Walls · NBC 2016 (SP 7:2016) Volume 1

31.1 General

Reinforced concrete walls subjected to direct compression or combined flexure and direct compression should be designed in accordance with Section 5 or Annex B provided the vertical reinforcement is provided in each face. Braced walls subjected to only vertical compression may be designed as per empirical procedure given in 31.2. The minimum thickness of walls shall be 100 mm.

31.1.1 Guidelines or design of walls subjected to horizontal and vertical loads are given in 31.3.

31.2 Empirical Design Method for Walls Subjected to In-Plane Vertical Loads

31.2.1 Braced Walls

Walls shall be assumed to be braced if they are laterally supported by a structure in which all the following apply:

a) Walls or vertical braced elements are arranged in two directions so as to provide lateral stability to the structure as a whole.

b) Lateral forces are resisted by shear in the planes of these walls or by braced elements.

c) Floor and roof systems are designed to transfer lateral forces.

d) Connections between the wall and the lateral supports are designed to resist a horizontal force not less than,

the simple static reactions to the total applied horizontal forces at the level of lateral support; and
2.5 percent of the total vertical load that the wall is designed to carry at the level of lateral support.

31.2.2 Eccentricity of Vertical Load

The design of a wall shall take account of the actual eccentricity of the vertical force subject to a minimum value of $0.05t$ .

The vertical load transmitted to a wall by a discontinuous concrete floor or roof shall be assumed to act at one-third the depth of the bearing area measured from the span face of the wall. Where there is an in-situ concrete floor continuous over the wall, the load shall be assumed to act at the centre of the wall.

The resultant eccentricity of the total vertical load on a braced wall at any level between horizontal lateral supports, shall be calculated on the assumption that the resultant eccentricity of all the vertical loads above the upper support is zero.

31.2.3 Maximum Effective Height to Thickness Ratio

The ratio of effective height to thickness, $H_{\mathrm{we}} / t$ shall not exceed 30.

31.2.4 Effective Height

The effective height of a braced wall shall be taken as follows:

a) Where restrained against rotation at both ends by:

floors : $0.75H_{\mathrm{w}}$ or
intersecting walls or : 0.75 $L_{1}$ similar members, whichever is the lesser.

b) Where not restrained against rotation at both ends by:

floors : 1.0 $H_{\mathrm{w}}$ or

31.2.5 Design Axial Strength of Wall

The design axial strength $P_{\mathrm{uw}}$ per unit length of a braced wall in compression may be calculated from the following equation:

P _ {\mathrm{uw}} = 0. 3 \left(t - 1. 2 e - 2 e _ {\mathrm{a}}\right) f _ {\mathrm{ck}}

31.3 Walls Subjected to Combined Horizontal and Vertical Forces

31.3.1 When horizontal forces are in the plane of the wall, it may be designed for vertical forces in accordance with 31.2 and for horizontal shear in accordance with 31.4. In plane bending may be neglected in case a horizontal cross-section of the wall is always under compression due to combined effect

of horizontal and vertical loads.

31.3.2 Walls subjected to horizontal forces perpendicular to the wall and for which the design axial load does not exceed $0.04f_{ck}A_{g}$ , shall be designed as slabs in accordance with the appropriate provisions under 23, where $A_{g}$ is gross area of the section.

31.4 Design for Horizontal Shear

31.4.1 Critical Section for Shear

The critical section for maximum shear shall be taken at a distance from the base of $0.5L_{\mathrm{w}}$ or $0.5H_{\mathrm{w}}$ , whichever is less.

31.4.2 Nominal Shear Stress

The nominal shear stress $\tau_{\mathrm{vw}}$ in walls shall be obtained as follows:

31.5 Minimum Requirements for Reinforcement in Walls

The reinforcement for walls shall be provided as below:

a) The minimum ratio of vertical reinforcement to gross concrete area shall be:

0.0012 for deformed bars not larger than 16 mm in diameter and with a characteristic strength of 415 N/mm² or greater.
0.0015 for other types of bars.
0.0012 for welded wire fabric not larger than $16\mathrm{mm}$ in diameter.

b) Vertical reinforcement shall be spaced not farther apart than three times the wall thickness nor 450 mm.

c) The minimum ratio of horizontal reinforcement to gross concrete area shall be:

0.002 0 for deformed bars not larger than 16 mm in diameter and with a characteristic strength of 415 N/mm² or greater.
0.0025 for other types of bars.
0.002 0 for welded wire fabric not larger than 16 mm in diameter.

d) Horizontal reinforcement shall be spaced not farther apart than three times the wall thickness nor 450 mm.

NOTE — The minimum reinforcement may not always be sufficient to provide adequate resistance to the effects of shrinkage and temperature.

31.5.1 For walls having thickness more than 200 mm, the vertical and horizontal reinforcement shall be provided in two grids, one near each face of the wall.

31.5.2 Vertical reinforcement need not be enclosed by

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

transverse reinforcement as given in 25.5.3.2 for column, if the vertical reinforcement is not greater than 0.01 times the gross sectional area or where the vertical reinforcement is not required for compression.

Chapter 31Walls