The meteorological loading of structures

Build. Sci. Vol. 6, pp. 17-23. Pergamon Press 1971. Printed in Great Britain

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The Meteorological Loading of Structures This article originally appeared in the Construction Industry Handbook* J. K. P A G E t

The effective design of structures to resist applied meteorological loads, acting both singly and in combination, is clearly an extremely important topic in building design and construction, for, the public, not unexpectedly, demand a high level of competence in all matters affecting public safety. A fundamental task for the building designer is to identify the point of balance in structural design, where the economic penalties of overdesign for meteorological loads are balanced properly against the safety risks implied in underdesign against such meteorological extremes. It is very difficult to identify this point of balance. This article attempts to clarify some of the basic principles which must be considered in selecting meteorological data for loading studies.

THE DESIGN PROCESS FOR STRUCTURAL STABILITY IN RELATION T O FAILURE

cycled. Further new hypotheses can then be formulated until convergence takes place on an acceptable range of solutions.

Practical causes of failure at times of extreme meteorological events

The design process The design process needed to secure reasonable meteorological safety involves an almost overfamiliar chain of inter-related decisions. Basically some structural form has first to be postulated as a hypothetical solution to a particular construction problem. Then the meteorological loadings on this postulated form have to be derived for different combinations of extreme weather likely to cause failure in combination with other loadings. This involves postulating modes of failure, in order to identify the meteorological inputs associated with those specific modes of failure. These meteorological loadings combined into selected adverse loading combinations with other loadings, both live loads and dead loads, have then to be fed into a subsequent analysis to study consequent stresses and deflections of the structure and its components. The simplifications in the analytical process may or may not introduce conservative elements into the design at this stage. The predicted stresses can then be compared with the permissible stresses in the materials being used in the structure. These permissible stresses may be set relatively high or low according to the codes of practice of the country concerned. The predicted deflections can be compared with the permissible deflections of the various elements. The design can then be modified, on the basis of the analysis, either to provide additional strength, if required, or to reduce strength in the interest of economy, if there is surplus strength. A new structural hypothesis can thus be formulated, and the analysis can be re-

Failures occurring at the time of a relatively extreme meteorological event may be due to a number of design and construction weaknesses. Such weaknesses may originate from defects in any stage in the design chain which may undermine the overall validity of the designer's original analysis. Extreme weather may be the event that produces failure, but is not necessarily the cause of the failure in fundamental design terms. Clearly it is possible that inappropriate meteorological loadings or live loadings may have been used in the case of such failure, or the wrong combinations of meteorological loads and live loads selected. Furthermore, the designer may not have correctly anticipated the mode of failure, hence the meteorological loading conditions that caused failure may not have been specifically identified in the original analysis. In addition, however, there may have been deficiencies in the rest of the analytical process concerned with the prediction of stresses and deflections which may have led to serious underestimates of strength and movement, for example the neglect of resonance phenomena in dynamic loading. Furthermore, the design may not have been constructed precisely in the way anticipated by the designer in his original design analysis. The materials used may not have had the strength predicted, either due to inadequate materials specification, defective manufacture, or site construction shortcomings. Finally, substantial deterioration may have taken place since construction, due to factors like corrosion and weathering. Frequently many of these factors are present in combination in the case of practical failures occurring at time of meteorological extremes. It is

* Published by M.T.P. Ltd., February 1971, 85/-. t Professor of Building Science,The University of Sheffield. 17

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J. A. Pa~e

never easy to unravel the precise causes of failure in any particular failure associated with an extreme meteorological event. Absence of failure under extreme conditions clearly does not imply that the meteorological input data is necessarily conservatively selected, for the overall security may have been achieved as the consequence of overdesign in some other stage in the total design decision-making chain, e.g. use of relatively high load factors, adoption of excessively conservative permissible strengths, and so on. Field evidence of the presence or absence of failures unsupported by detailed studies of the various links in the design chain thus can only give an indication of the overall statistical level of safety implicit in the total design decision making-construction chain. There is, at the moment, a lack of detailed studies of the inter-relationships of the components of the design chain. Recently much attention unfortunately has been concentrated on only onelink in the design chain, namely the meteorological Ioadingcodes, which can only be considered properly in relation to the total design process.

Codilication Ofdesign systems Codifications of the various stages of the design process have been made over long periods by official bodies like the British Standards Institution in an attempt to provide a system of up-to-date inter-related codes, each one of which tends to cover one aspect of the necessarily interlinked, and sequential design process. Safety is partly covered, for example, by specific guidance on reasonable load factors, maximum permissible stresses and deflections in structural design codes. Appropriately selected meteorological data, which has to be fed into the first stage of the design process to estimate loads, demands substantial simplifications of very complex fluid dynamical situations. These involve interlinking the aerodynamic effects of buildings placed in the considerably disturbed wind boundary layer flow characteristic of towns with national meteorological data on selected extreme wind velocities measured over open ground in various parts of the country at standard heights. It is hardly surprising in view of the scientific complexity of the problem, that formidable difficulties have been encountered in reaching agreement in codifying bodies concerned with wind loadings. Clearly the stage of complexity has been reached where codification on the traditional, relatively simple lines to which the building industry has become accustomed in the past, is likely to become increasingly more difficult, especially now building forms have become so

diverse, and the three dimensional nature oi" citic~ so complex. The difficulties of code revision in the UK have, in fact, proved so formidable thai the building industry has had to work with scientifically o u t - o f date meteorological codes for a very long time because of the difficulty of reaching agreement on revisions. The risks have in practice been small because the total design process has usually given a substantial safety factor. There is quite a I(/L of evidence, however, to suggest we still do not yet have a particularly reliable set of meteorological inputs for systematic building design in the UK, and a considerable amount of further research is needed, especially for low buildings. In the meantime, public concern in recent years has been awakened by a number of failures of important structures. One may cite the Ferrybridge cooling towers, the Ronan Point disaster, the ITA mast failure at Emley Moor, to mention only a few. Furthermore, extremely widespread gale damage to housing occurred in Sheffield and surrounding areas in 1962, and in Scotland in 1968, especially in Glasgow and in Central Scotland. The 1952 version of the Code of Practice on loading in the UK has consequently came in for considerable public discussion. Official attention has so far been directed towards an attempted revision of the sections on wind loading. The Code sections on snow loading and ice loading also remain unsatisfactory, but little systematic attention appears so far to have been given to revision of these aspects. The recently proposed revisions of the meteorological data incorporated in the draft loading Code[l] have been critically received in many sections of the professions concerned with structural design. This is because the application of upward revisions of meteorological loadings, in isolation from any reconsideration of the total design process to eliminate possible overdesign in other stages, has demanded substantially greater strength than formerly and hence less economic systems of construction. This has led to substantial professional pressure to reduce the proposed meteorological loadings, and there is a consequent danger that the new Code now adopted may underestimate basic meteorological forces, it appears, therefore, an appropiate moment to review the philosophies behind the selection of meteorological data lbr loading studies. M E T E O R O L O G I C A L I N P U T S TO THE DESIGN P R O C E S S

Concept of weather design value A meteorological input to a design process may be called a weather design value. Thorn[2] has

The Meteorological Loading o f Structures

defined a weather design value as the magnitude of a meteorological variable which, when used in the design of an engineering system, non-meteorological factors having been accounted for, jointly or independently, will assure with a given probability that the system will meet adequately a set of prescribed design requirements. The choice of the meteorological variable depends on assigned design conditions and on the prescribed value of the probability being surpassed. Thorn suggests the assignment of the design requirements and the choice of the probability acceptable is clearly the building designer's responsibility, while the analysis to determine the set of meteorological probabilities containing the one chosen by the engineer, or from which he can make a choice, is the problem of the meteorologist, who must use the techniques of statistical climatological analysis. Concept o f mean recurrence period

The proper statistical description of rare events in climatology depends on the use of the theory of extreme value analysis, and a consistent statistical series must first be selected[3]. The description of rare events usually is expressed in terms of the magnitude of a stated meteorological event that is likely to occur in a given mean recurrence period. For wind loading studies in the UK, for example, a mean recurrence period of once in 50 years has been widely adopted[4]. There is a probability in any one year of 1 in 50 of these design values being exceeded, and over a period of 50 years there is a probability of 0.64 that it will be exceeded at least once. There is also a probability of 0.36 that it will not be exceeded even once in 50 years. Table 1 gives the probability of the number of years in a 50 year period in which the 50 year mean recurrence event,

Table 1. Probability of the number of occasions in a 50 year and 20 year period on which a meteorological event is likely to equal or exceed the 50 year mean recurrence value of that event ii

No. of occasions mean recurrence value is equalled or exceeded Not at all At least once Once only At least twice Twice only At least three times Three times only At least four times

Probability over 50 years

Probability over 20 years

0.36 0.64 0'37 0-27 0.19 0.078 0.061 0.017

0.67 0.33 0"27 0.061 0.053 0.009 0.007 0.002

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for example wind speed, will be exceeded. The designer's first job is to select an acceptable probability of the recurrence of meteorological events in a stated period considering both the life of the structure and the practical consequences of the meteorological design value associated with that probability being exceeded. In doing so, he must take account of the fact that he is dealing with a statistical process. There is, for example, a probability of 2 per cent that the event exceeded on average once in 50 years will occur in the first year of the life of a structure. There may be serious dangers associated therefore with the reduction of meteorological design inputs against short life situations. The Building Research Station[5] has given guidance on the probabilities of different maximum gust wind velocities at a height of 10 m over open country being exceeded over different periods, and suggested the possibility of some relaxation of design wind velocities for short life structures by applying a building life factor correction. Designers who wish to use such reductions, should however recognize the finite possibility of the reduced meteorological inputs being exceeded within a relatively short period. The concept of mean recurrence period is central to any meteorological analysis, and the first stage in attempting to analyse any meteorological loading problem is to establish a weather design value for the location under consideration for the desired mean recurrence period which must be selected in relation to the consequences of failure. Some recently used values of mean recurrence period for various aspects of meteorological loading design are given in Table 2. Proceeding systematically in this way, it is possible for the meteorologist to describe horizontal spatial variations of meteorological variables associated with loading in terms of data at standard height obtained from standard meteorological sites. These sites are located as far as possible on open level ground covered with short grass with instruments mounted at standard heights. Such statistical results can be mapped, and can then form the first link in the systematic design process [6]. The next stage in the process is the attempt to derive meteorological loadings related to such weather design values, which must be first of all corrected to allow for local topographic effects and for height. Such corrections may be substantial. Fundamental principles in the derivation o f engineering loads meteorological data

The derivation of the estimated loads on a proposed structure from basic meteorological data involves the use of a relationship function to relate

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J. K. P a g e Table 2. Some selected values o f mean recurrence periods used in loading studies in various countries

Country Loading Meteorological event UK France

Wind Wind

3 second gust at 10 m 0.92 ofmaximum gust recorded by standard anemometer

USA

Wind

Fastest mile of wind

Mean recurrence period Oncein 50 years Normalloadings three times/1000 days Extreme 1.75x normal Once in 50 years

Canada Snow

Once in 30 years Max. ground snow depth + maximum 24 h rainfall for period when snow depths are greatest

USA

Snow

Max. annual weight of Not known snow pack on ground expressed in depth of water

USSR

Snow

Depth of snow and Approximately density of snow used oncein 10-15 to determine ground years snow load

the engineering design load tL to the weather design variable tw. Thus, following Thom[2], the engineer needs to have available a reliable relationship f u n c t i o n f t o be used in an expression of the form: tL = J t t w )

where tL is the derived applied load and t w is the weather design variable, expressed as a random variable with an associated distribution function. t w may be multidimensional, e.g. cloud water drop content, cloud drop size, temperature, and wind velocity in the case of ice loading from rime ice. It is usually very difficult to derive this relationship function. In practice a particular statistical value of t w is often selected, only exceeded with a selected low probability, and the designer does not work with the full set of statistical data. Dynamic oscillation problems cannot be handled in this way. The selection of a design value of tw from a set of statistical data frequently involves an important decision about time averaging of the basic meteorological data, which is linked to the nature of the relationship function. This requires engineering knowledge about the time response of the system to a given set of meteorological inputs. Some components of a building may fail over a very short period of time, for example a glass window, while overall structural failure due to over turning may take a far longer time. Different meteorological averaging periods are appropriate for the two types of failure. The best averaging period to adopt can only be settled on engineering arguments. Spatial

correlations in the statistical wdues Oftw at differem points in space are also important for design decisions especially in the case of large structures, e.g. only part of a structure may be immersed in an extreme gust. Thus large structures, where the spatial integration of design loads occurs over a large surface area, may be designed to lower average gust velocities. The fundamental problem is to decide on the permitted reductions in relation to the scale of the structure. An important class of problems insufficiently considered in recent years are the dynamic oscillation problems where the time response characteristics of the structure are interlinked with the temporal pattern of the loading inputs. Such fluctuating load inputs may be externally generated in the oncoming wind flow pattern, or self generated due to eddy shedding from the structure itself. The meteorological relationship functions become particularly complex in the case of such dynamic situations[7]. In practice much design proceeds on a nondynamic basis on the concept of the equivalent static load generated by a stated wind velocity averaged over a stated period of time. Thus, in current standard wind loading practice, the relationship function discussed above takes the form: p = ½K(p/,q)V 2

where p is the applied pressure, K is the local pressure coefficient, p is the density at the stated atmospheric pressure and temperature, V is the selected extreme wind velocity at the height appropriate to the wind pressure coefficient studies averaged over a selected period of time t. The application of the relationship function demands an adjustment in the weather design variable to allow for the height of the structure. The height selected has to be related to the height used for assessing wind velocity in the model tests. The effects of ground roughness influence the vertical correction. K is normally derived from wind tunnel studies on models whose scale is small compared with the real structure on the assumption that, at least for bluff bodies, the relationship function is approximately independent of Reynolds number. Thus the determination of K for design studies depends on the use of wind tunnels to determine empirically the spatial and temporal variations of K for range of typical shapes of structures with wind incident from a range of directions. Studies on such a range of typical examples of building shapes can be built-up into a systematic published set of wind tunnel results. The designer then compares the shape of his building with this set of published

The Meteorological Loading of Structures examples, and interpolates a set of pressure coefficients to apply to his specific design and calculates the loads using an appropriate averaging period for wind velocity. If the shape of his building is radically different from the set of examples, the designer has no choice but to guess conservatively or to seek special studies of the new proposed shape in the wind tunnel. Errors in estimated wind loadings can thus result from the use of inappropriate relationship functions, as well as from incorrect selection of the weather design variable. Present building aerodynamic research work centres on the derivation and elaboration of the relationship functions. The Building Research Station for example have studied the relationship between full scale wind loading and wind tunnel wind loading in a tall building with interesting results[8], which have shown that a big unknown factor in many designs is the permeability of the structure. There is consequently a considerable lack of knowledge of internal pressures in buildings in times of high wind and hence ignorance of the pressure drop across building elements. Recent studies appear to indicate that in permeable structures with opening windows most of the pressure drop takes place across the windward wall instead of being distributed uniformly between the windward and leeward wall. Further BRS full scale studies are being undertaken in a series of inter-related low buildings. One therefore may state that our scientific knowledge of wind relationship functions for full scale buildings is still extremely scanty. Another fundamental difficulty in devising wind loading codes is the problem of deciding on the influence of the surrounding environment on the estimated loadings. Three scales of perturbation may be identified, perturbation due to topographical features, perturbation due to urban structure (i.e. other structures) and perturbation due to the structure itself. Some crude knowledge exists about perturbations due to topographical features like hills and valleys and has been classified by BRS. Knowledge of perturbations due to other buildings is more scanty, though Wise, among others, has recently published some preliminary data in this field[9]. In the majority of situations, wind loads on structures situated close to the ground in the boundary layer in an urban environment are less than those on buildings on less obstructed country sites where the surface drag is less. There are urban situations, however, where strong funnelling may occur between buildings. Strong winds for example can occur between tall slab blocks arranged in a Y formation. The set of wind tunnel examples

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available in standard codes are frequently not safely applicable to such situations. Our present state of knowledge of the perturbation of the loading relationship function due to urban building form is poor, and hence it is difficult to suggest a safe way of taking account of reductions of wind velocity which occur statistically for large numbers of buildings sites in towns. Furthermore, future changes of urban form may produce marked loading changes in existing buildings many years ahead due to future perturbations in the urban wind field. The safe adoption of any code of practice with substantial reductions for urban shelter effects must depend on two essential facts: (a) The existence of adequate predictive theory ~o cover variations in the loading relationship function with urban form. (b) A town planning policy that ensures the legal enforcement of a three dimensional form pattern that prevents new developments creating dangerously adverse wind situations on any existing structures designed to reduced loading standards to take account of urban shelter effects. While many engineers have pressed strongly for such loading reductions to be made for urban situations to allow for urban shelter effects, they have not demonstrated that the necessary conditions for public safety outlined above are fulfilled. In the present state of knowledge, there would seem to be no choice but to be relatively conservative in the choice of the weather design variable, at least in all areas of design where risk to human life exists. At present the conservatism seems to exist in the rest of the design chain, and not in the selection meteorological wind loads in the U K Codes of Practice. This may Jnot involve much practical risk as long as safety factors in structural design remain high. It could however becomeavery serious matter, if safety factors in other parts of the design chain were lowered. There is clearly a need forengineers to give a higher priority to loading studies linked with meteorological forces.

Snow and ice loading Not only are there difficulties with wind loading, but snow and ice loading in the present U K Codes also are based on very unsatisfactory loading relationship functions. Most advance in the development of improved relationship functions has been made in snow loading, and readers are referred to the important work completed in Russia and Canada. A useful summary may be found in a joint study by Schriever and Otstavnov [10]. These studies are based on relationship functions of the following form:

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J. K. Page WL = K ' W ~ R

where WL is the snow load per unit area at a given point, Wa is the design ground snow load, K ' is the snow loading coefficient for that area o f r o o f o f a given shape, R is a reduction factor for aerodynamic exposure. Usually several aerodynamic snow loading conditions have to be covered, ranging from uniform deposit for falls associated with low wind velocities to marked asymmetrical loading for snow deposited in periods o f relatively high winds. Mitchell o f BRS [11] is conducting a systematic national survey o f r o o f snow loads in the U K which should help in the evolution o f more realistic loading relationship functions for this country. A preliminary study o f ice loading has been conducted by the author o f this article in connection with the large n u m b e r o f icing failures encountered in Northern England in 1969112]. More detailed studies, which will be published in due course,

have shown that the relationship function for ice loading at present used in UK loading practicc (25-4 mm ice deposited uniformly) is not scientilically based. As aerodynamic factors play a big part in ice deposition, ice loading in practice tends to be strongly asymmetric. The rate o f deposition is a function o f size o f the elements o f a structure, and under adverse exposed conditions ice from clouds can accumulate with remarkable rapidity, especially on all surfaces with a small radius o f curvature [13]. The problem o f the inter-relationship between different aspects o f meteorological loading, snow and wind loading, ice loading and wind loading requires systematic study. It will present severe problems in statistical climatology to evaluate various probabilistic combinations o f weather likely to cause failure in practice. Clearly the effort that goes into structural meteorology needs to be increased, for there is no point in having the capacity to design structures with precision, if the statistical meteorological loads on such structures can only be determined with great imprecision.

REFERENCES l, BRITISHSTANDARDSINSTITUTION,Draft British Standard Code o f Practice .[or Loading: Windloads [Pt. 2 of the revision of CP 3, Chapter 5] (1968).* 2. H. C. S. THo~, Application of climatological analysis to engineering design data, WMO Symposium on Urban Climates and Building Climatology Brussels. W M O Tecb. Note

no. 109 Geneva WMO (1970). 3. Some methods of climatological analysis. WMO Tech. Note no. 81 Geneva: WMO (1966). 4. H. C. SHELLARD,The estimation of design wind speeds. NPL Symposium no. 16, Wind effects on buildings and structures. Volume l, p. 30 London : HMSO (1965). 5. BUILDINGRESEARCHSTATION,Wind loading on buildings--2. BRS Digest (Second series) no. 119 London: HMSO. (1969). 6. - - - , Wind loading on buildings--1. BRS Digest (Second series) no. l l9 London: HMSO (1969). 7. A. G. DAVENPORT,Gust loading factors. Proe. ASCE, ST3 0967). 8. C. W. NEWBERRY,K. J. EATON,and J. R. MAYNE, Wind loading of a tall building in an urban enviroment, a comparison of full scale and wind tunnel tests. Proe. Syrup. Wind Effects on Buildings and Structures, Volume 1 Loughborough: The University of Technology (1968). 9. A. F. E. WISE, Effects due to groups of buildings. Symposium on Architectural Aerodynamics London: The Royal Society (1970). 10. W. R. SCHRIEVER,and V. A. OTSXAVNOV,Snow loads: preparations of standards for snow loads on roofs in various countries with particular reference to the USSR and Canada. Report no. 9, Methods of Load Calculation (Rotterdam: CIB) (1967). 11. G. R. MITCHELL, Loadings on buildings--a review paper. BRS Current Paper 50/69. London: HMSO (11970). 12. J. K. PAGE, Heavy glaze in Yorkshire--March 1969. Weather 24, 486 (1969). 13. WMO Commission for Climatology (1969) Abridged final report of the fith session. WMO report no. 260, RP 84, pp. 61-64 (Geneva). Note added in press:

This article was prepared before the recent publication of the new U.K. Code of Practice on Wind Loading. The data in this new Code are much less conservative than in the earlier draft. Substantial reductions are included for low buildings. Such reductions have yet to be statistically verified. Designers will have to be especially careful of interactive situations, using this new Code, tall and low buildings mixed. It is not clear how the town planning difficulties are going to be solved involving tall buildings erected after low buildings have been designed to reduced wind loadings. * The Code has now been published, since this article was originally drafted B.S.C.P. CP3 : chapter V, part 2 (1970). Loading Part 2 Wind Loads, BSI, London.

The Meteorological Loading of Structures Le dessin effectif des structures destin6es h r6sister aux charges m6t6orologiques agissant ~t la fois s6par6ment et en combinaison est 6videmment un probl6me tr6s important dans le domaine de la conception du bfitiment et de ses plans, car la tendance g6n6rale impose un haut niveau de comp6tences dans tousles domaines concernant la s6curit6 collective. La t~che fondamentale du dessinateur consiste b, identifier le point d'6quilibre dans le dessin de la charpente pour laquelle les p6nalit6s 6conomiques d'un dessin 6xag6r6ment calcul6 en vue de r6pondre aux charges m6t6orologiques sont compens6es par les risques de s6curit6 r6sultant d'un plan trop faiblement calcul6. I1 est tr6s difficile d'identifier ce point d'6quilibre. Cet article a pour but de clarifier certains principes de base qui doivent ~tre consid6r6s dans la s61ection des donn6es m6t6orologiques h appliquer ~ l'6tude des charges. Der gegen vorkommende, meteorologische Belastungen, in Einzelwirkung oder in Kombination, wirksame Entwurf von Bauwerken ist bestimmt ein/iusserst wichtiges Thema im Entwurf und in der Konstruktion von Bauten, denn, es ist nicht/iberraschend, dass die Offentlichkeit in allen Angelegenheiten der tiffentlichen Sicherheit einen hohen Grad fachlicher Tauglichkeit verlangt. Es ist eine grundlegende Aufgabe des Baukonstrukteurs, den Schwerpunkt des Bauwerkentwurfs festzustellen, wobei die finanziellen Nachteile des tOberentwurfs gegen meteorologische Belastungen einwandfrei gegenfiber dem Sicherheitsrisiko ausgeglichen werden, das sich aus dem Unterentwurf gegen diese meteorologischen Extreme ergibt. Es ist sehr schwer, diesen Punkt des Gleichgewichts festzulegen. Dieser Bericht versucht, einige der grundlegensen Prinzipien klarzustellen, die bei der Auswahl meteorologischer Daten for Belastungszwecke ber/Jcksichtigt werden miissen.

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