G. Solari 1, G. Bartoli 2, V. Gusella 3, G. Piccardo 1, P. Pistoletti 4, F. Ricciardelli 5, A. Vintani 6

Transcription

1 EACWE 5 Florence, Italy 19 th 23 rd July 2009 Flying Sphere image Museo Ideale L. Da Vinci The new CNR-DT 207/2008 Guidelines on Actions and Effects of Wind on Structures G. Solari 1, G. Bartoli 2, V. Gusella 3, G. Piccardo 1, P. Pistoletti 4, F. Ricciardelli 5, A. Vintani 6 1 DICAT University of Genoa, Italy, solari@dicat.unige.it, piccardo@dicat.unige.it 2 DICeA University of Florence, Italy, gbartoli@dicea.unifi.it 3 DICA University of Perugia, Italy, guse@strutture.unipg.it 4 SETECO Ingegneria srl, University of Genoa, Italy, ufficiotecnico@setecoge.it 5 DIMET University of Reggio Calabria, Italy, friccia@unirc.it 6 BCV Progetti, Milan, Italy, vint@bcv.it Keywords: wind loads, wind effects, codes of practice ABSTRACT The Italian National Research Council is a Governmental Institution whose mission is to promote, carry out and disseminate research activities in the main areas of knowledge and its applications for the scientific, technological, economic and social development of the Country. In 2002 CNR started promoting the drafting of documents in areas of Engineering where recent achievements have made the existing Codes obsolete, or where no Codes exist at all. In this framework, in 2005 a Panel was established with the goal of drafting a completely new set of Guidelines for the evaluation of actions and effects of wind on structures. In January 2008 a first draft of the document was presented and public discussion was opened, aimed at receiving any comment or suggestion before release of its final version. Finally, in spring 2009, the modified version of the document was issued with the name CNR-DT 207/2008 Guidelines on Actions and Effects of Wind on Structures. The Italian version is available for download from CNR web-site while the English version will soon be available. In this paper the main aspects of the Guidelines are presented, together with the new approach adopted. 1. EVOLUTION OF THE ITALIAN CODES OF PRACTICE ON WIND LOADS The first Italian document providing guidance for the evaluation of wind loads on structures was released by the Italian National Research Council (Consiglio Nazionale delle Ricerche, CNR) in 1964 (CNR 1964) and revised in The basic wind load was given in terms of wind velocity pressure for the five areas in which Italy was divided. In addition, the variations of the wind velocity pressure with the distance from the coast and with the altitude were defined. The document

2 also contained a simple description of the external pressure coefficients for the upstream and downstream faces, as well as for the roofs of buildings, and of the internal pressure coefficients for buildings with openings. In its whole, this document was rather consistent with similar documents published in the same period in many other countries. In 1968 it was also translated into requirements released by the Ministry of Public Works (M.LL.PP.1968). Such requirements were made compulsory for public works. In 1978 the Ministry of Public Works published the first Italian Code on Structural Loads (M.LL.PP. 1978a), compulsory for all structures, together with a commentary (M.LL.PP. 1978b). These documents were substantially the same described above, with the only addition of an equation aimed at evaluating the dynamic factor, as a function of the building natural frequency, height and construction material. In 1981 a new release appeared of the National Research Council Guidelines (CNR1981), which soon triggered an update of the Italian Code on Structural Loads (M.LL.PP. 1982a) and of its commentary (M.LL.PP. 1982b). No relevant advance was introduced however in these new documents, with respect to the original issue of the National Research Council. It happened, thus, that while many countries updated their original Guidelines to take into account the progress occurred in Wind Engineering, Italy remained almost 20 years behind the state-of-the-art. This situation suggested the National Research Council to appoint a Committee to fill such a gap through the release of new Guidelines which were finally issued in 1985 (CNR1985). Among several relevant advances, the new Guidelines contained a refined definition of the mean wind speed and turbulence profiles, pressure and force coefficients covering a wide variety of structural types and secondary elements, and a new definition of the equivalent static loads involving the concept of gust response factor. However, due to the lack or reliable wind data, a single value was given for the reference wind velocity of 30m/s at 10m height, to be used for the Italian mainland, as well as for the islands. Such a coarse definition of the design wind velocity was somehow in agreement with the ECCS Recommendations appeared in the same year (ECCS 1985), giving a wind map of Europe, in which no values were given for Italy. The above reference wind velocity increased substantially the wind loads, with the effect of raising criticism within the Italian Consultancy. The lack of a consistent definition of the Italian wind climate triggered a study that brought to the publication in 1992 of an Italian wind map (Ballio et al. 1992). Italy was divided into nine zones, with reference wind velocities at 10m height ranging between 25 m/s and 31 m/s. This map also defined the variation of the reference wind velocity with the distance from the coast and with the altitude. The new wind map was acknowledged by all the wind codes then since, and is still in use with minor amendments. In 1994 the ENV version of Eurocode 1 was published (CEN 1994), a document way too advanced and complex when compared to the Italian past practice. So, awaiting for the publication of the EN version, in 1996 a new Code on Structural Loads appeared (M.LL.PP. 1996a), together with its commentary (M.LL.PP. 1996b), with an attempt to slowly move towards the Eurocode approach. The wind velocities from the Italian wind map of 1992 were given as compulsory values, while only criteria were provided for the choice of the aerodynamic and dynamic coefficients for simple structural types. It was expected that in the following years a convergence between the Italian standard and Eurocode 1 would have been achieved, with the acknowledgement of the recent progress in Wind Engineering, which however did not occur. In 2005 the EN version of Eurocode 1 was released (CEN 2005, which did not bring any simplification to the ENV version of 1994 and, even more, it did not incorporate the impressive progress in Wind Engineering of the last decade. In the same year the draft of the new Italian Technical Standard for Constructions was issued by the Ministry of Infrastructures and Transports (M.II.TT. 2005), adopting a performance-based approach, which however made a step back in the definition of the wind loads with respect to the modifications introduced in the 1996 code. Just to mention one among many, return periods of 500 and 1000 years (cumulated with partial coefficients of the order of 1.5) were introduced for the design wind

3 velocity, in disagreement with all the other international and national standards. In particular, steel contractors greatly suffered from this choice.. It happened, therefore, that in 2005 Italy had three different documents concerning wind (and other) loads: the still compulsory 1996 code, the 2005 EN version of Eurocode 1, and the 2005 draft of the new Italian Technical Standard. Being these documents endowed by fully different concepts and rules, this almost put into panic the Italian Consultancy. Acknowledged this situation, the Italian National Research Council decided to issue the Guidelines described in this paper. To complete this intricate frame, in 2008 the Italian Ministry of Infrastructures and Transports published a new version of the Italian Technical Standard for Constructions (M.II.TT. 2008), conceptually in agreement, with both Eurocode 1 and CNR Guidelines, but in 2009, the same Italian Ministry of Public Works issued a new Commentary containing a set of rules in full disagreement with Eurocode 1, CNR Guidelines, and with the state-of-the-art in Wind engineering. 2. AIMS AND STRUCTURE OF THE DOCUMENT Early in 2005, the Italian National Research Council recognized the necessity to provide the national technical-scientific community a document containing the state-of-the-art criteria for defining the wind loading of structures and the structural response. A draft of CNR-DT 207/2008 was the outcome of the activity of a Workgroup established within the Committee for the drafting and analysis of technical standards on constructions of CNR, carried out over a period of about three years and with contributions coming from the Academia and the Consultancy; the acronym DT used in the title stands for Technical Document (Documento Tecnico in Italian). The aim of this new document is twofold. On one side, it contains a set of rules for establishing wind loads and the consequent structural response. On the other side, it provides concepts for the understanding of such rules and worked examples to illustrate their application. CNR-DT 207/2008 is consistent with the Technical Standards for Constructions (M.II.TT. 2008) and, on the whole, it is also consistent with the Eurocodes. In some few cases, CNR-DT 207/2008 departs from these, to incorporate recent developments in Wind Engineering, with the ambitious aim to collect, in a homogenous and unitary text, all the principles and rules needed to Consultants for the analysis of the structural response to wind. On the other hand, CNR-DT 207/2008 is also meant to help the reader to interpret and apply its contents, instead of simply offering a set of rules without any explanation. Therefore, CNR-DT 207/2008 should not be meant as set of coercive rules, but rather as a help to Consultants in finding their way through the design of wind-exposed structures. The document is organized in four Chapters. Chapter 1 contains some preliminary remarks: it sets the aims of the document in the Italian tradition and in the international context (1.1), it defines the field of application of the document (1.2), it illustrates the text organization and it provides a rational guide to its use (1.3), it reports a list of the main International Codes and Standards (1.4) and of the symbols adopted (1.5). Chapter 2 states the general concepts about wind actions and effects on structures ( ), together with a brief report of the core literature in Wind Engineering (2.9). Chapter 3 contains the Guidelines for the evaluation of the wind loading and structural response and is the main section of the document. It is structured in two parts: the main text ( ) and a set of 15 Annexes (A-Q). The main text gives explicit criteria to evaluate the wind velocity and wind velocity pressure (3.2); moreover, it gives general concepts on aerodynamic actions (3.3), and on aeroelastic and dynamic effects (3.4). Following the tradition of Eurocodes, the presentation is organized by points divided in principles (denoted by the symbol P ) and in rules. The Annexes provide indications and criteria updated to the state-of-art concerning the detailed evaluation of the oncoming wind (A-F), the aerodynamic actions (G-H) and the dynamic and aeroelastic effects (I-P). The Annex Q, related to the use of wind tunnel, is transversal to the previous subjects. The scheme adopted has three main advantages: it simplifies the use of the text, limiting it to the main part and to the relevant Annexes; it allows incorporating in the calculations procedures not included in the document by simply substituting them to one single Annex; it makes

4 maintenance and updating of the text simpler. Finally, Section 4 contains ten worked examples of common structural types (a low-rise building, three multi-storey buildings, a gas tank, a canopy roof, a reinforced concrete chimney, a steel stack, a railway bridge and a road bridge, all of them supposed to be placed at the same location) and is meant to guide the Consultant through the use of the document. Fig. 1 presents a schematic of the organization of Section 2 (Fundamentals) and Section 3 (Principles and rules). The Annexes can be naturally grouped into 4 homogenous sets: (1) Main Annexes, such as A (Design return period) and G (Global aerodynamic coefficients), which are essential for all applications; (2) Specific Annexes, such as D (Topography coefficient), H (Local and detailed aerodynamic coefficients), I (Dynamic characteristics of structures), L (Longitudinal actions and accelerations), M (Transversal and torsional actions and accelerations), N (Acceleration and serviceability), O (Vortex shedding on slender structures) and P (Other aeroelastic phenomena), which are required for specific applications; (3) Informative Annexes, such as Q (Wind tunnel tests), which is an aid for Consultants who have to deal with wind tunnel tests; (4) Special purpose Annexes, such as B (Reference velocity), C (Wind representation), E (Atmospheric turbulence) and F (Wind peak velocity), which provide more refined tools for defining the characteristics of the wind flow. FUNDAMENTALS 2.1 Introduction PRINCIPLES AND RULES MAIN TEXT APPENDIXES 3.1 Guidelines 2.2 Atmospheric circulation 2.3 Wind modelling 3.2 Wind velocity and kinetic pressure Appendix A-F 2.4 Buildings aerodynamics 3.3 Aerodynamic actions Appendix G-H 2.5 Dynamic response 2.6 Vortex shedding 3.4 Dynamic and aeroelastic phenomena Appendix I-P 2.7 Other aeroelastic phenomena 2.8 Interference Appendix Q Figure 1: structure of Sections 2 e 3 of the CNR-DT 207/ WIND CLIMATE AND MODELLING OF THE WIND FLOW The peak wind velocity pressure is referred to as the expected value of the maximum wind velocity pressure over ten minutes. The design values of the mean wind velocity, of the turbulent fluctuations and of the peak velocity pressure depend on the geographic location and on the altitude above sea level of the structure site, on local roughness and topography and on the return period. The evaluation of the wind design velocity and of the peak wind velocity pressure goes though the following steps: (a) the definition of the geographic location and of the altitude of the construction, which permits the computation of the basic wind velocity v b ; (b) the design reference velocity v r can be calculated by setting the design return period T R ; (c) the exposure category is determined from the local roughness of the ground; (d) the topography coefficient is calculated by

5 defining the local topography; (e) if necessary (e.g., for the calculation of aerodynamic actions on bodies with rounded surfaces or for the analysis of dynamic and aeroelastic phenomena), the wind mean velocity v m is evaluated; (f) if necessary (e.g., for the analysis of dynamic and aeroelastic phenomena), the turbulence intensity I v and the turbulence integral length scale L v are calculated; finally, (g) the peak wind velocity pressure of the wind q p is evaluated. Some noteworthy points are introduced in the document CNR-DT 207/2008. Entering briefly into some details, these Guidelines propose the evaluation of the reference wind velocity through a classical simplified method, but detailed analyses are also permitted and even more recommended through the application of three sequential steps: (a) acquisition, check and correction of measured data of mean velocity and of wind direction, representative of the site; (b) transformation of measured data into values consistent with the definition of the reference velocity; (c) probabilistic analysis of transformed data. Annex B provides some guidance for the development of detailed probabilistic analyses. Concerning the wind design reference velocity v r, it is defined as: vr = vb cr (1) in which v b is the basic wind velocity associated with a return period of 50 years, and c r is the return coefficient defined by: cr = 0.75 for TR = 1 year c = ln T for 1 year T <5 years c c ( ) r R R r r 1 = ln ln 1 for 5 years TR <50 years TR 1 = ln ln 1 for TR 50 years TR where T R is the design return period expressed in years. Annex A provides guidance to select the appropriate design return period for different analyses. Figure 2 shows a representation of Eqs. (2). 1,3 (2) 1,2 1,1 c r c r 1 0,9 0, T R (anni) T R (years) Figure 2: return coefficient c r as a function of the return period T R. The mean wind velocity, the turbulence intensity and the peak wind velocity pressure are calculated through simplified procedures, common to other international standards and implemented for the Italian territory and wind climate, however the possibility is given to use more refined procedures. Annex C illustrates detailed wind models, through a brief presentation of experimental measurements and numerical or analytical calculations. Annex D illustrates a detailed procedure form evaluating the topography coefficient for isolated hills, ridges and valleys. Annex E

6 introduces the local and spatial spectral properties of turbulence in neutral atmosphere. Moreover, some guidance is given towards Monte Carlo simulations of turbulent fields and their possible use. Finally, Annex F expresses the wind peak velocity as: v ( z) = v ( z) G ( z) (3) p m v where v m is the mean velocity, and G v is the gust factor of the wind velocity, given by the following equation: G ( z) = 1 + g ( z) I ( z) P( z) (4) v v v v g v being the peak factor, I v the turbulence intensity and P v a coefficient taking into account the reduction of the turbulence intensity due to the time interval over which the peak wind velocity is estimated (typically 3 sec). In the Guidelines, the wind peak wind velocity pressure is first transformed into peak aerodynamic actions, then into equivalent static actions through the introduction of factors that have both aerodynamic and dynamic origin. 4. AERODYNAMIC COEFFICENTS With the purpose of making the document as self contained as possible, the choice was made of incorporating a wide variety of aerodynamic coefficients. For simplicity, these are given in two separate Annexes. Annex G contains global aerodynamic coefficients for a number of structural types, while Annex H contains detailed and local pressure coefficients for buildings and detailed net force coefficients for canopies. In addition, the coefficients are always presented in the form of graphs and of equations. The first can be used for simple hand calculation and allow immediate visualisation of the variation of a coefficient as a function of the relevant parameters. The latter lend themselves to the implementation in computer software and automated procedures. In selecting the values of the aerodynamic coefficients the choice was made of keeping these as close as possible to those suggested by Eurocode 1 and departing from those only when the different approach of CNR- DT 207/2008 from Eurocode 1 required it, or when advanced information and data were found in the technical literature. The coefficients given in Annex G are divided into pressure coefficients, net pressure coefficients, total force and moment coefficients, force and moment coefficients per unit length and friction coefficients. External and internal pressure coefficients are given for rectangular and circular plan constructions. Net pressure coefficients are given for walls and parapets. Total force and moment coefficients are given for canopies, signboards, compact bodies (such as spheres and parabolic antennas) and for plane and 3-D lattice structures. Force and moment coefficients per unit length are given for a variety of structural shapes and for bridge decks. Finally, friction coefficients are given as a function of the surface roughness. For rectangular plan buildings, the global external pressure coefficients are given as a constant value to be applied to each vertical or roof face of the building. The values have been calculated as an envelope of the possible cases obtained by applying the pressure coefficients of Eurocode 1 to a variety of combinations of building width, depth and height. Besides the values given for the vertical walls, coefficients are given for flat, monopitch, duopitch, multipitch, hipped and vaulted roofs, evaluated following the same envelope procedure. As an example, in Fig. 3 the variation of the pressure coefficient for monopitch roofs is shown as a function of the roof slope. For the range of 0 to 45 a positive and a negative value of the coefficient are given and it is recommended that both values be used in the calculation, to make sure that the most stringent condition is considered. For circular constructions the global external pressure coefficients are given for the vertical walls as a function of the Reynolds number, and for dome roofs only as a function of geometry. It was chosen to separate the pressure coefficient on the surface of cylindrical bodies from the drag force coefficient (which instead in Eurocode 1 are presented in the same section) as it is believed that though referring to the same geometry, these two sets of coefficients are often used for different

7 applications. For the evaluation of internal pressures, the influence of the amount of openings and of their location is accounted for considering three different possibilities. The first is that of a construction featuring openings for more that 30% on at least two faces. In this case the aerodynamic actions are evaluated considering the roof as a canopy, if the large openings are located on the vertical walls and considering the construction as an assemblage of walls, if the large openings are located on the roof. For constructions with a lower percentage of openings the two cases are considered of the presence or of the absence of a dominating surface. A dominating surface is defined as a surface with a total opening area at least twice the sum of the openings on all the remaining surfaces. In this case, the internal pressure equalises to the external pressure on the dominating surface, being a fraction of that dependent on the ratio of the openings on the dominating surface to those on the remaining surfaces. The third case is that in which the construction has an evenly distributed porosity. In this case the internal pressure is influenced by the external pressure on all the surfaces and is therefore calculated as a weighted average of these, with respect to opening areas. Figure 3: global pressure coefficient for monopitch roofs as a function of the roof slope. For canopies, global net force coefficients have been evaluated following the same envelope procedure used for the walls and roofs of rectangular buildings. This led to graphs showing the global net force coefficients for monopitch and duopitch canopies as a function of the pitch slope. Global net force coefficients for multipitch canopies are derived from those of monopitch and duopitch canopies. When dealing with the total force coefficients on compact bodies, to the common case of spheres considered by many Codes of Practice, the case of parabolic antennas has been added. This, in fact, has in recent years become a quite common shape. Total force coefficients in the direction of the axis of the parabola and perpendicular to that are given, together with the eccentricity of the aerodynamic force, as a function of the antenna geometry. For lattice structures the total longitudinal force coefficients given in Eurocode 1 are given also in the CNR-DT 207/2008, considering the two cases of lattices with sharp-edged members and those with round-edged members. In addition, for plane trusses correction factors are given for the cases in which the flow is not orthogonal to the truss plane and for the case in which the truss is sheltered by a similar truss located upstream. For slender structures and structural members force coefficients per unit length are given for a variety of shapes: sharp- and round-edged square sections with flow perpendicular and at a 45 angle with the face, rectangular sections with different side ratios, polygonal sections, steel profile sections, circular sections and cables. For steel profiles, force coefficients in the direction of the

8 section axes are given. In the case in which the section is symmetric with respect to the oncoming flow, small deviation of the flow directions are considered, bringing non-zero values of the transverse force coefficient. For cables the longitudinal force coefficient is given as a function of the ratio between the strand and the cable diameters and of the Reynolds number. Special attention was devoted to the aerodynamic coefficients for bridge decks. These are given in the form of drag, lift and torque coefficients for isolated decks. A criterion for choosing the appropriate value of the force coefficients for twin decks is also given. For isolated decks, the following coefficients are suggested, referred to the deck width: c fx 1,85 0,10 2 d/ htot 5 d/ htot = 1, 35 d/ htot > 5 d/ htot (5) c fy d ± 0, 7 + 0,1 0 d/htot 5 = htot 1, 2 d/htot > 5 (6) c mz = ± 0,2 (7) The values suggested for the aerodynamic coefficients derive from the envelope of values available for a number of orthotropic and box decks and are globally conservative. D G Y 1 X 1 Y 2 X 2 h tot G 1 G 2 d 1 d 2 d 0 D Figure 4: geometry of twin decks. For twin decks the geometry sketched in Fig. 4 is considered, and the three cases are presented (a) in which the separation d 0 of the decks is larger than ¼ the largest of the two deck widths d 1 and d 2, (b) in which it is smaller and the two decks are not connected and (c) in which it is smaller but the two deck are connected. In the first case, the aerodynamic actions are evaluated separately on the two decks as each of them was isolated. In the second case, the aerodynamic actions are evaluated separately on each of the two decks as if it was isolated (subscript 1 in eqs. 8-10), then the total actions on the twin deck are evaluated based on a width D (subscript 2 in eqs. 8-10), finally, the aerodynamic actions on each of the two decks are evaluated as: f X 1 f X = max (8) 0, 75 f X 2

9 fy1 = ± max 0, 5 m m fy Z 2 Z1 fy 2 + DG (9) m Z = m Z1 (10) Finally, Annex H contains detailed and local pressure coefficients for rectangular plan buildings and detailed net force coefficients for canopies. These reflect the values given in Eurocode 1, therefore give the user the possibility of complying with Eurocode 1. In addition, Annex H contains some simple criteria for evaluating the external pressure coefficients for a number of building geometries that can be obtained as the assemblage of rectangular blocks. These are buildings of uniform height and with a plan resulting from the combination of rectangles and buildings with rectangular plan and two different heights. Examples of the latter case are given in Fig. 5. 4/5 e 1 e 1 /5 b h r1 C 4/5 e 1 e 1 /5 B A C h 1 B A h 2 =h r2 a hr1 C B C A B h 1 A h 2 =h r2 4/5 e 2 e 2 /5 d-e 2 e 2 Figure 5: examples of irregular buildings. 4/5 e 2 e 2 /5 d-e 2 e 2 5. EVALUATION OF THE STRUCTURAL RESPONSE CNR-DT 207/2008 addresses background concepts on the dynamic and aeroelastic response related to the actions and effects of the wind, underlining the aspects related to the non synchronicity of loading, the influence of structural parameters and the action-response interaction. With reference to dynamic effects (Annexes I, L and M), the main aspect consists in introducing several approaches where an increased level of difficulty corresponds to a more advanced design. Starting from the peak aerodynamic action, the equivalent static actions are defined as the product of the peak aerodynamic action and the dynamic coefficient c d. This coefficient can be assumed, on the one hand, equal to unity for stiff structures with limited height and sufficient damping. On the opposite hand, CNR-DT 207/2008 permits the checking the response through analytical and numerical methods, starting from an adequate description of the wind velocity and the aerodynamic loads. Moreover, for particular cases (buildings with rectangular plan, horizontal line-like structures, point-like structures) CNR-DT 207/2008 suggests, similarly to other Codes of Practice, simplified rules for assessing the along-wind dynamic coefficient. The CNR document contains an exhaustive Annex where the relevant dynamic proprieties of constructions can be found, with particular attention to natural frequencies and damping ratios of multi-storey buildings; these are also given in then form of diagrams together with several summary tables showing the steps of their implementation. The same approach is used also for the across-wind and torsional response and the resulting wind loads, which represents a novelty for the Italian standards. The procedures are based on those of AIJ

10 (2005) and of ISO TC 98/SC (2005), modified for application in connection with dynamic coefficients instead of gust factors. Moreover, similarly to the along-wind response and for the special case of tall buildings with square cross section, graphs are proposed which allow a direct conservative estimation of the cross-wind and torsional dynamic factor c d (e.g. Fig. 6). h (m) b (m) Figure 6: across-wind dynamic coefficient for steel square-plan buildings (grey areas indicate situations in which it is not strictly necessary to assess the effects of cross-wind actions as h < 3b) Annex O is devoted to the problem of shedding induced response. It illustrates the physical parameters governing this phenomenon and it specifies the correct application fields of the calculation methods for the determination of the peak transversal displacement and the related equivalent static loads. The first parameter to be considered is the Strouhal number St, which governs the dominant vortex shedding frequency n s by the well-known law: n St v b m s = (11) where v m is the mean wind velocity and b is a characteristic size of the structural cross-section in the position where the phenomenon occurs. St is a dimensionless parameter which is primarily function of the shape of the cross-section and of the Reynolds number. If the structure has a natural frequency n L,i, associated with a crosswind vibration mode close to the shedding frequency n s, the crosswind vibration becomes resonant with the vortex shedding: in these cases it is possible to define the critical velocity of vortex shedding, simply solving the Eq. (5) as regards the velocity setting n s =n L,i. The second, essential, dimensionless parameter in a vortex shedding evaluation is the Scruton number Sc: 4π mei, ξli, Sc = (12) 2 ρ b m e,i being the equivalent mass per-unit-length of the structure in the i-th across-wind vibration mode, ξ L,i is the damping factor related to i-th across-wind vibration mode (excluding aerodynamic damping), ρ is the air density. Since these parameters depend on the vibration mode with which critical shedding is associated, Sc is in turn a function of the i-th vibration mode. In resonant

11 conditions, the smaller the Scruton number (and therefore the more the structure is lightweight and/or with low damping) the greater the response. On the basis of experience with real cases, the following situations can be identified: (a) if the Scruton number is higher than 30, the risk of lock-in is almost non-existent and the vortex shedding phenomenon, in general, is not a particularly demanding load case; (b) if the Scruton number is between 5 and 30, the vortex shedding phenomenon is very sensitive to various parameters, first of all turbulence intensity, and it must be analysed to ensure that vibrations do not induce excessively high stresses and that fatigue limits are not exceeded; (c) if the Scruton number is less than 5, the vibrations induced by vortex shedding may be very intense and significantly hazardous, therefore it is advisable to address the problem with the utmost attention and prudence. The effect of cross-wind vibrations induced by a resonant vortex shedding on mode i can be represented through the application of an equivalent static force per unit length: 2 Li, () ()(2 Li, ) Li, () pli, TRi, F s = m s π n Φ s y C (13) where s is the current structural coordinate, m(s) is the mass of the structure per unit length, Φ L,i (s) is the i-th mode shape in the crosswind direction (normalized to 1 in the coordinate s of maximum displacement, Φ L,i ( s )=1), y pl,i is the peak value of the deflection of the structure at station s, C TR,i is a dimensionless parameter associated with critical values of mean wind velocity for long return periods T R. The technical-scientific literature currently includes many procedures for calculating the peak deflection caused by vortex shedding. Most of these procedures have complementary advantages and disadvantages; none may claim to be fully shared and recognised. The spectral method and the harmonic method are the best-known and most applied in the structural field. The spectral method, derived from the model of Vickery and Basu, is calibrated on experimental data derived from tests on cantilever structures (for example, chimneys, towers and masts), with regular variation of the section along its axis, and oscillating in the first vibration mode. In these cases, in which application of this method is recommended, it generally provides conservative solutions and, in some cases, lead to an unjustified overdesign.. The harmonic method, derived from the model of Ruscheweyh, is calibrated on experimental data referring to a broader class of structures, oscillating in the first and higher modes. It leads to results which in average agree with the actual observed behaviour; at times, however, it also leads to underestimate the effects of vortex shedding. For this reason, it is advisable to use it especially in cases in which the spectral method cannot be applied. In other situations, application of this method is suggested together with the spectral method in order to estimate, for example, the uncertainties affecting the response. When analysis of the response to critical vortex shedding is performed by the spectral method and/or the harmonic method, the most conservative calculation values given by the two methods should be adopted. Coefficient C TR,i is a dimensionless parameter introduced for two purposes. The first is to consider critical phenomena at mean wind velocities with a return period longer than the design return period adopted for routine safety checks. The second is to reduce the response amplitude for high values of mean wind velocity, since in these situations the flow tends to become significantly turbulent and therefore less favourable to regular vortex shedding (Fig.7). The equations proposed in Annex O have been checked against a database of 46 chimneys for which experimental results (observed or measured) are available. Other relevant aspects of this Annex are: (1) the introduction of a Strouhal number for circular cross-section as a function of the Reynolds number (revising data from ESDU 96030, 1996); (2) a new definition for the peak factor in the spectral method; (3) a new definition of the forcing dimensionless parameter in the spectral method, taking into account suggestions from theoretical elaborations and from CICIND (1999) code (in this way it will be possible to consider the response due to the second vibration mode, as desirable in a next future); (4) a complete revision of the application steps of the harmonic method, with the attempt of explaining all the aspects sometimes not totally clear in the Eurocode procedure; (5) a new definition for the dimensionless parameter of aerodynamic damping in the spectral method:

12 K = K C (14) a a,max I where K a,max is the maximum value of the aerodynamic damping parameter in smooth flow (Fig. 8.a) and C I is the turbulence factor, which is calibrated on a chimney database in non neutral atmosphere (Fig. 8b). 1 C TR,i 0,5 0 v m,0 v cr,i Figure 7: values of coefficient C TR,i ; v m,0 and v m,l are the mean wind velocities for design return periods equal to the reference or to 10-times the reference return period, respectively. v m,l 2 1,3 1,5 1 K a,max C I 1 0,7 0, , v cr,1 (m/s) Re Figure 8: maximum value of the aerodynamic damping parameter for circular cylinders (left) and turbulence factor (right). Moreover, the method for assessing ovalling has been completely revised introducing, together with the classical computation reported from several Codes, an aeroelastic criterion deduced from the literature as a function of the damping ratio of the n-th ovallig mode (Laneville & Mazouzi 1996). Finally, Annex P deals with other aeroelastic phenomena (galloping, torsional divergence and flutter). This Annex is also aimed at giving a physical interpretation of each phenomenon, therefore allowing a more aware application of the equations provided. For flutter, some experimental relationships are also proposed, which define the field of occurrence of such instability, for bridges having a span lower than 200 m.

13 6. CONCLUDING REMARKS In this paper some of main novelties of CNR DT 207/2008 Guidelines on Actions and Effects of Wind on Structures have been described. Like all similar documents issued by CNR, these Guidelines aim at combining two different aspects: on one side, the document contains specifications for use of Consultants, for the evaluation of wind loads and wind effects on structures; on the other side, the document addresses background concepts on the characteristics of wind and on the description of the interaction between the wind field and a wide set of structural types. This was done with the goal of stimulating Consultants towards a more advanced and more refined assessment of the wind loading and wind induced response of structures, by providing them with an understanding of the behaviour of the mechanisms of the wind loading and by guiding them through the steps to be made for the analyses. This twofold approach has been adopted consistently throughout the document. For each topic, in fact, both general principles and application rules are presented. The latter are aimed at covering the widest possible variety of practical cases, and an attempt has been made to not only provide equations and coefficients to be adopted in practical calculations, but also to provide explanation for their understanding. This approach could be considered as a model for a new generation of codes of practice, which provide not only rules but also guidance for applications and for understanding of underlying principles. 7. REFERENCES AIJ (2005). Recommendations for loads on buildings. Architectural Institute of Japan. Ballio G., Lagomarsino S., Piccardo G., Solari G. (1999). Probabilistic analysis of Italian extreme winds: Reference velocity and return criterion. Wind & Structures, 2, 1, CEN (1994). Eurocode 1: Basis of design and actions on structures. Part 2-4: Wind actions. ENV CEN (2005). Eurocode 1: Actions on structures - General actions. Part 1-4: Wind actions. EN CICIND (1999). Model code for steel chimneys. CNR (1964). Ipotesi di carico sulle costruzioni (in Italian). CNR-UNI CNR (1981). Azioni sulle costruzioni (in Italian), Technical document N /81. CNR (1985). Azioni sulle costruzioni (in Italian), Technical document N /85. ECCS (1985). Recommendations for calculating the effects of wind on constructions. European Convention for Constructional Steelwork, N. 52. ESDU 9630 (1996). Response of structures to vortex shedding: structures of circular or polygonal cross section. ISO TC 98/SC (2005). Wind actions on structures. ISO/CD 4354 (draft version, ). Laneville A., Mazouzi A. (1996). Wind-induced ovalling oscillations of cylindrical shells: critical onset velocity and mode prediction. Journal of Fluids and Structures 10, M.LL.PP. (1968). Ipotesi di carico sulle costruzioni (in Italian). Circolare LL.PP. N M.LL.PP. (1978a). Criteri generali per la verifica della sicurezza delle costruzioni e dei carichi e sovraccarichi (in Italian). D.M.LL.PP M.LL.PP. (1978b). Istruzioni relative ai carichi e ai sovraccarichi ed ai criteri generali per la verifica di sicurezza delle costruzioni (in Italian). Circolare M.LL.PP. N , M.LL.PP. (1982a). Criteri generali per la verifica della sicurezza delle costruzioni e dei carichi e sovraccarichi (in Italian). D.M.LL.PP M.LL.PP. (1982b). Istruzioni relative ai carichi e ai sovraccarichi ed ai criteri generali per la verifica di sicurezza delle costruzioni (in Italian). Circolare M.LL.PP. N , M.LL.PP. (1996a). Criteri generali per la verifica della sicurezza delle costruzioni e dei carichi e sovraccarichi (in Italian). D.M.LL.PP M.LL.PP. (1996b). Istruzioni relative ai carichi e ai sovraccarichi ed ai criteri generali per la verifica di sicurezza delle costruzioni (in Italian). Circolare M.LL.PP. N. 156, M.II.TT. (2005). Norme tecniche per le costruzioni (in Italian). D.II.TT M.II.TT. (2008). Nuove norme tecniche per le costruzioni (in Italian). D.II.TT