Compressive Strength of Masonry Walls
Masonry is most effective when resisting axial compressive loads normal to the bedding plane. This is, clearly, the way in which most load-bearing walls function but also the way that arches and tunnels resist load since an inward force on the outside surface of a curved plate structure such as a tunnel will tend to put the material into radial compression, as in Fig. 1.
If a load or force is put on a wall at a point, it would logically spread outward from the point of application in a stretcher-bonded wall since each unit is supported by the two units below it. This mechanism, shown diagrammatically in Fig. 2 by representing the magnitude of the force by the width of the arrows, leads to some stress being spread at 45° in a half-bond wall, but the stress still remains higher near the axis of the load for a height of 2 m or more.
Such a compressive force causes elastic shortening of the masonry. As a result of Poisson’s ratio effects, a tension strain and hence a stress is generated normal to the applied stress. In bonded masonry the overlapping of the units inhibits the growth of cracks, which are generated in the vertical joints by tension, until the stress exceeds the tensile resistance of the units.
The compressive strength of masonry is measured by subjecting small walls or prisms or larger walls of storey height (2–3 m) to a force in a compression test machine. Loading is usually axial but may be made eccentric by offsetting the wall and loading only part of the thickness. Masonry is not so good at resisting compression forces parallel to the bedding plane because there is no overlapping and the bed joints fail easily under the resultant tensile forces.
Additionally, most bricks with frogs, perforations or slots are usually weaker when loaded on end than on bed because the area of material resisting the load is reduced and the stress distribution is distorted by the perforations. . The data in Table 1 for prisms show that the strength does vary with loading direction, although not to the same extent as for units, because the aspect ratios of the masonry specimens are all similar.
The axial strength of masonry might be expected to depend on the strength of the units and of the mortar and, to a first approximation, the contribution of each to the overall strength should be related in some way to the volume proportion of each. This gives a reasonable model for the behaviour of squat structures. A complication is that most masonry comprises units that are much stronger than the mortar and the three-dimensional confining restraint increases the effective strength of the thin mortar beds.
Figure 3, based on data for wallettes tested in compression, shows the minor effect of mortar strength (i) and (iii) on the compressive strength of masonry made with a range of strengths of bricks, but the much greater influence of the brick strength is clear. The third line shows the different behaviour of blocks tested to BS EN 772-1.
As we know, platen friction in compression tests results in restraint of the material nearest to the platen and therefore in the standard test method inhibits tension failure in the zones shown shaded in Fig. 4, thus enhancing the measured strength of squat units (units wider than they are high) more than slender units.
This anomaly is taken account of in BS EN 1996-1-1 by correcting all strengths measured using units of varying shape to a value representing the strength of a specimen with a square cross section (with a height:breadth ratio of 1) using a conversion table based on test data. The process, called normalisation, also corrects to a standard air-dry state whatever the test condition. The former British Code, BS5628: Part 1, uses a different table for each unit shape. To calculate masonry strength, BS EN1996-1-1 uses an equation of the form:
fk = K.fba.fmb (35.1)
where fk, fb and fm are the strength of the masonry, the normalised strength of the units and the strength of the mortar, respectively, K is a factor that may vary to take into account the shape or type of the units, a is a fractional power of the order of 0.7–0.8 and b is a fractional power of the order 0.2–0.3.
Stability: Slender Structures and Eccentricity
If a structure in the form of a wall or column is squat, so that the ratio of height to thickness (slenderness ratio) is small, then the strength will depend largely on the strength of the constituent materials. In real structures the material will be stiffer on one side than the other, the load will not be central and other out-of-plane forces may occur.
This means that if the slenderness ratio is increased, at some point the failure mechanism will become one of instability and the structure will buckle and not crush. Loads on walls, typically from floors and roofs, are commonly from one side and thus stress the wall eccentrically.
Figure 5 illustrates, in an exaggerated form, the effect of an eccentric load in reducing the effective cross-section bearing the load and putting part of the wall into tension. This is recognised in practice and usually a formula is used to calculate a reduction factor for the design load capacity, which is a function of the slenderness ratio and the net eccentricity of all the applied loads. BS EN 1996-1-1 gives a formula for the reduction factor, f, which varies between the bottom, middle and top of the wall.
Many loads are fairly uniform in nature, being derived from the weight of the super-structure or more locally on floors. There will always be, however, some point loads, termed concentrated loads, in structures at the ends of beams, lintels, arches, etc.
In general the masonry must be capable of withstanding the local stresses resulting from the concentrated loads, but the designer may assume that the load will spread out in the manner of Fig. 2 so that it is only critical in the first metre or so below the load. Additionally, because the area loaded is restrained by adjacent unloaded areas, some local enhancement can be assumed.
Figure 6a shows the condition for a small isolated load applied via a pad where there is restraint from four sides and Fig. 6b, c and d show further conditions with reducing restraint. The local load capacity of the masonry in the patches compared to the average load capacity (the enhancement factor) can vary from 0.8 for case (d) to as much as 4 for case (a) (Ali and Page, 1986; Arora, 1988; Malek and Hendry, 1988).
The enhancement factor decreases as the ratio of the area of the load to the area of the wall increases, as the load moves nearer the end of a wall and as the load becomes more eccentric. Formulae describing this behaviour are proposed in many of the references at the end of this chapter, and are codified in BS EN 1996-1-1 Clause 6.1.3.
Cavity Walls in Compression
If the two leaves of a cavity wall share a compression load equally then the combined resistance is the sum of the two resistances provided that their elastic moduli are approximately the same. If one leaf is very much less stiff than the other, the stress is all likely to predominate in the stiffer wall and be applied eccentrically and then the combined wall may have less capacity than the single stiff wall loaded axially.
It is common practice to put all the load on the inner leaf and use the outer leaf as a rain screen. In this load condition more stress is allowed on the loaded wall because it is propped (its buckling resistance is increased) by the outer leaf.