Hollow Sections in Structural Application _2nd_edt (1)

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HOLLOW SECTIONS IN

STRUCTURAL APPLICATIONS

J. Wardenier, J.A. Packer, X.-L. Zhao and G.J. van der Vegte

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ISBN 978-90-72830-86-9

© CIDECT, Geneva, Switzerland, 2010

The publisher and authors have made careful efforts to ensure the reliability of the data contained in this publication, but they assume no liability with respect to the use for any application of the material and information contained in this publication.

Printed by Bouwen met Staal

Boerhaavelaan 40

2713 HX Zoetermeer, The Netherlands

P.O. Box 190

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Tel. +31(0)79 353 1277 Fax +31(0)79 353 1278 E-mail [email protected]

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PREFACE

The global construction market requires a world-wide coordination of product-, testing-, design- and execution-standards, so that contracts for delivery of products and for engineering- and construction services can be agreed on a common basis without barriers.

The mission of CIDECT is to combine the research resources of major hollow section manufacturers in order to create a major force in the research and application of hollow steel sections world wide. This forms the basis of establishing coordinated and consistent international standards.

For the ease of use of such standards, it is however necessary to reduce their content to generic rules and to leave more object-oriented detailed rules to accompanying non-conflicting complementary information, that have the advantage to be more flexible for the adaptation to recent research results and to be useable together with any international code.

The book by J. Wardenier, J.A. Packer, X.-L. Zhao and G.J. van der Vegte "Hollow sections in structural applications" is such a source, developed in an international consensus of knowledge on the topic. It incorporates the recently revised design recommendations for hollow sections joints of the International Institute of Welding, IIW (2009) and CIDECT (2008 and 2009). Both are consistent with each other and are the basis for the Draft ISO standard for Hollow Section Joints (ISO 14346) and may form the basis for future maintenance, further harmonisation and further development of Eurocode 3 (EN 1993-1-8), AISC (ANSI/AISC 360) and the CISC recommendations.

For the use together with EN 1993-1-8 and ANSI/AISC 360, both being based on the previous IIW (1989) recommendations, the main differences to these rules are highlighted.

The authors are all internationally recognized experts in the field of tubular steel structures, three of them having been chairmen of the IIW-Subcommission XV-E on "Tubular Structures" since 1981. This committee is the pre-eminent international authority producing design recommendations and standards for onshore tubular structures.

This book should therefore be an invaluable resource for lecturers, graduate students in structural, architectural and civil engineering, explaining the important principles in the behaviour of tubular steel structures. It is also addressed to designers of steel structures who can find in it the special items related to the use of hollow sections, in particular joints, their failure modes and analytical models as supplements to more general design codes.

Aachen, Germany, August 2010

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ACKNOWLEDGEMENTS

This book gives the background to design with structural hollow sections in general and in particular for joints to hollow sections. For the latter, the recently updated recommendations of the International Institute of Welding (IIW, 2009) and CIDECT (2008 and 2009) are adopted.

The background to design recommendations with the relevant analytical models is especially important for students in Structural and Civil Engineering, whereas the design recommendations themselves serve more as an example. Since the available hours for teaching Steel Structures, and particularly Tubular Structures, vary from country to country, this book has been written in a modular form. The presentation generally follows European codes, but the material is readily adapted to other (national) codes.

Since the first edition of this book was used not only by students but also by many designers, this second edition was needed due to the recent update of the recommendations by IIW and the subsequent revision of the CIDECT Design Guides Nos. 1 and 3 in 2008 and 2009.

The new IIW (2009) recommendations and the revised CIDECT Design Guides Nos. 1 and 3 (2008 and 2009) are consistent with each other and are the basis for the Draft ISO standard for Hollow Section Joints (ISO 14346). Although the current Eurocode 3 (EN 1993-1-8, 2005) and AISC (2010) recommendations are still based on the previous IIW (1989) and CIDECT (1991 and 1992) recommendations, it is expected that in the next revision these will follow the new IIW and CIDECT recommendations presented in this book.

Besides the static design recommendations and background for hollow section joints, information is given for member design in Chapter 2, composite structures in Chapter 4, and fire resistance in Chapter 5. These chapters fully comply with the latest versions of the Eurocodes (EN 1993 and EN 1994). Further, fatigue design of hollow section joints is covered in Chapter 14.

We wish to thank our colleagues from the IIW Sub-commission XV-E "Tubular Structures" and from the CIDECT Project Working Group and the CIDECT Technical Commission for their constructive comments during the preparation of this book.

We are very grateful that Prof. J. Stark and Mr. L. Twilt were willing to check Chapters 4 and 5 respectively on composite members and fire resistance.

Appreciation is further extended to the authors of CIDECT Design Guides Nos. 1 to 9 and to CIDECT for making parts of these Design Guides or background information available for this book.

Finally, we wish to thank CIDECT for the initiative to update this book.

Delft, The Netherlands, September 2010

Jaap Wardenier Jeffrey A. Packer Xiao-Ling Zhao Addie van der Vegte

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1. INTRODUCTION

Design is an interactive process between the functional and architectural requirements and the strength and fabrication aspects. In a good design, all these aspects have to be considered in a balanced way. Due to the special features of hollow sections and their joints, it is here even of more importance than for steel structures of open sections. The designer should therefore be aware of the various aspects of hollow sections.

Many examples in nature show the excellent properties of the tubular shape with regard to loading in compression, torsion and bending in all directions, see Figs. 1.1 and 1.2. These excellent properties are combined with an attractive shape for architectural applications (Figs. 1.3 and 1.4). Furthermore, the closed shape without sharp corners reduces the area to be protected and extends the corrosion protection life (Fig. 1.5).

Another aspect which is especially favourable for circular hollow sections is the lower drag coefficients if exposed to wind or water forces. The internal void can be used in various ways, e.g. to increase the bearing resistance by filling with concrete or to provide fire protection. In addition, heating or ventilation systems sometimes make use of the hollow section columns. Although the manufacturing costs of hollow sections are higher than those for other sections, leading to higher unit material cost, economical applications are achieved in many fields. The application field covers all areas, e.g. architectural, civil, offshore, mechanical, chemical, aeronautical, transport, agriculture and other special fields. Although this book will be mainly focused on the background to design and application, in a good design not only does the strength have to be considered, but also many other aspects, such as material selection, fabrication including welding and inspection, protection, erection, in service inspection and maintenance.

One of the constraints initially hampering the application of hollow sections was the design of the joints. However, nowadays design recommendations exist for all basic types of joints, and further research evidence is available for many special types of joints. Based on the research programmes carried out, CIDECT (Comité International pour le Développement et l'Etude de la Construction Tubulaire) has published Design Guides Nos. 1 to 9 for use by designers in practice. Since these nine Design Guides are all

together too voluminous for educational purposes and do not give the theoretical background, it was decided to write this book especially to provide background information for students and practitioners in Structural and Civil Engineering.

This book is written in a limit states design format (also known as LRFD or Load and Resistance Factor Design in the USA). This means that the effect of the factored loads (the specified or unfactored loads multiplied by the appropriate load factors) should not exceed the factored resistance of the joint or member. The factored resistance expressions, in general, already include appropriate material and joint partial safety factors (γM) or joint resistance (or capacity) factors (). This has been done to avoid interpretation errors, since some international structural steelwork specifications use γM values  1,0 as dividers (e.g. Eurocodes), whereas others use  values  1,0 as multipliers (e.g. in North America and Australia). In general, the value of 1/γM is almost equal to .

1.1 HISTORY AND DEVELOPMENTS

The excellent properties of the tubular shape have been recognised for a long time; i.e. from ancient time, nice examples are known. An outstanding example of bridge design is the Firth of Forth Bridge in Scotland (1890) with a free span of 521 m, shown in Fig. 1.6. This bridge has been built up from tubular members made of rolled plates which have been riveted together, because at that time, other fabrication methods were not available for these sizes.

In the same century, the first production methods for seamless and welded circular hollow sections were developed. In 1886, the Mannesmann brothers developed the skew roll piercing process (Schrägwalzverfahren), shown in Fig. 1.7, which made it possible to roll short thick walled tubulars. This process, in combination with the pilger process (Pilgerschrittverfahren, Fig. 1.8), developed some years later, made it possible to manufacture longer thinner walled seamless hollow sections.

In the first part of the previous century, an Englishman, Whitehouse, developed the fire welding of circular hollow sections. However, the production of welded circular hollow sections became more important after the development of the continuous welding process in 1930 by the American, Fretz Moon (Fig. 1.9). Especially after the Second World War, welding processes have been perfected, which made it possible for hollow sections to be easily welded

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together.

The end cutting required for fitting two circular hollow sections together was considerably simplified by the development of a special end preparation machine by Müller (Fig. 1.10).

For manufacturers who did not have such end cutting machines, the end preparation of circular hollow sections remained a handicap.

A way of avoiding the connection problems was the use of prefabricated connectors, e.g. in 1937 Mengeringhausen developed the Mero system. This system enabled the fabrication of large space structures in an industrialized way (Fig. 1.11).

In 1952, the rectangular hollow section was developed by Stewarts and Lloyds (now Corus Tubes). This section, with nearly the same properties as the circular hollow section, enables the connections to be made by straight end cuttings.

In the fifties, the problems of manufacturing, end preparation and welding were all solved and from that point of view the way to a successful story was open. The remaining problem was the determination of the strength of unstiffened joints.

The first preliminary design recommendations for truss connections between circular hollow sections were given by Jamm in 1951. This study was followed by several investigations in the USA (Bouwkamp, 1964; Natarajan & Toprac, 1969; Marshall & Toprac, 1974), Japan (Togo, 1967; Natarajan & Toprac, 1968), and Europe (Wanke, 1966; Brodka, 1968; Wardenier, 1982; Mang & Bucak, 1983; Puthli, 1998; Dutta, 2002).

Research on joints between rectangular hollow sections started in Europe in the sixties, followed by many other experimental and theoretical investigations. Many of these were sponsored by CIDECT.

Besides these investigations on the static behaviour, in the last 25 years much research was carried out on the fatigue behaviour and other aspects, such as concrete filling of hollow sections, fire resistance, corrosion resistance and behaviour under wind loading.

1.2 DESIGNATION

The preferred designations for structural applications are:

- Circular hollow sections (CHS) - Rectangular hollow sections (RHS) - Square hollow sections (SHS)

In Canada and the USA, it is common to speak about Hollow Structural Sections (HSS), whereas in Europe also the term Structural Hollow Sections (SHS) is used.

1.3 MANUFACTURING OF HOLLOW

SECTIONS

As mentioned, hollow sections can be produced seamless or welded. Seamless hollow sections are made in two phases, i.e. the first phase consists of piercing an ingot and the second step considers the elongation of this hollow bloom into a finished circular hollow section. After this process, the tube can go through a sizing mill to give it the required diameter. More information about other processes, most of them based on the same principle, is given by Dutta (2002). Nowadays, welded hollow sections with a longitudinal weld are mainly made employing either electrical resistance welding processes or induction welding processes, shown in Fig. 1.12. A strip or plate is formed by rollers into a cylindrical shape and welded longitudinally. The edges are heated, e.g. by electrical resistance, then the rollers push the edges together, resulting in a pressure weld. The weld protrusion on the outside of the tube is trimmed immediately after welding.

Rectangular hollow sections are made by deforming circular hollow sections through forming rollers, as shown in Fig. 1.13. This forming process can be done hot or cold, using either seamless or longitudinally welded circular hollow sections. Although it is common practice to use longitudinally welded hollow sections, for the very thick sections, seamless sections may be used.

Square or rectangular hollow sections are sometimes made by forming a single strip to the required shape and closing it by a single weld, preferably in the middle of a face.

Large circular hollow sections are also made by rolling plates through a so-called U-O press process shown in Fig. 1.14. After forming the plates to the required

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shape, the longitudinal weld is made by a submerged arc welding process.

Another process for large tubulars is to use a continuous wide strip, which is fed into a forming machine at an angle to form a spirally formed circular cylinder, see Fig. 1.15. The edges of the strip are welded together by a submerged arc welding process resulting in a so-called spirally welded tube.

More detailed information about the manufacturing processes and the limitations in sizes can be obtained from Dutta (2002).

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Fig. 1.1 Reeds in the wind

Fig. 1.3 Airport Bangkok, Thailand

Fig. 1.2 Bamboo

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Fig. 1.5 Paint surface for hollow sections vs open sections

Fig. 1.7 Skew roll piercing process (Schrägwalzverfahren)

Fig. 1.6 Firth of Forth Bridge, Scotland

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forming rollers heating

welding rollers welded CHS

heating coil

forming rollers heating

welding rollers welded CHS

heating coil

Fig. 1.9 Fretz Moon process

Fig. 1.11 Mero connector

Fig. 1.10 End cutting machine

Pressure rollers inductor Welded CHS Pressure rollers inductor Welded CHS Pressure rollers inductor Welded CHS

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Fig. 1.13 Manufacturing of rectangular hollow sections

Fig. 1.14 Forming of large CHS

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2. PROPERTIES OF HOLLOW

SECTIONS

2.1 MECHANICAL PROPERTIES

Hollow sections are made of similar steel as used for other steel sections, thus in principle there is no difference in mechanical properties.

Tables 2.1a and 2.2a show, as an example, the mechanical properties according to the European standard EN 10210-1 (2006) for hot finished structural hollow sections of non-alloy and fine grain structural steels. The cold formed sections are given in EN 10219-1 (2006): Cold formed welded structural hollow sections of non-alloy and fine grain structural steels (see Tables 2.1b and 2.2b). As shown, the requirements of EN 10210-1 and EN 10219-1 are almost identical.

Hollow sections can also be produced in special steels, e.g. high strength steel with yield strengths up to 690 N/mm2_{or higher, weathering steels and steel} with improved or special chemical compositions, etc.

Generally, the design of members is based on the yield strength. In this chapter the recommended γM0 and γM1 factors of 1,0 are adopted for the design yield strength fyd.

In statically indeterminate structures, sufficient deformation capacity or rotation capacity is required for redistribution of loads. In this case, yielding of members or yielding in the joints may provide the required rotation capacity. A tensile member made of ductile steel can be brittle if a particular cross section is weakened, e.g. by holes, in such a way that this cross section fails before the whole member yields. It is therefore required that yielding occurs first. This shows that the yield-to-ultimate tensile strength ratio is also important, especially for structures with very non-uniform stress distributions, which is a situation that occurs in tubular joints. Some codes, such as Eurocode 3 (EN 1993-1-1, 2005), specify the following requirement for the minimum ratios:

1 , 1 f f yd u _ _(2.1a)

The IIW (2009) recommendations and many offshore codes require a higher ratio between fu and fyd:

8 , 0 f f or 25 , 1 f f u yd yd u _ _ _(2.1b)

This is only one aspect for ductility. In the case of impact loading, the steel and members should also behave in a ductile manner. Hence, Tables 2.1a and 2.2a also give requirements based on the standard Charpy test to ensure adequate notch toughness. Nowadays, more refined characterisation methods exist to describe the ductility of cracked bodies, e.g. the CTOD (Crack Tip Opening Displacement) method. These characterisation methods are generally used for pressure vessels, transport line pipes and offshore applications, which are beyond the scope of this book. Another characterisation is sometimes required for thick walled sections which are loaded in the thickness direction. In this case, the strength and ductility in the thickness direction should be sufficient to avoid cracking, called lamellar tearing, see Fig. 2.1. This type of cracking is caused by non metallic manganese-sulphide inclusions. Thus, if the sulphur content is very low or the sulphur is joined with other elements such as calcium (Ca), such a failure can be avoided. Indirectly this is obtained by requiring a certain reduction of area RAZ in the tensile test. For example, RAZ = 35 means that in the tensile test the cross sectional area at failure has been reduced by 35% compared to the original cross sectional area. In most structural steel specifications the minimum required yield strength, ultimate tensile strength, elongation and the Charpy V-notch values are specified. Design standards or specifications give further limitations for the fu/fy ratio, whereas depending on the application, more restrictive requirements may be given related to CTOD values or the properties in the thickness direction (Z quality).

Another aspect is the effect of cold forming on the mechanical properties of the parent steel. In the case of cold forming of hollow sections, the yield strength and to a lesser extent the ultimate tensile strength are increased, especially in the corners, as shown in Fig. 2.2. Further, the yield-to-ultimate tensile ratio is increased and the elongation slightly decreased.

If the standards, e.g. EN 10210-1 and EN 10219-1, specify the properties at a particular cross section location based on the finished product, these properties have been already partly taken into account. Thus, this generally applies in Europe.

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However, some standards outside Europe specify the material properties of the parent material. In this case, the increased yield strength can be taken into account for design. A small corner radius produces a small cold formed area with a large cold forming effect and consequently a large increase in yield strength, while a large corner radius does just the opposite. According to research work of Lind & Shroff (1971), the product of area and increase in yield strength can approximately be taken as constant. Lind & Shroff assumed that in every corner of 90 the yield strength of the parent material fyb is increased over a length of 7t to the ultimate tensile strength of the parent material fu. The total increase over the section 4(7t)t(fu -fyb) can be averaged over the section, resulting in a design yield strength fya, as shown in Fig. 2.2.

It is noted that the cold formed sections should satisfy the requirements for minimum inside corner radius to guarantee sufficient ductility, see Table 2.3 for fully aluminum killed steel (steel with limited Si content).

Part 10 of Eurocode 3 (EN 1993-1-10, 2005) specifies the material selection. Here, a permissible thickness can be determined based on a reference temperature, the steel grade and quality and the stress level. The reference temperature covers, besides the air temperature, also cold forming effects, strain rate, etc. However, the current rules cannot be adopted to cold formed hollow sections because the determination of the effect of cold forming for cold formed hollow sections is not yet clearly specified. Based on the data obtained by Soininen (1996), Kosteski et al. (2003), Björk (2005), Kühn (2005), Puthli & Herion (2005) and Sedlacek et al. (2008), presently a proposal is being worked out for an amendment of EN 1993-1-10. In this proposal of CEN/TC 250/SC 3-N 1729 (2010), it is recommended that for cold formed hollow sections according to EN 10219, the procedure for hot formed material can be used provided that for the cold forming effects the reference temperature is reduced by Tcf. For CHS, Tcf varies from 0 C to 20 C depending on the thickness and the d/t ratio. For RHS with steel qualities according to EN 10219, Tcf varies from 35 C to 45 C depending on the thickness and the ratio between the inside corner radius and the thickness. For cold formed hollow sections with Charpy impact strengths significantly exceeding the requirements of EN 10219, a lower value of Tcf is allowed.

2.2 STRUCTURAL HOLLOW SECTION

DIMENSIONS AND DIMENSIONAL

TOLERANCES

The dimensions and sectional properties of structural hollow sections have been standardised in EN (EN 10210-2, 2006; EN 10219-2, 2006) and ISO standards (ISO 657-14, 2000; ISO 4019, 2001) for hot finished and cold formed structural hollow sections respectively.

The two applicable standards in Europe are EN 10210-2 (2006) "Hot finished structural hollow sections of non-alloy and fine grain steels – Part 2: Tolerances, dimensions and sectional properties" and EN 10219-2 (2006) "Cold formed welded structural hollow sections of non-alloy and fine grain steels – Part 2: Tolerances, dimensions and sectional properties". However, the majority of manufacturers of structural hollow sections do not produce all the sizes shown in these standards. It should be further noted that other sizes, not included in these standards, may be produced by some manufacturers.

The majority of the tolerances given in EN 10219-2 are the same as those in EN 10210-2, see Tables 2.4a and 2.4b.

Internationally, the delivery standards in various countries deviate considerably with respect to the thickness and mass tolerances (Packer, 1993). In most countries besides the thickness tolerance, a mass tolerance is given, which limits extreme deviations. However, in some production standards, e.g. in the USA, the thickness tolerance is not always compensated by a mass tolerance. This has resulted in associated design specifications which account for this, by designating a lower "design wall thickness" of 0,9 or 0,93 times the nominal thickness t. In Eurocode 3, where design is based on nominal thicknesses, the thickness tolerances in EN 10210-2 and EN 10219-2 are (partly) compensated by the mass tolerance. It is foreseen that in the next revision these tolerances will be tightened.

Although the circular, square and rectangular hollow sections are the generally-used shapes; other shapes are sometimes available. For example, some tube manufacturers deliver the shapes given in Table 2.5. Of these, the elliptical hollow sections have become more popular for architectural designs. These shapes are not dealt with further in this book. However, more information about elliptical hollow sections can be found in Bortolotti et al. (2003), Chan & Gardner (2008), Choo et al. (2003), Martinez-Saucedo et al.

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(2008), Packer et al. (2009b), Pietrapertosa & Jaspart (2003), Theofanous et al. (2009), Willibald et al. (2006) and Zhao & Packer (2009).

2.3 GEOMETRIC PROPERTIES

2.3.1 Tension

The design capacity Nt,Rd of a member under tensile loading depends on the cross sectional area and the design yield strength, and is independent of the sectional shape. In principle, there is no advantage or disadvantage in using hollow sections from the point of view of the amount of material required. The design capacity is given by:

yd Rd ,

t Af

N  (2.2)

If the cross section is weakened by bolt holes or slots, the net cross section should also be checked, in a similar way as for other sections, e.g. according to Eurocode 3 (EN 1993-1-8, 2005): 9 , 0 f A N 2 M u net Rd , t  _ (2.3)

where the partial safety factor M2 = 1,25.

The factor 0,9 may vary from country to country depending on the partial factor M used. Where ductile behaviour is required (e.g. under seismic loading), the yield resistance shall be less than the ultimate resistance at the net section of fastener holes.

2.3.2 Compression

For centrally loaded members in compression, the critical buckling load depends on the slenderness λ and the section shape.

The slenderness λ is given by the ratio of the buckling length ℓb and the radius of gyration i.

i b  

 (2.4)

The radius of gyration of a hollow section (in relation to the member mass) is generally much higher than that for the weak axis of an open section. For a given length, this difference results in a lower slenderness for hollow sections and thus a lower mass when compared with open sections.

The buckling behaviour is influenced by initial eccentricities, straightness and geometrical tolerances as well as residual stresses, non-homogeneity of the steel and the stress-strain relationship.

Based on extensive investigations by the European Convention for Constructional Steelwork (ECCS) and CIDECT, "European buckling curves" (Fig. 2.3 and Table 2.7) have been established for various steel sections including hollow sections. They are incorporated in Eurocode 3 (EN 1993-1-1, 2005). The reduction factor  shown in Fig. 2.3 is the ratio of the design buckling capacity Nb,Rd to the axial plastic capacity. yd Rd , b Rd , pl Rd , b f f N N    (2.5) where: fb,Rd = A Nb,Rd _(2.6)

The non-dimensional slenderness  is determined

by: E     (2.7) where: y E f E    (Euler slenderness) (2.8)

The buckling curves for the hollow sections are classified according to Table 2.6. Most open sections fall under curves "b" and "c". Consequently, for the case of buckling, the use of hot formed hollow sections generally provides a considerable saving in material.

Fig. 2.4 illustrates, for a buckling length of 3 m, a comparison between the required mass of open and hollow sections for a given load. It shows that in those cases in which loads are small, leading to relatively slender sections, hollow sections provide a great advantage (considerably lower use of material). However, if loads are higher, resulting in low slenderness, the advantage (in %) will be less.

The overall buckling behaviour of hollow sections improves with increasing diameter- or width-to-wall

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thickness ratio. However, this improvement is limited by local buckling. To prevent local buckling, d/t or b/t limits are given e.g. in Eurocode 3 (EN 1993-1-1, 2005), see Table 2.7. In the case of thin walled sections, interaction between global and local buckling should be considered.

In addition to the improved buckling behaviour due to the high radius of gyration and the enhanced design buckling curve, hollow sections can offer other advantages in lattice girders. Due to the torsional and bending stiffness of the members in combination with joint stiffness, the effective buckling length of compression members in lattice girders can be reduced (Fig. 2.5). Eurocode 3 (EN 1993-1-1) recommends an effective buckling length for hollow section brace members in welded lattice girders equal to or less than 0,75ℓ, in which ℓ represents the system length, see also Rondal et al. (1992).

For chords, 0,9 times the system length for in-plane buckling or 0,9 times the length between the supports for out-of-plane buckling, is taken as the effective buckling length.

These reductions are also based on the fact that the chord and brace members are generally not fully optimised. If for example the chord would be fully utilized with different members for every panel then these reductions would not be allowed.

Laterally unsupported compression chords of lattice girders (see Fig. 2.6) have a reduced buckling length due to the improved torsional and bending stiffness of the tubular members (Baar, 1968; Mouty, 1981). These factors make the use of hollow sections in girders or trusses even more favourable.

2.3.3 Bending

In general, I and H sections are more economical under bending about the major axis (Imax larger than for hollow sections). Only in those cases in which the design resistance in open sections is largely reduced by lateral buckling, hollow sections offer an advantage.

It can be shown by calculations that lateral instability is not critical for circular hollow sections, square hollow sections and for the most commonly used rectangular hollow sections with bending about the strong axis. Table 2.8 shows allowable span-to-depth ratios for the most commonly used sections (EN 1993-1-1, 2005). According to a study of Kaim (2006)

these limits can be taken considerably larger.

It is apparent that hollow sections are especially favourable compared to other sections if bending about both axes is present.

Hollow sections used for elements subjected to bending can be more economically designed by using plastic design. However, then the sections have to satisfy more restricted conditions to avoid premature local buckling. Like other steel sections loaded in bending, different moment-rotation behaviour can be observed.

Fig. 2.7 shows various moment-rotation diagrams for a member loaded by bending moments.

The moment-rotation curve "1" shows a moment exceeding the plastic moment Mpl and a considerable rotation capacity. Moment-rotation curve "2" shows a moment exceeding the plastic moment capacity Mpl, but after the maximum, the moment drops immediately, so that little moment-rotation capacity exists. Moment-rotation curve "3" represents a capacity lower than the plastic moment capacity, which, however, exceeds the yield moment capacity Mel. In the moment-rotation curve "4" the capacity is even lower than the yield moment capacity Mel and failure is by elastic buckling. The effect of the moment-rotation behaviour is reflected in the classification of cross sections as shown in Fig. 2.8 and Table 2.7. The cross section classification is given in limits for the diameter- or flat width-to-thickness ratio.

The limits are based on experiments and can be expressed as: yd f 235 c t d  for CHS (2.9) f 235 c 3 t b yd   for RHS (2.10a) yd f 235 c 3 t h_ _ for RHS (2.10b)

with fyd in N/mm2 and c depending on the section class, the cross section and the loading. For RHS, it is conservatively assumed that the width of the "flat" is equal to the external width b or depth h of the RHS minus 3t.

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The cross section classes 1 and 2 can develop the plastic moment capacity up to the given b/t or d/t limits with bi-linear stress blocks, whereas the moment capacity of the cross section classes 3 and 4 is based on an elastic stress distribution (see Fig. 2.8). The difference between the cross section classes 1 and 2 is reflected in the rotation capacity. After reaching the plastic moment capacity, the cross section class 1 can keep this capacity after further rotation, whereas the capacity of the cross section class 2 drops after reaching this capacity. As a consequence, the moment distribution in the structure or structural component should be determined by elastic analysis for structures made of sections with cross section classes 2, 3 or 4. For structures made of sections with cross sections in class 1 a plastic moment distribution can be adopted, but an elastic moment distribution is still permissible (and in some countries more common).

For a class 1 beam fully clamped at both ends and subjected to a uniformly distributed loading q, the plastic moment distribution implies that after reaching the plastic moment capacity at the ends, the beam can be loaded until a further plastic hinge occurs at mid span (see Fig. 2.9).

For the class 4 cross section, the maximum stress is determined by local buckling and the stress in the outer fibre is lower than the yield strength fy. Alternatively, an effective cross sectional area based on the yield strength may be determined.

Detailed information about the cross sectional classification is given by Rondal et al. (1992).

Research by Wilkinson & Hancock (1998) showed that especially the limits for the side wall slenderness of RHS need to be reduced considerably. E.g. for class 1 sections, they suggest reducing the Eurocode 3 limits (EN 1993-1-1) for the side wall slenderness to:

6t ) 2r 2t 5(b 70 t 2r) 2t (h  _ _   _(2.11) with 30 t r 2 t 2 b  _

For r = t, this can be simplified to:

t b 83 , 0 77 t h_ _ _with ₃₄ t b  (2.11a) In the absence of shear forces or if the shear forces

do not exceed 50% of the shear capacity Vpl,Rd, the effect of shear may be neglected and the bending moment capacity about one axis is given by:

yd pl Rd ,

c W f

M  for cross section classes 1 or 2 (2.12)

yd el Rd ,

c W f

M  for cross section class 3 (2.13)

yd eff Rd ,

c W f

M  for cross section class 4 (2.14)

When the shear force exceeds 50% of the shear capacity, combined loading has to be considered, see Eurocode 3 (EN 1993-1-1).

2.3.4 Shear

The elastic shear stress in circular and rectangular hollow sections can be determined with simple mechanics by: 3 f t I 2 S VEd _ yd   (2.15)

Fig. 2.10 shows the elastic stress distribution. The design capacity based on plastic design can be easily determined based on the Huber-Hencky-Von Mises criterion by assuming the shear yield strength in those parts of the cross section active for shear.

3 f A V yd v Rd , pl  (2.16) where: h b h A Av  _ for RHS (2.17)

(or just 2 h t) with V in the direction of h.

A 2 Av   for CHS (2.18)

2.3.5 Torsion

Hollow sections, especially CHS, have the most effective cross section for resisting torsional moments, because the material is uniformly distributed about the polar axis. A comparison of open and hollow sections of nearly identical mass in Table 2.9 shows that the torsional constant of hollow sections is about 200 times that of open sections.

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The design capacity for torsional moments is described by: 3 f W M yd t Rd , t  (2.19)

or circular hollow sections: F t ) t d ( 2 t d I 2 W t t      (2.20) here: w



d t



t 4 I 3 t    (2.21)

or rectangular hollow sections (Marshall, 1971): F A m t t _A 2 t I W    (2.22) here: w A A 3 t t A 4 3 t I 2 m     (2.23)



(2.24) (2.25) or thin walled rectangular hollow sections, eq. (2.22)

(2.26) he first term in eq. (2.23) is generally only used for

he exact, more complicated equations for the cross

.3.6 Internal pressure

he design capacity per unit length, shown in Fig.











 2 hm bm 2r m 4 A 







 b h r 4 A 2 m m m m F

can be approximated by: t b h 2 Wt  m m T

open sections. However, research by Marshall (1971) showed that the given formula provides the best fit with the test results.

T

sectional properties are given in EN 10210-2 (2006) and EN 10219-2 (2006).

2

he circular hollow section is most suitable to resist T an internal pressure p. T 2.11, is given by: t 2 d t 2 f p yd _ (2.27) M0 ectional classification,

.4 DRAG COEFFICIENTS

hollow sections,

.5 CORROSION PROTECTION

tructures designed in hollow sections have a 20 to eq. (2.27),  = 1,0, but for transport pipelines, the In

M0 value may be considerably larger than for other cases, depending on the hazard of the product, the effect of failure on the environment and inspectability. The design capacities for RHS sections subjected to internal pressure are much more complicated; reference can be made to the Deutscher Dampfkesselausschuβ (1975).

2.3.7 Combined loadings

arious combinations of loadings are possible, e.g. V

tension, compression, bending, shear and torsion. Depending on the cross s

various interaction formulae should be applied. Reference can be made to Eurocode 3 (EN 1993-1-1). It is beyond the scope of this book to deal with all possible formulae; however, the interaction between the various loads in the cross section can be based on the Huber-Hencky-Von Mises stress criterion (Roik & Wagenknecht, 1977). For the member checks, other interaction formulae apply, see e.g. EN1993-1-1.

2

ially circular Hollow sections, espec

have a striking advantage for use in structures exposed to fluid currents, i.e. air or water. The drag coefficients are much lower than those of open sections with sharp edges, see Fig. 2.12 (Schulz, 1970; CIDECT, 1984; Dutta, 2002).

2

tructures made of hollow sections offer advantages S

with regard to corrosion protection. Hollow sections have rounded corners (Fig. 2.13) resulting in a better protection than that for sections with sharp corners. This is especially true for the joints in circular hollow sections where there is a smooth transition from one section to another. This better protection increases the protection period of coatings against corrosion. S

50% smaller surface to be protected than comparable structures made of open sections. Many

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investigations, summarized by Tissier (1978), have been conducted to assess the likelihood of internal corrosion. These investigations, carried out in various countries, show that internal corrosion does not occur in sealed hollow sections.

Even in hollow sections which are not perfectly

.6 USE OF INTERNAL VOID

he possibilities of using the internal space are briefly

.6.1 Concrete filling

wall thicknesses are not

very important reason for using concrete filled

oncrete filling of hollow sections contributes not only

.6.2 Fire protection by water circulation

nother method for fire protection of buildings is to

he columns are interconnected with a water storage

order to prevent freezing, potassium carbonate

.6.3 Heating and ventilation

he inner voids of hollow sections are sometimes

.6.4 Other possibilities

ometimes hollow section chords of lattice girder

.7 AESTHETICS

rational use of hollow sections leads in general to sealed, internal corrosion is limited. If there is concern

about condensation in an imperfectly sealed hollow section, a drainage hole can be made at a point where water can drain by gravity.

2

he internal void in hollow sections can be used in T

various ways, e.g. to increase the compressive resistance by filling with concrete, or to provide fire protection. In addition, heating or ventilation systems are sometimes incorporated into hollow section columns. T described below.

2

the commonly-available If

sufficient to meet the required load bearing resistance, the hollow section can be filled with concrete. For example, it may be preferable in buildings to have the same external dimensions for the columns on every floor. At the top floor, the smallest wall thickness can be chosen, and the wall thickness can be increased with increasing load for lower floors. If the hollow section with the largest available wall thickness is not sufficient for the ground floor, the hollow section can be filled with concrete to increase the load bearing resistance.

A

hollow sections is that the columns can be relatively slender. Design rules are given in e.g. Eurocode 4 (EN 1994-1-1, 2004).

C

to an increase in load bearing resistance, but it also improves the fire resistance duration. Extensive test projects carried out by CIDECT and the European Coal and Steel Community (ECSC) showed that reinforced concrete filled hollow section columns without any external fire protection like plaster, vermiculite panels or intumescent paint, can attain a fire life of even 2 hours depending on the cross

section ratio of the steel and concrete, reinforcement percentage of the concrete and the applied load, see Fig. 2.14 (Twilt et al., 1994).

2

A

use water filled hollow section columns. T

tank. Under fire conditions, the water circulates by convection, keeping the steel temperature below the critical value of 450 C. This system has economical advantages when applied to buildings with more than about 8 storeys. If the water flow is adequate, the resulting fire resistance time is virtually unlimited. In

(K2CO3) is added to the water. Potassium nitrate is used as an inhibitor against corrosion.

2

T

used for air and water circulation for heating and ventilation of buildings. Many examples in offices and schools show the excellent combination of the strength function of hollow section columns with the integration of heating or ventilation systems. This system offers maximization of floor area through elimination of heat exchangers, a uniform provision of warmth and a combined protection against fire.

2

S

bridges are used for conveying fluids (pipe bridge). In buildings, the rain water downpipes may go through the hollow section columns or in other cases electrical wiring is located in the columns. The internal space can also be used for prestressing a hollow section.

2

A

structures which are cleaner and more spacious. Hollow sections can provide slender aesthetic columns, with variable section properties but flush external dimensions. Due to their torsional rigidity, hollow sections have specific advantages in folded structures, V-type girders, etc.

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often made of hollow sections directly connected to one another without any stiffener or gusset plate, is often preferred by architects for structures with visible steel elements. However, it is difficult to express aesthetic features in economic comparisons. Sometimes hollow sections are used only because of aesthetic appeal, see e.g. Fig. 2.15, whilst at other times appearance is less important.

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Table 2.1a Hot finished structural hollow sections – Non-alloy steel properties (EN 10210-1, 2006)

Minimum yield strength (1)

(N/mm2) Minimum tensile strength (N/mm2) Longitudinal (2) minimum elongation (%) on gauge o o 5,65 S L  Charpy impact strength (10 x 10 mm) Steel designation t  16 mm 16 < t  40 mm 40< t  63 mm t < 3 mm 3  t  100 mm 3 < t  40 mm 40 < t  63 mm Temp. C J S235JRH 235 225 215 360-510 360-510 26 25 20 27 S275J0H S275J2H 275 265 255 430-580 410-560 23 22 0 -20 27 27 S355J0H S355J2H S355K2H 355 345 335 510-680 470-630 22 21 0 -20 -20 27 27 40 (3)

(1)_{For thicknesses above 63 mm, these values are further reduced.} (2) _{In transverse direction 2% lower.}

(3)_{Corresponds to 27 J at -30 C.}

Table 2.1b Cold formed welded structural hollow sections – Non-alloy steel (EN 10219-1, 2006) – Steel properties different from EN 10210-1 (2006)

Steel designation Minimum longitudinal elongation (%), all thicknesses, tmax = 40 mm

S235JRH 24 (1) S275J0H S275J2H 20 (2) S355J0H S355J2H S355K2H 20 (2) (1)_{For t > 3 mm and d/t < 15 or} ₁₂_,₅ t 2 h

b _ _{the minimum elongation is reduced by 2 to 22%; for t  3 mm the minimum} elongation is 17%.

(2)_{For d/t < 15 or} ₁₂_,₅

t 2

h

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Table 2.2a Hot finished structural hollow sections – Fine grain steel properties (EN 10210-1, 2006)

Minimum yield strength (N/mm2) Minimum tensile strength (N/mm2₎ Minimum elongation (%) on gauge o o 5,65 S L  t  65 mm Charpy impact strength (10 x 10 mm) Steel designation t  16 mm 16 < t  40 mm 40 < t  65 mm t  65 mm Long. Trans. Temp. C J S275NH S275NLH 275 265 255 370-510 24 22 -20 -50 40 (1) 27 S355NH S355NLH 355 345 335 470-630 22 20 -20 -50 40 (1) 27 S420NH S420NLH 420 400 390 520-680 19 17 -20 -50 40 (1) 27 S460NH S460NLH 460 440 430 540-720 17 15 -20 -50 40 (1) 27 (1)_{Corresponds to 27 J at -30 C.}

Table 2.2b Cold formed welded structural hollow sections – Fine grain steel (EN 10219-1, 2006) – Steel properties different from EN 10210-1 (2006)

Feed stock condition M (1) Steel designation _{Minimum tensile strength}

(N/mm2) Minimum longitudinal elongation (%) (2) S275MH S275MLH 360 - 510 24 S355MH S355MLH 450 - 610 22 S420NH S420NLH 520 - 660 19 S460NH S460NLH 530 - 720 17

(1)_{M refers to thermal mechanical rolled steels.} (2)_{For d/t < 15 or} ₁₂_,₅ t 2 h b  

the minimum elongation is reduced by 2, e.g. from 24% to 22% for S275MH and S275MLH.

Table 2.3 Minimum inner corner radii of cold formed RHS according to EN 1993-1-8 (2005)

Maximum wall thickness t (mm) General

r/t Strain due to cold forming (%)

Predominantly static

loading Fatigue dominating

Aluminium-killed steel (Al  0,02%)  25  10  3,0  2,0  1,5  1,0  2  5  14  20  25  33 any any 24 12 8 4 any 16 12 10 8 4 any any 24 12 10 6

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Table 2.4a Hot finished structural hollow sections – Tolerances (EN 10210-2, 2006)

Section type Square/rectangular Circular

Outside dimension the greater of ± 0,5 mm and ± 1% (1) the greater of ± 0,5 mm and ± 1% but not more than 10 mm

Welded -10% Thickness

Seamless -10% and -12,5% at maximum 25% cross section Welded ± 6% on individual lengths

Mass

Seamless -6%; +8%

Straightness 0,2% of the total length and 3 mm over any 1 m length

Length (exact) +10 mm, -0 mm, but only for exact lengths of 2000 to 6000 mm Out of roundness - 2% for d/t  100 Squareness of sides 90 ± 1 -

Corner radii Outside 3,0t maximum - Concavity/convexity ± 1% of the side -

Twist 2 mm + 0,5 mm/m (1) -

(1)_{For elliptical hollow sections with h  250 mm, the tolerances are twice the values given in this table.}

Table 2.4b Cold formed welded structural hollow sections (EN 10219-2, 2006) – Tolerances different from EN 10210-2 (2006)

Section type Square/rectangular Circular

Outside dimension

b < 100 mm: the greater of ± 0,5 mm and ± 1% 100 mm  h, b  200 mm: ± 0,8% b > 200 mm: ± 0,6% ± 1%, minimum ± 0,5 mm maximum ± 10 mm Thickness Welded t  5 mm: ± 10% t > 5 mm: ± 0,5 mm for d  406,4 mm: t  5 mm: ± 10% t > 5 mm: ± 0,5 mm for d > 406,4 mm: ± 10% with maximum ± 2,0 mm Mass ± 6% ± 6%

Straightness 0,15% of the total length and 3 mm over any 1 m length

Outside corner radii (profile)

t  6 mm: 1,6 to 2,4t 6 mm < t  10 mm: 2,0 to 3,0t t > 10 mm: 2,4 to 3,6t

-

Concavity/convexity maximum 0,8% with a minimum of 0,5 mm -

Table 2.5 Special shapes available

Triangular Hexagonal Octagonal Flat - oval Elliptical Half-elliptical

Shape

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Table 2.6 European buckling curves according to manufacturing processes (EN 1993-1-1, 2005)

Cross section Manufacturing process Buckling curves

Hot finished 420 N/mm2 < fy  460 N/mm2 a0 Hot finished fy  420 N/mm2 a Cold formed c h b Flange Web _t h b Flange Web h b h b Flange Web _t h b Flange Web _t h b Flange Web h b h b Flange Web _t

b

t

d t _d t

d

t

h b Flange Web _t h b Flange Web h b h b Flange Web _t h b Flange Web _t h b Flange Web h b h b Flange Web _t

b

t

d t _d t

d

t

hh

Table 2.7 Limits for b/t, h/t and d/t for cross section classes 1, 2 and 3 (EN 1993-1-1, 2005)

Class 1 2 3

fyd (N/mm2) fyd (N/mm2) fyd (N/mm2)

Cross

section Load type

Considered element 235 275 355 460 235 275 355 460 235 275 355 460 3 f 235 c t b yd   c = 33 c = 38 c = 42 RHS

b/t (1) Compression Top face

36,0 33,5 29,8 26,6 41,0 38,1 33,9 30,2 45,0 41,8 37,2 33,0 3 f 235 c t h yd   c = 72 c = 83 c = 124 RHS

h/t (1) Bending Side wall (2)

75,0 69,6 61,6 51,8 86,0 79,7 70,5 62,3 127,0 117,6 103,9 91,6 yd f 235 c t d  c = 50 c = 70 c = 90 CHS d/t Compression and/or bending

t

t 50,0 42,7 33,1 25,5 70,0 59,8 46,3 35,8 90,0 76,9 59,6 46,0

(1) _{For all hot finished and cold formed RHS, it is conservative to assume that the width-to-thickness ratio of the "flat" is}

3 t b t 2r -2t -b _ _ _or ₃ t h t 2r -2t -h _ _ _.

(2)_{Wilkinson & Hancock (1998) suggested reducing the Eurocode limits (EN 1993-1-1) for the side wall slenderness of RHS} considerably, e.g. for class 1 in a simplified form to:

t b 83 , 0 77 t h _ _ _with ₃₄ t b  .

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Table 2.8 Allowable span-to-depth ratios L/(h-t) to avoid lateral buckling based on EN 1993-1-1 (2005)   t h L t h t b   S235 S275 S355 S460 0,5 73,7 63,0 48,8 37,7 0,6 93,1 79,5 61,6 47,5 0,7 112,5 96,2 74,5 57,5 0,8 132,0 112,8 87,4 67,4 0,9 151,3 129,3 100,2 77,3 1,0 170,6 145,8 112,9 87,2 h b Flange b _t h b Flange b h b h b Flange b _t h b Flange b _t h b Flange b h b h b Flange b _t

b

h

t

h b Flange b _t h b Flange b h b h b Flange b _t h b Flange b _t h b Flange b h b h b Flange b _t

b

h

t

Table 2.9 Torsional strength of various sections

Section _(kg/m)Mass ₍₁₀Torsion constant I4_mm4_{) or (cm}4₎t

UPN 200 25,3 11,9 INP 200 26,2 13,5 HEB 120 26,7 13,8 HEA 140 24,7 8,1 140 x 140 x 6 24,9 1475 168.3 x 6 24,0 2017

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Fig. 2.1 Lamellar tearing

Actual f_ymean

after cold forming

Actual f_ymean

after cold forming

Actual f_ymean

after cold forming

Fig. 2.2 Influence of cold forming on the yield strength for a square hollow section of 100 x 100 x 4 mm

0 1,00 0,75 0,50 0,25 0 0 0,5 1,0 1,5 2,0 00 1,00 0,75 0,50 0,25 0 0 0,5 1,0 1,5 2,0

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B uck lin g st re ss ( N /m m 2) B uck lin g st re ss ( N /m m 2)

Fig. 2.4 Comparison of the masses of hollow and open sections under compression in relation to the loading

Fig. 2.5 Restraints for the buckling of a brace member

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M

_pl

M

_el

M

_e

M

_pl

M

_el

M

_e

Fig. 2.7 Moment-rotation curves Fig. 2.8 Stress distribution for bending

(33)

Fig. 2.10 Elastic shear stress distribution

tf

_yd

t f

_yd

t f

_yd

t

d - 2t

Fig. 2.11 Internal pressure

(34)

paint layers

steel steel

corner protection for RHS and open sections

paint layers

steel steel

paint layers

steel steel

paint layers

steel steel

Fig. 2.13 Painted corners of RHS vs. open sections

Fig. 2.14 Fire resistance of concrete filled hollow sections

RHS 304,8x304,8x9,5

111 min.

14 min.

only

RHS

non-reinforced

concrete

filling

50min.

steel fibre

reinforced

concrete

filling

working load (kN)

fire

lif

e

(m

in

.)

1650.

3150.

120.

60. RHS 304,8x304,8x9,5

111 min.

14 min.

only

RHS

non-reinforced

concrete

filling

50min.

steel fibre

reinforced

concrete

filling

working load (kN)

fire

lif

e

(m

in

.)

1650.

3150.

120.

60. RHS 304,8x304,8x9,5

111 min.

14 min.

only

RHS

non-reinforced

concrete

filling

50min.

steel fibre

reinforced

concrete

filling

working load (kN)

fire

lif

e

(m

in

.)

1650.

3150.

120.

60.

(35)

Fig. 2.15 Aesthetically appealing structures

(36)

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3. APPLICATIONS

The applications of structural hollow sections nearly cover all fields. Hollow sections may be used because of the beauty of their shape or to express lightness, while in other cases their geometrical properties determine their application. In this chapter, examples are given for the various fields and to show the possibilities of constructing with hollow sections.

3.1 BUILDINGS AND HALLS

In buildings and halls, hollow sections are mainly used for columns and lattice girders or space frames for roofs. In modern architecture, they are also used for other structural or architectural reasons, e.g. facades. Fig. 3.1 shows a 10-storey building in Karlsruhe, Germany with rectangular hollow section columns 180 x 100. Special aspects are that the columns are made of weathering steel and are water filled to ensure the required fire protection. The columns are connected with water reservoirs to ensure circulation. Besides the fire protection, a further advantage is that due to the water circulation in the columns, the deformation of the building due to temperature differences by sunshine is limited.

Fig. 3.2 shows an example of lattice girder trusses used in a roof of an industrial building. For an optimal cost effective design, it is essential that the truss joints are made without any stiffening plates.

An especially appealing application is given in Fig. 3.3, showing a tree-type support of the airport departure hall in Stuttgart, Germany. For the joints, streamlined steel castings are used.

Fig. 3.4 shows the roof of the terminal of Kansai International Airport in Osaka, Japan with curved triangular girders of circular hollow sections.

Fig. 3.5 shows a dome under construction, whereas Fig. 3.6 illustrates a special application using columns and beams in the façade for ventilation assuring clean windows in the swimming pool.

Fig. 3.7 shows a very nice architectural application in Bush Lane House in the city of London, UK. The external circular hollow section lattice transfers the façade loads and the loads on the floors to the main columns. The hollow sections are filled with water for fire protection.

Very attractive applications can be found in the halls and buildings for the Olympic Games in Athens, e.g. Fig. 3.8.

Elliptical hollow sections are becoming more and more popular among architects and already several examples exist, see for example Fig. 3.9, the airport building in Madrid.

Nowadays, many examples of tubular structures are found in railway stations (Figs. 3.10 and 3.11) and roofs of stadia and halls (Figs. 3.12 to 3.14).

Indeed, as stated by one of the former CIDECT vice presidents, Jim Cran, at the Tubular Structures Symposium in Delft (1977) "The sky is the limit", whilst presenting beautiful applications of structural hollow sections.

3.2 BRIDGES

As mentioned in the introduction, the Firth of Forth Bridge is an excellent example of using the hollow section shape for structural applications in bridges. Nowadays, many modern examples exist (IISI, 1997). Figs. 1.4, 3.15 to 3.17 and 3.20 show various examples of pedestrian bridges; some of these are movable bridges.

Circular hollow sections can also be used as a flange for plate girders, as shown in Fig. 3.17 for a triangular box girder.

A very nice example of a road-pedestrian bridge is illustrated in Fig. 3.18, being a composite steel-concrete bridge with hollow sections for the arch and braces and a concrete deck.

Fig. 3.19 shows a railway bridge near Rotterdam, The Netherlands with circular hollow section arches.

3.3 BARRIERS

There are a few aspects which make hollow sections increasingly suitable for hydraulic structures, such as barriers. Due to environmental restrictions, the maintenance of hydraulic structures requires severe precautions, making durability an important issue. Structures of hollow sections are less susceptible to corrosion due to the rounded corners. Furthermore, especially circular hollow sections have lower drag coefficients, leading to lower forces due to wave loading. Fig. 3.21 shows a barrier with a support

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structure of circular hollow sections. Fig. 3.22 shows the storm surge barrier near Hook of Holland with triangular arms made of circular hollow sections and a length (250 m) equal to the height of the Eiffel Tower in Paris.

3.4 OFFSHORE STRUCTURES

Offshore, many application examples are available; most of them in circular hollow sections. For the support structure, the jacket or tower, not only is the wave loading important, but also other aspects are leading to the use of circular hollow sections. E.g. in jackets, the circular hollow section piles are often driven through the circular hollow section legs of the jacket, thus the pile is guided through the leg. Sometimes the internal void is used for buoyancy. Further, the durability and easy maintenance in severe environments are extremely important.

Hollow section members are used in jackets, towers, the legs and diagonals in topside structures, cranes, microwave towers, flare supports, bridges, support structures of helicopter decks and further in various secondary structures, such as staircases, ladders, etc. Figs. 3.23 and 3.24 show two examples.

3.5 TOWERS AND MASTS

Considering wind loading, corrosion protection and architectural appearance, there is no doubt that hollow sections are to be preferred. However, in many countries, electric power transmission towers are made of angle sections with simple bolted joints. Nowadays, architectural appearance becomes more important, while stringent environmental restrictions make protection and maintenance increasingly expensive. These factors stimulate designs made of hollow sections (Figs. 3.25 and 3.26).

3.6 SPECIAL APPLICATIONS

The field of special applications is large, e.g. along the roads, petrol stations (Fig. 3.27), sound barriers (Fig. 3.28), traffic information gantries (Fig. 3.29), guard rails, parapets and sign posts.

Further, excellent application examples are found in radio telescopes (Fig. 3.30), in mechanical engineering, cranes (Fig. 3.31) and roller coasters (Fig. 3.32).

In the agricultural field, glass houses (Fig. 3.33) and agricultural machinery are typical examples. Also in transport, many examples exist but these are outside the scope of this book. Indeed, the sky is the limit.

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Fig. 3.2 Roof with lattice girders

Fig. 3.1 Façade of the Institute for Environment in Karlsruhe, Germany

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Fig. 3.4 Roof of Kansai International Airport in Osaka, Japan

Fig. 3.6 Façade with ventilation through the RHS columns and beams, Borkum, Germany

Fig. 3.8 Hall for the 2004 Olympic Games, Athens, Greece

Fig. 3.5 Dome structure in Gothenburg, Sweden

Fig. 3.7 Bush Lane House in London, UK

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Fig. 3.10 Railway station in Rotterdam, The Netherlands

Fig. 3.12 Barrel dome grid for the Trade Fair building in Leipzig, Germany

Fig. 3.13 Retractable roof for the Rogers Centre in Toronto, Canada

Fig. 3.11 TGV railway station at Charles de Gaulle Airport, France

Fig. 3.14 Stadium Australia for the 2000 Olympic Games, Sydney, Australia

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Fig. 3.15 Movable pedestrian bridge in RHS, The Netherlands

Fig. 3.17 Pedestrian bridge in Houdan, France

Fig. 3.19 Railway bridge with CHS arches, The Netherlands

Fig. 3.21 Eastern Scheldt barrier, The Netherlands

Fig. 3.16 Movable pedestrian bridge in RHS near Delft, The Netherlands

Fig. 3.18 Composite road bridge in Marvejols, France

Fig. 3.20 Movable pedestrian bridge in CHS near Delft, The Netherlands

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Fig. 3.23 Bullwinkle offshore structure, Gulf of Mexico

Fig. 3.25 Electric power transmission tower

Fig. 3.27 Petrol station, The Netherlands

Fig. 3.24 Amoco P15 offshore platform with jack-up, North Sea

Fig. 3.26 Mast, The Netherlands

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Fig. 3.29 Traffic information gantry, The Netherlands

Fig. 3.30 Radio telescope

Fig. 3.31 Cranes

Hollow Sections in Structural Application _2nd_edt (1)