7. LA FIGURA DE MERLÍN EN OTROS TEXTOS DEL AUTOR
6.1. LA AVENTURA DEL MAR
168 REINFORCED CONCRETE DESIGN
4.5 -Explanatory Handbook on Indian Code of Practice for Plain and Reinforced Concrete (IS Special Publication' Bureau of Indian Standards, New Delhi, 1983.
5.1 INTRODUCTION
In the previous chapter, the behaviour of concrete beams (and one-way slabs) was explained, and procedures given for the analysis of sections. of beam sections mav involve of
. .
stresses under known service load moments, (2) allowable service load moments (working stress method) and (3) ultimate moment of resistance (limit states method). It may b e noted that the results of analysis of a beam section are unique, being dictated solely by the conditions of equilibrium of forces and compatibility of strains. On basis of these computations, it is possible to decide whether or not the beam is 'safe' under known moments.The design problem is somewhat the of the analysis problem. The external loads (or load effects), material properties and the skeletal dimensions of the beam are given, and it is to arrive at suitable cross-sectional dimensions and details of the reinforcing steel, which would give adequate and
In for flexure, the distribution of moments along the length of the
, .
beam be known from structural analysis. For this, the initial cross-sectional dimensions have to be assumed in order to estimate dead loads: this is also reauired for the analysis of indeterminate structures (such as continuous beams). The adequacy of the assumed dimensions should be verified and suitable changes made, if required.
172 REINFORCED CONCRETE DESIGN DESIGN OF BEAMS AND ONE-WAY SLABS FOR FLEXURE 173
longitudinal bars. 40 mm in general) and in CI. 26.4.2.2 for footings (50 in general). These are discussed in Chapters 13 and 14.
In addition, the Code introduced nominal cover requirements, based on fire resistance (in terms of hours) required. These provisions have been apparently borrowed from BS 8110. They are described in CI. 26.4.3 of Code. In general, for a nominal 1 hour resistance, the nominal cover specified is 20 for beams and slabs, and 40 mm for Larger cover is required only if the structural element consideration has to be specially designed for fire
5.2.2 Spacing of Reinforcing Bars
The Code specifies minimum and maximum limits for the spacing between parallel reinforcing bars in a layer. The minimum limits are necessary to ensure the concrete can be placed easily in between and around the bars the of fresh concrete. limits are specified for
of controlling crack-widths and
minimum spacing limits can be met without difficulty in slabs in general,
While fixing the overall size of the beam or the thickness of the slab, it is desirable to use multiples of 5 for slabs and 5 0 mm (or 25 for beams. This will be convenient in the construction of the formwork. The requirements for placement of flexural reinforcement are in 26.3 of the Code.' The salient features of specifications in Fig. 5.1. The student is advised to read the relevant clauses in the Code, while studying Fig. 5.1. The requirements for singly reinforced beams, slabs and doubly reinforced beams are depicted in parts (a), (b) and
of Fig. 5.1.
Stirrups provided in beams serve as transverse shear reinforcement [refer Chapter In singly reinforced beams, they may bc provided as U-shaped stirmps, with two hanger bars at top [Fig. However, it is more common to provide fully closed rectangular stirmps [Rig. for both singly and doubly reinforced sections; this is mandatory in latter for the effective functioning of the compression steel. Stirrups required for resisting torsion also be of the closed form [refer Chapter
stirrup
vertical clear spacing to be not less than
bar 15 aggregateslze
clear be not less than
f a bar bar
cover
in Table 5.1) plus
(a) singly reinforced beam
MAIN not to exceed spacing not to 3dor 300
DISTRIBUTION BARS I
clear cover (nominal cover
spacing not to in Table 5.1)
exceed 450 (b) one-way slab
closed
bars
through
(bundled bars
doubly reinforced beam
(d) 5.1 Code requirements for flexural reinforcement placement
CONCRETE DESIGN
are
bars of larger diameter. For this reason, the Code
limits the maxtmum of , . bars in slabs to one-eighth of the total thickness of the and the maximum spacing of such main to 3d or 300 (whichever is less) [Fig. However, it may he noted large cover is provided, more stringent bar spacing may be required to achieve the desired crack control [Ref.
in relatively deep flexural members, a substantial portion of the web will be in Tension reinforcement properly distributed will, no doubt, control the crack width at its level: however, wider cracks may develop higher up in the Moreover, as explained Section 2.12, cracking can occur in large unreinforced exposed faces of concrete on account of shrinkage and temperature variations. In order to control such cracks, as well as to improve resistance against lateral buckling of the web [Ref. the Code 26.5.1.3) requires side face reinforcenrent to be provided along the two faces of beams with overall depth exceeding 750
5.2.3 Minimum a n d Maximum of Flexural Reinforcement
A minimum area of tension reinforcing steel is required in flexural members not only to resist possible load effects, but also to control cracking in concrete due to shrinkage and temperature
Minimum Flexural Reinforcement in Beams
In the case of beams, the Code 26.5.1.1) prescribes the following:
which gives values equal to 0.340, 0.205 and 0.170 for Fe 250, Fe 415 and Fe 500 grades of steel respectively. In the case of flanged the
width of the web should be considered in lieu of b.
can he shown that the given by Eq. 5.1 results in an ultimate moment of resistance that is approximately equal to the 'cracking moment' of an identical plain
The limits specified here (as per IS 456) are applicable to reinforced concrete flexural in general. However, far earthquakc-resistant ('ductile different limits are applicable; this is described in Chapter 16.
concrete section. Thus, the reinforcement requirement ensures a sudden failure is avoided at M
Mlnlmum Flexural in Slabs
As specified in 26.5.2, the (A,,),,,,, in either direction in slabs is given by
=
where denotes the gross area of the section
In the design of one-way slabs, this is also to be provided for the secondary (or (refer Section 4.8.1) along the direction perpendicular to the main reinforcementt, with the spacing of such not exceeding or 450 (whichever is less) [Fig. It may be noted that in the case of slabs, sudden failure due to overload is less likely owing to better lateral distribution of the load effects. Hence, the minimum steel requirements of slabs are based on considerations of shrinkage and temperature alone, and not on strength. Accordingly, the specified value of is somewhat smaller in the case of slabs, compared to beams. However, for exposure conditions where crack control is of special importance, in of that given by Eq. 5.2 should be provided.
Maximum Flexural Reinforcement Beams
Providing excessive reinforcement' in beams can result in congestion (particularly at junctions), thereby adversely aifecting the placetnent and compaction of concrete. For this the Code 26.5.1) restricts the area of tension reinforcement (A,,) as well as reinforcement (A,) in beams to a maximum value of 0.04 If both A, and A,, are provided at their maximum limits, the total area (A,, o i steel would be equal to 8 percent of gross area of beam section; this is rather It is recommended that such high reinforcement areas should be generally avoided by suitable design measures. These include:
increasing the beam size (especially depth);
improving the of concrete and steel.
that the direction of the secondary reinforcement need not be the as that of the This ease is encountered, in a slab supported an opposite edges, wilh the actual span dimension being larger the transverse dimension.
may be designed in doubly reinforced beam sections and in flanged beam sections, without resulting in sections.
176 REINFORCE0 CONCRETE