I. INTRODUCCIÓN
1.10. Variables
1.10.1. Identificación de variables
The design ultimate axial force in a plain wall may be calculated on the assumption that the beams and slabs transmitting forces into it are simply supported.
3.9.4.2 Effective height of unbraced plain concrete walls
The effective height of unbraced plain concrete walls is given as follows:
a) wall supporting at its top a roof or floor slab spanning at right angles: le = 1.5lo;
Wall condition Reinforcement Maximum value of le/h
Braced As given in 3.12.5 but < 1 % 40 Braced As given in 3.12.5 but U 1 % 45 Unbraced As given in 3.12.5 30
3.9.4.3 Effective height of braced plain walls
The effective height of braced plain walls is given as follows:
a) where any lateral support resists both rotation and lateral movement, le equals three-quarters of the
clear distance6) between lateral supports or twice the distance between a support and a free edge as
appropriate;
b) where any lateral support resists only lateral movement, le equals the distance between centres of
support, or two and a half times the distance between a support and a free edge, as appropriate.
3.9.4.4 Limits of slenderness
The slenderness ratio le/h should not exceed 30 whether the wall is braced or unbraced.
3.9.4.5 Minimum transverse eccentricity of forces
Whatever the arrangements of vertical or horizontal forces, the resultant force in every plain wall should be assumed to have a transverse eccentricity of not less than h/20 or 20 mm. In the case of a slender wall further eccentricity can arise as a result of deflection under load. Procedures allowing for this are given in 3.9.4.16 and 3.9.4.17.
3.9.4.6 In-plane eccentricity due to forces on a single wall
In-plane eccentricity due to forces on a single wall may be calculated by statics alone.
3.9.4.7 In-plane eccentricity due to horizontal forces on two or more parallel walls
Where a horizontal force is resisted by several walls, it should be assumed to be shared between the walls in proportion to their relative stiffnesses provided the resultant eccentricity in any individual wall is not greater than one-third of the length of the wall. Where the eccentricity in any wall is found to be greater than this, the wall’s stiffness should be considered as zero and an adjustment made to the forces assumed carried by the remainder.
3.9.4.8 Panels with shear connections
Where, in a wall, a shear connection is assumed between vertical edges of adjacent panels, an appropriate elastic analysis may be made provided the shear connection is designed to resist the design ultimate forces.
3.9.4.9 Eccentricity of loads from concrete floor or roof
The design loads may be assumed to act at one-third the depth of the bearing area from the loaded face. Where there is an in-situ concrete floor on either side of the wall, the common bearing area may be assumed to be shared equally on each floor.
3.9.4.10 Other eccentricity-applied loads
It should be noted that loads may be applied to walls at eccentricities greater than half the thickness of the wall through special fittings (e.g. joist hangers).
3.9.4.11 In-plane and transverse eccentricity of resultant force on an unbraced wall
At any level full allowance should be made for the eccentricity of all vertical loads and the overturning moments produced by any lateral forces above that level.
3.9.4.12 Transverse eccentricity of resultant force on a braced wall
At any level the transverse eccentricity with respect to the wall’s axial plane may be calculated on the assumption that immediately above a lateral support the resultant eccentricity of all the vertical loads above that level is zero.
3.9.4.13 Concentrated loads
When these are purely local (as at beam bearings or column bases) these may be assumed to be immediately dispersed provided the local design stress under the load does not exceed 0.6fcu for concrete strength
class 20/25 or above, or 0.5fcu for lower-strength concrete.
6)This distance is measured vertically if the lateral supports are horizontal (e.g. floors) or horizontally if the lateral supports are
3.9.4.14 Calculation of design load per unit length
Design load per unit length should be assessed on the basis of a linear distribution of load along the length of the wall, with no allowance for any tensile strength.
3.9.4.15 Maximum unit axial loads for stocky braced plain walls
The maximum design ultimate axial load per unit length of wall due to ultimate loads, nw, should satisfy
the following:
3.9.4.16 Maximum design ultimate axial load for slender braced plain walls
The maximum design ultimate axial load nw should satisfy equation 43 and the following:
3.9.4.17 Maximum unit axial load for unbraced plain walls
The maximum unit axial load for unbraced plain walls should satisfy the following:
3.9.4.18 Shear strength
The design shear resistance of plain walls need not be checked if one of the following conditions is satisfied: a) horizontal design shear force is less than one-quarter of design vertical load; or
b) horizontal design shear force is less than that required to produce an average design shear stress of 0.45 N/mm2 over the whole wall cross-section.
NOTE For concrete of strength classes lower than 20/25 and lightweight aggregate concrete, the figure of 0.30 N/mm2 should be
used instead of 0.45 N/mm2.
3.9.4.19 Cracking of concrete
Reinforcement may be needed in walls to control cracking due to flexure or thermal and hydration shrinkage. Guidance is given in 3.9.4.20, 3.9.4.21, 3.9.4.22 to 3.9.4.23. Wherever provided, the quantity of reinforcement should be in each direction at least:
nw k 0.3(h – 2ex)fcu equation 43
where
ex is the resultant eccentricity of load at right angles to the plane of the wall (with minimum
value h/20).
nw k 0.3(h – 1.2ex – 2ea)fcu equation 44 where
ex is as defined in 3.9.4.15;
ea is the additional eccentricity due to deflections which may be taken as le2/2 500h where le is the
effective height of the wall.
a) nw k 0.3(h – 2ex,1)fcu equation 45
b) nw k 0.3{h – 2(ex,2 + ea)}fcu equation 46
where
ea is defined in 3.9.4.16.
a) for grade 500: 0.25 % of the concrete cross-sectional area; b) for grade 250: 0.30 % of the concrete cross-sectional area.
3.9.4.21 “Anticrack” reinforcement in internal plain walls
It may be sufficient to provide reinforcement only at that part of the wall where junctions with floors and beams occur. When provided it should be dispersed half near each face.
3.9.4.22 Reinforcement around openings in plain walls
Nominal reinforcement should be considered.
3.9.4.23 Reinforcement of plain walls for flexure
If, at any level, a length of wall greater than one-tenth of the total length is subjected to tensile stress, resulting from in-plane eccentricity of the resultant force, vertical reinforcement to distribute potential cracking may be necessary. It needs to be provided only in the area of wall found to be in tension under design service loads. It should be arranged in two layers and conform to the spacing rules given in 3.12.11.
3.9.4.24 Deflection of plain concrete walls 3.9.4.24.1 General
The deflection of plain concrete walls should be within acceptable limits if the preceding recommendations of this clause are followed.
3.9.4.24.2 Shear walls
The deflection of plain concrete shear walls should be within acceptable limits if the total height does not exceed ten times the length of the wall.