its sufficiency is more doubtful. Effectively, designing in this way is asking for a very large redistribution in the support section. This means that, even under dead ci load, the support steel will yield if the concrete cracks, and it cannot therefore act effectively as anti-crack reinforcement. It may well be that cracks in the top surface of slabs over the supports are often not serious. the cracks being covered by floor finishes N or partitions. The engineer should nevertheless be aware that this.method of design does

have risks of serious cracking associated with it.

The necessity of providing internal ties in accordance with 3.12.3 may on occasions LI influence the application of these detailing rules.

Where waffle slabs have been designed as flat slabs. situations will arise where particular spacings of ribs will lead to those of the middle strip requiring more reinforcement than -those in the column strip for positive moments in the span (Table 3.21). In such ii circumstances the reinforcement required for the total moment should be spread evenlyacross the middle and column strips.

0 3.6.6.1 Curtailment of bars

13 3.6.6.2 Reinforcement in topping for ribbed or hollow block slabs 3.6.6.3 Links in ribs

See comment on 3.6.1.3.

U links to help control torsional cracking.The ribs along the external edges of waffle slabs should be provided with nominal

U 3.~ Flat slabs

3.7.1 General

The design of flat slabs by the empirical method given in CP11O was generally less conservative than that of more rigorous elastic methods. Research in recent years has

51

Handbook toBS8IJO:J985

shown that pattern loading for the rigorous methods is overconservative. For this reason

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the simplified single load case (3.5.2.3) has been introduced. In general. designs now carried out in accordance with the simplified rules will be more conservative than those made using equivalent frame or grillage methods.

New shear clauses have been introduced as a result of research by Regan~3•’9 and Long et alt30~. These clauses are considered internationally to provide the closest relationship with the test results of all the existing concrete codes.

3.7.1.1 Symbols 3.7.1.2 Design

In order to satisfy the serviceability criteria, elastic methods of analysis are likely to control the design of flat slabs. The use of computers enables equivalent frame and grillage methods to become increasingly popular for modelling flat slabs. Grillage methods can provide reasonable estimates of deflection provided care is taken in selecting section properties and due account is taken of the effects of cracking and creep. Detailed methods are given by Whittle~3181.

The distribution of moments across the width of slab for negative moment alters with

F

respect to the aspect ratio of the panel. Figure H3.24. taken from Regan43’91 shows the relationship.

F

-80

80 —

[

PERcENTAGE OF 50 LONG-SPAN

NEGATIvE

MOMENT —

[

20 —

10

r..

0.

L

1.0 15 20

ASPECT RATIO

Figure H3.24.- Distribution of long-span negative moment in internal panel of flat slab.

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3.7.1.3 Column head

3.7.1.4 Effective diameter of a column or column head (

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3.7.1.5 Drops

When checking the shear resistance. two critical perimeters should be considered. One

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LSdd from the face of the column and the otherI.Sd. from the outer edge of the drop (dd=effective depth of drop and d5=effective depth of slab).

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3.7.1.6 Thickness ofpanels

3.7.2 Analysis of flat slab structures

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Analysis of flat slabs is normally carried out by using one of three methods: yield line.

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Parr1: Section S equivalent frame, or grillage analogy. Yield line methods whilst providing the most economic solution do not provide information concerning the most suitable arrangement of reinforcement for working load conditions with consequent implications for cracking and deflection.

Elastic methods are more likely to predict the behaviour under working load conditions and they can be extended to an analysis at the ultimate limit state. The equivalent frame approach provides a reasonable representation of the behaviour of the floor by a system of columns and beams analysed separately in each span direction.

One misconception held by some engineers is to considera reduced load when analysing the slab in one direction. A flat slab supported on columns, other than perimeter beams.

can fail as a one-way mechanism just as a single-way slab, and it should be reinforced to resist the moment from the full load in each orthogonal direction.

The use of computers for the analysis of flat slabs is becoming more common and grillage programs are now often used to solve routine design problems. These can give reasonable predictions of deflection provided care is taken in selecting section properties and due account is taken of the effects of cracking and creep. Detailed methods are given by Whittle~~181.

3.7.2.1 General

The justification for a single load case of maximum design load on all spans or panels is given by Beebv~310~. Although it is a reasonable assumption for normal occupancy it cannot be rigorously proved. It is not considered valid for structures designed for storage where pattern loading may be a real possibility.

3.7.2.2 Analysis

It should be realised that although the equivalent frame method of analysis provides a reasonable set of moments and shear forces it will normally over-estimate the moments at the edge columns. The lateral distribution of moments at edges is normally very restricted as described in 3.7.4.2.

3.7.2.3 Division offlat slab structures into frames

The division into longitudinal and transverse frames gives design moments in two directions at right-angles. These moments must be provided for in full, as otherwise equilibrium will not be satisfied. The loads in the columns can be assessed from either the longitudinal or the transverse frames and the assumptions of simple support as defined in 3.8.2.3 may be used if desired for internal columns where appropriate.

Where drops are used account should be taken of them in determining the section properties of the slab if they project more than0.151 intothe span.

3.7.2.4 Frame analysis methods

The frame method gives satisfactory results for most orthogonal grids. However whereas this method is suitable for analysis at the ultimate limit state it does not provide accurate predictions of deflections at service loads.

3.7.2.5 Frame stiffness

3.7.2.6 Limitation of negative design moments

This clause provides a check to ensure that static equilibrium is obtained. The value of h~for the purposes of this clause should not exceed 1.5 times the size of the shorter side of a rectangular column.

3.7.2.7 Simplified method for determining moments

The coefficients given in Table 3.19 have been prepared taking due account of the necessary 20% downward redistribution of moments required under the single load case.

The value of the effective shear force (3.7.6) when using these coefficients may be determined from the simplified factors 1.15 for internal columns. 1.25 for corner columns and edge columns bent about an axis parallel to the free edge and 1.4 for edge columns bent about an axis perpendicular to the free edge.

Handbookto BS8IIO.-1985

3.7.2.8 Division of panels (except in the region of edge and corner columns)

The definition of column and middle strips for rectangular panels has been altered with respect toCPI10. This is as a result of work carried out by Regan’3 ~^- For aspect ratios greater than 2 the centre section of the longer span tends to span one way and should be reinforced with nominal steel only in the direction of the short span. The lateral distribution of moments is discussed in more detail by Regan~3’9~ and ‘vVhittle13’~~.

3.7.2,9 Column strips between unlike panels

3.7.2.10 Division of moments between column and middle strips

It should be noted that Table 3.21 does not give a suitable division of moments at edge columns. These areas require special attention as given in 3.7.4.2.

3.7.3 Design of internal panels 3.7.3.1 Column and middle strips

A distinction should be made between the design of flat slabs with flat soffits and those with drops (or waffle slabs with solid areas around columns). The presence of the drop or solid area causes stress concentration at the outer corners. This affects the way in which the top reinforcement should be distributed to resist the negative moment in the column strip. The concentration of reinforcement provided by this clause should-not apply in such situations and the reinforcement should be placed evenly across the column strip. Care should be taken to ensure that the top reinforcement extends over the corner of the drop or solid area to control cracking in this area.

3.7.3.2 Curtailment of bars 3.7.4 Design of edge panels

3.7.4.1 Positive design moments in span and negative design moments over interior edges 3.7.4.2 Design moments transferable between slab and edge or corner columns

Equation 24 gives a simplified formula providing a reasonable basis to assess the maximum moment of resistance of an edge joint. The reasons for restricting the effective width

are given by Regant3^{19), -}

F

The value of be for a circular column may be taken as that for a square enclosing the circle.

If the equivalent frame method is used for determining the edge transfer moment this clause allows up to 50% redistribution. The reason for this is that this method is known to give higher moments of transfer than actually take place. Long discusses this in ‘—

detail~320~.

Where the simple coefficients given in Table 3.19 or the single load case in conjunction

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with the equivalent frame method of analysis are used. the moments and shear forces I may be redistributed a further 30%. This is equivalent to approximately 50%

redistribution of the elastic values from these methods.

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In circumstances where. in spite of redistribution of the elastic moments to the limits given, they still exceed M~max consideration should be given to altering the structural configuration. Otherwise flexural cracking in the slab close to the edge columns could

become so excessive that it affects the shear capacity.

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3.7.4.3 Limitation of moment transfer

Although the limitation on moment transfer is severe it is unlikely that much can be

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gained by torsion reinforcement in slabs with a depth less than 300mm. ~~g~fl(3i9~and U Whittle~318~ discuss this in more detail.

3.7.4.4 Negative moments at free edge

It should be noted that for normal situations the total transter moment (slab/cOlumn) ^IS

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.

^{.. . —}

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—.-...-. .

Paul SeLtun3 resisted by edSe reinforcement in a narrow band (3.7.4.2). The remaining ed2e of the slab should be reinforced with nominal steel as described in this clause.

3.7.4.5 Panels with marginal beams or walls 3.7.5 Openings in panels

In document BS8110 structure use of concrete (Page 47-51)