Design objectives and general recommendations
19. Structural properties and analysis
19.1 Structural properties
19.1.1 Characteristic compressive strength of masonry, fk
19.1.1.1 General
The purpose of this warning in the Code is to draw attention to the fact that in a reinforced or prestressed element, the units may be loaded in a direction other than that which would normally occur in unreinforced masonry.
The compressive strength of masonry units is determined by applying loads through the platens of a testing machine normal to the bed faces of the unit. The strength so obtained is unique to that direction of loading. Even allowing for the adjustment necessary for the effect of changing the aspect ratio when the unit is tested in a different direction (for example, load normal to the header faces), the strength of the unit is still likely to be different, depending upon the type of unit.
*which replaces CP 3: Chapter V
In the case of solid aggregate blocks, variations in strength with unit orientation will be introduced by the method of manufacture, although these will generally be small. In many cases, vertical compaction and vibration during manufacture could lead to a variation in strength over the height of the unit, whereas a few machines mould blocks on end which could lead to variation in properties along the length of the unit. Autoclaved aerated blocks are cut to size from “cakes” of foamed concrete and here the properties of the units may depend on the orientation in which the units are cut from the “cake”.
Design objectives and general recommendations 29
For design purposes solid concrete units and hollow and cellular concrete units filled with concrete are assumed to have the same characteristic strength regardless of the direction of loading, even on end. When unfilled cellular or hollow blocks are employed loaded in directions other than “normal” the characteristic strength must be determined by test as discussed later.
In the case of some extruded wire cut bricks which have a number of perforations (20–
25% of bed area), the strength when loaded through the header faces may be of the order of 10–15% of that obtained when loaded through the bed faces. This is clearly related to the geometrical form of the unit, since when on end the brick is more slender than on bed and platen restraint is reduced. In addition, the perforations act as stress raisers and superimposed on these effects are any directional properties due to the extrusion process.
Although this reduction in strength is dramatic, the available test results indicate that when built into an element the strength of the reinforced clay brickwork when loaded parallel to the bed faces is at least 40% of that when loaded normal to the bed faces7. Brickwork made from some pressed bricks is stronger when loaded parallel to the bed faces than when normal to them.
The compressive strength of the unit is not, of course, the characteristic strength of the masonry, but the above hopefully illustrates how variations in performance with direction of loading are likely to occur in practice. In the following section the determination of characteristic compressive strength of masonry is discussed.
19.1.1.2 Direct determination of the characteristic compressive strength of masonry, fk
The “characteristic” masonry strengths presented in Table 3 of the Code are based on those presented in BS 5628: Part 1. Although these are termed characteristic they have not been determined statistically but are in general agreed lower bounds to the masonry strength based substantially on updated information from the permissible stress Code CP 1118. The designer may wish to directly determine a value of the characteristic compressive strength of a particular combination of units and mortar. This may be done by deriving a value statistically from test results (see Appendix D).
19.1.1.3 Value oƒ fk where the compressive force is perpendicular to the bed face of the unit
This section essentially reflects the information provided in Part 1 except that only mortar designations (i) and (ii) are considered. A new table, Table 3(B) and accompanying figure 1(b), have been added which cater for the use of units with a height to thickness ratio of 1.0. This information is useful for reinforced hollow block masonry with filled cores (remembering to use the nett unit strength unless the infill concrete is less strong than the compressive strength of the units, in which case the cube strength of the infill should be used to determine the characteristic compressive strength of the masonry).
19.1.1.4 Value oƒ fk where the compressive force is parallel to the bed face of the unit
This section requires no further detailed comment. Note that filled hollow blocks are treated as solid units and are not covered by this section.
19.1.1.5 Value of fk for units of unusual format or for unusual bonding patterns
This section requires no further detailed comment.
19.1.2 Characteristic compressive strength of masonry in bending
This clause indicates that the value derived for the characteristic compressive strength of masonry should be used for both direct and flexural compression. The reason for the statement is that designers familiar with CP 1118,9 or indeed other Codes based on permissible stress design, will be used to enhance the maximum permissible compressive stress when this is due to flexural compression. Such enhancements compensate for the inaccurate assumption that the stress distribution is linear across the section and are not necessary for the different assumptions made with limit state design.
19.1.3 Characteristic shear strength of masonry Further information on the provision for shear is given in Clause 22.5.
19.1.3.1 Shear in bending (reinforced masonry)
19.1.3.1.1 The value of the characteristic shear strength of masonry, fv, in which the reinforcement is placed in bed or vertical joints (including Quetta bond) or is surrounded by mortar and not concrete is 0.35 N/mm2. No enhancement in shear strength is given for the amount of tensile reinforcement since this type of section has been shown experimentally10 not to warrant such an enhancement when mortar is the embedment medium. It is not entirely clear why this should be so but is likely to be due to a reduction in the amount of dowel action which can be utilised in such reinforcement. Consequently, there is a reduction in the contribution by dowel action to the average shear strength across the section. It may be noted that 0.35 N/mm2 is also the characteristic shear strength assumed for unreinforced masonry.
For simply supported beams or cantilevers an enhancement factor of (with a limiting factor of 2) can be applied when a principal load (usually accepted as one contributing to 70% or more of the shear force as a support) is at a distance av from the support. This is again demonstrated in the work of Suter and Hendry11. The maximum factor of 2 implies a cut off in the shear strength at a ratio .
Design objectives and general recommendations 31
The Code suggests that in certain walls where substantial precompression can arise, for example, in loadbearing walls reinforced to enhance lateral load resistance, it is often more advisable to treat the wall as plain masonry, i.e., unreinforced, and design to BS 5628: Part 12.
19.1.3.1.2 For sections in which the main reinforcement is enclosed by concrete infill, an enhancement to fv is given depending upon the amount of tensile reinforcement, by the formula:
where
with an upper limit of 0.7 N/mm2.
19.1.3.1.3 For simply supported beams or cantilever retaining walls an enhancement in the shear strength as derived above is given by the formula:
Here the shear span is defined as the ratio of the maximum design bending movement to
the maximum design shear force, i.e., . This enhancement is similar to that in 19.1.3.1.1, but has been derived on a more rational basis reflecting the greater amount of more specific data on this subject. An upper limit of 1.75 N/mm2 is applied, i.e., a maximum enhancement of 2.5 when ; the enhancement factor equals 1.0 when . Much below , the masonry would act as a corbel not a beam, above , the failure mode would be flexural, shear failure being most unlikely. Between these values a “transition” occurs from shear to flexural failure. This behaviour in shear is analogous to that of reinforced concrete upon which much has been written. Values of fv
for various percentages of reinforcement and ratios are given in Table 3.1.