additional moment diagram
(a) restrained columns
(b) cantilever columns
23.3.2 Walls subjected to a combination of vertical loading and bending
These walls both short and slender as defined for columns, may be designed in the same way as columns subjected to combined loading. Although the sub-clause dealing with short walls refers only to analysis of the section using the assumptions given in 22.4.1, there is no reason why the design formulae given in 23.3.1.1 should not be used, or indeed the design charts, taking the width of the section, b, as the unit length of the wall.
Often, however, walls in reinforced masonry will be singly reinforced with the reinforcement placed approximately centrally in the section, and it is then necessary to amend the formulae.
23.3.2.1 The short wall sub-clause (23.3.2.1) refers specifically to the situation where the resultant eccentricity, ex, is greater than 0.5t. In this case the axial load may be neglected and the wall designed as a member in bending in accordance with Clause 22.
23.3.2.2 Slender walls are treated in the same way as short walls with the exception that the additional moment derived in the same way as for columns is included.
23.4 Deflection
This clause refers the designer to the limiting dimensions given in Clause 22.3, but does not make it clear whether Table 8 or Table 9 should be used. Some degree of judgement is required in this matter. Since Table 8 refers to walls, its use seems appropriate, but consideration should be given to the situation where a column of primary structural importance is subjected to a predominantly substantial bending moment, i.e., if the member is designed as a beam in accordance with 23.3.1.1, Table 9 should also be used.
23.5 Cracking
If the design vertical load of a wall or column exceeds , then the eccentricity of the load at a critical cross section is not likely to be great enough to cause cracking due to flexural tension. In more lightly loaded columns reinforcement may be provided to control cracking and this should be provided in the same way as for beams. The recommendations are given in Clause 26.
24. Reinforced masonry subjected to axial compressive loading
This clause deals with walls and columns which carry a design vertical load, the resultant eccentricity of which does not exceed 5% of the thickness of the member in the direction of the eccentricity.
As mentioned in Section 23 in this respect, BS 5628: Part 2 differs from CP 110. The CP 110 design equations for short reinforced concrete columns include an allowance for an additional moment due to erection tolerances based on an eccentricity of 5% of the depth of the section. Thus reinforced concrete column designs automatically assume a minimum eccentricity of 5% for columns with a nominal axial load. In BS 5628: Part 2, the designer is referred either to the equations appropriate for columns subjected to
Design of reinforced masonry 69
combined loading, or to the design method given in BS 5628: Part 1, making no allowance for the reinforcement. Recourse to Part 1 is also recommended for the design of walls subjected to concentrated loads, the implication being that the provision of special reinforcement is impractical.
25. Reinforced masonry subjected to horizontal forces in the plane of the element
Where walls are used to provide overall stability to a structure, significant horizontal loads can be applied in the plane of the walls. The capability of the element to resist these forces should be checked in respect of both the resistance to racking shear and the resistance to bending.
25.1 Racking shear
Walls which are subjected to in-plane horizontal forces and loaded to failure, crack typically in the manner illustrated in Figure 4.19. The cracks are caused by diagonal tension and, although there has been some research into the strength of brickwork when subjected to biaxial loading19, it is usual to treat the design of walls on the basis of the average stress over the plan area. Thus, if the total design horizontal force is V, the shear stress due to design loads is considered to be v, where:
and where t and L are the thickness and length of the wall respectively.
The Code states that adequate provision against the ultimate limit state being reached must be assumed if the average shear stress is less than the design shear strength, i.e.:
fv is the characteristic racking shear strength taken from 19.1.3.2, i.e., 0.35+0.6 gB
N/mm2, where gB is the design vertical load per unit area of wall cross section due to the vertical dead and imposed loads calculated from the appropriate loading condition. (The maximum value to be taken for fv is 1.75 N/mm2.)
Alternatively research20 has shown that for walls which are reinforced with the main reinforcement in pockets, cores or cavities, a lower bound for the shear resistance is 0.7 N/mm2 and this may be used as a characteristic value instead of 0.35+0.6 gB. The value of 0.7 N/mm2 was derived from tests on walls with a limited range of shapes and so the use of the value is limited to walls where the height/length ratio is not greater than 1.5.
Where v is greater than , horizontal shear reinforcement should be provided (but
in no case should v exceed N/mm2). This reinforcement should be provided according to Code equation:
Part of the applied shear force, V=vtL, is considered to be resisted by a com ponent of
force in the masonry, , and the remainder by the total area of horizontal steel acting in tension across any incipient crack. If the crack is assumed to be at 45°, the
number of points at which horizontal steel crosses the crack is then . The formula can then be written:
which, rearranged, gives:
Any vertical reinforcement will also help resist shear in racking by dowel action. This is not as effective as the horizontal reinforcement in tension, and so has been ignored. In any event, many shear walls will not require any horizontal steel specifically for shear resistance, particularly where some light horizontal distribution steel is already provided.
In any case of reinforced or unreinforced masonry where the designer is considering the use of shear walls, particular consideration must be given if any type of damp-proof course has been introduced which is likely to produce a plane at the base of the wall along which sliding could occur21.
Design of reinforced masonry 71