17

WALLS

17.1 INTRODUCTION

A wall is a vertical member with width greater than four times its thickness. If this condition is not satisfied it is a column. Based on functional aspects, walls may be classified under the following two broad groups:

1. Load-bearing walls
2. Non-load-bearing walls

The masonry types explained in the previous chapter may be used for load-bearing structures and as separators or partitioners.

17.2 LOAD-BEARING WALLS

In general, masonry used for exterior walls, central main walls and some cross walls in a building are of load-bearing walls. These walls should have adequate thickness such that it will sustain a slight eccentricity in loading. The mortar used for such walls should give adequate bonding to hold the structure even when there is a slight buckling. This load-bearing wall, apart from supporting the loads, subdivides the space, provides thermal and acoustic insulation, and affords fire and weather protection.

Load-bearing walls are of the following five types:

1. Solid wall
2. Solid wall with piers (Pilaster)
3. Cavity wall
4. Faced wall
5. Veneered wall

17.2.1 Solid Wall

It is a wall built of solid bricks or perforated bricks and designed to carry an imposed dead loads and live loads, including its self-weight (Fig. 17.1).

Figure 17.1 Solid wall

17.2.2 Solid Wall with Piers or Pilasters

These walls are similar to solid walls but the thickness of wall at intervals is increased (Fig. 17.2). The thickened portions are called as piers or pilasters. These walls are used for the following purposes:

1. To carry concentrated loads from roof or floor beams
2. To provide lateral support
3. To reduce the slenderness ratio by stiffening the walls.

Figure 17.2 Solid wall with piers

17.2.3 Cavity Wall

Cavity wall consists of two structural leaves separated by an uniform continuous space called cavity. The two leaves are interconnected by metal ties (Fig. 17.3). The provision of cavity forms a barrier against penetration of dampness through the internal wall and also keeps the room cool because of air column in cavity. In such walls, the inner leaf will be of load-bearing and the outer wall carries its weight only. But bending moment is carried by the stiffness of both the leaves.

Figure 17.3 Cavity wall

17.2.4 Faced Wall

It is similar to a solid wall but with a different facing material. These two materials are bonded together such that both take the load. The facing material may be of a different quality such that it may give a better aesthetic view. Generally dressed stone facing is done on brick walls (Fig. 17.4).

Figure 17.4 Faced wall

17.2.5 Veneered Wall

In veneered wall the facing is attached to the backing but need not be bonded. The entire load is taken by the backing. The facing is mostly for decoration purposes or to meet aesthetic needs (Fig. 17.5).

Figure 17.5 Veneered wall

17.3 NON-LOAD BEARING WALLS

In general, non-load bearing walls have adequate strength, stability, sound insulation and fire resistance. Non-load bearing walls may be interior walls or exterior walls. Different types of materials are used for non-load bearing walls, viz., brick, wood, hollow block, metal lath, corrugated sheet, etc. Following are some of the important non-load bearing walls:

1. Panel wall
2. Partition wall
3. Curtain wall
4. Free-standing wall

17.3.1 Panel Wall

It is an exterior wall in a structural frame construction. It forms into a unit in each storey (Fig. 17.6).

Figure 17.6 Panel wall

17.3.2 Partition Wall

Partition wall is an interior wall whose main function is to divide the space within a building to rooms and other areas of varied use.

Sometimes partition walls are required to support girders. In such a case, it is called a load-bearing partition wall. In this case, a portion of a floor is transferred to the partition wall through the girders.

In residential buildings, wood, brick, concrete and hollow block partitions are commonly used. Glass partitions are used in public buildings, hotels, recreation centres, etc. Hollow block partition provides adequate insulation (Fig. 17.7).

Figure 17.7 Partition wall

17.3.3 Curtain Wall

It is a wall carrying its self-weight but subjected to lateral loads. But it may be laterally supported by horizontal structural members wherever necessary (Fig. 17.8).

17.3.4 Free-Standing Wall

Parapet wall, compound wall, shear wall, buttress wall, counter fort wall are the examples of free-standing walls. These walls are expected to carry their self-weight and horizontal force due to wind and while supporting other structures.

Figure 17.8 Curtain wall

17.4 LOADS ON WALLS

Loads on walls may be classified based on the load–wall reaction. The load–wall interaction may be divided into the following two major groups:

1. Vertical
2. Lateral or Transverse

17.4.1 Vertical Loads

Vertical loads may be uniformly distributed load or concentrated load. Loads acting parallel and along the axis of wall cause axial stress. Loads may act eccentrically. In such cases, these loads will cause bending stress in addition to the axial stress. Thus, a uniformly distributed load or concentrated load may act axially or eccentrically. The design of such structural elements is different from that normally used in the case of walls with lateral or transverse loads.

17.4.2 Lateral Loads

Lateral or transverse loads may act parallel to the face of the wall or on the surface perpendicularly. This type of loads may be concentrated, uniformly distributed or triangularly distributed. The lateral or transverse loads cause bending stresses in addition to in plane or transverse shear force.

17.5 LATERAL SUPPORTS AND STABILITY

Masonry structures gain stability from support offered by cross walls, floors and roof. Load-bearing walls are structurally sound as long as the load is applied axially without any eccentricity.

Lateral support for load-bearing walls or columns limit the slenderness of the structure. Further the lateral supports reduce the possibility of buckling of member due to vertical loads and to resist horizontal forces. Thus in total the lateral support ensures stability against sliding and overturning.

It is mandatory that an RCC floor or roof slab, irrespective of the direction of space, has to bear on a wall or cross wall for a minimum length of 90 mm.

Stability of a wall or column subject to vertical and lateral loads should be ensured. The lateral support provided for a wall or column should be capable of resisting simple static reactions at the point of lateral support to all the lateral loads, plus 2.5% of total vertical load.

In case of load-bearing buildings up to four storeys, stability requirements are ensured when the height-to-width ratio of building does not exceed two.

Cross walls used as stiffening walls continuously from outer wall to outer wall or outer wall to load-bearing wall shall have the spacing and thickness as given in Table 17.1.

Table 17.1 Thickness and spacing of stiffness walls

Halls exceeding 8 m span have to be adequately laterally supported.

For basement walls the following stability requirements are needed:

1. 1. Bricks should have a minimum crushing strength of 5 N/mm².
2. 2. Mortar used in masonry should be of grade M1 or better.
3. 3. Clear height of ceiling in basement should not exceed 2.6 m.
4. 4. Adequate cross walls.
5. 5. Thickness of basement should be 300–400 m for spans up to 1.75 m and 2.5 m, respectively.

17.6 EFFECTIVE HEIGHT OF WALLS

If both lateral and rotational restraints are offered by a support, then the wall is said to be fully restrained at the support. It is said to be partial, if only lateral restraint is provided. Combination of these two restraint cases yields different boundary conditions depending on location. The effective height of a wall is based on the boundary conditions. Table 17.2 presents the condition of supports and the corresponding effective heights and H is the distance between the supports.

Figure 17.9 shows the effective height of standing in different situations.

17.7 EFFECTIVE LENGTH OF WALLS

While deciding the length of walls, the following end support conditions are considered:

1. Free end of the wall
2. Continuity of the wall
3. Support from cross walls or piers or buttresses
4. Openings

Various combinations of the above conditions and the effective length of a wall are presented in Table 17.3 (Fig. 17.10).

Table 17.3 Effective length of walls

Source: IS: 1905, 1987.

Note:

(i) H = actual height of wall between centre of cross wall/pier.

L = length of wall from or between centre of cross wall/pier.

(ii) If there is an opening taller than 0.75 H in a wall, then the ends of the wall at the opening are considered free.

Figure 17.10 Effective length of wall (Source: IS: 1905, 1987)

Figure 17.10 (Continued)

17.8 EFFECTIVE THICKNESS OF WALLS

Effective thickness of a wall is an idealised thickness which reflects the behaviour of the wall. Effective thickness is determined as detailed below.

1. Solid Walls and Faced Walls: Effective thickness is the same as actual thickness.
2. Cavity Walls with Uniform Leaves: Effective thickness is two-thirds of the sum of actual thickness of both the walls.
3. Solid or Faced Walls Stiffened by Piers or Cross Walls: Effective thickness is obtained by multiplying the actual thickness by a stiffening coefficient as given in Table 17.4.

where S_p = Centre to centre spacing of pier or cross wall

w_p = Width of pier in the direction of the wall or the actual thickness of cross wall

t_p = Thickness of pier

t_w = Thickness of wall proper

17.9 SLENDERNESS RATIO AND STIFFNESS

Slenderness ratio is the ratio of effective height or effective length to effective thickness of the masonry unit. Slenderness ratio is an important factor to be considered in the stability of a wall.

For solid walls, the effective thickness is the actual thickness of the wall. For the solid walls which are adequately bonded with piers, buttresses, etc., the effective thickness is determined using slenderness ratio. Here the slenderness ratio is based on effective thickness which is the actual thickness multiplied by stiffness coefficient values as given in Table 17.5.

Maximum slenderness ratio for walls should be taken as given in Table 17.5.

17.10 REINFORCED BRICK WALLS

Ordinary masonry walls are reinforced with iron bars or expanded metal mesh and such walls are called reinforced brick walls. Here, the reinforcement, iron bars or expanded metal mesh are provided at every third or fourth course (Fig. 17.11).

Figure 17.11 Reinforcement of brick wall with metal mesh

Alternately flat bars of sections about 25 mm × 15 mm may be used as loop iron reinforcement for walls (Fig. 17.12). They are hooked at corners and junctions. In order to increase the resistance against rusting, the bars are dipped in tar and sanded immediately.

Figure 17.12 Reinforcement of brick wall with loop iron

Reinforcement in vertical direction is provided by using special bricks or blocks as shown in Fig. 17.13. Mild steel bars of 6 mm diameter may also be used as longitudinal reinforcement in walls.

Figure 17.13 Reinforcement of wall by vertical rods

17.11 ECCENTRICALLY LOADED BRICK WALL

In general, walls in buildings are commonly loaded with some eccentricity. Eccentricity may be caused due to one reason or another. Thus, there is a little possibility of establishing an exact relationship between factors which may cause eccentricity.

Some of the factors which contribute for eccentricity on brick walls are:

1. Long floor edges
2. Magnitude of loads
3. Relative stiffness (of slab or beam and the wall)
4. Flexibility of the support
5. Geometry of the support
6. Unequal spans

Thus a designer has to use his judgment to assess the degree of eccentricity based on the situation. However, I.S. Code (IS: 1905, 1987) provides certain guidelines for determination of eccentricity which are discussed below.

17.11.1 Exterior Walls

1. When a span of concrete floor or roof is more than 30 times the thickness of the wall, then all eccentricity may be anticipated due to sagging. The eccentricity is given as one-sixth of the bearing width.
2. When the roofs or floors do not bear on the entire width of the wall, then there is a possibility for eccentricity even for normal span. In such cases, the eccentricity is taken equal to 1/12^th the thickness of the wall.
3. For timber and other light weight floors, eccentricity is assumed one-sixth the thickness even for full-width bearings.

17.11.2 Interior Walls

1. Eccentricity is caused by unequal span of roof or floor. In such cases a net bending moment is induced (Fig. 17.14). This bending moment is due to an eccentric load.
2. The load is considered axial if the difference between the two loads is within 15%. Otherwise, each floor load is assumed to act at a distance equal to one-sixth the thickness of the wall and then the overall eccentricity is computed.
3. In general, eccentricity of loading increases with the increase in the fixity of slabs/beams at the supports.

Figure 17.14 Eccentricity due to unequal span in interior wall

17.11.3 Stress Distribution Under Eccentric Loads

In an eccentrically loaded wall, there is an axial load and a bending moment. These two may be combined into a single resultant load acting at a distance. This is known as equivalent eccentricity (Fig. 17.15).

Figure 17.15 Equivalent eccentricity

The stress distribution due to axial load and the bending moment are combined to get the stress distribution due to the resultant load. The stress distributions for various eccentricities are shown in Fig. 17.16.

Figure 17.16 Variation of stress distribution (Source: IS: 1905, 1987)

It can be observed that with an increase in eccentricity, the net compressive stress in the tension face decreases. That is, the tensile stress due to bending moment decreases.

17.12 CRACKING IN WALLS

17.12.1 Causes

Cracks are frequently found in brick masonry walls due to some of the reasons given below.

1. Brick masonry behave differently when constructed in conjunction with concrete foundations and concrete framing.
2. Combination of brick masonry with other members having greater deflections and strains.
3. Effect of deflection and shrinkage of concrete slabs resting on walls.
4. Due to introduction of new types of construction.
5. Restraint of stresses developed inside the brick masonry due to moisture absorption, temperature variation, etc.

17.12.2 Preventive Measures

Following are the preventive measures which could minimise the cracks in brick masonry.

1. Foundation Design

Depending on the type of foundation soil, the foundation has to be designed and the supporting masonry walls should be designed with adequate stiffness. Such a design will help to control excessive shear of flexural stresses in the masonry.

2. Expansion Joints

Providing horizontal and vertical expansion joints in walls helps to reduce the cracks to a considerable extent. Horizontal and vertical expansion joints absorb vertical and horizontal movement respectively. In general, expansion joints have to be provided for every 15 m. The sealant used for joints are natural or cellular rubber, bitumen, expanded plastics, coconut pith, etc. The depth of sealant should not be more than half the width of joint and should not be less than 4 mm. Figure 17.17 shows some typical locations for joints.

Figure 17.17 Plan of locations of expansion joint

Typical expansion joints in brick masonry provided at different locations to avoid cracks are shown in Fig. 17.18 to 17.20.

3. Isolation Joint

Isolation joint is similar to expansion joint but provided under the following conditions:

1. When it is desired to separate the foundation of machines from the rest of the structure.
2. When one portion of a building is higher than the other.
3. When one portion of a building rests on rock and the adjacent portion on com pressible clayey soil.

Figure 17.18 Expansion at corner of walls

Figure 17.19 Expansion joint at roof level

4. Sliding Joint

Sliding joint is provided when one part of a structure has a tendency to slide over the other due to variations in temperature and moisture content. Figure 17.21 shows the details of a sliding joint at floor level.

5. Slip Planes

Smooth slip planes are provided between the roof slabs and brick walls. Because of this arrangement, cracks will be developed only at the re-entrant corners. These cracks can be easily covered after the complete construction is over.

Figure 17.20 Expansion joint at foundation level

Figure 17.21 Sliding joint

6. Spans

It is recommended to provide short spans for the floor slabs.

7. Quality of Concrete

For floor and roof slabs, it is desirable to use concrete of low shrinkage characteristics.

1. A wall is a vertical member with width greater than four times its thickness.
2. Walls are classified as load-bearing walls or non-load bearing walls.
3. Solid wall is a load-bearing wall which is built of solid bricks or perforated bricks and designed to carry an imposed dead load and live loads including its self-weight.
4. Solid wall with piers or pilasters is a load-bearing wall with thickness of wall being increased at intervals. The thickened portion is called a pier or pilaster.
5. Cavity wall is a load-bearing wall consists of two structural leaves separated by a uniform continuous space called cavity. The two-leaves are connected by metal ties.
6. Faced wall is a load-bearing wall similar to solid wall with a different facing material. These two materials are bonded together such that both take the load.
7. Veneered wall is a load-bearing wall in which the facing is attached to the backing but need not be bonded. The entire load is taken by the backing.
8. Panel wall is a non-load bearing wall and an exterior wall in a structural frame construction. It forms into a unit in each storey.
9. Partition is a non-load bearing and an interior wall whose main function is to divide the space within a building to rooms and other areas of varied use.
10. Curtain wall is a non-load bearing wall which carries its self-weight but subjected to lateral loads. It may be laterally supported by horizontal structural members wherever necessary.
11. Free-standing wall is one which carries its own weight and also the horizontal force due to wind. This is a non-load bearing wall.
12. Walls are subjected to vertical and horizontal forces. Vertical loads may be uniformly distributed load or concentrated load. Lateral or transverse loads may act on the face of the wall in any direction. This type of loads may be concentrated, uniformly distributed or triangularly distributed.
13. Masonry structures gain stability from supports offered by cross walls, floors and roof.
14. Load-bearing walls are structurally sound as long as the load is applied axially without any eccentricity.
15. Lateral support for load-bearing walls limit the slenderness of the structure.
16. If both lateral and rotational restraints are offered by a support, then the wall is said to be fully restrained at the support. It is said to be partial, if only lateral restraint is provided.
17. Length of a wall is decided based on the following conditions: (i) free end of the wall, (ii) continuity of the wall, (iii) support from cross walls or piers or buttresses and (iv) opening.
18. Effective thickness of a wall is an idealised thickness which reflects the behaviour of the wall.
19. Slenderness ratio is the ratio of effective height or effective length to the effective thickness of the masonry unit.
20. Ordinary masonry walls are reinforced with iron bars or expanded metal mesh and such walls are called reinforced brick wall.
21. Factors contributing for the eccentricity in walls are: (i) long floor edges, (ii) magnitude of load, (iii) relative stiffness, (iv) flexibility of the support, (v) geometry of the support and (vi) unequal span.
22. In an eccentrically loaded wall, there is an axial load and a bending moment. These two are combined into a single resultant load acting at a distance. This distance is known as equivalent eccentricity.
23. Cracks in walls may be minimised by adopting the following preventive measures: (i) foundation design (ii) providing expansion joints, isolation joints, sliding joint, slip planes, (iii) providing short spans and (iv) quality control of concrete.

1. Distinguish between load-bearing and non-loading bearing wall.
2. What are the advantages and disadvantages of cavity wall construction?
3. Distinguish the difference between a bearing and non-bearing portion?
4. Briefly discuss the types of load-bearing walls.
5. What are partition walls? List the materials used for partition walls.
6. What are the requirements of partition walls?
7. What are the advantages of concrete partitions?
8. Explain different types of wooden partitions commonly used.
9. How later support helps in the stability of a wall?
10. How the effective height of a wall is decided?
11. Discuss the end support conditions in deciding the effective length of walls.
12. How the thickness of a wall is designed?
13. Why damp-proofing courses are provided?
14. What are the ill-effects of dampness in building?
15. How damp-proof surface treatment is done?
16. What are the causes for cracking in walls?
17. Discuss the preventive measures to be taken to prevent cracks in walls.