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Design of Structural Steelwork Design of Structural Steelwork Second Edition PETER KNOWLES, MA, MPhil, CEng, MICE, FIHT Consulting Engineer Surrey University Press Glasgow and London Published by Surrey University Press Bishopbriggs, Glasgow G64 2NZ and 7 Leicester Place, London WC2H 7BP 1987 Blackie limitations of time, space and money generally restrict the latter aspect to calculation and drawing with perhaps the construction and testing of models. But much can be done with pencil and paper to inculcate a sound approach to the design of structures, provided the student is made aware of the fundamentals of design and the specific problems associated with the various structural materials. The aim of this publication is to present the essential design aspects of one structural materialsteel. The book is of an entirely introductory nature, demanding no prior knowledge of the subject, but readers are assumed to have followed or be following courses in structural analysis and mechanics of materials in sufficient depth to give them a confident grasp of elementary structural and stress analysis techniques. Although it has been written primarily with undergraduates in mind the book will be of use to young graduates who may be coming across the subject for the first time. For this reason the example calculations con as far as possible to practical requirements. The first chapter commences with a brief review of the historical development of the science of iron and steel making and the use of these two materials in structures, followed by a discussion of the important properties of structural steel, and the types of steel products available for structural use. Design philosophy and stability, outlined in Chapter 2, are followed by a detailed chapter on that most important structural element, the beam. After consideration of local and overall instability the chapter goes on to describe the design of a number of different beam types; rolled sections, compound beams, welded plate girders, gantry girders and composite beams. Chapter 4 is devoted to elements loaded in tension or compression, with or without bending, considering rolled and built-up members, concrete encasement and concrete filling, and the special problems of angle members. Connections are the subject of Chapter 5. Detailed treatment of the fundamentals of connection design is given, with emphasis on high- strength friction grip bolting and welding. Finally Chapter 6 introduces some very simple assemblies of elements. Mere manipulation of code of practice clauses is a poor preparation for a student; he must be aware of the theoretical background to present and future design practice. Yet codified ination needs to be used if comparisons are to be made and some discipline imposed on examples and rcises. In this case the current version of British Standard 5950 has been used as a basis for calculations. Extracts from BS5950 Part 11985 are reproduced by permission of the British Standards Institution. Complete copies can be obtained from BSI at Linford Wood, Milton Keynes, MK14 6LE. Design is an open-ended subject in which there are no unique solutions. Students often have difficulty in accepting this fact, accustomed as they are to finding the unique correct solution to an analytical problem. They must try to cultivate an attitude of mind which will help them to criticise their solution to design problems from economic and aesthetic points of view in so far as this is possible in a student environment. Finally, an intelligent interest in the world of engineering is essential. Visits to structures under construction, fabricating shops and steelworks are to be encouraged. At the very least students should read architectural and engineering journals to keep abreast of developments in steel construction. It must always be borne in mind that a textbook such as this one must of necessity always lag behind the most modern practice even though the fundamental ideas which it contains will still be valid. My particular thanks go to Norman Wootton, BSc, MICE, for his help in checking the example calculations. PK ix Notations and units The system of notation adopted follows that in British Standard 5950. The major symbols are listed here for reference; others are defined when used in the text. The units adopted are generally those of the SI system with the important variation that the centimetre which is not in the SI system has been retained for the steel section properties, radius of gyration cm, area cm2, modulus cm3 and second moment of area cm4. The mass of a cubic metre of steel is 7850 kilograms. 1 metric tonne9.81 kilonewtons AArea AeEffective area AgGross area AsShear area bolts AtTensile stress area bolts AvShear area sections aSpacing of transverse stiffeners or Effective throat size of weld BBreadth bOutstand or Width of panel blStiff bearing length DDepth of section or Diameter of section or Diameter of hole dDepth of web or Nominal diameter of fastener EModulus of elasticity of steel eEnd distance FcCompressive force due to axial load FsShear force bolts FtTensile force FvAverage shear force sections fcCompressive stress due to axial load fvShear stress GShear modulus of steel HWarping constant of section hStorey height IxSecond moment of area about the major axis IySecond moment of area about the minor axis JTorsion constant of section LLength of span LEEffective length MLarger end moment Max, MayMaximum buckling moment about the major or minor axes in the presence of axial load MbBuckling resistance moment lateral torsional Mcx, McyMoment capacity of section about the major and minor axes in the absence of axial load MEElastic critical moment MoMidspan moment on a simply supported span equal to the unrestrained length Mrx, MryReduced moment capacity of the section about the major and minor axes in the presence of axial load , Applied moment about the major and minor axes Mx, MyEquivalent uni moment about the major and minor axes mEquivalent uni moment factor nSlenderness correction factor PbbBearing capacity of a bolt PbgBearing capacity of parts connected by friction grip fasteners PbsBearing capacity of parts connected by ordinary bolts Pcx, PcyCompression resistance about the major and minor axes PsShear capacity of a bolt PsLSlip resistance provided by a friction grip fastener PtTension capacity of a member or fastener PvShear capacity of a section pbBending strength pbbBearing strength of a bolt pbgBearing strength of parts connected by friction grip fasteners pbsBearing strength of parts connected by ordinary bolts pcCompressive strength pEEuler strength xi psShear strength of a bolt ptTension strength of bolt pwDesign strength of a fillet weld pyDesign strength of steel qbBasic shear strength of a web panel qcrCritical shear strength of web panel qeElastic critical shear strength of web panel qfFlange dependent shear strength factor rx, ryRadius of gyration of a member about its major and minor axes Sx, SyPlastic modulus about the major and minor axes sLeg length of a fillet weld TThickness of a flange or leg tThickness of a web UsSpecified minimum ultimate tensile strength of the steel uBuckling parameter of the section VbShear buckling resistance of stiffened web utilizing tension field action VcrShear buckling resistance of stiffened or unstiffened web without utilizing tension field action vSlenderness factor for beam xTorsional index of section YsSpecified minimum yield strength of steel Zx, ZyElastic modulus about major and minor axes αeModular ratio βRatio of smaller to larger end momen γfOverall load factor γoRatio M/Mo, i.e. the ratio of the larger end moment to the midspan moment on a simply supported span δDeflection εConstant λSlenderness, i.e. the effective length divided by the radius of gyration λcrElastic critical load factor λLOLimiting equivalent slenderness λLTEquivalent slenderness λ0Limiting slenderness Slip factor xii A note on calculations The example calculations have been laid out in a similar to that adopted in a design office. Reference is made in the left-hand column of the calculation sheet to the relevant clause of the British Standard which affects the calculation in the centre column. Where a British Standard number is not quoted the reference is to British Standard 5950 1. The student is urged to carry out all calculations in a ical manner on prepared calculation sheets; in this way the possibility of error will be reduced and checking facilitated. In order to make the best use of the example calculations a copy of British Standard 5950 1 and tables of section properties 2 are necessary. Further example calculations are to be found in Reference 3. References 1. British Standard 5950. Structural use of Steelwork in Building Part 11985. Code of Practice for Design in Simple and Continuous Construction Hot Rolled Sections. Part 21985. Specification for Materials. Fabrication and Erection. 2. Steelwork Design. Guide to BS5950 Part 11985. Volume 1. Section Properties. Member Capacities. Constrado, London 1985. 3. Steelwork Design. Guide to BS5950; Part 11985. Volume 2. Worked Examples. The Steel Construction Institute, London 1986. xiii 1 Iron and steel 1.1 Production The basic constituent of structural steel is iron, an element widely and liberally available over the world’s surface but with rare exceptions found only in combination with other elements. The main deposits of iron are in the of ores of various kinds which are distinguished by the amount of metallic iron in the combination and the nature of the other elements present. The most common ores are oxides of iron mixed with earthy materials and chemically adulterated with, for example, sulphur and phosphorus. Iron products have three main commercial s; wrought iron, steel and cast iron in ascending order of carbon content. Table 1.1, which gives some physical properties of these three compounds, shows that as the carbon content of the metal increases the melting point is lowered; this fact has considerable importance in the production process. Modern steelmaking depends for its raw material on iron produced by a blast furnace. Iron ore is charged into the furnace with coke and limestone. A powerful air blast raises the temperature sufficiently to melt the iron, which is run off. The iron at this stage has a high carbon content; steel is obtained from it by removing most of the carbon. In the most modern processes decarburizing is done by blowing oxygen through the molten iron 1. Table 1.1 Some properties of iron and steel MaterialTypical carbon Melting point C Ultimate tensile stress N/ mm2 Pure ironnil1535335 Mild steelup to 0.25varies*450 High carbon steel1.4varies*900 Cast iron5.01140110 *Melting point decreases as carbon content increases 1.2 Mechanical working By the middle of the nineteenth century the practice of rolling iron sections had been established, iron rails being exhibited by the Butterley Company at the Hyde Park Exhibition in 1851. The first wrought-iron I section beams were rolled in 1845 in France under the direction of a French Iron-master, F.Zores. It appears that similar sections were first rolled in England in 1863. Dorman Long were rolling steel beams up to 400mm deep in 1885 but in the United States a tabular presentation of the section properties of rolled steel shapes had been published in 1873. 1.3 Steel in structure By the end of the eighteenth century all that was needed to inaugurate an era of building in iron was the courage which all pioneers require. The bridge at Coalbrookdale 1777–81 constructed by Abraham Darby III 1750–91 and the iron framed factories designed by William Strutt 1756–1830 from 1792 onwards appear to mark the beginning of this era. At first only cast iron columns were used in building but in 1801 James Watt devised a cast iron beam in the of an inverted T which could span 4.3m as a floor beam. In building construction the centre of pressure to adopt steel framing was located first in the United States. The reasons for this were complex; suffice it to say that in Chicago in the 1880s economic factors, stemming from the need to make the greatest use of expensive land in a cramped city centre, led to the adoption of the tall steel-framed building later known as the skyscraper. High building in traditional masonry construction was limited by the great thickness of material required at lower levels and the consequent heavy load imposed on the foundations. The personal physical problem of climbing stairs had been solved by the invention of the elevator in 1857 E.G.Otis. A six-storey wrought iron frame, the Cooper Union Building was completed in 1858. All these facts, coupled with an aggressive marketing attitude by the American steel makers, produced a climate in which in 1884 William le Barren Jenney 1832–1907 designed the nine-storey Home Insurance Building, the first skeletal iron and steel frame 2. Major steel frame construction in Great Britain is generally agreed to have commenced with the Ritz Hotel 1904 in London. A summary of significant dates in both building and bridge construction is given below. 2DESIGN OF STRUCTURAL STEELWORK Significant dates in iron and steel structural history DateDetails 1779Cast iron bridge at Coalbrookdale span 30.0m 1792Multi-storey iron-framed mill building at Derby William Strutt 1796Buildwas bridge span 43.0m 1796Sunderland bridge span 79.0m 1801James Watt cast iron beams to span 4.3m 1809Schuylkill bridge span 103m 1820Berwick bridge span 150m 1826Menai suspension bridge span 177m 1845Wrought iron beams rolled in France 1848Five-storey factory New York James Bogardus 1850Menai tubular rail bridge span 153m 1857Otis elevator invented 1853–58Cooper Union six-storey wrought iron frame 1856Bessemer steel-making process 1860Boat store Sheerness, four-storey cast iron frame 1863Butterley Co rolled wrought iron beams 1865Siemens Martin open hearth steel-making process 1877Board of Trade regulations changed to allow steel to be used in bridges 1880Siemens electric lift invented 1883Brooklyn Bridge span 486m 1884Home Insurance Building Chicago, ten-storey steel frame W.le Baron Jenney 1884Garabit viaduct span 180m Eiffel 1885Dorman Long opened constructional departments 1887Hexagonal steel columns used in Birmingham 1889Eiffel Tower 300m high 1890Forth Rail bridge span 521m 1896Robinsons, Stockton, first steel frame in England 1904Ritz Hotel London 1917Quebec Bridge span 549m 1931Bayonne Bridge span 510m 1932Sydney Harbour Bridge span 509m 1937Golden Gate Bridge span 1280m 1964Verrazano Narrows Bridge span 1298m 1981Humber Bridge span 1410m IRON AND STEEL3 1.4 Properties of structural steel 3 To the structural designer, certain properties of steel merit special consideration. As a general introduction to the behaviour of steel