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Essay Review: Perils ofCertitude in the StructuralAnalysis of Historic MasonryBuildingsSergio L. Sanabria aa Department of Architecture , MiamiUniversity , Oxford, Ohio, 45056, USAPublished online: 05 Nov 2010.

To cite this article: Sergio L. Sanabria (2000) Essay Review: Perils of Certitudein the Structural Analysis of Historic Masonry Buildings, Annals of Science, 57:4,447-453, DOI: 10.1080/000337900750013543

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A n n a l s o f S c i e n c e , 57 (2000), 447± 453

Essay Review

Perils of Certitude in the Structural Analysis of Historic MasonryBuildings

J a c q u e s H e y m a n , Arches, Vaults and Buttresses. Aldershot, Hampshire : Variorum

Ashgate, 1996. xii ­ 418 pp. Illustrations and indexes. £95.00, US$152.95, (cloth).

ISBN 0-86078-597-1 .

Reviewed by

S e r g i o L. S a n a b r i a , Department of Architecture, Miami University,

Oxford, Ohio 45056, USA

Structural analysis of historic buildings began with a heroic 1748 study by G.

Poleni of the dome of St Peter’s in Rome. Since the mid-nineteenth century,

architectural theorists such as G. A. Breymann, Viollet-le-Duc, Auguste Choisy,

Julien Guadet and Pol Abraham have investigated ancient Roman and Gothic

construction but, prior to the 1960s, few engineers specialized in studies of historic

structures. In that decade, three civil engineers, Rowland Mainstone, Jacques

Heyman and Robert Mark, invigorated the ® eld by applying new analytical tools to

this task.

Rowland Mainstone, the elder statesman of this group, relied heavily on his

powerful intuitive grasp of structural behaviour.1 Robert Mark deployed photoelastic

models and ® nite element analysis.2 Jacques Heyman used statical equilibrium

equations, reinforcing classic thrust line limit analysis with `plastic theory ’ which he

had helped to develop. The book under review is a collection of 26 papers published

by Heyman between 1966 and 1995, a few of which have attained canonical status

among architectural historians. They were not meant to be read in sequence, as most

begin with the same technical arguments, which allowed them to stand independently.

Heyman has revised this collection of papers into a short book published in 1995 by

Cambridge University Press under the title The Stone Skeleton. This booklet adds

nothing to the original papers, so it is appropriate to review Heyman’s papers in their

original format as published by Variorum Ashgate in this expensive photocopied

assembly.

Twelve of the papers discuss the theory of unreinforced arcuated masonry, the

subject of Heyman’s most interesting contributions. His ® rst, `The stone skeleton ’

(1966), outlines the topic broadly. At least ® ve others repeat the basic arguments, `On

shell solutions for masonry domes ’ (1967), `The safety of masonry arches ’ (1969),

`Chronic defects in masonry vaults : Sabouret’ s cracks ’ (1983), ’Poleni’s problem ’

1 Rowland J. Mainstone, Developments in Structural Form (Cambridge, Massachusetts : MIT Press,1975) ; `Structural analysis, structural insights and structural interpretations ’ , Journal of the Society ofArchitectural Historians, 563 (1997) 316± 40.

2 Robert Mark, Experiments in Gothic Structure (Cambridge, Massachusetts : MIT Press, 1982); Light,Wind and Structure (Cambridge, Massachusetts : MIT Press, 1990); Architectural Technology up to theScienti® c Revolution: the Art and Structure of Large-Scale Buildings (Cambridge, Massachusetts : MITPress, 1993).

Annals of Science ISSN 0003-379 0 print/ISSN 1464-505X online ’ 2000 Taylor & Francis Ltdhttp://www.tandf.co.uk/journals

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448 Essay Review

(1988) and `The collapse of stone vaulting ’ (1993). Masonry structures are very

strong in compression but extremely weak in tension because joints, whether dry or

mortared, have unreliable tensile strength. Shear failure, slippage along joints, is rare

because friction between stones is very high. Theoretically, masonry structures cannot

resist bending, which generates tension along an edge. They must be loaded more or

less axially, distributing loads broadly, which in turn usually reduces compression

stresses on the stone itself to a negligible level. Thus the elasticity equations that relate

stresses to internal deformations, and that are needed for designing steel or wood

beams are generally irrelevant. Heyman argues that masonry science can be reduced

to problems of statical equilibrium.

Most masonry structures are statically indeterminate: their internal stresses can be

distributed in in® nitely many ways and small changes in loading or support can

produce large unpredictable internal shifts, but a potentially simplifying factor is that

local bending is in fact possible and expresses itself in typically harmless open cracks.

Heyman incorporates this into his analysis. The venerable middle third law states that

lines of thrust, graphic depictions of summed forces, must be contained within the

inner third of any structural cross-section to avoid tension. If thrust lines go outside

this limit, bending simply concentrates loads on narrowed portions of the joints. In

1773, French physicist Charles de Coulomb realized that lines of thrust can approach

the edges of a joint and form hinges.3 Cracking is not intrinsically dangerous, but

hingeing can be either stable or unstable. Instability follows the formation of enough

hinges to turn a structure into a dynamic mechanism with thrust lines outside joints.

Stable hinged structures de® ne the limit of stability and are statically determinate.

Their thrust lines, determined from hinge points, become unique and thus calculable.

Arches accommodate small shifts and imperfections by forming hinges, turning

themselves into statically determinate structures. This is the conceptual basis of limit

analysis.

Heyman adduces a result from `plastic theory ’ , ® rst developed in the 1940s to

handle statically indeterminate steel structures. The `safe ’ or lower bound theorem of

limit analysis states that, if any line of thrust can be found that is in equilibrium with

external loads and lies wholly within the masonry, the structure is always safe. If an

analyst can ® nd a safe thrust line for a given loading, the structure can too, although

the two will not necessarily be the same. A corollary is that small shifts in foundations

or variations in wind loads do not lead to failure.

Heyman’s methods elucidate simple structural components, arches, ¯ ying

buttresses, or domes. For example, limit analysis of ¯ ying buttresses yields two limit

lines of thrust : passive thrust, as curved and drooping as the shape of the ¯ yer allows

and maximum active thrust, as straight as the same boundaries permit. Hingeing

under maximum active thrust reverses that of passive thrust. Heyman calculates the

passive thrust of typical ¯ yers to be about 3 tons, but that of the giant ¯ yers of Notre

Dame in Paris, about 14 tons, could distort the high walls. Some ¯ yers can handle

active thrusts of 1000 tons, but not those of the choir of Amiens, which could buckle

under wind loads of 20 tons. Heyman incorrectly claims that at Amiens ¯ yers with the

same weak design were used also along the nave and had to be replaced in the

® fteenth century. The early nave ¯ yers by master Robert de Luzarches (ca 1230) were

very diŒerent from the intentionally mannered ¯ yers by Regnault de Cormont in the

3 Charles de Coulomb, `Essai sur une application des re! gles de maximis et minimis a’ quelquesproble’ mes de statique, relatifs a’ l’architecture ’ , MeU moires de MatheU matique et de Physique preU senteU s al’AcadeU mie Royale des Sciences par Savants EU trangers (Paris, 1776), VII.

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Essay Review 449

choir (ca 1270). Heyman is unimpressed by French ¯ yers in general. Their frequently

pronounced curvatures induce drooping passive thrust lines near the clerestory wall

that can promote shear failures. Heyman speculates that this may explain the

colonnettes supporting lintels against which the ¯ yers thrust in late thirteenth century

examples. In straighter English ¯ yers, as at Lich® eld, the passive thrust line is almost

horizontal, making buttresses very e� cient when loaded. Heyman’ s view partially

agrees with that of Robert Mark, who pointed out in 1972 that the French never

imitated their most e� cient ¯ yers at Bourges (ca 1200± 1210).4

Isolated domes are also relatively simple and Heyman explains their behaviour

in various papers, including `The stone skeleton ’, `On shell solutions for masonry

domes ’ (1967) and `Poleni’s problem ’ (1988). He uses thin shell theory, diŒerentiating

membrane solutions where stresses are assumed to lie within an in® nitesimally thin

surface and slice solutions where thickness is allowed and thrust lines can shift from

the centre axis, causing bending. Domes develop vertical radial and horizontal hoop

stresses. Hoop stresses in thin hemispherical domes are compressive from the crown

down to 52 ° from the zenith and tensile down to 90 ° . Vertical cracks develop in that

38 ° range, varying depending on the shell thickness and on conditions at the crown.

Thus masonry domes do not act as shells but as a series of `orange slice ’ arches.

Poleni invented slice solutions for his 1748 analysis of the dome of St Peter’s in Rome.

In a paper of 1986, Robert Mark and Paul Hutchinson studied this kind of cracking

in the Pantheon in Rome, the largest dome of antiquity and show how it was

exploited in Late Antique and Byzantine domes.5 Heyman calculates that the shell

span: thickness ratio can safely exceed 45 :1. More slenderness requires hoop

reinforcements added to absorb tension, or a segmental pro® le to the arc of the dome,

or a thickened mass around the base. Heyman uses this to explain generically the

surcharging rings around the springing of the Pantheon and the heavy in® lls behind

pendentives at Hagia Sophia. His analysis of complex components and buildings is

less reassuring than that of simple elements.

For example, Heyman applies membrane solutions to Gothic rib vaults,

concluding that transverse and wall ribs are unstressed and carry only their own

weight, but diagonal ribs are necessary because at the `creases ’ , thin smooth shells

would be overstressed and fail. Roman and Romanesque ribless groin vaults work

because of their thickness. The result stems from his assumption, following Pol

Abraham’s, that forces ¯ ow along parallel sections of the web towards the ribs,

concentrating there. In a 1973 paper on photoelastic modelling and ® nite element

analysis of a quadripartite vault, Robert Mark, John F. Abel and K. O’Neill reported

forces that ¯ owed directly from webs to springings.6 Mark agrees with Heyman that

diagonal ribs conceal waving joints and simplify construction but rejects emphatically

their structural function.

Heyman counterargues in `Chronic defects in masonry vaults : Sabouret’s cracks ’

(1983):

4 Robert Mark, `The structural analysis of gothic cathedrals : a comparison of Chartres and Bourges ’ ,Scienti® c American, 227(5) (November 1972), 90± 9 ; Robert Mark and Maury Wolfe, `Gothic cathedralbuttressing: the experiment at Bourges and its in¯ uence ’ , Journal of the Society of Architectural Historians,33(1) (1974) 17± 26.

5 Robert Mark and Paul Hutchinson, `On the structure of the Roman Pantheon ’ , The Art Bulletin,68(1) (1986) 24± 34.

6 Robert Mark, John F. Abel and K. O’Neill, ’Photoelastic and ® nite element analysis of aquadripartite vault ’ , Experimental Mechanics, 13(8) (1973), 322± 9 ; Robert Mark, John F. Abel and K. D.Alexander, `The structural behavior of medieval ribbed vaulting ’ , Journal of the Society of ArchitecturalHistorians, 36 (1977), 241± 51.

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450 Essay Review

¼ it should be emphasized that the vault remains a highly redundant structure,

for which ¼ it is still not meaningful to ask for the `actual ’ distribution of

forces. What can be said is that (this) pattern ¼ is in fact reasonable and that,

much more strongly, whether the pattern is reasonable or not, calculations of

vault stability based upon that pattern are safe.

Yet the question Mark raised is not whether Heyman’s analysis is safe, but

whether it is optimal. If diagonal ribs are unnecessary, as Mark claims to demonstrate,

limit analysis reveals its limitations.

Problems with Heyman’s methods resurface in his paper, `Beauvais Cathedral ’

(1967± 8). Heyman examines the two collapses of this tallest Gothic church in 1284

and 1573. He argues that the choir collapsed in 1284 owing to an essentially trivial

detail or unforeseen event. Because it had stood 12 years, the lower bound theorem

guarantees that it would have continued to be safe, but his comparisons of this church

with those at Amiens, Reims and Paris yield only narrow insights. Robert Mark and

Maury Wolfe’ s photoelastic models of Beauvais, reported in 1976, pinpoint an area

of high stress concentrations where intermediate pier buttresses overhang a solid

buttress below, what the French call a porte-a[ -faux.7 Shear failure there would have

brought down the ¯ yers above and with them the high vaults. Heyman believes that

no possible failure could occur at the porte-a[ -faux, perhaps misled by his assumption

that stresses in masonry are almost always low. He turned instead to Viollet-le-Duc’s

ingenious explanation of the collapse as caused by fracturing of thin colonnettes

holding salient blocks of the tas-de-charge, the area of horizontal coursing at the

springing of Gothic vaults. Heyman views this as the expected kind of failure.

Regarding Jean Vast’s crossing tower of 1564± 9, that collapsed in 1573, he concludes

that it was never in equilibrium and needed the added buttressing recommended in

1571 by the French king’ s masons.

The Beauvais paper exposes the problematic character of Heyman’s analysis. It is

extremely di� cult to apply to complex systems and its results can have an

unsatisfying vagueness about them if a collapse mechanism is neither unique nor

obvious. The assumptions that neither compression nor shear failures are likely in

masonry and that elasticity considerations can be ignored can blind the unwary to

troublesome details. The puzzling assurances of the safe theorem apparently need

much expert quali® cation, since collapse and even instability are possible despite its

guarantees. Finite-element analysis software comes certainly not without its problems,

but for very good reasons it has eŒectively replaced such classic computations in most

practical applications.

Heyman himself is an important and perhaps late agent in the development of

limit analysis and he has investigated his subject both mathematically and historically.

Heyman’ s technical expertise is tightly focused, narrow perhaps, but his historical

work covers a broad ground, all seen from a fresh, and at times eccentric,

perspective. ``̀ Gothic ’ ’ construction in ancient Greece ’ (1972) reinterprets the

trabeated structure of Greek temples. As he notes, many standing ancient architrave

blocks are cracked and depend for their stability on ¯ at arch action. The architraves

of the un® nished temple of Olympic Zeus in Akragas, Sicily, are built as two abutted

overhanging slabs, abandoning any pretence that they are beams. By analysing

7 Robert Mark and Maury Wolfe, `The collapse of the Beauvais vaults in 1284 ’ , Speculum (1976),462± 76.

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Essay Review 451

classical entablatures as deep plate-bandes or ¯ at arches, Heyman establishes a

common ground between Gothic and Greek construction. Both depend on arcuated

structure for their stability.

`On the rubber vaults of the Middle Ages and other matters ’ (1968), addressed to

art historians, was an in¯ uential synopsis of conclusions from his previous papers.

The title derives from Nikolaus Pevsner, who retorted to Choisy’s remarks about the

¯ exibility of Gothic vaults that they ¼̀ were not made of rubber. ’ Heyman explains

how pinnacles increase stresses negligibly but can have a measurable impact on

stability and a still greater eŒect against shear slippage caused by the thrust of ¯ ying

buttresses. He frowns on Pol Abraham’s view, repeated by Paul Frankl, that neither

ribs nor ¯ ying buttresses are necessary.8 Heyman argues that ribs play many roles,

reinforcing weak creases, doubling as formwork, simplifying complex geometric

joints and `as a bonus the rib has been thought to be aesthetically satisfying. ’ Since

transverse and wall ribs only carry their own weight he criticizes the transverse ribs

in John Wastell’ s fan vaults at King’s College chapel, Cambridge, as `clumsy ¼

structurally meaningless ¼ a supreme example of ¼ irritation (for the structural

engineer’s eye)’ , but Heyman’ s most important conclusion is that since stability, and

not stress concentration, is the overwhelming structural design criterion, pro-

portioning of components using ratios is the right kind of structural rule for masonry.

This was precisely what concerned mediaeval architects. Progress in the re® nement of

ratios was possible as long as structural experiments unfolded in the twelfth and

thirteenth centuries. Heyman believes that stultifying Late Gothic rules ensured the

collapse of structural progress during the Renaissance.

In his earlier paper on `Beauvais cathedral ’ (1967± 8), Heyman had already

argued, somewhat incoherently, that Gothic architecture declined after the second

half of the thirteenth century. He claimed that Jean Mignot, a French expert advising

the masters of Milan cathedral ca 1400± 1, did not understand the rules that he used

and that he confused structure and aesthetics. Heyman puzzles over architects

worrying about both categories at once and resents the con¯ ation. He claims that

Milan posed no new structural di� culties and old rules, invented for much more

demanding structures such as Amiens, now su� ced. In a decidedly shaggy argument,

Heyman sees this relaxation leading down to the easy rules of Vitruvius and to a

structurally decadent Renaissance.

In several papers, Heyman tries to follow the later life of these mediaeval

structural ratios and the birth of modern analysis. In `Calculation of abutment sizes

for masonry bridges ’ (1982), he traces what is in his view a stunted tradition of

geometric sizing rules. The Spanish architect Rodrigo Gil de Hontan4 o! n, the

Frenchman Fran†ois Blondel and Christopher Wren all proposed geometric

constructions yielding a ratio between arch abutments and span of 1 :4.9 Heyman

misdates to 1743 the famous treatise on stereotomy of Fran†ois Derand, published in

Paris in 1643, where a similar formula was included.10

8 Pol Abraham, Viollet-le-Duc et le rationalisme meU dieU val (Paris, 1934), 88 ; Paul Frankl, The Gothic :Literary Sources and Interpretations (Princeton, New Jersey, 1960), 807.

9 For Rodrigo Gil de Hontan4 o! n see Simo! n Garcõ !a, Compendio de Arquitectura y Simetrõ Ua de losTemplos (Salamanca, 1681) (Ms 8884, Biblioteca Nacional de Madrid), facsimile edition and transcriptionby Cristina Rodicio, Valladolid, 1992 ; Sergio Sanabria, `The mechanization of design in the 16th century:the structural formulae of Rodrigo Gil de Hontan4 o! n ’ , Journal of the Society of Architectural Historians,41 (1982) 281± 93. For Blondel and Wren see Fran†ois Blondel, Cours d’Architecture EnseigneU dansl’Academie Royale d’Architecture, 2 vols (Paris, 1675± 83); Christopher Wren, Parentalia (London, 1750).

10 R. P. Fran†ois Derand (S. I.), L’Architecture des Voutes, ou l’Art des Traits et Coupe des Voutes(Paris, 1643), 16± 7.

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452 Essay Review

In `Couplet’ s engineering memoirs, 1726± 1733 ’ (1976), Heyman discusses the late

writings of Pierre Couplet, whose analysis of the action of a masonry arch laid the

basic foundations for limit analysis. Couplet’s results were adopted in Ame! de! e-

Fran†ois Fre! zier’ s three-volume stereotomic treatise published in Strasbourg in

1737± 9.11 They were apparently unknown to Coulomb, who did his own analysis of

hingeing in 1773. Couplet wrote other papers, on the overturning pressure of earth on

a retaining wall, on hydraulics and on the loads imposed on diŒerent members of

horse teams, that Heyman ® nds less signi® cant.

Theoretical progress in eighteenth-century analytical mechanics saw erratic

application in architecture. Heyman’s `The crossing piers of the French Pantheon ’

(1985) examines several memoirs on Jacques Germain Sou‚ ot’s precariously slender

Pantheon by the engineer E. M. Gauthey and the architects Pierre Patte and Jean-

Baptiste Rondelet. Heyman does not comment on similarities between the triple-

shelled Parisian dome and Wren’s St Paul’ s, London, but focuses on disputes about

thrust and the causes of stone distress that started when centering was struck. Even

at this late date and to Gauthey’s despair Rondelet, who knew Couplet’s work,

attempted to demonstrate that domes do not thrust, but Gauthey seemed equally

insensitive to the dangers of porte-a[ -faux, `false ’ work, loading a great dome on four

slender piers, arguing that Sou‚ ot’ s design allowed exterior walls to carry the dome.

Both agreed that a major cause of stone distress was the construction vicieux endorsed

by Patte where joints were trimmed so that only their edges carried load. Heyman

notes wryly that neither acknowledged responsibility for overseeing construction.

Mediaeval timberwork has some analogies with traditional masonry. Unlike

stone, wood is almost as strong in tension as in compression and wooden members

such as beams and rafters tolerate bending well. But unreinforced wood joints, strong

in compression, are usually weaker in tension and shear. Timber has a vexing

combination of properties that appear to have tested many Gothic master carpenters.

Three papers in Heyman’ s anthology investigate English carpentry. `An apsidal

timber roof at Westminster ’ (1976) discusses the problems faced by an inexperienced

English carpenter roo® ng the ® rst French-style apsidal choir in England ca 1260. He

leaned all the turning trusses on the unbraced king post of the last straight truss,

causing the roof to sag. A modern restoration has deployed diagonal battens to stiŒen

the roof, thereby preserving the `bad ’ mediaeval design while relieving structural

distress. Heyman and E. C. Wade undertake a much more di� cult analysis in `The

timber octagon of Ely Cathedral ’ (1985). This stupendous crossing vault, begun in

1328 by William Hurley, is a partially triangulated timber frame with one degree of

static indeterminacy and one degree of mechanical freedom. Heyman and Wade show

that the curved ribs and the posts framing the central lantern were overloaded in

bending and the structure fell gradually into ruin. Again, new braces introduced in

restorations by Essex in 1757± 62 and by G. G. Scott, beginning in 1862, have

preserved the mediaeval form while ameliorating structural distress.

In `Westminster Hall roof ’ (1967) Heyman attempted to determine how the

monumental hammerbeam trusses built ca 1395± 1400 by Hugh Herland actually

work. After summarizing restoration reports and structural interpretations from

Viollet-le-Duc to Pevsner, Heyman calculates stresses on all members under dead and

wind loads using a free body diagram assuming pinned joints. He concludes that the

11 Ame! de! e-Fran†ois Fre! zier, TraiteU de SteU reU otomie a[ l’Usage de l’Architecture; TheU orie et Pratique dela Coupe des Pierres et des Bois pour la Construction des VouW tes et Autres Parties des BaW timents Civils etMilitaires, 3 vols (Strasbourg, 1737± 9).

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Essay Review 453

rafters and the lower collar carry all dead loads. According to Heyman all other

members are largely redundant : the great arch, the curved braces, the hammerpost

and hammerbeam and the upper collar. He calculates that the great arch only carries

wind loads and assumes that the wall post simply notches the truss against the wall

and that the corbel that supports it plays no supporting role. The angels carved on

the ends of the hammerbeams, suggesting that they carry the roof on their outspread

wings, are according to Heyman just an architectural joke.

Heyman’s conclusions diŒer substantially from those of Rowland Mainstone

(1967), Robert Mark, Yun Huang and Lynn Courtenay (1987), and E. T. Morris, R.

G. Black and S. Tobriner (1995).12 Mainstone disagreed with Heyman’s assumption

of pinned joints and no role for the corbels. He thought the great arched rib, the lower

arched brace and the hammerpost were the principal carrying members. The

experiment by Mark and Huang using an instrumented timber model showed that in

fact the arched rib carried most of the load, as Mainstone held, but that the lower

brace, hammerpost, hammerbeam, lower rafter and wall post were also very active.

The angels indeed carried a load. The ® nite-element model of the hammerbeam truss

by Morris, Black and Tobriner allowed a more detailed and realistic loading than in

any previous investigation. Their results con® rm those of Mark and of Huang, but

with the unexpected outcome that the decorative tracery, which was not part of

Herland’s original work, reduces bending stresses substantially in many members. In

the end Heyman’s paper on Westminster Hall in London served to inspire a

generation of new researchers using new tools destined to supplant it.

Other papers in the anthology discuss stairs, various bridges, church spires and the

structural restoration of speci® c monuments. Many testify to Heyman’s extensive

work in preservation. Although some of Heyman’s papers have been published in

other anthologies, they remain widely scattered, so this is a welcome publication, even

if reading the uninterrupted collection seemed to this reviewer less satisfactory than

® rst encounters with the originals. The publication serves also to signal esteem for his

pioneering eŒorts in the study of the structural engineering of pre-industrial

architecture.

Still there are many problems with the book. There is no editing, no commentary,

nor any updating of articles to acknowledge new work and current bibliography.

Heyman’ s brief preface seems unaware of any critique of his work, or of the

widespread mistrust of classic limit methods. The preface mentions that calculations

encompassing elasticity can be so intensive that computers are necessary. It does not

point out that ® nite-element analysis allows a much less reductive approach so that

complex problems can be addressed more realistically, without assuming away shear,

or compression failure. Such assumptions may be sound, but they must be tested,

especially when attempting to learn how a historic failure occurred. The intellectual

horizon of this book is ® xed in the 1960s.

12 Rowland J. Mainstone, discussion of Heyman’s paper on the Westminster Hall roof in Proceedingsof the Institution of Civil Engineers, 38 (1967) 788± 92 ; Lynn Courtenay and Robert Mark, ’TheWestminster Hall roof : a historiographic and structural study ’ , Journal of the Society of ArchitecturalHistorians, 46 (1987) 374± 93 ; E. T. Morris, R. G. Black and S. Tobriner, ’Report on the application of® nite element analysis to historic structures ’ , Journal of the Society of Architectural Historians, 54(4) (1995)336± 47.

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