Annex B(Clauses 17.2.2, 21.3.1, 21.7, 25.2.1.1 and 31.1) STRUCTURAL DESIGN (WORKING STRESS METHOD) B-1 General

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 867

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B-1.1 General Design Requirements

The general design requirements of Section 5A (C) shall apply to this annex.

B-1.2 Redistribution of Moments

Except where the simplified analysis using coefficients (see 21.5) is used, the moments over the supports for any assumed arrangement of loading, including the dead load moments may each be increased or decreased by not more than 15 percent, provided that these modified moments over the supports are used for the calculation of the corresponding moments in the spans.

B-1.3 Assumptions for Design of Members

In the methods based on elastic theory, the following assumptions shall be made:

a) At any cross-section, plane sections before bending remain plain after bending.

b) All tensile stresses are taken up by reinforcement and none by concrete, except as otherwise specifically permitted.

c) The stress-strain relationship of steel and concrete, under working loads, is a straight line.

d) The modular ratio m has the value $\frac{280}{3\sigma_{cbc}}$

where $\sigma_{\mathrm{cbc}}$ is permissible compressive stress due to bending in concrete, in $\mathrm{N / mm^2}$ , as specified in Table 25.

NOTE — The expression given for m partially takes into account long-term effects such as creep. Therefore this m is not the same as the modular ratio derived based on the value of $E_{c}$ given in 5.2.3.1.

B-2 Permissible Stresses

B-2.1 Permissible Stresses in Concrete

Permissible stresses for the various grades of concrete shall be taken as those given in Tables 25 and 27.

NOTE — For increase in strength with age, 5.2.1 shall be applicable. The values of permissible stress shall be obtained by interpolation between the grades of concrete.

B-2.1.1 Direct Tension

For members in direct tension, when full tension is taken

by the reinforcement alone, the tensile stress shall be not greater than the values given below:

Grade of Concrete	M10	M15	M20	M25	M30	M35	M40	M45	M50 and Above
Tensile Stress N/mm $^{2}$	1.2	2.0	2.8	3.2	3.6	4.0	4.4	4.8	5.2

The tensile stress shall be calculated as:

\frac {F _ {\mathrm{t}}}{A _ {\mathrm{e}} + m A _ {\mathrm{st}}}

where

$F_{\mathrm{t}} =$ total tension on the member minus pretension in steel, if any, before concreting;

$A_{e}$ = cross-sectional area of concrete excluding any finishing material and reinforcing steel;

$m =$ modular ratio; and

$A_{st}$ = cross-sectional area of reinforcing steel in tension.

B-2.1.2 Bond Stress for Deformed Bars

In the case of deformed bars conforming to accepted standard [6-5A(36)], the bond stresses given in Table 25 may be increased by 60 percent.

B-2.2 Permissible Stresses in Steel Reinforcement

Permissible stresses in steel reinforcement shall not exceed the values specified in Table 26.

B-2.2.1 In flexural member the value of $\sigma_{st}$ given in Table 26 is applicable at the centroid of the tensile reinforcement subject to the condition that when more than one layer of tensile reinforcement is provided, the stress at the centroid of the outermost layer shall not exceed by more than 10 percent the value given in Table 26.

B-2.3 Increase in Permissible Stresses

Where stresses due to wind (or earthquake), temperature and shrinkage effects are combined with those due to dead and imposed load, the stresses specified in Tables 25, 26 and 27 may be exceeded up to a limit of $33\frac{1}{3}$ percent. Wind and seismic forces need not be considered as acting simultaneously.

Table 25Permissible Stresses in Concrete (Clauses B-1.3, B-2.1, B-2.1.2, B-2.3 and B-4.2) All values in N/mm².

Sl No.	Grade of Concrete	Permissible Stress in Compression		Permissible Stress in Bond (Average) for Plain Bars in Tension
Sl No.	Grade of Concrete	Bending $\sigma_{\text{cbc}}$ (3)	Direct $\sigma_{\text{cc}}$ (4)	$\tau_{\text{bd}}$ (5)
(1)	(2)	Bending $\sigma_{\text{cbc}}$ (3)	Direct $\sigma_{\text{cc}}$ (4)	$\tau_{\text{bd}}$ (5)
i)	M 10	3.0	2.5	—
ii)	M 15	5.0	4.0	0.6
iii)	M 20	7.0	5.0	0.8
iv)	M 25	8.5	6.0	0.9
v)	M 30	10.0	8.0	1.0
vi)	M 35	11.5	9.0	1.1
vii)	M 40	13.0	10.0	1.2
viii)	M 45	14.5	11.0	1.3
ix)	M 50	16.0	12.0	1.4
x)	M 55	18.0	13.5	1.5
xi)	M 60	20.0	15.0	1.6
NOTES
1 The values of permissible shear stress in concrete are given in Table 27.
2 The bond stress given in col 4 shall be increased by 25 percent for bars in compression.

B-3 Permissible Loads in Compression Members

B-3.1 Pedestals and Short Columns with Lateral Ties

The axial load $P$ permissible on a pedestal or short column reinforced with longitudinal bars and lateral ties shall not exceed that given by the following equation:

P = \sigma_ {\mathrm{cc}} A _ {\mathrm{c}} + \sigma_ {\mathrm{sc}} A _ {\mathrm{sc}}

where

$\sigma_{cc} =$ permissible stress in concrete in direct compression,

$A_{c}$ = cross-sectional area of concrete excluding any finishing material and reinforcing steel,

$A_{\mathrm{sc}} =$ cross-sectional area of the longitudinal steel.

NOTE — The minimum eccentricity mentioned in 24.4 may be deemed to be incorporated in the above equation.

B-3.2 Short Columns with Helical Reinforcement

The permissible load for columns with helical reinforcement satisfying the requirement of 38.4.1 shall be 1.05 times the permissible load for similar member with lateral ties or rings.

Table 26Permissible Stresses in Steel Reinforcement (Clauses B-2.2, B-2.2.1, B-2.3 and B-4.2)

Sl No.	Type of Stress in Steel Reinforcement	Permissible Stresses N/mm2
Sl No.	Type of Stress in Steel Reinforcement	Mild Steel Bars Conforming to Grade 1 of Accepted Standard [6-5A(42)]	Medium Tensile Steel Conforming to Accepted Standard [6-5A(42)]	High Yield Strength Deformed Bars Conforming to Accepted Standard [6-5A(36)] (Grade Fe 415)
(1)	(2)	(3)	(4)	(5)
i)	Tension ( $\sigma_{st}$ or $\sigma_{sv}$ )
	a) Up to and including 20 mm	140	Half the guaranteed yield	230
	b) Over 20 mm	130	stress subject to a maximum of 190	230
ii)	Compression in column bars ( $\sigma_{sc}$ )	130	130	190
iii)	Compression in bars in a beam or slab when the compressive resistance of the concrete is taken into account	The calculated compressive stress in the surrounding concrete multiplied by 1.5 times the modular ratio or $\sigma_{sc}$ whichever is lower
iv)	Compression in bars in a beam or slab where the compressive resistance of the concrete is not taken into account:
	a) Up to and including 20 mm	140	Half the guaranteed yield	190
	b) Over 20 mm	130	stress subject to a maximum of 190	190
NOTES
1 For high yield strength deformed bars of Grade Fe 500 the permissible stress in direct tension and flexural tension shall be 0.55 $f_y$ . The permissible stresses for shear and compression reinforcement shall be as for Grade Fe 415.
2 For welded wire fabric conforming to accepted standard [6-5A(43)], the permissible value in tension $\acute{o}_{st}$ is 230 N/mm2.
3 For the purpose of this standard, the yield stress of steels for which there is no clearly defined yield point should be taken to be 0.2 percent proof stress.
4 When mild steel conforming to Grade II of accepted standard [6-5A(42)] is used, the permissible stresses shall be 90 percent of the permissible stresses in col 3, or if the design details have already been worked out on the basis of mild steel conforming to Grade 1 of accepted standard [6-5A(42)]; the area of reinforcement shall be increased by 10 percent of that required for Grade 1 steel.

PART 6 STRUCTURAL DESIGN — SECTION 5 CONCRETE: 5A PLAIN AND REINFORCED CONCRETE