THE AUSTRALIAN MASONR Y CODE

S.J. Lawrencel and A.W. Page2

1. ABSTRACT

In 1988 a completely revised and unified Australian Masonry Code (AS 3700) was published. The code is performance based as far as possible and is expressed in limit state terms using characteristic values. It provides a foundation for rational design of masonry similar to that available for other structural materiais. Its publication constituted a significant improvement in the basis for design and construction of masonry buildings in Australia.

AS 3700 applies to ali sizes and shapes of masonry units, whether of cIay, concrete, calcium silicate or sawn natural stone. The code structure and the supporting body of research, much of it carried out in Australia in the last two decades, make it one of the most up to date and soundly based masonry codes in the world.

An outline of the new code is presented and the areas of significant difference from previous Australian brickwork and blockwork codes are indicated. Areas discussed in greater detail are design for lateral loading, design for compression loading, site control testing and design for fire resistance.

2. INTRODUCTION

In 1982 the Standards Association of Australia (SAA) formed a new committee, replacing earlier brickwork and blockwork committees, and charged it with the responsibility of producing a unified masonry code to apply to both solid and hollow cIay, concrete, calcium silicate (or sand lime) and sawn stone masonry. The emphasis was on recognizing the basic similarities between various types of masonry, rather than

Keywords: Masonry; Codes; Design

I Project Manager - Masonry Structures, Division ofBuilding, Construction & Engineering, CSIRO, PO Box 310, North Ryde, NSW 2113, Australia. (Chairman, Standards Australia Masonry Code Committee BD/4)

2 CBPI Professor in Structural Clay Brickwork & Head ofDepartment, Department of Civil Engineering & Surveying, The University ofNewcastle, NSW 2308, Australia. (Chairman, Standards Australia Masonry Code Design Subcommittee BD/4/4.)

1509

promoting divisions in the industry by having separate codes with sometimes conflicting provisions.

It was also apparent that there was an increasing recognition of masonry as an engineering material - a trend begun in Australia in the 1960's. There is a need for masonry to be seen as a legitimate structural material, with appropriate guidance for rational engineering design, while retaining its traditional use for those simple structures that do not require complex rules. These aims can conflict at times and the maintenance of an appropriate balance in the document was the most difficult task facing the committee.

The unified code was published in 1988 as AS 3700(1) and was followed by the publication of a commentary<2) in 1991. Three amendments have been issued and work is currently under way to produce the next revision of the code. Work is also well advanced in producing a separate code for masonry in housing. This document will provide rules for masonry not usually requiring engineering design such as houses, small sheds, boundary fences and the like. It will be illustrated with diagrams and make extensive use of charts and simple tables.

Other work under way at present is aimed at producing a masonry unit standard applying to ali masonry units - clay, concrete, calcium silicate, autoclaved aerated concrete (AAC) and natural stone. This will be the first time in Australia that the various material standards supporting the masonry code have been combined into one document with common test procedures and performance based requirements. The wall tie standard is also being extensively revised and expanded to cover ali types of connectors, ties and accessories for masonry.

3. BACKGROUND TO AS 3700

3. 1 Previous Masonry Codes

The SAA Brickwork Code was first published in 1969 and was based on British codes of the time, although some of the data and provisions were changed to suit Australian conditions. Although it was metricated in 1974 there were no changes of content until 1983, when an amendment concerning lateralload design was issued. The provisions for reinforced brickwork were particularly outdated. The code as a whole remained virtually unchanged for twenty years.

Part 1 of the SAA Blockwork Code was based primarily on North American research and first published in 1963. It was revised in 1967 and metricated in 1977. Part 2 covering reinforced masonry was added in 1983. Apart from the lateral loading amendment to Part 1 in 1983 and some mino r corrections, this code was not revised for over twenty years. In addition to the obvious need for revision of these outdated codes, changes (in 1984) to the various specifications for units, mortar and ties provided further incentive for a major revision.

3.2 Structure OfThe New Code

Unlike the previous brickwork and blockwork codes, AS 3700 is a limit states code based on characteristic values for properties and capacity reduction factors. In conjunction with the relevant Australian loading codes, which are also in limit states format and provide common load factors for ali the material codes, it attempts to provide

1510

a reliability for masonry structures consistent with that applying to other engineered structures. The items covered by the capacity reduction factors include:

(a) simplifications and inaccuracies in analysis;

(b) differences between the strengths of members and control specimens constructed in a laboratory and those constructed on site;

( c) differences in the variability of materiais from that accounted for in the assessment of characteristic values;

(d) the structural nature ofthe member and the mode offailure under consideration;

(e) the importance ofthe structure and the consequences offailure.

Because of the use of factored loads it has only been possible to calibrate the new code with the previous codes at one load combination. Hence the capacity reduction factors have been determined such that there is usually only one point at which the new code gives similar results to previous codes, while all other designs will give slightly different results. But the advantage of limit states design is that ali designs provide the same levei ofreliability. This was not the case with previous codes. For lateralload the calibration has been carried out for wind load only and for compressive load it has been carried out for equal dead and live loading. For the next revision ofthe code a full reliability analysis will be carried out for cornrnon design situations to determine the true reliability of the design in terms of acceptable leveis of safety and the leveis of safety pertaining to other structural materiais.

It was recognized by the committee from the outset that the new co de would be more complex than those it replaced. Since the code covers ali masonry it is used by engineers, architects, builders, bricklayers and building inspectors. Unlike other structural materiais, only a small proportion of masonry construction is designed and supervised by engineers and the structure of the co de therefore provides 'streams' for these different users.

The code refers to 'masonry' and 'special masonry'. The latter is a category for engineered masonry where quality is required to be controlled by site testing and for which the designer may use higher strengths than the basic leveis permitted by the code. Control testing must verifY that these strengths are achieved during construction and it is left to the designer to specify which parts of a structure (if any) should be special masonry and require site control testing.

3.3 Significant Changes In The Code

The new code is a significant advance in that, for the first time, it covers ali types of masonry in a single document. Areas of the code which incorporate major changes from previous codes are compressive strength design, robustness, concentrated loading, design for fire, reinforced masonry, material strengths and procedures for quality control and site testing Some details of the changes are given later in the paper.

4. SCOPE AND GENERAL

The masonry code covers ali types of masonry unit : clay, concrete, calcium silicate and natural stone, in ali shapes: solid, cored and hollow and in all sizes. Units with up to 25% of the bed face area taken up by perforations are referred to as cored units and are assumed to behave in the same way as solid units (that is, a gross area basis is used for calculations). For more than 25% perforations in the bed face the units are called hollow

1511

units and are treated on a bedded area basis. These units are always assumed to be bedded on the face shells onJy, typical of Australian practice.

While reinforced masonry is covered, the code does not provide guidance on the use of prestressed or prefabricated masonry or on design for composite action of masonry with steel or concrete. But the code is certainJy not intended to discourage the use of these forms of construction and recourse is always available to materials and methods not specifically mentioned, provided that their adequacy can be demonstrated by proof tests, appraisals or accreditation. Another area where information is lacking is the design of panels with door and window openings for lateral loading. Despite a detailed exarnination by the committee, no reliable method for such design could be found, although research is continuing. A future revision of the code will probably incorporate provisions in this area.

Also absent is detailed information on design for earthquakes. The committee has now commenced work on preparing clauses for earthquake design which will be incorporated in the next revision.

The new code has been developed with a strong emphasis on performance rather than prescriptive specifications. In general, the committee has not included detailed mies in the code where guidance on specific topics is readily available in the literature, for example the Australian Masonry Manual(3). To include such details where altematives exist would unnecessarily restrict a designer's choice ofmethod.

5. MATERIALS

5.1 General

Design in AS 3700 is based on ultimate strength and the use of capacity reduction factors. Ali material properties are 95% characteristic values. Simple tables and values are provided to give lower bound (conservative) material properties for most masonry, while the properties of new materials can be derived by testo A designer always has the freedom to base design on measured performance incorporating higher strength values, with site control testing to verify achievement ofthese leveis of strength on the job.

5.2 Masonry Units

Masonry units of various materiais are required to comply with the relevant Australian standards. For compressive strength AS 3700 is based on a characteristic unconfined strength. A uniform height-to-thickness ratio of 5 has been adopted to determine the unconfined strength, that is, the strength when platen restraint is eliminated.

For the design of masonry subjected to flexure and spanning horizontally the code requires a property known as lateral modulus of rupture. A test procedure for determining this property has been recently introduced to the masonry unit standards. The method involves applying a uniform moment to a unit, acting in the same direction as the flexural stresses when the unit is in a wall subjected to lateralloading.

5.3 Mortar

The provisions for mortar in the new co de are a good example of its emphasis on performance rather than prescriptive specification. Mortar is required to have adequate workability and durability and the ability to impart the necessary compressive and flexural tensile strengths to the masonry. This is in contrast to the previous codes which

1512

specified, in an appendix, how mortar should be mixed and what properties of flow and compressive strength were required. It seems unlikely that these rules were generally complied with and, given the wide range of sands, mortar mixes and additives in use in Australia, a performance specification allows the freedom for a designer to use available materials to best advantage. A c1assification of mortars has been introduced, with four divisions based on the cement content in the mixo This is to allow for a simplified specification of mortars where small differences in composition have a negligible effect on properties.

Additives are restricted to certain types and it is now mandatory to demonstrate that the use of any particular additive will not reduce the compressive and flexural strengths below the requirements for the masonry. Specific reference is made to the use of water thickeners (modified methyl cellulose compounds) which have been shown to give higher tensile bond strength with calcium silicate and concrete units.

5.4 Reinforced Masonry

The most significant materials aspects relating to reinforced masonry concem grout and the corrosion resistance of the reinforcing.

Grout is required to have a pouring consistency such that cores and cavities can be completely filled. It is required also to have a compressive strength from cylinder tests of at least 12 MPa. But irrespective of the actual grout strength, because of the likely effects of shrinkage and differential behaviour, the strength adopted in design cannot exceed 1.3 times the unit compressive strength.

The code has introduced a corrosion-resistance rating for steel, inc1uding embedments and reinforcing, and a c1assification of exposure conditions. The corrosion-resistance rating is used in conjunction with the exposure c1assification to design for durability.

6. DESIGN FOR FLEXURE

The first requirement in designing for flexure is to determine the section modulus of the wall cross section based on bedded area. For unreinforced walls of solid or cored units this is derived from the gross section dimensions. For hollow units it is derived from the face-shell thickness. For cavity walls it is possible to assess the load sharing between the two leaves based on their relative flexural stiffnesses and the stiffness of the ties; altematively, it may be assumed that ali load is taken by one leaf Because of the complexity of a full design for load sharing based on stiffness, the code inc1udes a formula which is acceptable for medium and heavy duty ties.

The second requirement is knowledge of the material properties and in the case of unreinforced walls the critical property is the flexural tensile strength f 'mt. The whole basis for flexural tension is the strength of a single joint in vertical bending, that is, with stresses normal to the bed joint. The previous codes were based on the strength of small beams and it has been shown that, because of variability, these beams are weaker than the average strength of the joints making them up. More importantly, the measured strength depends on the geometry of the loading arrangement. On the other hand, the bond wrench test inc\uded in the code measures directly the flexural strength of individual joints. For unreinforced special masonry the value of f 'mt can be as high as 1 MPa and for normal masonry it can be assumed up to 0.2 MPa. Achieving high values can be difficult and a prudent designer will not assume a value exceeding 0.2 without

1513

supporting test evidence. Tlús levei of characteristic strength is approximately equivalent to the average ofO.28 MPa for beams used in previous codes.

Masonry in vertical flexure fails by cracking at the bed joints and it is a sudden, brittle failure . The design equation in the code requires that the design bending moment be less than the tensile moment capacity (Mcv), with an allowance for superimposed compressive force on the cross-section. An upper limit is placed on tlús allowance for compressive loading corresponding to the point at wlúch a member should be checked for compressive strength.

In designing for horizontal bending it is required that the design horizontal bending moment be less than the bending capacity (Melo)' wlúch is calculated as the least of three expressions. Two of the expressions result fIom an empirical fit to the results of tests on horizontal masonry beams and the third is an approximate analysis of the horizontal flexural action of stretcher-bonded masonry. An allowance is also made for the strengthening effects of superimposed compressive stress on the bed joints. A perpend spacing factor allows for stretcher-bonding other than the common half-bonding.

Design of walls without openings is based on an empirical strip method(4) The basis of tlús approach is to calculate the load resistances of horizontal and vertical strips through the centre of the wall panel and simply add these load capacities together. The method has been shown to agree well with a range of experimental data gathered in Australia on full-scale wall specimens.

The design wind pressure is given by:

W d :s; 1O(bvMcv1H2 + bhMchlL 2) (1)

where H and L are the height and length of the wall, Mcv and Mch are the vertical and horizontal bending capacities and bv and bh are factors which allow for different support conditions.

As a result of measurements on full scale walls there is a general 50% increase in the horizontal bending coefficient (bh) for hollow units over that applicable to solid and cored units. Tlús phenomenon has not yet been explained. Due to uncertainties and critical dependence on worlemanship the in-plane arclúng forces wlúch might develop due to building-in between stiff supports are not taken into account. Research is continuing on methods of design for walls with door and window openings and it can be expected that the next revision of the code will include provisions for these cases.

7. DESIGN FOR COMPRESSION

The design of a masonry member for compression is determined by its slendemess ratio and cross-section properties, the characteristic compressive strength of the masonry and the effective eccentricities ofloading at the ends ofthe member.

7.1 Cross-Sectional Area

As discussed previously AS 3700 uses the concept of bedded area, which is the area in contact with the bed joint mortar. For solid units this is the gross cross-sectional area less any depth of raking of the joints. For face-shell bedded hollow units it is the miIÚmum face-shell area of the unie

1514

7.2 Characteristic Compressive Strength

Strengths of masonry can be obtained from tables of lower bound values in the code or by testo The tabulated values have been derived from an assessment of ali available tests on Australian materials and represent a conservative estimate of the 95% characteristic strength. Entry to the tables is by the characteristic unconfined compressive strength of the units and the composition of the mortar. In practice, the unit strength would usually be obtained from the manufacturer since units are specitied by compressive strength. A correction factor is applied for the ratio of unit height to joint thickness, since it is well established that the greater the relative thickness of the mortar joints the lower the compressive strength ofthe masonry.

Testing of masonry specimens will usually only be used for the case of special masonry, where higher design stresses are desired and more efficient use of the material is required. In this case the tests must be carried out on stack-bonded prisms built in the same manner as the job. Plywood capping is used to obtain an even bearing and if face-shell bedding is used for hollow units the prisms must be tested between strips of plywood placed on the face shells on1y. A correction factor to allow for the effects of platen restraint is applied, bringing the compressive strength in alI cases to the 'unconfined' basis of a height-to-thickness ratio of tive.

7.3 Slendemess Ratio

The slendemess ratio of the member must be considered when estimating its buckling capacity, as well as checking its robustness under the serviceability lirnit state (overall lirnits on slendemess apply to both loadbearing and non-Ioadbearing walls). Because most masonry members have a sim pIe rectangular cross section, slendemess ratios are expressed in terms of height-to-thickness rather than radius of gyration. Effective height is determined from the actual height of the wall using factors to account for end restraint conditions. Allowance is also made for the increased buckling resistance of walls supported on their vertical edges due to pane! action. Effective thickness is the overall thickness, modified to account for engaged piers when these are present.

A significant change from previous codes is that cavity walls can no longer be treated in terms of an effective thickness of two-thirds the sum of the separate leaf thicknesses. Because tests have shown that the leaves do not give significant mutual support in a buckling mode, AS 3700 requires that each leaf be assessed separately for compressive load. However, the concept of an effective thickness of the leaves in combination has been retained for consideration of overall robustness.

7.4 End Eccentricity

Eccentricity of loading is always present at the ends of compression members due to normal building tolerances and as 'effective eccentricity' due to the combined action of compressive force and bending moment. These eccentricities produce non-uniform compressive stresses and can reduce the buckling load.

Eccentricity factors are provided, based on the ratio of the maximum eccentricity (e l ) to wall thickness (tw), the ratio of eccentricities at the two ends (el /e2 and the wall slendemess. This ratio (el/e2) ranges from -1 (corresponding to double curvature) to + 1 (single curvature). Double curvature produces significantly greater strength than single curvature. The ratio (el /tw ) is lirnited to 0.33 which leads to tensile cracking for walls composed of solid or cored units but ensures that walls composed of hollow units remain

1515

uncracked at the ends. A minimum value ofO.05 is assumed for (edtw) to account for normal construction tolerances and uncertainties.

7.5 Concentrated Loading

The strength enhancement which occurs under a concentrated load is determined by the use of an enhancement factor which allows for the loaded area ratio (the relative size of the concentrated load) and its location along the wal1. This enhancement can be substantial, but has been limited to a factor of2 in AS 3700. The design provisions are a significant improvement on previous codes and the result of recent Australian and British research(5).

8. WORKMANSHIP AND SITE CONTROL

The section on workmanship and site control has been extensive1y re-worked trom previous codes and inc1udes provisions for mortar, as these are not covered by the mortar standard.

For special masonry the sampling rate for site control testing has been fixed at one sample per storey or 400 square metres ofwall, with a minimum oftwo samples. There is no point in testing compressive strength for masonry which has been designed for tension, or bond strength for masonry which has been designed for compression, and the designer must therefore specify which tests are required in each case.

The concept of target strength has been introduced to allow quality control procedures to be followed on larger jobs. The target strengths, which are means of test results, are specified as 1.25 times the characteristic compressive strength and IA times the characteristic flexural tensile strength. The average strength of the last four samples must be greater than or equal to 90% of the appropriate target strength, otherwise remedial action must be taken during construction.

In addition, there are rejection criteria, and masonry is deemed not to comply if the average strength of a sample is less than 80% of the design characteristic strength, or if two consecutive samples give an average strength less than the design characteristic strength.

8.1 Strength Testing

In addition to test procedures for compressive and bond strengths the code sets out the method of assessing characteristic strength trom a set of test results - ensuring that a consistent reliability is obtained for various sample sizes. This approach is necessary because ofthe high degree ofrandom variation inherent in masonry.

It has proved difficult to provide a prism compressive strength test method for ali shapes of solid and hollow units. The main problem is the limitation imposed by the distance between platens on many compression testing machines. Although three courses are desirable, a minimum of two courses is allowed by AS 3700 in a compression test specimen, so that the effect of a joint is inc1uded. Ali compressive strength results are reduced to a common height-to-width ratio of 5, to provide an estimate of strength tree of platen restraint effects. The laying procedure specified is intended to simulate the laying of masonry on site, where a bed of mortar is strung out for a number of units at once - the specimens should not be constructed individually. At least three specimens are required for each determination of compressive strength

1516

The code provides for flexural strength tests to be carried out either with beams or with a bond wrench which applies a bending moment to individual joints in tum. The bond wrench method is preferred. The most convenient way of testing the bond strength of masonry is to build specimens four units high, as for compressive strength tests, and to use the bond wrench to test each of the three joints in tum.

9. DESIGN FOR FIRE RESIST ANCE

Three criteria are generally accepted as being appropriate for determining the resistance of walls to fire. They are insulation, being the ability of the wall to resist temperature rise on the side not exposed to the fire such that finishes will not ignite; integrity, being the ability of the wall to remain intact to the extent that flames and hot gases will not penetrate; and structural adequacy, being the ability of the wall to continue to perform a structural function (such as supporting a beam or fioor) .

In the past, wall characteristics for fire resistance were determined by looking up tables of sizes deemed to comply with the building regulations or on the basis of a test certificate reporting the performance of a prototype testo For a wall to have a one hour 'rating' it was necessary for it to have a resistance for each of insulation, integrity and structural adequacy of at least one hour. From recent research it has beco me possible and desirable to develop a design method which can take account of the primary factors deterrnining performance and provide a rational basis for sizing members . AS 3700 incorporates this approach and is probably the first code in the world to do so.

The code allows three ways of designing against fire. Firstly, a designer may work from first principies and a knowledge of the material properties and the structural response to a fire. Secondly, a designer may look up a resistance in tables for both insulation and structural adequacy. No information is provided for integrity resistance because this has not been studied sufficiently but the code allows a designer to assume a certain integrity resistance if that resistance is provided for both insulation and structural adequacy. The tables for insulation and structural adequacy resistance are derived from test data accumulated over many years at the Experimental Building Station(6) The relationships derived from these accumulated test data are conservative lower bounds. It will be some time, if ever, before it is possible to speak in terms of 95% characteristic values for these properties .

The third option for design is to use known test information, either directly if the member being designed exactly matches that tested, or by interpolation and extrapolation where the member characteristics differ from those tested. Extrapolation is allowed only within strict limits and the process is illustrated by example in the AS 3700 commentary(2) and the Australian Masonry Manual(3)

For many years it was believed that cavity masonry walls could be designed for structural adequacy by combining the characteristics of the two leaves into an equivalent thickness. Recent research(7) has demonstrated however that the behaviour of a cavity wall where the two leaves are loaded differently (the common case) depends very strongly on the direction from which the fire impinges on the wall . If the non-Ioaded leaf is subjected to the fire the wall has considerably greater resistance than a single leaf wall. In the opposite situation, where the fire impinges on the loadbearing leaf, the cavity wall does not survive longer than a single leaf.

1517

The provisions for design against fire are inc\uded in the code in recognition of the fact that fire is potentially the most severe event to which a masonry wall can be subjected and in many cases will be the lirrúting factor in designo The code writers hope that the framework provided will help to increase understanding of the importance of design for fire and, in particular, will lead engineers to a greater awareness of the need for design against this, as against other types of, loading.

10. SUMMARY

The Australian Masonry Code (AS 3700) is a Iirrút states, performance based co de which applies to alI types of masonry. The user has the options of adopting the lower bound characteristic strength values given in the Code or using higher design values provided they are confirmed by appropriate testing ('special' masonry). The Code contains appropriate acceptance/rejection criteria for this procedure to be followed.

The Code is a significant advance on previous brickwork and blockwork codes, particularly in the areas of compressive and lateral loading, fire resistance, and procedures for quality control and site testing. It has played a major role in rationalising masonry design procedures in Australia.

11 . REFERENCES

1. SAA Masonry Code, AS 3700-1988, Standards Australia, Sydney, 1988.

2. SAA Masonry Code - Commentary, AS 3700 Supplement 1, Standards Australia, Sydney, 1991.

3. Baker, L.R., Lawrence, SJ. & Page, AW., Australian Masonry Manual, Deakin University Press, 1991 .

4. Baker, L.R. Flexural Strength of Brickwork Panels, Proceeding of the Third International Brick Masonry Conference, Essen, 1973, pp.378-383 .

5. Page, AW. & Hendry, AW. Design Rules for Concentrated Loads on Masonry, The Structural Engineer, Vo1.66, No.! 7, SepU988, pp.277-281.

6. Lawrence, S.J. & Gnanakrishnan, N. The Fire Resistance of Masonry Walls - An Overview, Proceedings of the First National Structural Engineering Conference (Melbourne, 1987), Institution ofEngineers Australia, Sydney, 1987, pp.431-437.

7. Gnanakrishnan, N.; Lawrence, SJ. & Lawther, R. Behaviour of Cavity Brick Walls Exposed to Fire, Proceedings of the 8th International BrickIBlock Masonry Conference (Dublin, 1988), Elsevier Applied Science, London, pp.981-990.

1518