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15 Load-bearingMasonry

R J M SutherlandFEng, BA, FICE, FIStructE

Contents

15.1 Introduction 15/3

15.2 Material properties 15/3

15.3 Codes of practice 15/3

15.4 Limit state principles 15/4

15.5 Unreinforced masonry 15/515.5.1 The mechanism of failure in

compression 15/515.5.2 Slenderness 15/815.5.3 Eccentricity of loading 15/815.5.4 Concentrated loads 15/915.5.5 Lateral loads on masonry panels 15/915.5.6 Stability and robustness 15/1015.5.7 Accidental forces 15/11

15.6 Reinforced and prestressed masonry 15/1115.6.1 General 15/1115.6.2 Structural performance of reinforced

masonry 15/1215.6.3 Uses for reinforced masonry 15/1315.6.4 Durability of reinforced masonry 15/13

15.7 Dimensional stability of masonry 15/13

15.8 Application of masonry and scope for future use 15/1315.8.1 High-rise (small-cell) residential

buildings 15/1315.8.2 Low-rise (large-cell) buildings 15/1415.8.3 Boundary walls 15/1415.8.4 Retaining walls 15/1415.8.5 Bridges 15/16

15.9 Conclusions 15/17

15.10 Acknowledgements 15/17

References 15/17

15.1 Introduction

In this chapter the word 'masonry' has been used to describeeither brickwork or blockwork as well as natural stone. Today,natural stone is seldom used except as a decorative facing andthe advice which engineers need on masonry relates primarily tobrickwork and concrete blockwork.

The use of brickwork in the UK has changed quite markedlyin the 20 to 25 years following its rebirth as a structural materialin the early 1960s. Full exploitation of its strength for slenderload-bearing walls in high-rise flats has now been halted by thesharp social reaction against this form of building. In thedomestic field what once looked like the greatest justification for'engineered' brickwork has given way to the limited demands oftraditional housing. However, there are now new challenges atleast as great as those of high-rise housing.

The structural use of concrete blockwork largely dates fromthe 196Os and its fortunes have followed the same path as thoseof brickwork.

One of the new structural challenges for masonry lies in theconstruction of buildings for sport, education, manufacturingand storage. Here the economy of masonry is being usedincreasingly in unframed buildings, often single-storey, withlarger spans than in domestic construction, taller walls and fewpartitions or returns to brace the whole assembly. As a result ofthese changes, today's engineering problems with masonry inbuildings are largely wind resistance, overall stability and com-posite action with floors and roofs. Crushing strength takessecond place.

Another field where masonry is finding increasing favour is inthe cladding of framed construction especially in large industrialunits built in steelwork. In this case not only are there problemsof the lateral strength of large thin panels but there are complexquestions of movement and of the compatibility of the differentmaterials.

All these are very much engineering problems and not mattersof architectural opinion.

In civil engineering, the once dominant place of masonry wastaken about a century ago first by mass concrete and then byreinforced concrete. Concrete may be more in keeping with amechanized age than a labour-dominated material like masonrybut its appearance is increasingly being criticized and nowdoubts are arising as to its durability, especially when rein-forced. What is more, with growing appreciation of the struc-tural performance of masonry, especially when reinforced orprestressed, concrete has a rival both in slenderness and load-bearing capacity. This makes masonry particularly attractive forstructural use in retaining walls, bridge abutments and othercivil engineering works, particularly in areas which are visuallysensitive. There is also a good case for the revival of the masonryarch.

Engineers need to keep in touch with developments in the useof masonry. Today it is not just a craft material for houses ordecorative facings, as was thought 30 years ago, but a majorstructural element and one benefiting increasingly from engi-neering understanding.

15.2 Material properties

Before embarking on any structural design in masonry it isimportant to distinguish between the physical properties of thedifferent materials of which the units are made and to appreciatethe limitations of each.

Table 15.11 shows the types of masonry units normallyavailable with their materials, sizes, unit strengths and approxi-mate share of the UK market. It also gives the numbers of thecurrent British Standards which define the acceptable quality ofeach.

Table 15.2 gives some indication of the dimensional stabilityof each type of masonry unit, i.e. its response to changes intemperature, load and moisture content. Equivalent figures arealso given for other materials commonly used in construction.The most essential factors to note are the initial moisturemovements:

(1) Clay units are fired at a high temperature and expand, forthe most part irreversibly, as they take up moisture fromthe atmosphere. The expansion is greatest immediatelyafter firing but continues at a diminishing rate for effecti-vely about 10 to 20 years.

(2) Concrete units (bricks or blocks) are cast wet and shrink asthey dry out, again largely irreversibly. The shrinkage isgreatest immediately after casting but continues at a dimi-nishing rate for effectively about 10 to 20 years.

(3) Calcium silicate bricks, which are of sand and lime, hyd-rated, pressed and then autoclaved, behave similarly toconcrete units.

Not only are the initial moisture movements generally greaterthan any subsequent cyclic ones due to change of atmosphericconditions, but those of clay and concrete are of comparablemagnitude and in opposite directions.

This simple distinction between the behaviour of clay andconcrete has frequently been ignored in the past with resultswhich have sometimes caused major disruption. Today, nowthat the different properties of the materials are better under-stood, there is a tendency to over-react to the problems ofmovement and sometimes to take precautions which are un-necessary and could even be harmful. The question of pre-cautions against movement is discussed in section 15.7.

15.3 Codes of practice

In the UK, the accepted guidance on the way in which masonryshould be designed is given in the British Standard Code ofPractice BS 5628. The first part of this Code dealing withunreinforced masonry was issued in 1978.2 This part is thesuccessor to the greater part of the earlier code CP 111 and dealsessentially with walls and piers.

The second part of BS 56283 which covers the structural use ofreinforced and prestressed masonry was not published until1985. It makes good the wholly inadequate treatment of rein-forced masonry in CP 111 and also puts prestressed masonry onan 'official' basis for the first time. This part of the Code coversthe design of all types of spanning structures in masonry as wellas walls and piers.

The third part of BS 5628,4 also published in 1985, givesadvice on various aspects of detailing with masonry and onworkmanship, durability and similar topics. It could be said tobe more architecturally slanted than the first two parts of thisCode and is the successor to the earlier Code CP 121.

Since the issue of all three parts of BS 5628, masonry in theUK has been on a parallel basis to concrete in up-to-date andofficially recognized design methods. This does not mean that allan engineer needs to know about masonry is in the three parts ofthis Code - far from it. However, anyone designing masonrystructures, in the UK at least, should be aware of the contents ofthis Code and, whether experienced in masonry or a newcomerto it, would do well to consult the handbooks to Parts 1 and 2.References to these handbooks and to a selection of otherpublications on the structural design of masonry are given at theend of this chapter.5-6

While BS 5628, together with the relevant material standards,will be used as anchor points for the advice in this chapter, thisCode should be used for checking design rather than as a

* Equivalent based on 102.5 mm wall

starting-point. The wide variety of forms of structural brick-work and blockwork make their use even more of a designmatter, needing individual judgement, than almost any othermaterial.

It is the aim in this chapter to point to these design aspectsand to emphasize both the great opportunities for the use ofmasonry and some of the pitfalls, rather than to provide anotherhandbook to the BS Code.

Reference is made throughout this chapter to BS 5628.Readers working in countries other than the UK will need to

be aware of the local codes, which may differ quite markedlyfrom BS 5628. The following information may be helpful in thisrespect.

USThe most widely used code in the US is the Uniform BuildingCode. Chapter 24 of the 1985 edition deals with masonry on alinear elastic (working stress) basis.CanadaThe current code CAN-S304-M84 issued by the CanadianStandards Association covers both design by rules and designby full engineering analysis. This is still on a working stressbasis. A limit state code is planned for 1990.

AustraliaA unified code incorporating AS 1640-1974 (SAA BrickworkCode) and AS 1475 (SAA Blockwork Code, Part 1: unrein-forced blockwork and Part 2: reinforced blockwork) is aboutto be issued. This is written in ultimate strength format andwill be converted to a full limit state form in the next edition.New ZealandReferences to existing codes may be misleading but two newcodes are in draft DZ 4229 for masonry not requiring specificdesign and DZ 4210 for designed masonry.

The information given above is considered as a starting-pointonly. Readers are advised to check directly with the appropriateauthority in each country.

15.4 Limit state principles

The design guidance in BS 5628: Parts 1 and 2 for unreinforced,reinforced and prestressed masonry follows the same limit, stateprinciples, with partial safety factors, as are used with reinforced

Table 15.1 Types of masonry units normally available.

Clay brick(BS 3921)

Calciumsilicatebrick (BS187)

Concrete(BS 6073)

AggregateConcreteblock (BS6073)

Autoclaved(aerated)concreteblock (BS6073)

Material andmanufacture

Clay firedgenerally at> 100O0C toachieveceramic bond

Sand and lime;hydrated,pressed andautoclaved

Aggregate andcementhydrated andmoulded withpressure andvibration

Cement andground sandor PFA withaeratingagent hydratedand mouldedin largeblocks andthen cut

Normal (actual)dimensions ofunit(mm)

Standard 215x 102.5 x 65high

Metric modular(small demand)190x90x65(BS 6649)

Varies widely:length 390-590height 140-290thickness60-250

Type of unit

Solid,froggedorperforated

Solid, orfrogged

Solid orfrogged

Solid orhollow

Solid only

Characteristic strength of unit(N/mm2)

Range in RangeBritish commonly usedStandard

7-100 14-100

14^8.5 20.5-34.5

7-40 7^0

2.8-35 3.5-21

>2.8 2.8-7.0

Approximate shareof UK market1985(106m2of wall)

58*

1.75

4.5

Dense 30.4lightweight 22

23

* Concrete bricks similar

and prestressed concrete. Most engineers in the UK are nowfamiliar with these principles but, regrettably, there is still a lackof uniformity in the terminology used in the different BSmaterial codes.

In BS 5628, the phrases 'design load' and 'design strength' areused to denote the factored loads and strengths which need to becompared to check adequacy. Thus, for the ultimate limit state,if yf is the partial factor of safety for loading and ym is the partialfactor of safety for material strength, adequacy is achieved if:

Ultimate (characteristic) strength ^ characteristic load x 7f

In all cases (direct load, bending, shear, etc.) the partial factorsof safety are expressed separately in BS 5628 and never lostwithin the characteristic values quoted.

The same principle and the same terminology are used in BS5628 for the serviceability limit states and for precautionsagainst disproportionate collapse following a major explosionor other accident, but in these cases the partial factors of safetyare different.

Table 15.3 shows the principal factors for each limit state andhow these compare with the factors of safety used in BS 81107

for concrete. With unreinforced masonry the serviceability limitstates of deflection and cracking are seldom if ever relevant but,

when considering the behaviour of reinforced or prestressedmasonry sections in bending, they can be vital.

15.5 Unreinforced masonry

15.5.1 The mechanism of failure in compression

Rather than just accept the characteristic strengths and 'Code'factors of safety for masonry, designers are advised to considerwhat influences its strength and to try to visualize the actualmechanism of failure.

Table 15.4 lists some of the major factors affecting thestrength of a masonry wall.

The mechanism of failure seems to be generally agreed.Because the mortar is almost always weaker than the masonryunits it tends to be squeezed out of the joints. This movement ofthe mortar is restrained by the bricks or blocks, which are thussubjected to lateral tensile stresses which lead first to splittingand finally to collapse. This mechanism is shown diagrammati-cally in Figure 15.1.

Even under absolutely uniform downward loads, masonrywalls - brick or block - fail first due to vertical splitting. This isvirtually universal. With brickwork, the wall strength averagesabout 0.3 times (0.15 to 045 times) the brick strength and with

Table 15.2 Dimensional stability of masonry compared with reinforced concrete and steel

Claybrickwork

Calciumsilicatebrickwork

Aggregateconcreteblockwork*

Aeratedconcreteblockwork

Reinforcedconcrete

Steelwork

Coefficient ofthermalexpansion(per 0C XlO" 6 )

5-8

8-14

6-12

Approximately 8

7-14

Approximately 12

Movementas resultof 2O0Cchange(%)

0.010-0.016

0.016-0.028

0.012-0.024

0.016

0.017-0.028

0.024

Unrestraineddrying shrinkage(partlyreversible)(%)

Shrinkage ofmortarallowed for inexpansionfigures (right)

0.01-0.04(BS limit 0.04)

0.02-0.06(BS limit 0.09maximum)

0.02-0.09(BS limit 0.09maximum)

0.02-0.10

Unrestrainedmoistureexpansion(%)

Depends ontype of clayand firingtemperature.Probably0.02-0.12%.Precisefiguresuncertain.Too few long-term tests

-

—

-

;

Elasticmodulus(kN/mm2)

4-26

14^-18

4-25

1.5-4.0

15-36

175-210

Creep with time:creep factor =final strainjelastic strain(for stress ^0.5x uh.)

1.2-4.0

Approximately 2.5

2.0-7.0

No test resultsavailable

1.0-4.0

Figure 15.1 Simplified failure mechanism for vertical loads onmasonry

concrete blockwork it averages about 0.8 times the blockstrength.1

The difference between the apparent performance of bricksand blocks in walls compared with their individual strengths isprimarily due to the shape of the units. The 'cube' strength ofthe concrete in the blocks is generally well below the equivalentstrength of the fired clay in bricks but, when tested as units, thetaller blocks fail at a stress nearer to that in a wall than the squatbricks, which are more fully restrained laterally by the platens ofthe testing machine. This is shown in Figure 15.2. The logicalclimax is that a storey-high unit should fail at the same load asthe wall into which it is built.

Figure 15.3 shows the relationship, as given in BS 5628, Part1, of the characteristic compressive strength of different types ofmasonry to that of the individual masonry units. This followsthe principles outlined above.

Other important factors affecting the capacity of a wall orpier to support vertical loads, apart from those shown in Table15.4, are slenderness, eccentricity and concentration of loading.

Mortar squeezed outby vertical pressurebut restrained laterallyby brick/block

Lateral tensilestress in brick/blockbalances restrainton mortar but eventuallysplitting takes place

Note: Partial factors of safety for load (y f) with masonry similar to those for concrete (basically 1.4 for dead load and 1.6 for superimposed load with variations forcombinations and different limit states)

Table 15.3 Partial factors of safety (material) for masonry compared with concrete (BS 5628 and BS 8110)

BS 5628: 1978,Part 1

Y mm (compression)

ymv (shear)

7 m (wall ties)

( Unreinforced masonry)

Control levelManufacturing and site specialManufacturing normal and site specialManufacturing special and site normalManufacturing and site normal

BS 5628:1985, Part 2 (Reinforced and prestressed masonry)

Y mm (compression)

Y mv (shear)

Y mb (bond to steel)

7 ms (steelreinforcement)

Control levelManufacturing and site specialManufacturing normal and site special

BS 8110:1985 Parts 1 and 2 (Concrete)

Y m (compressionor bending

Y m (shear)

Y m (bond to steel)

Y m (steelreinforcement)

-

Ultimatelimit state

2.52.83.13.5

2.5

3.0

2.02.3

2.0

1.5

1.15

1.5

1.25

1.4

1.15

Accidentaldamage

1.251.41.551.75

1.25

1.5

1.01.15

1.0

1.0

1.0

1.3

1.0

Serviceabilitylimit state

1.51.5

1.0

1.05

1.05

1.05

1.05

Notes

No shearreinforcementassumed

Table 15.4 Major factors influencing the strength of masonry walls

Source: Sutherland (1981) 'Bride and block masonry in engineering'. Proc. Instr. Civ. Engrs, 70, Table 2.

Variable factor

Strength ofmasonry unit

Strength ofmortar

Thickness ofmortar bed

Geometry ofmasonry units

Bond

Poor fillingof bed joints

Poor fillingof perpendicularjoints

Effect on wall strength

Brickwork

Most dominant factor: wall strength proportional tosquare root of brick strength

Not very significant: wall strength proportional to cuberoot of mortar strength for middle range of brickstrengths

Fairly critical: 17 mm bed instead of 10 mm gives 30%reduction; with ground faces and no mortar, wallstrength approaches brick strength

Ratio of wall strength to brick strength little affectedwhether wirecut, deeply frogged or perforated

English (50% cross-bonded material)

No noticeableFlemish (33% cross- difference in

bonded material) strength

Stretcher (100% cross- Up to 40% strongerbonded material) than English or

Flemish

Collar j ointed 1 0- 1 5 % weaker(steel ties only than English orbetween skins of Flemishstretcher bond butno cavity)

Tests show 30% reduction in strength common,and more possible

No reduction found in tests even with whollyunfilled perpendiculars

Concrete blockwork

Most dominant factor

Little effect on wall strength

No experimental data; effect probably lesssignificant than with bricks

Ratio of wallto blockstrength about0.8

Ratio of wall to block strengthreduced to about 0.5 with normalbond because cross-webs do notline up; higher with stack bond

Seldom used other than in stretcher bond(or in stack bond with reinforcement in

horizontal joints)

No equivalent tests

Effect of not filling perpendicular jointsat all is small

Compressive strength of individual masonry unit: N/mm2

Figure 15.3 Relationship of characteristic compressive strength ofmasonry to the compressive strength of the units (BS 5628)

15.5.2 Slenderness

Tests on walls in recent years have shown that slenderness is lessof a problem than it was thought to be 30 years ago and, insuccessive British Codes, reductions in load-carrying capacityfor slenderness have tended to become smaller.

Reduction factors for slenderness tabulated in BS 5628 permitwalls with a slenderness (effective height/thickness) of up to 27.Thus, for instance, allowing for end fixity a half-brick (102.5-mm thick) wall could be 3.7 m high and still carry 40% of theaxial load appropriate for a low-height wall of the samethickness. Such a slim wall, as shown in cross-section in Figure15.4, must lack robustness and most designers would prefer notto extend slenderness to this limit even with purely axial loading.

15.5.3 Eccentricity of loading

Simple eccentricity can be dealt with conservatively by using theappropriate reduction factor for slenderness and eccentricity inBS 5628 and assuming that the eccentricity at the top of any wallreduces to zero at the next level of lateral support below asshown in Figure 15.5(a). In practice, the eccentricity at thebottom of the wall is likely to be as in Figure 15.5(b) which isnormally more favourable as it tends to counteract any furthereccentricity applied at that level.

As an alternative to this simple procedure, the whole structurecan be analysed rigorously as a frame.

Figure 15.5 Line of thrust due to eccentric loading: (a) simplifiedassumption on line of thrust (BS 5628); (b) more likely line ofthrust

Unless loaded heavily enough to provide fixity as in Figure15.6(a), the rotation of the ends of floor slabs can causeunsightly cracking as in Figure 15.6(b). This is particularly likelywith long-span concrete roofs or upper-floor slabs supported onmasonary walls. In such locations the cracking can usually beeliminated, or at least controlled, without over-stressing themasonry, by allowing rotation as shown in Figure 15.6(c).

Figure 15.2 Effect of platen restraint on failure of bricks andconcrete blocks under test

Block shape

Figure 15.4 Maximum slenderness of wall (BS 5628)

Brick shape

Chara

cteris

tic co

mpres

sive

stren

gthof

maso

nry (N

/mm^

)

Figure 15.6 Avoidance of cracking of masonry walls due torotation of floor slabs at supports, (a) Slab rotation resisted bycouple Wx. No cracking of masonry, (b) Load W not great enoughto resist slab rotation. Upper wall lifted by rotation of slab and crackinduced at point (C). (c) Load Wnot great enough to resist slabrotation, but soft pack as shown permits rotation without crackingof masonry walls

15.5.4 Concentrated loads

Traditionally, the need to spread concentrated loads fromcolumns or the ends of beams has been met by using padstonesof greater strength than the basic masonry of the walls or bylaying local courses of stronger brick or stone. Today, thispractice has been formalized in rules such as those in BS 5628.The problem is one of splitting and is most serious at the ends ofwalls or at piers where the splitting is most likely to lead tofailure.

Such splitting could also be caused - or accentuated - byhorizontal forces due to thermal or other movements. This is

(b) Beam ends carried well back along supporting wallFigure 15.7 Concentrated loads on ends of walls or on stiffeningpiers

shown in Figure 15.7(a). It is something which is not explicitlycovered in most Codes of Practice but which designers shouldconsider. With long-span beams especially, the bearing shouldbe carried back as far as practicable or a reinforced padstoneshould be used as indicated in Figure 15.7(b).

15.5.5 Lateral loads on masonry panels

The design of masonry walls so that they can be shown to resistwind forces is one of the major areas of doubt in the treatmentof the material.

The uncertainty is greatest with panel walls with no verticalload except their own weight, but it is present with most thin orlightly loaded walls.

Following an extensive series of tests by the British CeramicsAssociation8 some partly empirical and partly theoretical guid-ance has been incorporated in BS 5628, Part 1. This can be usedto demonstrate adequacy in many of the most common situa-tions but with vertical spanning in particular it is restrictivecompared with what has been common practice for many years.

Where it can be shown to be appropriate, design for arching isa very good method of proving lateral stability; it, too, isrecognized in BS 5628.

Real walls do sometimes blow out, or over, and those whichdo always seem to have a slenderness or lack of restraint faroutside recommended limits. Some unlikely 'successes' may wellbe due to arching - even unexpected arching - and some tomuch higher tensile strengths in mortar joints than thosegenerally assumed. In many cases the full 'Code' wind forcesmay never have occurred and may never occur in the future.

Much research is still being carried out on the resistance ofmasonry panels to lateral loads and it is hoped that furtherguidance may be given in future amendments to BS 5628. In themeantime, it is worth keeping in touch with the results of thisresearch.

Two points of detail which designers would do well toremember are these:

(1) Whatever the published bending strengths, bending in ahorizontal plane as in Figure 15.8(a) is much more reliable

Soft pack

Splitting caused byvertical load fromend of beam

Splitting dangerously accentuatedby horizontal force due toshrinkage or thermal movement

Figure 15.8 Resistance of masonry panels to lateral loads: (a)Strength in bending in horizontal plane; (b) greater and morecertain than in vertical plane

in practice than in a vertical plane as in Figure 15.8(b).Tensile strength across bed joints can easily be disruptedespecially during construction after the mortar has set butbefore it has gained its full strength.

(2) The edge support to panels will often be altered - some-times for better and sometimes for worse - by the deforma-tion of the structure, as can be seen from Figure 15.9. It isimportant to consider such possible movements and tomake sure that adequate fixings are used at all edgesassumed to provide restraint, even if there are relativemovements due to deflection, shrinkage or settlement.

15.5.6 Stability and robustness

The great bulk of the advice in the world's masonry codes isdevoted to stresses and compressive strength with only passingexhortations to consider stability. This seems curious when oneconsiders that almost all failures of masonry structures - few inpractice - have been due, not to overstressing, but to some formof instability. The reason may be that attempts to codifystability, while helpful in some circumstances, have tended to

Figure 15.9 Possible effects of deformation on lateral support formasonry cladding

lead to anomalies, unreasonable restrictions, or even newdangers, in others.

Stability and robustness are best seen as design matters. Theyneed thought rather than rules.

Stability is a particularly important factor with masonrybecause of its low tensile strength. Unless reinforced or pres-tressed, masonry should generally be planned to rely for stabi-lity on gravity forces and on the friction induced by such forces.

With today's thin walls, this stability depends on the interac-tion of walls, floors and roofs. A lateral force acting on the faceof a wall, such as that due to wind, is transferred to floors androofs, which act as rigid horizontal plates and in turn transferthe force to the ground through shear walls running in thedirection of the force. This is shown diagrammatically in Figure15.10.

Designers should always follow the forces through this routeto make sure that all the connections are adequate, that thefloors and roofs are stiff and strong enough and that the'racking' shear strength of the shear walls is great enough. Evenif the structure is fully sheltered from wind it is important toconsider a nominal lateral force acting in any direction and to

Roof braced in eitherhorizontal planes (B) and(C) or on slope (A)and horizontal plane (B)to form effectivelyhorizontal plates

Critical connections, or joints, to transfer forceto the groundLaterally loaded wall to horizontal plates: Joints 5 and 6Horizontal plate to shear walls: Joints 1,2 and 4Shear wall to lower horizontal plate: Joint 3

Figure 15.10 Diagrammatic representation of transfer ofdisturbing force on masonry structure to provide stability

Failure depends onbreaking of the masonryunits or shearing ofthe bed joint mortar

Failure depends onlyon the tensile strengthof the bed joint mortar

Worse than planned: creepdeflection of slab atlevel A reduces panel toone way span

Better than planned: creepdeflection of slab at Bloads panel (even saythrough window) and improveslateral stability

Figure 15.11 Relative strengths of three possible methods ofconnecting on to a shear wall

follow this to the ground. British Standard 5628 recommends anominal lateral force of 1.5% of the dead load above the levelbeing considered. A larger force may sometimes be preferable.

One of the most difficult questions with masonry is theassessment of the strength of connections. Figure 15.11 showsthree means of connecting on to a shear wall. Designers woulddo well to opt for the strongest connection which is compatiblewith cost and other requirements rather than accept the lowestlevel of apparent adequacy.

There are few absolutes in design for stability. The skill lies in

balancing requirements which are often in conflict. This isillustrated in Figure 15.12. There is no virtue in just achievingwhat appears to be adequate stability if an unnoticeable changecan make this much more certain. Accidental forces andmaterial defects are largely unpredictable.

15.5.7 Accidental forcesDesign to allow for accidental forces is just a particular case ofdesign for stability. Rules introduced following the partialcollapse of the large panel concrete structure at Ronan Point in1968 have largely blinded engineers to the broad issue ofaccidental forces. These rules, which apply only to buildings offive storeys and above, were made to guard against gas explo-sions and do not in themselves ensure safety against all hazards.

The thinking was rightly to limit damage but the tying forcesintroduced in the structural codes to satisfy the rules may insome cases actually spread the damage. This is illustrated inFigure 15.13 where continuous vertical ties could actually causeprogressive collapse as was shown in tests on a quarter-scalemodel at the Building Research Establishment.9

Accidental damage can be limited by planned sacrifice or bygreater structural strength. The choice is a design matteralthough the solution must satisfy the broad functional require-ments of the Building Regulations in most practical cases.

15.6 Reinforced and prestressedmasonry

15.6.1 General

Much of the previous section on unreinforced masonry stillapplies but, when reinforced or prestressed, the scope for the useof masonry is greatly extended. Reinforced or prestressed beamscan be made, as well as walls, following exactly the same generalprinciples as those used with concrete. This has been demon-strated in tests and in practice.10 The question to be decided iswhen such forms are advantageous, and this can be done onlywith some knowledge of the possible structural performance of

In situ two-way reinforcedconcrete

Composite precast(two-way)

Precast with lateraland longitudinalties

Timber board andjoist with staggeredjoints

Precast planks withno lateral ties andno continuity or tiebars at supports

Flooring options

Wall layout options

Figure 15.12 Comparison of wall and floor options for simplemasonry cross-wall construction

Moststurdy

Leaststurdy

Moststurdy

Leaststurdy

Incr

easin

g stu

rdine

ss o

f wal

ls

Incr

easin

g stu

rdine

ssof

floo

rs

Figure 15.13 Diagrammatic representation of how simple coderecommendations can spread accidental damage, (a) Horizontal tiesbroken by explosive force. Continuous vertical ties (as codes) keepwell intact which bows out dropping ends of several floors aboveand below, (b) Independent staggered ties would allow localsacrifice and confine damage

reinforced or prestressed masonry. It is convenient, in thiscontext, to compare the properties of masonry with those ofconcrete.

15.6.2 Structural performance of reinforcedmasonry

Figure 15.14 shows the relative bending strengths of reinforcedbrickwork (BS 5628, Part 2) and reinforced concrete in accord-ance with BS 8110. It can be seen that with moderate brickstrengths the bending capacity of brickwork matches thatrequired for most reinforced concrete and that, even with thelowest brick strength likely to be used (20 N/mm2 unit strength),there is a very useful level of bending strength. A similar

Figure 15.14 Comparison of bending strength of reinforcedbrickwork and reinforced blockwork

comparison can be made with concrete blockwork although thehighest practical bending strength with blockwork is not sogreat.

With shear, the comparison is not so favourable to masonryas it is with bending. Figure 15.15 shows - again for brickwork -that without shear reinforcement the shear strength of allstrengths of brickwork is well below that of reinforced concrete;the same is true of concrete blockwork. It is worth noting thatthe shear strength is virtually independent of the strength of themasonry units, being dependent on the tensile strength of themortar and its bond to the units.

One advantage of prestressing over reinforcing with anymaterial is that the prestress reduces the principal tensile stressand thus increases the shear capacity. With masonry this is avery real advantage but, curiously, no reference has been madeto it in BS 5628, Part 2. The omission does not mean thatdesigners cannot take advantage of this property of prestressing.

Design resistance moments

Figure 15.15 Comparison of shear strength of reinforcedbrickwork and reinforced concrete

Design shear strength

Notes:(1) No shearsteel(2) Shear

strengthvarieswithproportionof mainsteelIn some cases

with short shearspans this canbe increasedto a max. of0.87

Concrete (BS 8110:1985)Brick (BS 5628:Part 2)

Brick (BS 5628:Part 2) Concrete (BS 8110)

Designstrength(N/mm2)

Char, cubestrength(N/mm2)'cu

Design shearstrength(N/mm2)

Unitstrength(N/mm2)

Mortar (i) : specialmfg. control

Unitstrength(N/mm2)

DESIGN P.M.Md(N. mm)Explosion

It has been an accepted feature of prestressed concrete for atleast 40 years.

Compared with reinforced masonry, there has been littlepractical experience with prestressed masonry and there is aschool of thought which considers that it would have been bestto omit it altogether from BS 5628 at this time. Nevertheless, ithas been used successfully, as is shown later in this chapter, andmay be used more in the future.

15.6.3 Uses for reinforced masonry

Not only is the shear strength of masonry low, but it is verydifficult to incorporate shear reinforcement in it to increase this.For both these reasons, masonry compares unfavourably withconcrete for use in beams except possibly in special cases such asthat of a deep beam within the plane of a wall. However,reinforced masonry comes into its own in laterally loaded wallswhere the low shear strength is seldom a major problem.

Masonry is essentially a wall material, or one for arches andvaults. Traditionally, it used to depend for stability on its massbut today, with reinforcement, laterally loaded walls can bemade of comparable slenderness to those in reinforced concrete.Further, while concrete walls, if visible, are increasingly beingfaced with masonry, reinforced masonry has its elegance builtinto the structure. Figure 15.16 shows a number of ways ofreinforcing masonry walls using standard bricks and blocks.

Vertical-spanning reinforced masonry

carbon steel and to galvanized reinforcement in masonry fordifferent levels of exposure.

With stainless steel reinforcement there is no need for anycover to the steel specifically for durability. Although the supplycost of the steel is several times that of normal carbon steel, thepercentage extra on the whole project tends to be very smallonce fixing and all the other costs of the construction areincluded.

Stainless steel is being used increasingly for ties and fixings inmasonry as well as for reinforcement and is no longer theexorbitantly expensive material it used to be.

15.7 Dimensional stability of masonry

The order of unrestrained movements of clay and concreteproducts is shown in Table 15.2. Because of the restraints whichexist to some extent in all real buildings these movements tendto be less than one would calculate from the tabulated figures.The vital question is how much less, and the answer must be thatat present we do not know.

British Standard 5628, Part 3 recommends an allowance of1 mm of movement per metre for clay brickwork, with move-ment joints a maximum of 15 m apart. For calcium silicatebricks joints are recommended at 7.5 to 10m and at 6m forconcrete bricks and blocks.

It is worth remembering what these joints are for. In the caseof clay brickwork they are needed primarily for expansion andthus, to be effective, must be wide enough and filled withsomething soft enough to allow the expansion to take place. Inthe case of calcium silicate bricks and all concrete units thejoints are needed mainly for shrinkage and effective sealingbecomes most important.

As in the case of movement joints in concrete structures, thereis some evidence that the recommended cure for movementproblems has not always been effective. The cure may evenintroduce new problems of maintenance. A recent study for theConstruction Industry Research and Information Association(CIRIA)11 showed that, both for clay and concrete units,adherence to the spacing then recommended in CP 121 failed toeliminate noticeable defects in a significant number of cases,while in quite a large proportion of other cases no noticeabledefects were found even in walls well beyond recommendedlimits of unbroken length.

The problem of movement is too complex for simple rules.What is more, in many cases it may be most economical, and inthe long term most satisfactory, to reduce the number of joints,risk some cracking and repoint after a few years.

Designers would do well to study case histories, observe realbuildings and then try to recognize the situations where move-ment may be serious and those where damage, if it occurs, isonly of a cosmetic nature. Figures 15.17 and 15.18 show somekey factors but these should only be considered as examples.

15.8 Application of masonry andscope for future use

15.8.1 High-rise (small-cell) residential buildings

Unreinforced masonry has formed the sole vertical support toresidential buildings of up to at least 18 storeys while withvertical reinforcement it has been used in blocks of over 20storeys even in seismic zones. In most cases the design has beendominated by the need for resistance to lateral forces and theassessment of interaction between floors and walls to achievethis. Perhaps surprisingly, tension has often proved more of aproblem than compression.

* Essentially vertical span but secondary horizontal reinforcement often used in bed jointst Full two-way span possible (or horizontal only). (Grout is the term used in the USfor fine high slump concrete.)HORIZONTAL-SPANNING REINFORCED MASONRYNormally used for light loads only (except grouted cavity type).Reinforcement normally small bars or mesh set in bedding mortar.Equally suitable for brickwork and blockwork.

Figure 15.16 Typical methods of reinforcing masonry walls

15.6.4 Durability of reinforced masonry

As with reinforced concrete, durability is a factor which isreceiving increasing attention today. British Standard 5628, Part2 gives clear and full recommendations on cover to normal

Modifiedquetta bond*Bars set infine concrete

Brick

Used inair-raidshelters

Quetta bond*

Bars normallyset in mortar

Brick

Firstdevelopedfor earth-quake resist-ance

Grouted cavityt

Bars set in fineconcrete or'grout'Brick (orconcrete block)225 mm wall alsopossible withbars in mortar(stainless steeladvisable ifexposed)

Pocket wall*

Bars set innormalconcreteBrick

Mainly usedfor earthretaining walls

Filled hollowblock*Bars set infine concrete

ConcreteblockWidely usedin the US

Figure 15.17 Typical serviceability problems due to horizontalmovements in masonry

With the increased development of computer programs in thelast 15 years the design of such buildings should be much easierthan it was in their heyday in the mid 1960s. There are also somesigns of a revival in the popularity of high-rise flats and hostels.

15.8.2 Low-rise (large-cell) buildings

The challenge of extending the economic use of masonry, asproved in domestic construction, to larger-cell buildings such assports halls or warehouses has been mentioned at the beginningof this chapter.

Here the downward loads tend to be small but the walls mayneed to span vertically 2 or 3 times as far as in housing. This hasbeen achieved in frameless construction with deep ribs at regularintervals, emphasized architecturally, or by making the wallscellular. In such cellular walls - popularly known as 'dia-phragm' walls - the masonry is placed in the most efficient wayto resist lateral forces (Figure 15.19a). The walls are usuallydesigned to span from the ground to a roof which is braced totransfer the load to shear walls as already discussed. Sometimessuch walls are prestressed with vertical cables either bonded orin voids as shown in Figure 15.19(b), or they can be reinforced.Care is needed during construction to make sure that the wallsdo not fail in wind before the bracing of the roof is provided.

15.8.3 Boundary walls

Masonry has proved itself for boundary walls over a very long

period. Such walls, either of constant section or with whollyinadequate stiffening piers, frequently defy all probability ofstability but have given good service for decades or centuries;some such walls survive even in spite of considerable bulges ortilts. Nevertheless, from time to time they do blow over andthere is no justification for building unstable boundary wallstoday.

Stability can be achieved by an irregular planform or byreinforcing vertically or prestressing or by any combination ofthese. Reliance on the tensile strength of the masonry across thebed joints is unwise except on a very small scale. The forms ofreinforcement shown in Figure 15.16 can be used or stable wallsof very slender sections can be built with special or cut bricks asshown in Figure 15.20. The use of stainless steel is advisable inmost boundary walls because of their extreme exposure.

Shear strength is virtually never a problem with boundarywalls.

15.8.4 Retaining walls

Reinforced masonry has proved to be particularly suitable forretaining walls, either on a small scale associated with housingor on major civil engineering works. It is hard to understandwhy it has not been used more. Reinforced concrete walls areoften faced in masonry for appearance. Why not use themasonry structurally? The answer to this must be that it islargely habit which prevents engineers from thinking of rein-forced masonry, or fear which leads to unduly high pricing of

HORIZONTAL MOVEMENT (CLAY UNITS)Movement may cause sliding ond.p.c. (often scarely noticeable.)Movement may cause cracking(if not cyclic can be repairedlocally)Restraint at d.p.c. will resistsliding but may accentuatetendency to cracking as in (2)aboveShort 'returns' in plan a majorsource of cracking due toexpansion (most in need of'protection' by vertical jointsto absorb movement).Movement often reduced oreliminated by restraints ofmany sorts, but local restraintsat movement planes, as due tofailure to continue cavity trayDPCs around corners, a frequentcause of cracking.Cracking here due to thermal

movements of roof slab can usuallybe eliminated by sliding joint locallybetween roof and wall

Load-bearingcross-walls

Concreteslab

HORIZONTAL MOVEMENT (CONCRETE ORCALCIUM SILICATE UNITS)

With concrete blockwork theneed for vertical movementjoints is generally greaterthan with clay brickwork.Load-bearing blockwork wallsare frequently disrupted by

cyclic movements at roof levelunless local sliding jointsare provided.Marked changes of section asover doorways (A) are afrequent source of shrinkagecracking. Full height doorframes as (B) are preferablewith concrete brickwork.

Crack DPC stops short

Overhangsay 5-15mm

DETAILX

Figure 15.18 Relative movements of masonry cladding andconcrete frames: diagrammatic representation based on two storeys.Real problems tend to be confined to multi-storey building

the slightly unfamiliar. However, today there is enough evidenceof successful construction of retaining walls in masonry to provethat they are easy to build, and perform well.

Brick retaining walls have been built successfully in Quettabond, in grouted cavity construction and using the 'pocket wall'technique. With concrete blockwork the fixing of reinforcingbars in the filled hollows in the blocks has become quitecommon. The planforms of such walls are shown in Figure15.16.

Unlike boundary walls, retaining walls normally need onlyresist lateral forces in one direction. Thus, for cantilevers thereinforcement should be as near to the loaded face as possible.The pocket-type of wall is ideal in this respect. Figure 15.21shows how such walls are formed, with the thickness increasingto match the increasing bending moment and the reinforcementas close to the rear face as practicable. In some cases the steps inthickness have been repeated two or three times.

In pocket-type walls the reinforcement is surrounded in denseconcrete whose compaction can be checked once the small backshutter to the pocket is removed. Thus, the durability is equiva-

lent to that of reinforced concrete but the compressive force isresisted not by the concrete but by the brickwork.

Pocket-type retaining walls have been built in the US withheights of up to 7.3 m.12 There are now quite a few major wallsof this type in the UK used for bridge wing walls and similarpurposes. One British pocket-type retaining wall approximately4 m high was monitored for 517 days after which the deflectionwas only 16mm. As expected, the movement was apparentlycontinuing but tailing off.13

The design of pocket-type walls is covered in BS 5628, Part 2which deals with concrete cover, pocket spacing and workman-ship as well, of course, as with structural design. In somecircumstances, the characteristic shear strengths in BS 5628,Part 2 may prove a restriction on the performance of such walls,although tests on actual walls have almost all shown failure inbending. There is a good case for revising the shear clauses in BS5628 especially in relation to retaining walls.

One objection which is sometimes raised to the use ofmasonry retaining walls on civil engineering projects is speed ofbuilding. This objection may or may not be real but with

OUTER SKIN SUPPORTEDON NIBS

MASONRYWITHINFRAME

No disruption likely Possible severe disruption Disruptionuncertain

Disruptionunlikely

Original level

Claybrickouterskin

Concreteblockinnerskin

Outerskinsupportedon nib

Brickslips Concrete

nib

Concentratedand eccentricforce onouter skinprior todisruption

Brick strong,stiff andwith highexpansionLoad largelytransferredto, andcarried by,brickwork.

Overallexpansionpossiblebut unlikely

Brick weakand compressiblewith lowexpansionBrickworkcompressed andload, initiallytransferredto brick, largelyreturning toframe.Overall verticalcontraction.

SECTIONALELEVATIONASBUILT(Concreteframe andcavitycladdingin mixedmaterials)

Frame and concrete blockworkboth shrink while brickworkexpands. Most vertical loadshifts - eccentrically - on toouter skin only.Forces tend to push off slipbricks, crack nib and bowcladding outwards.

As M butcladding allclay brick

Behaves as (a)or (b) providedthe inner skincan supportwhole loadtransferred fromframe.

As(c)but claddingall concreteblock.All materialsshrink anddifferentialmovementssmall.

Figure 15.20 Thin but stable boundary walls formed with specialor cut bricks (concrete blocks also as in Figure 15.16)

Sectional elevationof wall

Figure 15.21 Typical pocket-type retaining wall

pocket-type walls, prefabrication is a very real possibility. Thiswas demonstrated by a trial some years ago.1

15.8.5 BridgesMasonry bridges have generally proved more durable than

Figure 15.19 Typical forms of cellular (diaphragm) wall, (a)Unreinforced (wall spans from ground to roof), (b) Prestressed (orreinforced). Wall acts as vertical cantilever

Reinforced orprestressed

Wall proppedby roof

Sectionalplan B-B

Prestressed(cables could beunbonded in thecells but groutedcables built inpreferable)

Sectionalplan A—A

Earthpressure

Sectional plan B-B

Sectional plan A—AStainlessbar

Detail X

Speciallymouldedbrick

Standardbrickcut

those of iron, steel or concrete. There are thousands of themunder roads and railways and over canals which have hadminimal maintenance during 100 years or more. They haveadapted themselves to settlement without distress and still lookattractive. Tests have been made in recent years, both in thelaboratory14 and by the Transport and Road Research Labora-tory on actual arch bridges. Our understanding of the behaviourof masonry bridges is better today than it ever was but in spite ofthis we continue to use slab or beam bridges under highwayswhich are subject to corrosion due to rain and de-icing salt.

In the future, there seems a clear case for using more masonryarches, possibly combining mass concrete with brickwork. Sucharches would be particularly appropriate for medium spanswhere piped culverts are too small but major long spans are notneeded. Useful guidance on the appraisal of existing masonrybridges is given in the Departmental Standard BD 21/84 and theassociated Advice Note BA16/84 both issued by the Depart-ment of Transport.15 These documents are also relevant, at leastin part, to new construction.

15.9 Conclusions

With the publication of all three parts of BS 5628, the UK isprobably leading the world in recognized guidance on masonrydesign. Further, in research, the UK has taken a leading role for20 years. It is now up to design engineers to make full use of thisrediscovered material.

15.10 Acknowledgements

Tables 15.1 to 15.3 were first published in the Author's paper1

but have been brought up to date. Figures 15.1, 15.3, 15.16 and15.21 and parts of Figures 15.19 and 15.21 have been adaptedwith the permission of Thomas Telford Ltd from those in thispaper. Figures 15.10, 15.11, 15.12 and 15.13 have been adaptedwith the permission of the Institution of Structural Engineersfrom those already published in the Author's paper.9 TheAuthor wishes to thank those concerned for permission toreproduce this material.

References

1 Sutherland, R. J. M. (1981) 'Brick and block masonry inengineering.' Proc. Instn Civ. Engrs, Part 1, 70, 31-63. [Tables15.1, 15.2 and 15.3 are updated versions of tables first publishedin the above paper.]

2 British Standards Institution (1978) Code of practice for use ofmasonry. BS 5628, Part 1: 'Unreinforced masonry.' BSI, MiltonKeynes.

3 British Standards Institution (1985) Code of practice for use ofmasonry. BS 5628, Part 2: 'Structural use of reinforced andprestressed masonry.' BSI, Milton Keynes.

4 British Standards Institution (1985) Code of practice for use ofmasonry. BS 5628, Part 3: 'Materials and components, design andworkmanship/ BSI, Milton Keynes.

5(a) Haseltine, B. A. and Moore, J. F. A. (1981) 'Unreinforcedmasonry.' In: R. G. D. Brown (ed.) Handbook to BS 5628:structural use of masonry, Part 1: Unreinforced Masonry BrickDevelopment Association, Windsor, (b) Roberts, J. J., Edgell, G.J. and Rathbone, A. J. (1986) 'Palladian.' Handbook to BS 5628,'Structural use of reinforced and prestressed masonry.' ViewpointPublication No. 13.028, London.

6(a) Hendry, A. W. (1981) Structural brickwork. Macmillan, London,(b) Curtin, W. G. et al. (1982) Structural masonry designers'manual. Granada, St Albans; (c) Gage, M. and Kirkbride, T.(1980) Design in blockwork, 3rd edn. Architectural Press, London,(d) Orton, A. (1986) Structural design of masonry. Longman,Harlow.

7(a) British Standards Institution (1985) Structural use of concrete. BS8110, Part 1: 'Code of practice for design and construction.' BSI,Milton Keynes; (b) British Standards Institution (1985) Structuraluse of concrete. BS 8110, Part 1: 'Code of practice for specialcircumstances.' BSI, Milton Keynes.

8 West, H. W. H., Hodgkinson, H. R. and Haseltine, B. A. (1977)The resistance of brickwork to lateral loading; Part 1:Experimental methods and results of tests on small specimens andfull-sized walls.' Struct. Engr 55, 10, 411^*21.

9 Sutherland, R. J. M. (1978) 'Principles for ensuring stability.'Symposium on stability of low-rise buildings of hybrid construction.Institution of Structural Engineers, 5 July 1978, London, pp. 28-33.

10 Bradshaw, R. E., Drinkwater, J. P. and Bell, S. E. (1983)'Reinforced brickwork in the George Armitage office block, RobinHood, Wakefield.' Struct. Engr. 61A, 8, 247-254.

11 Construction Industry Research and Information Association(1987) Movement and cracking in long masonry walls. CIRIAPractice note. (To be published.)

12 Abel, C. R. and Cochran, M. R. (1971) 'A reinforced brickmasonry retaining wall with reinforcement in pockets.' In:H. W. H. West and K. H. Speed, (eds) SIBMAC Proceedings,International Brick Masonry Conference. British Ceramic ResearchAssociation, Stoke-on-Trent. pp.295-298.

13 Maurenbrecher, A. H. P. (1977) A pocket-type reinforcedbrickwork retaining wall. Structural Clay Products, Potters Bar,SCP Publication No. 13.

14 Sawko, F. and Towler, K. (1982) 'Load-bearing brickwork:structural behaviour of brickwork arches.' Proc. Br. Ceramic Soc.,30, 7, 160-168.

15(a) Department of Transport (1984) The assessment of highwaybridges and structures. Roads and Local Transport Directorate.Advice Note No. BA16/84. HMSO, London, (b) Department ofTransport (1984) The assessment of highway bridges and structures.Roads and Local Transport Directorate. Departmental StandardBD21/84. HMSO, London.