Annex C(Clauses 5.3.3 and 5.4.1.5)

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 761

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Guidelines for Design of Masonry Subjected to Concentrated Loads

C-1 Extent of Dispersal of Concentrated Load

For concentric loading, maximum spread of a concentrated load on a wall may be taken to be equal to $b + 4t$ (b is width of bearing and t is thickness of wall), or stretch of wall supporting the load, or centre-to-centre distance between loads, whichever is less.

C-2 Increase in Permissible Stress

C-2.1 When a concentrated load bears on a central strip of wall, not wider than half the thickness of the wall and is concentric, bearing stress in masonry may exceed the permissible compressive stress by 50 percent, provided the area of supporting wall is not less than three times the bearing area.

C-2.2 If the load bears on full thickness of wall and is concentric, 25 percent increase in stress may be allowed.

C-2.3 For loading on central strip wider than half the thickness of the wall but less than full thickness, increase in stress may be worked out by interpolation between values of increase in stresses as given in C-2.1 and C-2.2.

C-2.4 In case concentrated load is from a lintel over an opening, an increase of 50 percent in permissible stress may be taken, provided the supporting area is not less than 3 times the bearing area.

C-3 Criteria of Providing BED Block

C-3.1 If a concentrated load bears on one end of a wall, there is a possibility of masonry in the upper region developing tension. In such a situation, the load should be supported on an RCC bed block (of M20 Grade) capable of taking tension.

C-3.2 When any section of masonry wall is subjected to concentrated as well as uniformly distributed load and resultant stress, computed by making due allowance for increase in stress on account of concentrated load, exceeds the permissible stress in masonry, a concrete bed block (of M20 Grade) should be provided under the load in order to relieve stress in masonry. In concrete, angle of dispersion of concentrated load is taken to be $45^{\circ}$ to the vertical.

C-3.3 In case of cantilevers and long span beams supported on masonry walls, indeterminate but very high edge stresses occur at the supports and in such cases it is necessary to relieve stress on masonry by providing concrete bed block of M20 Grade concrete. Similarly when a wall is subjected to a concentrated load from a beam which is not sensibly rigid (for example, a timber beam or an RS joist), a concrete bed block should be provided below the beam in order to avoid high edge stress in the wall because of excessive deflection of the beam.

ANNEX D

(Clause 5.5.5)

Guidelines for Approximate Design of Non-load Bearing Wall

D-1 Panel Walls

A panel wall may be designed approximately as under, depending upon its support conditions and certain assumptions:

a) When there are narrow tall windows on either side of panel, the panel spans in the vertical direction. Such a panel may be designed for a bending moment of PH/8, where P is the total horizontal load on the panel and H is the height between the centres of supports. Panel wall is assumed to be simply supported in the vertical direction.

b) When there are long horizontal windows between top support and the panel, the top edge of the panel is free. In this case, the panel should be considered to be supported on sides and at the bottom, and the bending moment should depend upon height to length ratio of panel and flexural strength of masonry. Approximate values of bending moments in the horizontal direction for this support condition, when ratio ( $\mu$ ) of flexural strength of wall in the vertical direction to that in horizontal direction is assumed to be 0.5, are given in Table 32.

Table 32Bending Moments in Laterally Loaded Panel Walls, Free at Top Edge and Supported on Other Three Edges

Height of Panel, $H$ Length of Panel, $L$	0.30	0.50	0.75	1.00	1.25	1.50	1.75
Bending Moment

\frac{P . L}{25}

NOTE — For $H / L$ ratio less than 0.30, the panel should be designed as a free-standing wall and for $H / L$ ratio exceeding 1.75, it should be designed as a horizontally spanning member for a bending moment value of $PL / 8$ .

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c) When either there are no window openings or windows are of 'hole-in-wall' type, the panel is considered to be simply supported on all four edges. In this case also, amount of maximum bending moment depends on height to length ratio of panel and ratio ( $\mu$ ) of flexural strength of masonry in vertical direction to that in the horizontal direction. Approximate values for maximum bending moment in the horizontal direction for masonry with $\mu = 0.50$ , are given in Table 33.

Table 33Bending Moments in Laterally Loaded Panel Walls Supported on All Four Edges [Clause D-1(c)]

Height of Panel, $H$ Length of Panel, $L$	0.30	0.50	0.75	1.00	1.25	1.50	1.75
Bending Moment	$\frac{P.L}{72}$	$\frac{P.L}{36}$	$\frac{P.L}{24}$	$\frac{P.L}{18}$	$\frac{P.L}{15}$	$\frac{P.L}{13}$	$\frac{P.L}{12}$

NOTE — When $H / L$ is less than 0.30, value of bending moment in the horizontal direction may be taken as nil and panel wall may be designed for a bending moment value of $PH / 8$ in the vertical direction; when $H / L$ exceeds 1.75, panel may be assumed to be spanning in the horizontal direction and designed for bending moment of $PL / 8$ .

D-2 Curtain Walls

Curtain walls may be designed as panel walls taking into consideration the actual supporting conditions.

D-3 Partition Walls

D-3.1 These are internal walls usually subjected to much smaller lateral forces. Behaviour of such wall is similar to that of panel wall and these could, therefore, be designed on similar lines. However, in view of smaller lateral loads, ordinarily these could be apportioned empirically as follows:

a) Walls with adequate lateral restraint at both ends but not at the top :

The panel may be of any height, provided the length does not exceed 40 times the thickness; or
The panel may be of any length, provided the height does not exceed 15 times the thickness (that is, it may be considered as a free-standing wall); or
Where the length of the panel is over 40 times and less than 60 times the thickness, the height plus twice the length may not exceed 135 times the thickness;

b) Walls with adequate lateral restraint at both ends at the top :

The panel may be of any height, provided the length does not exceed 40 times the thickness; or
The panel may be of any length, provided the height does not exceed 30 times the thickness; or
Where the length of the panel is over 40 times and less than 110 times the thickness,

PART 6 STRUCTURAL DESIGN — SECTION 4 MASONRY

the length plus three times the height should not exceed 200 times the thickness; and

c) When walls have adequate lateral resistant at the top but not at the ends, the panel may be of any length, provided the height does not exceed 30 times the thickness.

D-3.2 Strength of bricks used in partition walls should not be less than $3.5 \, N/mm^{2}$ or the strength of masonry units used in adjoining masonry, whichever is less. Grade of mortar should not be leaner than M2.

D-4 Cantilever Projections

The provisions given under 5 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code shall be applicable. As a general guide, the details of connection between parapets and slabs for existing structures shall be as given in Fig. 57.

NOTES

1 Hole shall be made in bricks in advance to accommodate the reinforcement bars of slab.

2 Bottom bar of slab continued in the parapet at spacing of 600 mm c/c.

FIG. 57 PARAPET WALLS