Walls and Columns Subjected to Vertical Loads · NBC 2016 (SP 7:2016) Volume 1

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.

$0.14\mathrm{N} / \mathrm{mm}^2$ 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

5.5.1 Walls and Columns Subjected to Vertical Loads

Walls and columns bearing vertical loads shall be designed on the basis of permissible compressive stress. Design consists in determining thickness in case of walls and section in case of columns in relation to strength of masonry units and grade of mortar to be used, taking into consideration various factors, such as slenderness ratio, eccentricity, area of section, workmanship, quality of supervision, etc, subject further to provisions of 5.5.1.1 to 5.5.1.4.

5.5.1.1 Solid walls

Thickness used for design calculation shall be the actual thickness of masonry computed as the sum of the average dimensions of the masonry units specified in the relevant standard, together with the specified joint thickness. In masonry with raked joints, thickness shall be reduced by the depth of raking of joints for plastering/pointing.

5.5.1.2 Cavity walls

a) Thickness of each leaf of a cavity wall shall not be less than $75\mathrm{mm}$ .

b) Where the outer leaf is half masonry unit in thickness, the uninterrupted height and length of this leaf shall be limited so as to avoid undue loosening of ties due to differential movements

between the two leaves. The outer leaf shall, therefore, be supported at least at every third storey or at every 10 m of height, whichever is less and at every 10 m or less along the length.

c) Where the load is carried by both leaves of a wall of a cavity construction, the permissible stress shall be based on the slenderness ratio derived from the effective thickness of the wall as given in 4.5.4 or 4.5.5. The eccentricity of the load shall be considered with respect to the centre of gravity of the cross-section of the wall.

d) Where the load is carried by one leaf only, the permissible stress shall be the greater of values calculated by the following two alternative methods:

The slenderness ratio is based on the effective thickness of the cavity wall as a whole as given in 4.5.4 or 4.5.5 and on the eccentricity of the load with respect to the centre of gravity of the cross-section of the whole wall (both leaves) (This is the same method as where the load is carried by both the leaves but the eccentricity will be more when the load is carried by one leaf only).
The slenderness ratio is based on the effective thickness of the loaded leaf only using 4.5.1 and 4.5.2, and the eccentricity of the load will also be with respect to the centre of gravity of the loaded leaf only.

In either alternative, only the actual thickness of the load bearing leaf shall be used in arriving at the cross-sectional area resisting the load (see 5.5.1.1).

5.5.1.3 Faced wall

The permissible load per length of wall shall be taken as the product of the total thickness of the wall and the permissible stress in the weaker of the two materials. The permissible stress shall be found by using the total thickness of the wall when calculating the slenderness ratio.

5.5.1.4 Veneered wall

The facing (veneer) shall be entirely ignored in calculations of strength and stability. For the purpose of determining the permissible stress in the backing, the slenderness ratio shall be based on the thickness of the backing alone.

5.5.2 Walls and Columns Mainly Subjected to Lateral Loads

5.5.2.1 Free standing walls

a) Free standing walls, subjected to wind pressure or seismic forces shall be designed on the basis of permissible tensile stress in masonry or stability as in 4.2.2.4. However, in seismic Zones II, free-standing walls may be apportioned without making any design calculations with the help of Table 12 provided the mortar used is of grade not leaner than M1.

b) If there is a horizontal damp-proof course near the base of the wall, that is, not capable of developing tension vertically, the minimum wall thickness should be the greater of that calculated from either,

the appropriate height to thickness ratio given in Table 12 reduced by 25 percent, reckoning the height from the level of the damp-proof course; or
the appropriate height to thickness ratio given in Table 12 reckoning the height from the lower level at which the wall is restrained laterally.

Table 12Height to Thickness Ratio of Free Standing Walls Related to Wind Speed [Clause 5.5.2.1(a) and (b)]

Sl No.	Design Wind Pressure N/mm2	Height to Thickness Ratio
(1)	(2)	(3)
i)	Up to 285	10
ii)	575	7
iii)	860	5
iv)	1 150	4

NOTES

1 For intermediate values, linear interpolation is permissible. 2 Height is to be reckoned from $150\mathrm{mm}$ below ground level or top of footing/foundation block, whichever is higher, and up to the top edge of the wall.

3 The thickness should be measured including the thickness of the plaster.

5.5.2.2 Retaining walls

Normally masonry of retaining walls shall be designed on the basis of zero-tension, and permissible compressive stress. However, in case of retaining walls for supporting horizontal thrust from dry materials, retaining walls may be designed on the basis of permissible tensile stress at the discretion of the designers.

5.5.3 Walls and Columns Subjected to Vertical as well as Lateral Loads

For walls and columns, stress worked out separately

PART 6 STRUCTURAL DESIGN — SECTION 4 MASONRY

for vertical loads as in 5.5.1 and lateral loads as in 5.5.2 shall be combined and elements designed on the basis of permissible stress.

5.5.4 Walls Subjected to In-Plane Bending and Vertical Loads (Shear Walls)

Walls subjected to in-plane bending and vertical loads, that is, shear walls shall be designed on the basis of no tension with permissible shear stress and permissible compressive stress.

5.5.5 Non-Load Bearing Walls

Non-load bearing walls, such as panel walls, curtain walls and partition walls which are mainly subjected to lateral loads, according to present state of art, are not capable of precise design and only approximate methods based on some tests are available. Guidelines for approximate design of these walls are given in Annex D.

Chapter 5Structural Design

5.1 General

5.2Design Loads

5.3Load Dispersion

5.3.1 General

5.3.2 Arching Action

5.3.3 Lintels

5.4Permissible Stresses