Chapter 5Structural Design

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 715

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5.1 General

The building as a whole shall be analysed by accepted principles of mechanics to ensure safe and proper functioning in service of its component parts in relation to the whole building. All component parts of the structure shall be capable of sustaining the most adverse combinations of loads, which the building may be reasonably expected to be subjected to during and after construction.

5.2Design Loads

Loads to be taken into consideration for designing masonry components of a structure are:

a) dead loads of walls, columns, floors and roofs;

b) imposed loads of floors and roof;

c) wind loads on walls and sloping roof; and

d) seismic forces.

NOTE — When a building is subjected to other loads, such as vibration from railways; machinery, etc, these should be taken into consideration accordingly to the best judgment of the

designer (see also Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code).

5.2.1 The design loads and other forces to be taken for the design of masonry structures shall conform to Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code.

NOTE — During construction, suitable measures shall be taken to ensure that masonry is not liable to damage or failure due to action of wind forces, back filling behind walls or temporary construction loads.

5.3Load Dispersion

5.3.1 General

The angle of dispersion of vertical load on walls shall be taken as not more than $30^{\circ}$ from the vertical.

5.3.2 Arching Action

Account may also be taken of the arching action of well-bonded masonry walls supported on lintels and beams, in accordance with established practice. Increased axial stresses in the masonry associated with arching action in this way, shall not exceed the permissible stresses given in 5.4.

5.3.3 Lintels

Lintels that support masonry construction shall be designed to carry loads from masonry (allowing for arching and dispersion where applicable), and loads received from any other part of the structure. Length of bearing of lintel at each end shall not be less than 90 mm or one-tenth of the span, whichever is more, and area of the bearing shall be sufficient to ensure that stresses in the masonry (combination of wall stresses, stresses due to arching action and bearing stresses from the lintel) do not exceed the stresses permitted in 5.4 (see Annex C).

5.4Permissible Stresses

5.4.1 Permissible Compressive Stress

Permissible compressive stress in masonry shall be based on value of basic compressive stress ( $f_{b}$ ) as given in Table 9 and multiplying this value by factors known as stress reduction factor ( $k_{s}$ ), area reduction factor ( $k_{a}$ ) and shape modification factor ( $k_{p}$ ) as detailed in 5.4.1.1 to 5.4.1.3. Values of basic compressive stress given in Table 9 take into consideration crushing strength of masonry unit and grades of mortar, and hold good for values of slenderness ratio not exceeding 6, zero eccentricity and masonry unit having height to width ratio (as laid) equal to 0.75 or less.

Alternatively, basic compressive stress may be based on results of prism test given in Annex B on masonry made from masonry units and mortar to be actually used in a particular job.

NOTE — Linear interpolation between values is permissible.

Table 9 Basic Compressive Stresses for Masonry (After 28 days)(Clauses 5.4.1 and 6.3.1)
Sl No.	Mortar Type (Ref Table 1)	Basic Compressive Stresses, in N/mm $^{2}$ , Corresponding to Masonry Units of which Height to Width Ratio does not Exceed 0.75 and Crushing Strength, in N/mm $^{2}$ , is not Less than
Sl No.	Mortar Type (Ref Table 1)	3.5(3)	5.0(4)	7.5(5)	10(6)	12.5(7)	15(8)	17.5(9)	20(10)	25(11)	30(12)	35(13)	40(14)
i)	H1	0.35	0.50	0.75	1.00	1.16	1.31	1.45	1.59	1.91	2.21	2.5	3.05
ii)	H2	0.35	0.50	0.74	0.96	1.09	1.19	1.30	1.41	1.62	1.85	2.1	2.5
iii)	M1	0.35	0.50	0.74	0.96	1.06	1.13	1.20	1.27	1.47	1.69

5.4.1.1 Stress reduction factor

This factor, as given in Table 10, takes into consideration the slenderness ratio of the element and also the eccentricity of loading.

Table 10Stress Reduction Factor for Slenderness Ratio and Eccentricity (Clause 5.4.1.1)

Sl No.	Slenderness Ratio	Eccentricity of Loading Divided by the Thickness of the Member
Sl No.	Slenderness Ratio	0(3)	1/24(4)	1/12(5)	1/6(6)	1/4(7)	1/3(8)
i)	6	1.00	1.00	1.00	1.00	1.00	1.00
ii)	8	0.95	0.95	0.94	0.93	0.92	0.91
iii)	10	0.89	0.88	0.87	0.85	0.83	0.81
iv)	12	0.84	0.83	0.81	0.78	0.75	0.72
v)	14	0.78	0.76	0.74	0.70	0.66	0.66
vi)	16	0.73	0.71	0.68	0.63	0.58	0.53
vii)	18	0.67	0.64	0.61	0.55	0.49	0.43
viii)	20	0.62	0.59	0.55	0.48	0.41	0.34
ix)	22	0.56	0.52	0.48	0.40	0.32	0.24
x)	24	0.51	0.47	0.42	0.33	0.24	—
xi)	26	0.45	0.40	0.35	0.25	—	—
xii)	27	0.43	0.38	0.33	0.22	—	—
NOTES1 Linear interpolation between values is permitted.2 Where in special cases the eccentricity of loading lies between 1/3 and 1/2 of the thickness of the member, the stress reduction factor should vary linearly between unity and 0.20 for slenderness ratio of 6 and 20, respectively.3 Slenderness ratio of a member for sections within 1/8 of the height of the member above or below a lateral support may be taken to be 6.

5.4.1.2 Area reduction factor

This factor takes into consideration smallness of the sectional area of the element and is applicable when sectional area of the element is less than $0.2 \, m^{2}$ . The factor $k_{a} = 0.7 + 1.5A$ , A being the area of section, in $m^{2}$ .

5.4.1.3 Shape modification factor

This factor takes into consideration the shape of the unit, that is, height to width ratio (as laid) and is given in Table 11. This factor is applicable for units for crushing strength up to $15 \, N/mm^{2}$ .

Table 11Shape Modification Factor for Masonry Units (Clause 5.4.1.3)

Sl No.	Height to Width Ratio of Units (As Laid)	Shape Modification Factor ( $k_p$ ) for Units Having Crushing Strength N/mm2
Sl No.	Height to Width Ratio of Units (As Laid)	Up to 5.0 (3)	7.5 (4)	10.0 (5)	15.0 (6)
i)	Up to 0.75	1.0	1.0	1.0

5.4.1.5 Increase in permissible compressive stress for walls subjected to concentrated loads

When a wall is subjected to a concentrated load (a load being taken to be concentrated when area of supporting wall equals or exceeds three times the bearing area), certain increase in permissible compressive stress may be allowed because of dispersal of the load. Since, according to the present state of art, there is diversity of views in regard to manner and extent of dispersal, design of walls subjected to concentrated loads may, therefore, be worked out as per the best judgment of the designer. Some guidelines in this regard are given in Annex C.

5.4.2 Permissible Tensile Stress

As a general rule, design of masonry shall be based on the assumption that masonry is not capable of taking any tension. However, in case of lateral loads normal to the plane of wall, which causes flexural tensile stress, as for example, panel, curtain partition and free standing walls, flexural tensile stresses as follows may be permitted in the design for masonry:

a) Grade M1 1) $0.07\mathrm{N} / \mathrm{mm}^2$ for bending in the vertical direction where tension developed is normal to bed joints.

1. $0.14\mathrm{N} / \mathrm{mm}^2$ for bending in the longitudinal direction where tension developed is parallel to bed joints, provided crushing strength of masonry units is not less than .

5.4.3 Permissible Shear Stress

In case of walls built in mortar not leaner than Grade M1 (see Table 1) and resisting horizontal forces in the plane of the wall, permissible shear stress calculated on the area of bed joints, shall not exceed the value obtained by the formula given below, subject to a maximum of $0.5 \, N/mm^{2}$ :

f _ {\mathrm{s}} = 0. 1 + f _ {\mathrm{d}} / 6

m2

5.4.1.4 Increase in permissible compressive stresses allowed for eccentric vertical loads, lateral loads under certain conditions

PART 6 STRUCTURAL DESIGN — SECTION 4 MASONRY

In members subjected to eccentric and/or lateral loads, increase in permissible compressive stress is allowed as follows:

a) When resultant eccentricity ratio exceeds 1/24 but does not exceed 1/6, 25 percent increase in permissible compressive stress is allowed in design.

b) When resultant eccentricity ratio exceeds 1/6, 25 percent increase in permissible stress is allowed but the area of the section under tension shall be disregarded for computing the load carrying capacity of the member.

NOTE — When resultant eccentricity ratio of loading is 1/24 or less, compressive stress due to bending shall be ignored and only axial stress need to be computed for the purpose of design.

10 N / mm^{2}

b) Grade M2 1) 0.05 N/mm² for bending in the vertical direction where tension developed is normal to bed joints.

0.10 N/mm² for bending in the longitudinal direction where tension developed is parallel to bed joints, provided crushing strength of masonry units is not less than 7.5 N/mm².

1 No tensile stress is permitted in masonry in case of water-retaining structures in view of water in contact with masonry. Also no tensile stress is permitted in earth-retaining structures, in view of the possibility of presence of water at the back of such walls.

2 Allowable tensile stress in bending in the vertical direction may be increased to $0.1\mathrm{N} / \mathrm{mm}^2$ for M1 mortar and $0.07\mathrm{N} / \mathrm{mm}^2$ for M2 mortar in case of boundary walls/compound at the discretion of the designer, since there is not much risk to life and property in the event of failure of such walls.

+

fd/6

$f_{d}$ = compressive stress due to dead loads, in N/mm $^{2}$ ; and

$f_{\mathrm{s}} =$ permissible shear stress, in $\mathrm{N / mm^2}$

5.4.4 If there is tension in any part of a section of masonry, the area under tension shall be ignored while working out shear stress on the section.