Structural Masonry Elements

Transcription

1 Structural Masonry Elements Despite the use of masonry for construction during many centuries, design techniques based on wellestablished scientific principles have only been developed during the latter part of the 20th century. The mechanisation and development of brickmaking occurred in the mid-19th century. Prior to this time the firing of bricks had always been in intermittent kilns. Using this technique, moulded and partially dried bricks were loaded into a kiln and fired. On completion of the firing the fire was put out, the kiln opened and the bricks allowed to cool. This process was then repeated for the next batch. 1

2 Modern brickmaking is carried out using a continuous process in which batches of bricks are loaded, fired, cooled and removed in permanent rotation. The shaping of clay to produce bricks is carried out either by extrusion or by moulding/pressing. The strength of masonry/brickwork is dependent on a number of factors, one of which is the unit strength. (Note: the distinction between brickwork an assemblage of bricks and mortar and brick the individual structural unit. In this text in general, reference to bricks and brickwork also implies blockwork, stonework, etc.) In civil engineering projects which require high strength characteristics, high density engineering bricks are frequently used, whilst in general construction common bricks (commons) are used. Where appearance is a prime consideration facing bricks are used combining attractive appearance, colour and good resistance to exposure. Bricks which are non-standard size and/or shape are increasingly being used by architects and are known as specials The design of structural masonry/brickwork in the U.K. is governed by the requirements of BS 5628 Code of Prac;ce for the Use of Masonry Parts 1, 2 and 3. 2

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4 Materials Masonry can be regarded as an assemblage of structural units, which are bonded together in a particular pattern by mortar or grout. Structural Units There are seven types of structural unit referred to in BS 5628: The specification for each of these unit types is given in the appropriate British Standard as indicated. The selection of a particular type of unit for any given structure is dependent on a number of criteria, e.g. strength, durability, adhesion, fire resistance, thermal properties, acoustic properties and aesthetics. The structural units may be solid, solid with frogs, perforated, hollow or cellular, as indicated in Figure 4

5 Dimensions and Sizes The sizes of bricks are normally referred to in terms of work sizes and co-ordinating sizes, as shown in Figure. When using clay or calcium bricks the standard work sizes for individual units are 215 mm length mm width 65 mm height. In most cases the recommended joint width is 10 mm, resulting in coordination sizes of bricks equal to 225 mm mm 75 mm. Mortar Mortar is the medium which binds together the individual structural units to create a continuous structural form, e.g. brickwork, stonework, etc. Mortar serves a number of functions in masonry construction, i.e. to: 5

6 Present-day mortars are specifically manufactured to suit the type of construction involved. In most cases they are mixtures of sand, cement and water. The workability is often improved by the inclusion of lime or a mortar plasticiser. Lime is used in mortar for several reasons: Bonds Walling made from regular-shaped units is constructed by laying the units in definite, specific patterns called bonds, according to the orientation of the long sides (stretchers) or the short sides (headers). The method of laying structural units is specified in Section 8 of BS 5628:Part 1 and detailed in Section 32 of Part 3 of the Code. Normally all bricks, solid and cellular blocks are laid on a full bed of mortar with all cross joints and collar joints filled. 6

7 It is essential when constructing brickwork walls to ensure that the individual units are bonding together in a manner which will distribute the applied loading throughout the brickwork. This is normally achieved by laying units such that they overlap others in the course below. The resulting pattern of brickwork enables applied loads to be distributed both in the horizontal and vertical directions as shown in Figure 7

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9 The final appearance of brickwork is dependent on the finish of the joints between individual units (perpend/vertical-cross joints), and the bed joints between the courses. Various joint finishes can be created depending upon the desired aesthetic effect. The type of joint finish selected will be influenced by a number of factors such as exposure conditions and aesthetics. The most effective types to resist rain penetration are weather and keyed finishes. In situations where wind-driven rain is likely, finishes which produce a water-retaining edge, e.g. recessed, should be avoided. 9

10 If the finishes are created during construction then the process is called jointing; if this is done after completion of the brickwork it is called pointing. 10

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