AJ Handbook of Building Structure EDITED
BY
Allan
Hodgkinson
The Architectural Press, London
AJ Handbook of Building Structure Structure Introduction
This handbook
Allan Hodgkinson
Consultant editor and authors The consultant ed itor for the Handbook is Allan Allan Hodgkinson Principal of Allan MEng, FICE, FIStructE, MConSE, Principal of Allan Hodgkinson & Associates, consulting civil and structural engineers. Allan Hodgkinson has been the AJ consultant for structural design since 1951; he is a frequent AJ contributor and is the author of various sections of this handbook. The authors of each section will be credited at the start of the section of the Handbook in which their material appears. The original Architects' Journal articles were edited by Esmond Reid, BArch, and John McKean, BArch, MA, ARIBA, ACIA, ARIAS.
The frontispiece illustration shows one, of the most magnificent building structures from the era of the Eiffel Tower, the Forth Bridge and the great railway stations. The Palais des Machines for the Paris Exhibition of 1889 (Contamin, Pierron & Charton, engineers) was a pioneer example of three hinged arches.
Preface to the second edition There have been considerable changes in some British Standards, Codes of Practice, and Building Regulations since 1974; and unlike the reprints of 1976 and 1977, this is a substantially revised and updated re-issue of the now well-established AJ Handbook of Building Structure. The principal changes are in the sections on Masonry (rewritten to take account of the 1976 Building Regulations, and the new BS 5628 'limit state' code of practice); and on Timber (substantially revised to take account of the new timber gradings). Steel handbooks have been replaced for all types of structural sections; and technical study Steel 3 has therefore been revised accordingly. In general, the new 'limit state' approach to design is discussed (eg in the section on Masonry); but in view of the rejection of the limit state Codes and draft Codes in their present form, by the majority of practical designers, it has been thought prudent to retain the allowable stress methods of design as the basis of the handbook. Finally, it should be mentioned that the opportunity has been taken to bring all references in this Handbook up to date; and t o cor rect a number of misprints of the first edition. ISBN 0 85139 0 85139 273 273 3 (paperbound)
First published in book form in 1974 by The Architectural Press Limited: London
Reprinted 1976, 1977 Second edition 1980, 1982, 1983 Printed in Great Britain by Mackays of Chatham Ltd
Scope There are two underlying themes in this new handbook on building structure. First, the architect and engineer have complementary roles which cannot bo separated. A main object of this handbook is to allow the architect to talk intelligently to his engineer, to appreciate his skills and to understand the reasons for his decisions. Second, the building must always be seen as a whole, where the successful conclusion is the result of optimised decisions. A balance of plannin g, s truc ture or services, services, decisions decisions may not necessarily provide the cheapest or best solution from any of these separate standpoints, but the whole building should provide the right solution within both the client's brief and his budget. The handbook provides a review of the whole structural field. It includes sections on movement in buildings, fire protection, and structural legislation, where philosophy of design is discusssed from the firm base of practical e xpe rience. Foundations and specific structural materials are also covered, while sufficient guidance on analysis and design is given for the architect to deal with simple structures himself. Arrangement The handbook deals with its subject in two broad parts. The first deals with building structure generally, the second with the main structural materials individually. The history of the structural designer and a general survey of his field today is followed by a section on basic structural analysis. The general part of the handbook concludes with sections on structural safety—including deformation, fire and legislation— legislation—and and on the the sub-struc ture: foundations and retaining structures. Having discussed the overall structure, the sections in the second part of the handbook discuss concrete, steelwork, timber and masonry in much greater detail. Finally there are sections on composite structures and on new and innovatory forms of structure.
Presentation Information is presented in three kinds of format: technical studies, information sheets and a design guide. The technical studies are intended to give background understanding. They summarise general principles and include information that is too general for direct application. Information sheets are intended to give specific data that can be applied directly by the designer. Keywords are used for identifying and numbering technical studies and information sheets: thus, technical study STRUCTURE 1, information sheet FOUNDATIONS 3, and so on. The design guide is intended to remind designers of the proper sequence in which decisions required in the design process should be taken. It contains concise advice and references to detailed information at each stage. This might seem the normal starting point, but the guide is published at the end of the handbook as it can be employed only when the designer fully understands what has been discussed earlier. The general pattern of use, then , is first to read the relevant technical studies, to understand the design aims, the problems involved and the range of available so lutions. The information sheets then may be used as a design aid, a source of data and design information. The design guide, acting also as a check list, ensures that decisions are taken in the Tight sequence and that nothing is is loft loft out.
Section
1
Building structure: General Scope The first section of this handbook consists of two technical studies which provide an introduction to the subject of building structure. The first study shows the growth of the structural designer through history and the role of architect and engineer up to the present day. The second study provides a wide review of the subject today, giving the background on which the architect’s knowledge of structure can build. It provides a frame of reference and guide to the remainder of the handbook, while also offering knowledge from practical experience which has not previously been contained in a structures textbook.
Authors The authors for Section 1 are W. Houghton -Evans and Herbert Wilson. W. Houghton -Evans AMTPI, RIBA runs a course in architectural engineering in Leeds University’s Department of Civil Engineering. His buildings include Leeds Playhouse ( AJ 22.12.71 p1428) and his research is in planning and industrialised building. Herbert Wilson CEng, FICE MCOnSE, is a consulting civil engineer and for many years was a partner of Norman & Dawbarn, architects and engineers. Both authors are enthusiastic advocates of active collaboration between architect and engineer in the design of building structures.
Illustration on previous page is a section of Milan Cathedral from Caesariano’s Vitruvius (1521)
The structural designer
The major part of this handbook deals in detail with current structural theory and practice. But first it is helpful to understand the role of the structural designer and to
1
Architects’
lack of specialist knowledge
1-01 A biulding may be regarded simultaneously as a system of spaces for specific uses; a system to control local climate; a system to distribute services and take away wastes; a stuctural system capable of carrying its own and applied loads to the ground. Each of these is capable of subdivision and elaboration. To be built, a building must be conceived as a constructional system, and during its life may have to facilitate maintenance alteration, or even removal. In detail it will make its presence: felt on those that use it, and as a whole it will affect the town and landscape. 1.02 The uniquc task of the architect is to propose a solution which simultaneously and in an adequate and appropriate manner satisfies all these roles. The solution is unlikely to ‘spring fully-armed’ in every detail from his head, and he may need thc help of others to develop it in detail. The basic strategy must, be capable of tactical elaboration in respect of each role the building will havc to play, and therefore the architect must understand every aspect. 1.03 Because of tho many sides of any design problem, specialism in designers poses a difficulty. A specialist will tendl to seo first only that aspect of the task which falls within his specialism, and ignore the others. While special-
1 Analytical section of S Sophia
appreciate the development of structural forms. This is the purpose of these first two articles. This first study, by w. HOUGHTON-EVANS, describes the role of the early architect/engineers an.d shows how modern structural theory evolved. The second article reviews the various structura1 forms now available and acts as a guide to the remainder of the handbook
isin in architecture is still less well-defined than elsewhere, architects may also be lop-sided in their approach. Their failings will most probably result from an inadequate understanding of specialist matters. It is especially difficult for them to be inventive and to think creatively where their knowledge is superficial and confined to stock solutions. No aspect of design in recent times has created greater difficulties in this regard than structural engineering. This is the more surprising in that throughout history engineering in general- and structural engineering in particular-has been intimately bound up with architecture. 2
Origins
of
engineering
science
Vitruvius and Archimedes 2.01 Vitruvius’ de Architectura (c 1st century AD) records almost all that is known from antiquity of technical design, and historians of engineering customarily acknowledge Vitruvius as tho first writer in their field. In Byzantium, the architects of S Sophia 1 (6th century AD) were Anthcmius, a leading mathematician of his age, and lsodorus who wrote a commentary on tho works of Archimedes. We are as much indebted to them for the preservation of Hellenic mechanical scienco as for the most, audacious practical demonstration of its validity. 2.02 It was within those very ecclesiastical establishments which embodied thc miracle of medieval vaulting, that scholars were appropriately taking up afresh the rediscovered works of Archimedes, and were beginning to make tho first significant advances in science since Classical times. Our only architect/author from the Middle Ages, Villard de Honnecourt, shows as lively an interest in machines as in building, and it is a commonplace of architectural history that medieval architecture displays an inventive mastery of structural design 2, 3. Leonardo, Alberti and Wren 2.03 The pristine audacity of Renaissance Man, confidentlydetermined to put the entire universe under thc sway of human reason, is epitomised in tho work of Leonardo da Vinci-painter, sculptor, musician, poet, scientist, inventor: designer of everything from fortifications to birds. He, like Vitruvius, is never absent from histories of architecture and engineering. To his near-contemporary. the architect Alberti, we owe the first great scientific treatises of tho age, his mature masterpiece being the 10 books de re Aedificatoria. As did Vitruvius, he reviews the entire technology of his time and tr ies to bring tho whole within the scope of scientific principle. Unlike the savants of Antiquity, moreover, this new Universal Man did not affect a patrician disdain of manual craftmanship. Alberti, we are told, ‘would
Technical study Structures 1 para 2.03 to 2.04
4 2 Rheims cathedral buttresses, as seen by Villard de Honnecourt (c. 1230) 3 Medieval limber truss structure at Westminster Hall 4 Sketch by Domenico Fontana (1543 -1607) of his lowering the 327 -ton monolith in Rome. Lifting it from plinth strained technical resources of the time to the limit
learn from all, questioning smiths, shipwrights and shoe makers lest any might have some uncommon secret knowledge’. So we find recorded in his work some usable rules for dimensioning structural members. But, like rules-of -thumb still in use today, his formulae, eg the thickness of voussoirs and of bridge piers, were probably based upon the accumulated experience of centuries, and structural engineering was as yet little more than craft lore. Nonetheless, Alberti did not doubt the responsibility of the architect in technical matters. His definition of the architect as one ‘. . . who, by sure and wonderful art and method, is able . . . to devise, and, . . . to complete all those works which, by the movement of great weights, and the conjunction and amassment of bodies, can, with the greatest beauty, be adapted to the uses of mankind . . .’ compares strikingly with the purpose of‘ civil engineering describcd by Thomas Tredgold in 1828 as ‘. . . the art of directing the great sources of powers in nature for the use and convenience of man . . .’. 2.04 For Alberti and Leonardo, and for a century or more‘ after them, the ‘movement of great weights and the con junction and amassment of bodies’ continued to be under taken with little more understanding than in previous centuries 4. For practical advance, the precision and empiric ism of modern engineering science was lacking. Brunelles chi’s dome at Florence notwithstanding, the High Renais-
11
Technical Study Structures 1 para 2.04 to 2.10 ornamental and so lightly pass over the geometrical, which is the most essential part of architecture. For instance, can an arch stand without butment sufficient? If the butment bo more than enough, ‘tis an idle expence of materials; if‘ too little it will fall; and so for any vaulting . . . the design . . . must be regulated by the art of staticks . . . without which a fine design will fail and prove abortive . . .’. As precise a statement as one could find of what today would be thought the engineer’s view of the matter 6. Foundations of modern technology 2.06 With Wren and Hookc at meetings of the newly-created Royal Society sat Newton, the scientist and mathematician whose theories and mathematical procedures were to prove an adequate basis for the whole subsequent development of modern technology. Also, in the late 17th commerce and manufacture, there were those
tendencies
architecture provinces 2.07 The France
which
and
in
a
century
engineering
of distinct first formal
where,
within
civil
were
established
professions. steps in this
1716
century world of rapidly growing
(following
direction the
to
its were
earlier
see
distinct taken
success
of
in a
corps of specially trained military engineers), a civilian corps of engineers for highways and bridges was formed. The word ‘engineer’ which then begins to enter into common use, derives mainly from the military connotations of the Latin ingenium: originally a clever device, later also a war-like instrument. It was not until the latter half of the 18th century that military engineering had a recognisable civilian counterpart in this country*. From these beginnings has grown the modern with its many branches
profession of engineering which, and subdivisions. has taken over
from architecture much of its classical territory and most of its pursuit of science. 2.08 The schism has never been complete, however, and to this
day
there
remain
professionals
who
are
qualified
and
equally at home in both fields. In some countries, architects and engineers are united in a single professional institution, and in some, elements of joint education persist. Thoughout the lust 200 years, there have been many who, like Telford, described
6
Wren's attempt at scientific analysis of domes and vaults
(from
his
second
Tract
on
architecture)
themselves
sometimes
as
architect,
sometimes
its
engineer. Works on engineering, such as de Belidor's 17th century classics, often used the word 'architecture' to describe their contents, and we still speak of thc engineer who designs ships as a ‘naval architect’. But in spite of continuing attempts to heal the breach or belittle its
sance failed to match in construction its theoretical and
significance
spiritual innovations*. 2.05 For some time yet, architecture was able to retain its traditional interest in technology. Palladio, writing in the mid-16th century, although, no longer Vitruvian in his range,
ture today are only too real, and must be understood if further progress is to be made. 2.09 Early work in modern civil engineering was largely
includes
an
excellent
exposition
of
trussed
bridges
among
differences
between
engineering
and
architec-
confined to canals and other means of communication essential to developing commerce. Its practitioners were often recruited from tho upper level of craftsmen who put
his palaces, temples and piazzas 5. When, after 1600, Stovinius and Galileo had laid a sound foundation for the whole subsequent development of modern experimental science, architects like Christopher Wren and Robert Hooke (whose celebrated Law is still a corner-stone of structural
their expertise inventively to use. It was not long, however, before new and unfamiliar tasks in the design of hydraulic systems, mills and machinery obliged them to look to
theory)
to tho 19th century, almost all building was accomplished within the use of a few constructional materials stone (with
were
remarkable in and professor
among
its
first
devotees.
It
was
not
considered
any way that Wren, a ‘natural philosopher’ of mathematics, should be asked to design
harbour works and fortifications at Tangier as readily as he was commissioned to design St Paul’s. Wren writes im patiently
of architects
who
‘.
.
. dwell so much
upon
contemporary
science
for
aid.
In
buildings
generally,
prior
its mud-based substitutes; bricks and mortar, plaster, concrete) and wood (with reeds, rushes and so on). Even today,
most
of
our
building
remains
within
this
range.
this
*It may indeed be that the much-admired art of the period is the most striking expression of precisely its technical limitations. Unable to achieve in reality perfection promised by omniscience and rational outlook, the men of the time sought it most brilliantly in products removed from intractable realities, to the untrammelled realm of the spirit. In his celebrated letter to the Duke of Milan Leonardo speaks of himself more as inventor than painter, and perhaps without too much exaggeration it may be said that Leonardo the artist is da Vinci the trust rated engineer, creating on paper the mastery over nature which could not as yet be achieved in the real world
The new material-iron 2.10 For a time therefore, architecture could be well content, to allow others to take ov er such work-a-day things as
*John
Smeaton
(1724-92)
was
first
Englishman
to
call
himself
‘civil
engineer‘
12
Technical study Structures 1 para 2.10 to 3.01
roads and machines, while it pursued more elegant goals easily attainable with traditional means. The change was to come only after the engineers began to perfect for their use the great new constructional material of modern times: wrought iron and its derivative, structural steel 7, 8. 9. Within 50 years of its introduction into fire-proof mill construction, engineers were confidently using the newmaterial to roof great railway station concourses and to span ravines and estuaries. No rule-of-thumb was adequate here. For tasks such as these, full and inventive use had to be made of modern knowledge in the mechanical sciences, and modern structural analysis and design was created as a fundamental component of civil engineering. By 1850, a young man aspiring to be an engineer would have to acquire a scientific and mathematical education if he wished to proceed towards full professional competence. 2.11 What was learnt in terms of iron and steel could with advantage also be applied to other materials. Basing themselves on Galileo's pioneering work, scholars in 18th century France and elsewhere had solved most of the fundamental theoretical problems in structural mechanics. Gregory had shown that the catenary and not the neoPlatonic semi-circular ideal was the 'perfect' curve for the voussoir arch 10. Mariotte had set the seal upon the classical theory of the bending of beams. Euler had solved the
problem of the buckling of columns. Behind the back of architecture there was already developing new experience and knowledge which was to invalidate much of the theoretical basis of its practice. With the coming of the Steam Age, engineers everywhere were quick to use their science to give new shapes to masonry viaducts and timber frameworks. It only needed the invention of reinforced concrete, and the widespread introducti on of rolled steel after 1880 to consolidate the claim of structural engineering to re-enter the main stream of architecture after a gap of 200 years during which it made its most frui tf ul and rapid advances (see cover illustration). 2.12 Today, engineering and architecture confront one another as estranged members of a once-united family. During their long years apart they have acquired strange habits and, if there is to be renewed association, patience, tact and understanding will bo called for on both sides. Collaboration is now essential in the many fields they still have in common: structural design, construction technique, environmental science, town planning, the servicing and engineering equipment of buildings. In some of these we may hope for a new breed of architect/engineer which is again capable, across a wide design spectrum. Whatever the outcome of future development, it is necessary for the architect to understand something of modern engineering science, the structural aspects of which are the special concern of this handbook.
3 The engineer's approach 3.01 From the point of view of its behaviour as a structural system, a fully adequate understanding of a building would take account of the way in which all elements contributed to strength and stability. Ideally, therefore, it might seem that structural design would tend towards simple, fully give it the appearance of
fig. 319, it is also bent i n one general curve in the direc tion of its length, so as to gi\ e it the appearance of fig. 350, we have t hen an arch of great strength, capable
of serving as a roof, without rafters, or any description of support, except at the eaves or abutments. It is evident that, the span of any roof being given, seg-
ments of (corrugated iron may be riveted together, so as to form such an arch as may be deemed proper for covering it. To every practical man, it will be furt her evident, that a roof of extraordinary span, say 100
feet, which could not be covered by one arch of corrugated iron wit hou t the aid of rafters, might be covered by two or three, all resting on, and tied together
by, tie-rod s, fig 351
•Further, that in
the case of roofs of a still larger span, _ _ say 200 feet, a tie-rod might be combined wit h a trussed iron beam, fig. 352 ; by which
8b
7 Interior of Palm House, Kew, by Decimus Burton (1845) 8a. b J. C. London's Encyclopaedia of villa, farm and cottage architecture (1833) recommends folded plate roofs of corrugated iron
integrated solutions. But structural design is concerned with protection of probable structural behaviour, and needs to be reasonably certain of its ground. So it is necessary to identify dements likely to play a predictable role throughout the life of the building and regard only them as ‘structural’. These will then be analysed as components of a ‘structural arrangement’ which (to make analysis possible) will transmit loads primarily in one of a limited number of definable ways. In practice, much modern building has tended rather to forms in which ‘structural’ and ‘nonstructural’ elements are separately expressed. 3.02 But the structural engineer is aware that his analytical procedures are necessarily simplifying idealisations, and always keeps his mind open for opportunities to recognise and exploit possible alternatives. He will be aware that inflexible thinking is a disadvantage in structural matters and that over-simple idealisation can lead to wasteful and stodgy design. 3.03 Many new materials are now appearing, and now ways are found of using the old. New techniques of analysis — especially those employing computers - allow prediction to approach much closer to performance and, as a consequence, permit engineering design to move closer to the limit of structural potential. 3.04 But engineering and building are a practical affair. There is no point in designing to impracticable limits or unrealisable tolerances. What is designed must be capable of being built. Construction technique lies always been us great a limitation as structural behaviour, and throughout history as much design significance has been attached to the problem of how a building was to be built as to any other factor. For instance skeletal framework were used from primitive times as permament scaffolding to support men and materials during as well as after construction, and they have had A profound influence on both architecture and structural engineering. The size of members and assemblies must always be related to transport and lifting capacity. Stability during construction (as most structural failures testify) is as important as upon completion. The sequence of operations and the joining of members have always posed difficult problems in design. 3.05 There will also be important constraints arising from weathering, corrosion and fire: resistance. The efficient performance: of a structure over a period of time will be dependent upon its protection and maintenance, and for this accessibility will be necessary. Many such considerations are
9a, b Iron trusses with tension cables, from London's Encyclopaedia 10 Gregory's analysis of voussoir (from Polini's Memorie istorichc della Gran Cupola del Tempio Vaticano,1748). What was learnt in terms of iron and steel could be applied to other materials
11 Palazzetto dello Sport, Rome, by P. L. Nervi 12 Mining materials and Metallurgy Building, Birmingham, by Arup Associates
12 reflected in codes and regulations, and much is embodied in legislation. 3.06 While matters of strength and stability pose problems less acute in buildings than in massive works of civil engineering, their involvement in a complex totality can make a satisfactory solution more difficult to achieve. The eloquent clarity of a suspension bridge will rarely be achievable in buildings, and in many it may be appropriate ‘that the structural system should pass unnoticed in the finished product. Structural engineering can nonetheless claim with pride to have descended from the achievements of S Sophia and the Gothic master -masons, and can point to today’s achievement in the works of engineer/architects such as Nervi 11, or architect/engineers’ collaboration in the more forward -looking practices 12.
15
Technical study Structures 2
Structural forms, design and materials
1 Introduction 1.01 The peculiar division now existing between architect and structural engineer has its roots in the Industrial Revolution; most particularly in this country where 'engineering' became synonymous with 'progress' and where science was omniscient. The engineer, whoso impact on society became so intense and diverse during the 19th century, has been reluctant to forgo privilege. However the architect can achieve reciprocal integration and collaboration today through an understanding of the structural engineering objective. 1.02 The words of Thomas Tredgold quoted in the previous study, 'the art of directing the great sources of power in nature for the use and convenience of man', have been embodied in the constitution of the Institution of Civil Engineers to define the role of the engineer. To extend 'use and convenience' into building terms, we have material advantage (maximum worth and pleasure) and personal comfort (efficient environmental properties). The architectural objective cannot be less than this.
2 Maximum worth 2.01 The most recurrent word in all writing or speeches made about building in this century is 'economy'. Overworked and frequently misused it can lead to the demand for minimum first cost rather than maximum worth overall. 2.02 Maximum worth is the supreme test of cohesion both within the professional team, and between the team and its client. The resolution of the individual solutions in which this can be achieved depends upon experience and skill. Design parameters are so complex that a complete scientific or mathematical evaluation is impossible, but attempts are being made to provide more scientific assistance and guidance. 2.03 Reference can be made to recent work at the Imperial College of Science and Technology towards the development of computer-based procedure for economic evaluation and comparison of multi-storied buildings and potential use of findings as a design tool. Although the range of interdependent variables is large, considerable progress has been
Technical study Structures 2 para 1.01 to 3.03
Section 1 Building structure: General
Having discussed the development of the structural designer, the handbook turns to a general appraisal of his field today. This article by H E R B E R T W I L S O N discusses structural form and design principles, and adds an introduction to the various structural materials. This study acts as a framework and guide for the remainder of the handbook
made and results illustrate the effect design decisions have on running costs, income and capital costs. The project directors consider that 'the main criteria by which any structural system should be judged and towards which any design should be directed are speed of construction and maximum ratio of usable (or lettable) area to gross floor area.' 2.04 Although this investigation is restricted to multistoried office-type buildings of simple plan shape, it serves to illustrate that the economies of a structural system are not necessarily an expression of maximum building worth, as the choice of an appropriate economic criterion of building has a radical effect on the selection of a structural system 4. 2.05 This type of analysis, based on a comparison of fluctuating costs of the building elements, is not absolute. The analysis indicates that the most significant cost effectiveness for structure (considered in isolation) is ease and therefore speed of construction. However, the overall building worth depends on decisions made at its inception, rather than on subsequent refinements.
3 Structural forms—solid structures 3.01 The primary building decision is one of structural form. There are (permissibly over-simplified) three basic divisions of structural form: solid construction 1, skeletal construction 2 and surface construction 3, 5. 3.02 Solid is the most int uitive form, from cave and rock temple to loadbearing brickwork. During the historical stage of experimentation, the builders of solid structure fully utilised the virtues of stone and its ability to contain compressive loads. Great skill and ingenuity was employed in enclosing space by the transfer of non-vertical reactions through arches, vaults, domes and abutments to vertical forces at foundation level. Solid construction relies on a heavy homogeneous wall mass within which, in the ideal state, compressive forces are uniformly distributed. 3.03 Solid structures perform the function of enclosure, support and protection; but this benefit carries the disadvantage of the highest ratio of mass to unit of enclosure. To some extent this disadvantage is balanced by economies in materials and labour costs, but it could be a significant
Technical study Structures 2 para 3.05 to 4.02
16
6 Skeletal constructio n, spec ial ty pe s wi th stressed membran es; a umbrella; b basic ten t; c Mun ic h Ol ym pic tent by Otto fact or if choice of foundation is a criterion in t he selection of stru ctur al form. Modern loadbearing buildings are not t rue solid struct ures as they are usually composites of loadbearing walls in the form of perimetcr malls (continuous or in sections) with internal walls. These may be cross walls or spine walls, diaphragms and tower structures used in conjunction with slabs, slab beam floors or roofs in various materials. (Structure s built u p from large flat pane ls used horizontally and vertically are not ‘solid’ structures because of the special function of panels as elem ents of a surface structure , discussed la ter.) 3.04 ‘Solid’ buildings have structural limitations. Usually they ar e of modest heights and have short span s (say u p to 7.6m). If tall, the ir forms ar e confined to those in which each storey has an idontical plan. Special consideration is necessary for problems of crack control and differential movements, and these problems will be dealt vith in detail in ot her sections of thi s handbook. Fire resistance and thermal-insulation properties are good; bu t their insulation against noise requires specific investigat ion because of mass transmission effect.
4
Skeletal structures
4.01 Skeletal forms are also traditional, having developed from experience and the availability of materials. The tent is an early, special form of skeletal construction in which the enclosing membrane was stressed in conjunction with an internal framework. Modern t ent structures have succeeded in liberating the skeleton from within the skin 6. 4.02 The trabeated architecture of classical Greece is skeletal in the form of post an d beam from which is deri ved th e frame and slab structures (of similar natures but of varying techniques), which are the structural forms of most modern building s 7.
9
10
4.03 The structural elements of struts, ties and beams were
Olympic tent of Professor Behnisch which has now been Constructed in Munich 6c (see AJ 16..2.72 p338).
extended to frames by the triangulation of struts and ties, and inevitably to the three-dimensional space frame 8. 4.04 Skeleton forms avoid the limitations of enclosure imposed on solid forms, and have much more spatial freedom in that the framework can be within the space confined, or between the li mits of interna l space and external form, or even completely outside the external form. Tho most outstandingly visual development in skeletal structures has been the exploitation of th e qualities of th e flexible tie or tendon as used in suspension and tented structures 9. These range from Nervi's workshops a t Mantova (built like a suspension bridge) and cable supported roofs to many aeroplane hangars, 10; from tendon suspended multi -storey office buildings and cable structures as expressed by Dr Buckholdt, David Jawerth and others, to the controversial
4.05 The greater use of metals and the growth of appropriate design skills and understanding of the strengths of materials have developed skeletal structures from traditional stone and timber. (The transition from empirical rules to a theoretical statics approach of solid structures was not achieved until the middle of the 18th century, and even modern design methods for such structures retain an empirically -based crudity.) 4.06 The design of skeletal structures has progressed on scientific principles with less restraint from traditional methods. But inevitably there have been pitfalls and one of the most common errors today is the assumption that if the mathematics are in order, then the st ructure is in order. This is particularly significant when basic assumptions, on
which mathematical design is built, are tainted by the arbitrary character imposed on such hypothesis. For these assumptions are the work of earlier compilers of rules and regulations who could not divorce themselves from tradi tion. Modern design is bold and imaginative, backed by research and aided by modern tools of calculation that enable th e designer to consider the building a nd its foundation as a complex but integral structure. 4.07 The extension of t he skeletal frame from horizontal and vertical planes to the three-dimensional frame enabled designers such as Fuller and Zeiss-Dywidag to create structures which almost came within the next category to be described, surface st ruct ures 11.
5 Surface structures 5.01 I n these structures the loadbearing surface both defines the space an d provides support. I n the design of a surface structure an exact understanding of its behaviour and an appropri ate scientific analysis is required. Such structure s have only recently (in building ter ms) become practical building realities because of the availability of new materials (reinforced concrete in parti cula r). So there is no background of long experience and therefore no in tui tiv e confidence, but theory and skill of execution are advancing to the stage where our understanding of surface structures is probably more refined than our understanding of any other form of structure. 5.02 The most obvious example is the shell structure in i ts multiple forms, but such structures are not confined to curved surfaces. The horizontal plate, used as a slab, or the vertical plate used as a wall, panel or beam 12 are forms of surface structures which can be used as 'folded' plate surface structures 13, or in conjunction with other forms of structure 14. (See technical study 1 8 for an early example.) 5.03 Surface structures can be constructed in most of the building materia ls but the y are generally limi ted to th e enclosure of space in a single cell or a series of such cells. They are subjected to limitations of construction and architectural conception, engineering theory and practical construction may not be entirely compatible, eg Sydney Opera House.
5.04 Most structure s ar c composites of different forms. It , is logical t o resolve a parti cular build ing problem by hav ing a sur fac e roof form supported by a skeleton superstructure with a 1oadbearing substructure but it is very important to check th e str uctura l logic of the points of con tin uit y from one system t o another, and where possible to avoid any abrupt changes in t he load flow pat ter n.
6 Structural design
-loading
6.01 With established design criteria for structural form based on si te limit ation s, funct ional requi rement s an d architectural conception, the structural designer moves to the second stage which is concerned with the evaluation of loading and the analysis of loading patterns. The dead Ioads, or self weights of building and structural elements can be determined with reasonable accuracy at this stage b ut t h e values of imposed lo ads ar e a matter of judgment. Unless special requirements ha ve to be met, an d the client is specific about loading in t he building, t he use to which he will pu t the building will classify it und er Regulations which prescribe floor loadings. 6.02 I t should be kept in mind tha t statutory loadings are attempts to classify a wide variety of buildings load. They govern a minimal condition, and for convenience are expressed as uniformly distributed loads, although this condition is rare in practice. The anticipated disposition of loads in a build ing should be studied before a rationalised Regulation of uniformly distributed loading is adopted. The capacity of a floor design to spread loads over areas greater than the virtual area of loading application helps towards this rationalisation. A pri nti ng machin e, a telephone exchange a magnetometer or a safe are very concentrated loads but require clear working areas around them which can be considered in the spread of load. 6.03 However, in general terms, experience has shown t ha t the classified loadings are not likely to be exceeded unless the building usage changes. For instance, office loading at 2 . 4 kN/m 2 plu s th e accep table minimu m allowance for par tit ion s has pro ved t o be totally inadequate to cover the frequent internal replanning to which such buildings are sub jec ted . The likelihood of th ese lar ger load s occurring \vi11 have t o be assessed, and th e probability factor in loading is
becoming more prevalent in design practice. 6.04 Not only the size of the, loads, bu t t he ir nature, requires consideration. For instance, the effects of dead load which is constant, a n d superimposedl load, which is transient, are different in character. Tho sustained dead load can give rise to creep (ie continuing stress deformation without increase in load) in the materials of construction. Some superimposed loads suchas loads from stores area s are usually a t a constant level and can contribute to this creep effect, which is particularly noticeable and detrimental on long span floor constructions. 6.05 A fluctuating superimposed load can result in vibration, sway, flutter, instability and fatigue. The acceptance of these variable loads and consideration of their effect on overall safety undo ubtedl y complicates the design processes, bu t research an d exper ience a re c reati ng new guide lines for rationalised design with a greater understanding of what is happening to the building. 6.06 The loads due to natural phenomena which lncludc wind, snow and scismic loads are essentially of uncertain character. Within this category of loading, thc user and indeed the legislator are unable to provi de an y comple tely satisfactory dictates. Th e gap between normal loads arising from natural phenomena at fairly frequent intervals, and the exceptional loads of disastrous but, infrequent, events, is too large to be covered by statutory Regulations and remains a mat te r of engineering judgmen t. 6.07 Only actual observations, and tests on complete build ings over a long period of time can provide the information necessary to redraft the Regulations. Any available Regul a tion notwithstanding, the conditions in the specific locality must always be studied, as, among other things, the effect of wind loading is complex. No t only does i t re late to th e building str uct ure as a whole in matt ers of general stabili ty, bu t also to t ho severe local effects on claddings, fastenings a nd struct ural elements. I n conditions of severe exposure, with t all buildings or complex shape, with buildings having large holes or tunnels through them, and with buildings likely to be effected by the juxtapositioning of other large buildings (as in the case of th e cooling towers which collapsed a t Ferrybr idge), the structural effects can be resolved by intensive investigation only.
-
6.08 Snow loadings, particularly in northern and exposed areas where high winds and low temperatures can prevail, are not adequately covered by Regulations. Efficient thermal insulation can prolong the life of snow on some roofs and this and the retention of frozen snow on concave roof shapes and valleys could extend into a period of exceptional wind load. These combined loadings are probabilities clearly related to the building location. 6.09 Special oases of floor loads may occur not covered by statutory loadings and these may arise from storage, plant, machinery and equipment during construction, installation and the future use of the building. Some of these loadings will give rise to dynamic effects and it should be noted that these can be in vertical and horizontal planes. The accelera tion, deceleration and braking of vehicles and cranes will create surge or lateral forces in the structure. The energy release of suddenly applied loads or falling loads create stress effects several times larger than those induced by an equivalent static load 15. 6.10 Fortunately these effects are, in most cases, of ex tremely short duration, but they must be considered. The dynamic effects of wind load have been mentioned, and there are classic examples of flutter and resonance in tall structures and long span bridges. The dynamic effects of seismic disturbance are well known, and because of the catastrophic results, much research has been undertaken in this field. The findings of these investigations have been incorporated into design guides for classified earthquake areas, which include the probability factor. 6.11 But there are many more buildings, outside earthquake zones, having complex loadings in which large static loadings are combined with dynamic effects. In industry heavy stamps and presses can produce earthquake-like shocks. Unbalanced reciprocating machines and compressors can induce destructive vibrations and resonance. The release of energy, when a testing machine fractures a specimen, can be transferred through the framework of the machine into the floor and the structure so that isolation or insulation is essential. 6.12 The Building (Fifth Amendment) Regulations 1970 cover the problem of collapse loading in a building over four storeys (including basements) which receives damage due to an incident (the amendment makes no reference to explosion). The purpose of this amendment is to ensure stability if a structural member, ie a section of a beam, a column, a floor slab or a wall, is removed by an ‘incident’ 16. Alternatively a pressure of 34 kN/m2 in any direction must be considered to act in combination with the dead load plus one third the live load and one third the wind load. Floors below the level of the incident must also be capable of carrying the load of debris from above. Under these conditions of loading, a significant reduction in the factor of safety is allowed. For reinforced concrete and steelwork 1.75 times normal stresses may be used and for brickwork 3.5 to 4 times the stresses given in CP 111. 6.13 The fifth amendment of the Building Regulations is brief but is carefully worded to cover all types of structure. However, because of its brevity, it is possible for a number of interpretations to arise. To provide a simple solution early agreement between designer and the responsible authorities is necessary, but the effects of the fifth amend ment on fairly substantial buildings have been found less severe than was at first feared.
7
Structural
design
-foundations
7.01 The ultimate respository of all loads, dead and super imposed, is the ground; and the primary function of struc -
Technical study Structures 2 para 7.01 to 10.03
21 ture (of which the fouridation is part) is to carry the loads safely and transfer them efficiently to the ground. Within the qualifications of superimposed loads previously referred to, the loads on the structure can be determined. With a properly conducted and efficient subsoil investigation, plus local information on general experience of conditions, the structural qualities of the ground can also be determined. 7.02 However, further decisions must be made in connec tion with the foundation which could affect the structure. Examples are: 1 structures on shrinkable clays or earths in which loads may have to be collected by foundation beams and/or slabs and then concentrated on piers or piles which will transmit the loads down to levels below the region of climatic change 2 structures over poor or weak loadbearing soils where the total loads have to be widely dispersed by means of rafts or grillages 3 structures requiring piled foundations where column layouts and piling layouts have to be compatible 4 structures of great inherent rigidity which have to be shielded from the effects of normal differential settlement and which require equally rigid foundation units, and possible subdivision into monoliths 5 structures designed to have a large degree of flexibility within isolated units, to accommodate, without structural failure, the effects of subsidence. 7.03 Basements have the dual function of retaining struc tures and foundation structures which generally are of rigid construction (solid structure). This generates problems in the control of cracks, particularly when located below a water table. Constructional methods, movement joints and water tightness are to be integrated in the design. (Base ments to simple buildings will provide efficient foundations on poor soils, but a lightly loaded basement within a watcr table should be checked for flotation risks, particularly during construction.)
8 Structural services
design
-environment and
8.01 The environmental requirements of modern buildings have a great influence on structural design. Some require ments are statutory, others either arise from activities within the bidding, or they may be directives of the client. In general terms the services can be classificd in four groups: 1 Environmental scrvices; those directly concerned with control of physical environment, heating, mechanical ventilation, lighting 2 Supply services; those concerned with providing physical matorials to meet the needs of building users, hot and cold water, gas electricity and so on 3 Disposal services; those concerned with removing waste products, refuse, foul arid surface water drainage 4 Central plant to provide or generate or motivate the services described above. 8.02 Service layouts have an effect on the structural Design, and thefollowing design problems are characteristic: 1 Large ducts and pipes required for ventilation antl drainage, are not only space consuming but inflexible, iri that tight bends or changes in direction are not always acceptable. Moreover, main distribution lines might have to be planned with considoration for future change of the building’s use. 2 Most services, particularly those carrying fluids, arc potential noise generators and could be noise transmitters. The transmission of noise and vibration from services and plant will havo to be checked against acceptance levels, and
where necessary structural devices incorporates for insula tion or isolation of sources of nuisance. 3 Thermal movement of heating and steam pipes can impose heavy loads both on anchorage: points arid at other points of intentional or accidental restraint. Such movements and loads must bo integrated in tho structure. Conversely large structural movements, such as deflection of large span beams or floors, must not be transmitted to inflexible services. 4 The choice of structural arid cladding materials relates to the standards of thermal insulation necessary in the building envelope, and similar evaluations are required for ventilation, both natural and mechanical, natural lighting and sound insulation. 5 The mechanical devices by which material and people are moved within the building have obvious structural influence in the shape of cranes, hoists, conveyors, lifts and elevators.
9
Structural
design
-resources
9.01 The question of resources in labour and materials does not frequently arise in the UK in sufficient degree to influence choice of structure. In other countries, however, particularly in ‘emergent’ countries, inaccessible areas, or areas subject to extreme natural phenomena, the question of resources may be extremely relevant. Local building methods and matorials are sometimes worthy of adoption, particularly when ‘low-cost’ buildings arc to bo erected. Experience will indicate the most suitable materials and methods to be used, and these will vary from place to place steel, concrete, bricks and blocks.
10
Structural
design—design
methods
10.01 During architectural conception of the building, tho limits of structural choice have been set by external para meters and tho skill and experience of the engineer. Up to this point, mistakes can bo expensive and irrevocable if not recognised. But future progress in the structural design is a technical process within the major decisions already taken. 10.02 Tho structural aims can now be restated and checked against tho design concept as: 1 the most efficient structural mechanism, and structural matorial, with minimum spatial demands within tho structural form 2 the best use of structural elements within the chosen mechanism. 3 the most efficient use of the chosen material 4 tho durability of the structural material 5 the behaviour of the structure in fire 6 tho considerations of the site and of construction 7 the economy of the structure. 10.03 To achieve an efficient mechanism, possible patterns of load -flow to the ground must be examined and a system established. In general terms, cost, and the spatial demands of structure are related directly to the complexity of tho load -flow pattern and the distances covered. Tho planning decisions already made will help determine the spans, and unobstructed plan arms required. Before an analysis of tho relationship of slabs, beams arid columns in the load -flow pattern can be made, the matter of structural subdivision conditioned by thermal and shrinkage effects will havo to bo resolved. Buildings up to say 60 m long may not require complete separation for shrinkage arid expansion, but within this length some structural elements such as parapets antl brick panels may require special provisions. Buildings of odd shape, or liquid containers and some special purpose . buildings (such as cold stores) require further consideration.
Technical study Structures 2 para 10.03 to 10.08
22
Such consideration is also necessary in buildings of com framework such as a roof truss, or the general stiffening posite construction (eg brick walls and concrete slabs have effects of cladding in resisting wind loads. differing characteristics of volumetric change under shrink 10.08 The new approach to design is not a rejection of age and thermal effects). earlier principles which are quite sound; but because there 10.04 Planning demands may have set limiting or critical is now a better understanding of behaviour of structure, dimensions, such as floor thickness and depths of beams many of the old restrictions are condemned. The existence vertically, or wall thickness and column sizes horizontally of the plastic state beyond the elastic state has been These considerations have to be balanced against structural established for a long time but it is only comparatively deformations under load, which should be checked against recently that elasto-plastic design has been generally statutory limits, but again, acceptance of recommendations accepted and has resulted in a more efficient distribution of should be balanced against experience. Statutory limitations structural material with considerable progress in the design of deflections of slabs and beams are no guarantee against of highly indeterminate structures. This development has the cracking of walls and partitions carried by those slabs been assisted by research which demonstrated that the and beams. previously assumed simple relationships between working 10.05 Structural analysis is the subject of this handbook's stress (in the elastic state) and stress at failure are not valid next section, but an appraisal of design methods is necessary because of plastic deformation, and in place of the so-called for any understanding of structure. To reconcile safety 'factor of safety' a more meaningful relationship (or ratio) with economy a correct evaluation of all loads must be is now considered to be that between the working load and followed by a precise evaluation of critical stresses for the actual load at failure, ie the 'load factor', see 17, 18, 19. the structure or its component parts, the relationship between load and critical stress is a measure of safety. Until recently the understanding of the behaviour of materials under load was, in a scientific sense, more the province of the mechanical engineer than of the structural engineer The traditional knowledge of building was resistant to the new knowledge of materials and because of this a great deal of simplification and rationalisation of structural design, ie practical structural design was inevitable. Some of the assumptions, which were conveniences of design methods, have persisted today and relate directly to timber beams 17 Stress and strain: a elastic; b elastic and plastic and steel joists. deformation 10.06 Robert Maillart said in an article about the development of flat slab design: 'Previously, rolled steel and timber were available for the construction of long span flat frameworks. Both are materials which cannot be shaped arbitrarily but are only available in beam form where the main dimension is linear and is determined with rolled steel because of the rolling process and with timber because of the growth process. With these materials the singledimensional basic element struts, beams, piers became so familiar to engineers that any other solution appeared foreign to their minds. Also the calculation process was very simple. Reinforced concrete entered the field and at first nothing changed, one constructed just as with steel and timber with beams spanning from wall to wall or column to column. Set transversely to these main beams were a for loading up secondary beams and the spans between were spanned by 18 Rectangular steel beam stress diagrams: to yield stress (elastic); b at yield stress; c rectangular slabs. But instead of being designed and used for their unique structural characteristics, slabs were designed as representation of stress at yield for purposes of calculation single strips and were then considered as beams in the old-fashioned manner. Only the steam-ship* designer was in a position to consider the slab as a structural element for which he used the deductions made by Grashof; the structural engineer for the time did not do so.' 10.07 Maillart's point is still valid and structural design follows the convenient assumption that structural elements can be designed in isolation. Furthermore they are completely elastic and this behaviour is ideal. It is convenient to assume that an element will behave as one wishes it to 19 a Failure under simple point load forming a 'plastic' behave—without regard for the behaviour of the complete hinge; b plastic hinge failure in a rigid frame structure. Such assumptions are not always valid, sometimes because of the structural participation of what are considered non-structural elements; for instance the effect of infill panels of brickwork within a column and beam framework, or the effect of cladding on a lightweight *A more recent analogy is the disassociation between designers of aeronautical structures and building structures. The phenomenon and understanding of collapse of box girder bridges is more familiar to aeroplane designers than to
bridge designers
23
10.09 From stress follows strain and it is absolutely essential to have a clear understanding of strain arid structural doformation for tho proper use of modern design methods. Structural safety is a prime obligation but a structurally adequate building will not be acceptable if structural refinement results in cracked partitions, distorted window and door frames, broken and detached cladding panels, and fractured service mains. Not only strain (the deformation arising as the direct consequence of load) but the dimen sional changes in structural materials owing to temperature and chemical changes must be considered. Although unrestrained expansion and contraction because of tem perature changes are reversible and cause no damage within the normal range of building temperatures, there will exist a temperature gradient throughout a building mass that will cause differential movements in addition to the differential movements between materials having differing thermal characteristics. The chemical changes in concrete and loss of free moisture results in shrinkage which will persist over the first year of life of the building. The gaps created by such shrinkage will be kept alive thereafter by thermal effects and by changes in the moisture content from atmos pheric causes. Other chemical reactions such as the growth of rust on metal, and the expansion of unsound lime can
Technical study Structures 2 para 10.09 to 10.12
create large volumetric changes as well as unsightly blem ishes. 10.10 Stress and strain are mathematical in derivation but 'stability' problems are a combination of theory, practice and engineering skill. The general overall stability of a building is recognised easily and resolved by the principles of equilibrium, but the stability of building elements is loss readily understood although it is a familiar phenomenon. Stability controls the design of compression members. Buckling of a strut is a function of its length and cross sectional stiffness, and failure can occur owing to initial buckling at loads well below the critical compressive stress 20, 21, 22. 10.11 Once there were many diverse academic theories about the strength of columns, so that by choosing an appropriate one almost any design could bc justified. But research has consolidated theory, and rationalised design rules now prevail. The compression zones of beams, canti levers and plates, acting as vortical girders or diaphragms, are subject to instability. Engineering skill is necessary for the recognition and containment of these physical effects. 10.12 The linear beam (the element of skeleton construction as described by Maillart) in a simply supported condition, and the ideal pin- jointed (or hinged) strut or tie as fabricated
Technical study Structures 2 para 10.12 to 11.04
into frames are theoretical concepts rarely achieved in practice. A simple support or pin joint implies that the members at the support or joint are free to rotate relative to each other without restraint, but this is obviously an ideal situation and in practice joints have restraint, the members interact with each other (in addition to the purely statical reaction) and moments are developed. The consequences of rigid or partly rigid joint s, or in other words the 'continuity' of the structure, have long been recognised and adopted in structural design, but design methods were very mathematical until the original approach of Hardy Cross, see 23. 10.13 It is significant that Hardy Cross described his method of 'moment distribution' as a physical concept, implying that the deformations of the structure under the various conditions imposed must be visualised. By such methods, and as the words 'moment distribution' imply, the benefits of continuity are a redistribution of the bending effects of load throughout the structure and therefore a more efficient use of the structural material. 10.14 In general terms continuity is a development of linear design extended to two dimensions. The more significant advance (and it is to this that Maillart was referring) was the adoption in structural design of the well known and understood isotropic qualities of homogeneous materials. The ideal structural material should be homogeneous, ie it should have uniform physical characteristics throughout its mass, and it should be isotropic, ie its behaviour under stress should be the same in all directions through the material. 10.15 From this point onwards the understanding of surface structures and the evolution of suitable design methods grew rapidly. The flat slab of Maillart is a classic example. A slab supported along two opposite edges will deform under load to a cylindrical shape, and for design purposes can be considered as a parallel series of linear beams. A slab of material having isotropic qualities supported at the four corners only (no beams) will deform to a shape similar to a spherical figure, and is thus an element of surface structure which cannot with any degree of accuracy be designed on a linear basis, see 24. Such structures have three possible systems of internal stress: direct loading in tension or compression; shear forces; and bending moments and torsional moments. All these are multi-directional and the shape of the shell governs the relative importance of the three systems 25. The computer as a design tool can take most of the mathematical load, from the design of these structures.
24
24 a Normal slab; b slab acting as an element of surface, structure
25 Three possible systems of internal stress in shell structures: a direct forces, b shear forces; c bending moments and torsional moments
structures. Disadvantages ofsteel are its relative inflexibility of shape, and the fact that generally protection is required against fire and corrosion attack.
Aluminium 11.03 Aluminium alloys are not at present acceptable substitutes for steel in large structural elements. Greater cost, and the considerable increase in deformation under direct and thermal loadings, outweigh the advantages of lightness in self-weight, and corrosion resistance. However, aluminium may be considered in special circumstances, and particularly when self-weight of structure is a major consideration and the other loads are incidental. It is reasonable to use alloy units of small cross section built up into space frames to roof over large column-free spaces.
11 Structural materials Steel 11.01 Steel is an indispensible material both in its own right as a basic structural element and in the supporting role of reinforcement: as a substitute for the homogeneous and isotropic qualities in which concrete and bricks and blocks are deficient. It is provided in varying chemical compositions to fulfil different strength and weathering requirements and, in form, from the thinnest sheet to heavy solid sections or shaped sections. 11.02 Further working can be applied to the steel in rod or bar shape to produce much higher ultimate strengths usually for the reinforcing and pre-stressing of concrete or for the cables of suspension structures. The development of automatic welding and cutting has allowed the fabrication of an even greater variety of shapes and sizes, while the advent of high-strength friction grip bolts has revolutionised methods of jointing, not only for steel work but for all
Timber 11.04 Timber is one of the oldest materials of construction because it has been readily available, and is easy to work. Although long confined in its use to relatively small linear elements of structure (other than in the construction of ships), it has been developed more recently as a major structural material. This is possible both through a greater understanding of its properties and the development of more comprehensive design methods and is aided by parallel improvements and inventions in the field of adhesives, structural connections and wood-shaping machinery. However, wood is neither a homogeneous nor an isotropic material and the greater expansion of its use, as in surface structures, is possible only because these disadvantages have been minimised by the ability to glue or fasten timber in successive layers with the gram running in different directions. However, it is subject to destruction by insects and fungocidal attack.
25
Concrete
11.05 Concrete as a mortar was fully understood
by the Romans, and it is interesting to speculate how building would have developed if other countries had acquired this skill and developed to the full its quality of unifyi ng small elements into a monolithic structure having considerable tonsile qualities But its structural potential was obliged to wait for the development of acceptable steel reinforcement to provide the special qualities of reinforced concrete, a material which can bo designed as though it w e r e homo geneous and isotropic, if imperfect Being a cast material it has complete freedom of shape but it is a multi trade material in construction, and under site conditions it demands effective supervision and is a relat ively slow building operation 11.06 These disadvantaged, have been recognised by the rapid development of the off site precast industry, but in this, as in all popular acceptance of good ideas, there has been much over optimism The period of reappraisal now in being will re establish the position of precast concrete in the building world The disadvantages of self weight and space con summing factors have been overcome by the science of prestress and the development of high strength concretes which havepractically createda newmaterial different in characteristics and behaviour from 'normal concrete
Masonry 11.07 Masonry in the form of natural stone is also out, of the oldest of building materials While stone is still used in minor structures in a loadbearing capacity, the definition now embraces all forms of brickwork and blockwork, unrein forced and reinforced Statutory Regulations still allow the proportioning of brick structures on an empirical basis but calculated masonry in accordance with the Code of Practice can provide an economic solution even in high buildings where there is a repeated prominent wall system on plan
Plastics 11.08 The use of plastics as a structural material is still in the experimental stage, and further research and experience is essential to its acceptance in competition with established structural materials But its development is rapid, and a breakthrough may be imminent
Conclusion 11.09
The choice of prime material for the structure is largely dependent on structural form But even here design ingenuity has removed barriers and the prime materials can be used m practically any desired shape Choice of material is more likely to be conditioned by factors other than structural efficiency 11.10 Structural form can dictate the material, as can the required foundation, the limitations of resource, site and constructional factors and time available Freedom of choice of materials where other conditions are equal is restricted to those buildings where the structure is hidden. Although experience, research and design skills may benefit from machine aids to design, such as the computer, they cannot be replaced.
26 John Hancock Center Chicago (SOM) Example of tapering steel frame building with exposed cross bracing as design feature
Technical study Structures2 para 11 O5 to
11 10
AJ Handbook of
Building Structure
Of
related
interest
edited by Allan Hodgkinson
In its first edition, this Handbook became a standard reference for both students and practitioners. Recent changes to British Standards, Codes of Practice and Building Regulations have generated demand for a new, updated edition; and unlike the reprints of 1976 and 1977, this is a radically revised and updated version of the original 1974 Handbook.
AJ Handbook of Building Enclosure edited by A J Elder and Maritz Vandenberg
The principle changes are in the sections on Masonry (totally rewritten to take account of the 1976 Building Regulations) and on Timber (substantially revised to take account of new timber gradings). In addition to many minor improvements, the opportunity has also been taken to bring up to date all the references quoted.
"A new and more integrated approach to construction techniques than the traditional textbook" Building Trades Journal
For the rest, this remains the widely acclaimed structural design handbook first published in 1974. Information is specific enough to be of practical value, yet presented in a way intelligible to users without engineering backgrounds.
"With its many references and general high quality of presentation, the handbook will be of use and interest to anyone concerned with the built environment" IHVE Journal
"The information is generally of a very high Standard . . - great care has been taken by the various section authors" Building "For the student, the handbook ought to be a 'set book' to take him through many years of use . . . it deserves widespread circulation" The Architects' Journal Paper edition ISBN 0 85139 282 2
Some press comment on previous editions:
"This admirable and useful volume deserves to be studied carefully by readers outside the architectural profession, as well as those within it. . . a well designed and thoroughly interesting book" Build International ''This handbook provides a review of the whole structural field" Building Technology and Management "All in all, a most useful and comprehensive text book which no self-respecting architect can afford to be without" Architect's News
Guide to the 1976 (Seventh A J Elder
Building Edition)
Regulations
This new 1982 edition of the Guide to the 1976 Building Regulations, coming on the heels of the Secretary of State's long-awaited Command Paper, incorporates two new appendixes: on the Proposed Second Amendment, and on The Future of Building Control in England and Wales. Some press comment on previous editions: "An invaluable source of guidance through the ver bal jungle of the Regulations" Building Technology and Management "The book provides a comprehensive reference on matters of everyday practice for all members of the building team and students and should act as a companion to the 1972 Regulations themselves" Building Trades Journal "Should provide a valuable reference book for the architect and for the builder in ensuring that their work complies with the Regulations" Construction News ISBN 0 85139 850 2
New Metric Handbook edited by Patricia Tutt and David Adler
ISBN 0 85139 273 3 The Architectural Press 9 Queen Anne's Gate, London SWlH 9BY
With sales approaching 100 000 over the past 10 years, the original Metric Handbook is an established drawing board companion. But now that the metrication programme in the UK is virtually com plete, the emphasis on conversion to metric which formed the basis of the old Metric Handbook is no longer appropriate. This radically revised and greatly expanded New Metric Handbook retains many of the features of the old, but concentrates much more strongly on planning and design data for all common building types. If ever there was a drawing board bible, this is it. 480 A4 pages. ISBN 0 85139 468 X
