Basics of Building Construction

Basics in Building Construction 4(2+2)

Lesson 07 : Walls

Design Considerations

Load bearing walls may be subjected to super imposed loads, dead loads, wind pressure, and so on. Super imposed loads or live loads and dead loads act in vertical direction. When the floor slabs transferring the loads to the wall are not supported through the full width of the wall, the loads act eccentrically, causing moments in the wall.

• Load bearing walls are structurally efficient when the load is uniformly distributed and when the structure is so planned that eccentricity of loading on the wall is as small as possible.
• The strength of a wall is measured in terms of its resistance to the stresses set up in it by its own weight, by super-imposed loads and by lateral pressure such as wind etc.
• The stability of a wall is measured by its resistance to overturning by lateral forces and bucking caused by excessive slenderness.
• In order to ensure uniformity of loading, openings in walls should not be too large and these should be, as far as possible, of ‘hole in wall’ type.
• Bearings for lintels and bed blocks under beams should be liberal in size.
• Heavy consideration of loads should be avoided by judicious planning and sections of load-bearing members should be varied with the loadings so as to obtain more or less uniform stresses in adjoining parts of members.
• One of the commonly occurring causes of cracks in masonry is wide variation in stress in masonry in adjoining parts.
• Eccentricity of loading on walls should be reduced by providing adequate bearing of floors/roofs on the walls and making them as rigid as possible consistent with economy and other considerations.
• The strength of a masonry wall depends primarily upon the strength of the masonry units and the strength of the mortar. In addition, the quality of workmanship and the method of bonding is also important.
• The thickness of a load-bearing wall should be sufficient at all points to ensure that the stresses due to the worst conditions of loading for which the structure is designed are within the limits prescribed for that particular type of wall. The thickness used for design calculations should be the actual thickness of the masonry and not the nominal thickness. In the case of modular bricks, thickness of one brick wall will be 19 cm actual and 20 cm nominal. Similarly, the thickness of 1 ½ brick wall will be 29 cm actual and 30 cm nominal (Figure 1.) The actual thickness of the wall is computed as the sum of the average dimensions of masonry units together with the specified joint thickness.
• When vertical loads act on the wall, either axially or at small eccentricity, the wall behaves like a column. Its strength of the same vertical load intensity, depends upon the slenderness ratio which is a function of
  • Height of the wall
  • Length of the wall
  • Thickness of wall and
  • Support conditions
• The slenderness ratio of a wall is the ratio of its effective height divided by the effective thickness or the effective length divided by the effective thickness, whichever is less. The effective height and effective length of the wall depend upon the lateral support to the wall.

Lateral support:

• Lateral support to a wall has to perform two important functions, i.e.
  1. to limit the slenderness so as to prevent bucking and
  2. to provide stability to the structure against over-turning on account of horizontal forces.
• A wall may be considered to be provided with adequate lateral support if the construction providing the support is capable of resisting the sum of following lateral forces:
  1. The simple static reactions to the total applied horizontal forces at the point of lateral support, and
  2. Two and a half percent of the total vertical load that the wall is designed to carry at the point of lateral support.
• A wall can be laterally supported either at vertical intervals by floor roof transmitting horizontal forces to cross-walls and then to the foundation or at horizontal intervals by cross-walls, piers or buttresses transmitting horizontal forces to foundation.
• The load-bearing capacity of a wall depends upon the spacing and effectiveness of lateral supports.
• If the slenderness ratio is based on height, a horizontal lateral support (i.e., floor/roof) may be deemed to be adequate if the RCC floor/roof bears on wall to the extent of at least 10cm.
• In case slenderness ratio is based on effective length, a vertical support will be deemed to be adequate if cross-wall, pier or buttress extends to the extent of one-sixth of the height of the wall, has a minimum thickness of half the thickness of supported wall or 100 mm whichever is more, and is bonded to the supported wall.
• National Building Code of India specifies that when the concrete slabs do not bear on a wall, non-corrodible metal anchorages shall be provided at intervals of not more than 2 m and built into concrete slabs to a minimum distance of 40 cm.
• Timber floors and roofs shall be anchored by non-corrodible metal anchors having a minimum cross-section of 30 mm wide and 6 mm thick securely fastened to the joist and provided with split and upset ends or other approved means for building into the walls.
• The anchors shall be provided at intervals of not more than 2 meters in buildings up to two storey and 1.25 meters for all storeys in other buildings.

Height and length of the load bearing wall:
The height and length of the load bearing walls which support floors and roof is based on calculations. The design of such walls is commonly known as calculated masonry method.