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Blockwork Diaphragm Walls in I reland

FINBAR MC SWEENEY Ove Arup & Partners Ireland

10 We llington Road , Dublin 4, I reland

ABSTRACT

This paper deals with the experience ga ined within Ove Arup & Partners Ireland wi th the des i gn and construction of masonry diaphragm walls for build i ngs duri ng the last 10 years. The system was developed in the UK initially using brickwork but most I ri s h examples employ concrete blockwork which is the most common construct i on medium . Various aspects of pract i cal design and detail are discussed and a selection of buildings illustrated. The experience has been reviewed and assessed . Future prospects for broader applications are considered in the light o f mo r e recent research findings and developments using prestress i ng .

INTRODUCTION

Masonry construction in Ireland has a ve r y l ong tradition, early examples of

which can be seen in the variety of medieval stone structures throughout the

country . The progression over the centuries involved developments in cut

stone and brickwork which were the universal pe rmanen t construction media

until the turn of this centur y . Steel and concrete developments then led

to the great strides i n building design which provide the basis of the wide

choice available today. Ire land i s well endowed with the ingredients for

concrete and the growth in the use of concrete b l ockwork was a natural

phenomenon particularly in respect of smaller and simpler building t ypes.

Brickwork also has a p lace but blockwork has held a predominent position in

recent years.

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The inherent limitations of eonerete bloekwork have restrieted the

seope of its use to si tuations whe r e i ts weakness to resisting tension

stresses was not an important eonstraint. In building construction terms,

therefore, blockwork has prineipally been employed in housing , low ri se

struetures, infilling to framed buildings and non loading partitions.

Since it is, without doubt, a very versatile and economie form of

const ruction widening its range of application would obvious l y be

beneficiaI.

Researeh in the UK by W. G. Curtin led to the evolution of a format

that extended the capaci ty of simple masonry construction to a new leveI .

It was found that disposition of the wall leaves at, say 400 mm, apart was

the key to exploitation of a basically straightforward building technique to

fulfilling a role whieh greatly extended previous eapabilities.

This paper reeounts the experienee of Ove Arup & Partners Ireland in

apply ing these principIes to the construction of a variety of buildings, in

alI cases single storey struetures in the range of 6 to 9 m high, using

diaphragm walls 600 mm wide.

CONCEPT AND DESIGN

The concept of the diaphragm wall is that, by increasing the width,

substantial height can be built as externaI walls of buildings using simple

construction techniques. The structural requirements are met by use of 100

mm inner and outer leaves, linked with similar ribs, bonded to the leaves at

interva ls , generally about 1.2 m. This can provide walls eomfortably

managing to serve as primary support and enelosure to building up to about

8 m high in a single storey. This building type is eou@on for factories,

warehouses , sports halls and churches, the normal areas of applica tion for

diaphragm walls.

Conventional construction of this forro of building would assume a

steel or concrete frame, infilled with masonry or elad in steel. Diaphragm

walling has been found to be eompetitive in this field, having a number of

advantages and few disadvantages.

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Particular circumstances, of course, could pre-empt the choice in

favour of framed construction but for many applications the balance favours

a diaphragm wall design approach.

The basis and technique of design are relatively simple and are well

documented elsewhere (1). They are therefore omi tted here except for

practical and detail aspects that have arisen on various jobs.

The ideal format would use a simple rectangular shape, with a flat

roof designed to prop the wall at the t op and resist overall wind effects.

Since the wall effectively spans vertically the incorporation of narrow

vertical windows is easily accommodated if required.

Var ious aspects of the practical design and construction arising from

experience gained in a number of buil t examples are reviewed below and a

selection of buildings i llustrated.

ECONOMICS

Diaphragm walls use nati ve materiaIs in a straightforward manner in

achieving both structure and enclosure in one operation. Simple

construction using masonry with strip footings, make it suitable for remote

sites or where avaiIabIe skill leveIs are moderate.

WEATHERTIGHTNESS

This is an area where scepticism has been voiced regarding reIiable

performance. The forro of construction used precludes the normal cavi ty

found as the usua l technique in walling to ensure weathertightness.

Since the outer leaf of both systems is similar,

assumed that rain pene trates the outer leaf.

bitumastic DPC at the interface of the rib and

say 100 mm, it must be

Use of a painted on

outer leaf has proved

effective. This detail requires careful atten~ion as it is sensitive to

workmanship standards and must be done properly.

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Horizontal DPCs are rel a tively easy to arrange and should fol1ow

conventional principIes of good practice. A type with high resistance to

horizontal shear is required at ground leveI to de aI with the considerable

forces generated at this point.

WESLEY COLLEGE SPORTS HALL

This was the first building in this medium, designed in 1978. It is a

classical case of the brief requirements fitting the favourable feature of

diaphragm wall; simple shape, no glazing and flat roof. The smooth

surfaces inside and out suited both the functional and architectural

requirements.

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ST. LAURENCE O'TOOLE CHURCH

A recent design for a suburban church using concrete brick internally

and externally. The use of simple masonry detailing with glazing, alI in a

quadrant shape, combine to produce a successful architectural result o

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GOULD FACTORY

This building is a good example of archi tectural potenti al in i ts use

of Forticrete blockwork throughout. The availability of standard

specials permitted window, door and roof edge details t o be designed

consistently and elegantly.

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TRABOLGAN HOLIDAY CENTRE SPORTS HALL

A sloping site, brief requirements for 2 separa te halls, and architectural

preference were instrumental in deciding the unusual form of this building.

Again Forticrete blockworks throughout produced a simple, pleasing and cost

effective building.

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ST. ANDREWS COLLEGE SPORTS HALL

This project will use exposed concrete blockwork to provide an economi c

solution to the multi-purpose hall for a large school.

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39 m

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REVEIW OF EXPERIENCE

Experience in Ove Arup & Partners Ireland has now extended over of a period

of some 10 years and the original expectations have been largeIy confirmed.

Buildings using this system have been built in most areas of the country 50

that design features used have been well tested in service. Rev iewing the

experience indicates that broadly speaking design intentions have been

reaIised and apart from minor refinements in detail the basic cbncept is

effective and economical.

However there are aspects that need careful attention in alI cases,

particularly in areas where weather condi tions may be unusuaIly severe.

Rendering would be considered more reliable than exposed masonry in areas

subject to severe weather conditions. It follows that in these conditions

DPCs, and joint sealing, should h ave extra care applied in their execution,

apart, of course, from a high standard in the masonry itself.

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Unfami liarity wi t h t he me t hod of construc tion may require c areful

ver ba l explana t ion t o avo id any misunder standing of detail requirements.

This is not t o say that there is any inhere nt t e chn ical difficulty , but

cr iticaI details must be pointed out to the builder.

The use of temporary support to the walls is, in theory, essential,

unt i l the wall has been braced by the roof. However the double scaffol d

with b racing , normally used, ha s i nvariably proved adequate to impart the

nece ssary stability.

The apparent conflict between continuity of perimeter r ing beam with

the need to provide joints in the walling has not proved to be a problem in

p r a ctice . Restriction of pour lengths in the beam helps to reduce

shrinkage of the concrete , and the masonry on which it rests does not seem

to suffer.

Installation of services including ducts i n the wall voids has proved

re l a tively easy and helps to ma intain simple and tidy internaI detailing.

It has been found best to use simple flat roofs. However where it

ha s proved necessary to use pitched roofs, ties should be used to avo i d any

horizontal forces at the wall topo

Simplicity, is the keynote for success, as it would be in any

s tructural design. Since the diaphragm wall in inherently simple and

economi c al it is inappropriate to employ complicated geometry in the shape

of the building .

FUTURE PROSPECTS

Experience to date has been confined to high single storey buildings in

s imple construction i. e. the most conventional and straightforward format

available. Increasing confidence in the performance of these walls

suggests that this method could be extended substantically using similar

p rincipIes with, of course , appropriately thicker walls. The opportunity

t o do so has, however, not arisen so far o

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Prestressing techniques have recently been developed and examples of

its use have been built in the UK. This offers exciting prospects enabling

a wider range of building and retaining wall problems to be tackled. In

this context prestressing involves the use of a simple vertical threaded bar

system, with manually tightened nuts, to achieve the required level of

compression. On this basis prestressing can be seen to introduce very

little complication, compared to the considerable additional benefit to be

gained.

CONCLUSION

The use of masonry diaphragm walls have developed in Ireland to the point

where it is becoming a common method for certain building types. These

tend to be mainly sports halls, for which the system appears to be well

suited, but also churches and some industrial type applications. The

performance has been found to be satisfactory and the potential exists for

further development along similar lines. The advent of prestessing appears

to add an extra dimension to the capabili ty of the concept which, i t is

expected, will lead to interesting projects in the future .

By these means i t should be feasible to design a wide range of

building types, hitherto requiring structural steel and reinforced concrete

walls, using simple elements and easily avaiable skills. All this should

be beneficial nationally in its use of natural materials exploited to

provide economical solutions to the design of buildings.

(1) "Design of Concrete Masonry Diaphragm Walls"

Concrete Society Technical Report No. 27.