Steel structures—structural engineering

 1.1 NEED FOR AND USE OF STRUCTURES: Structures are one of mankind’s basic needs next to food and clothing, and are a hallmark of civilization. Man’s structural endeavours to protect himself from the elements and from his own kind, to bridge streams, to enhance a ruling class and for religious purposes go back to the dawn of mankind. Fundamentally, structures are needed for the following purposes: • to enclose space for environmental control; • to support people, equipment, materials etc. at required locations in space; • to contain and retain materials; • to span land gaps for transport of people, equipment etc. The prime purpose of structures is to carry loads and transfer them to the ground. Structures may be classified according to use and need. A general classification is: • residential—houses, apartments, hotels; • commercial—offices, banks, department stores, shopping centres; • institutional—schools, universities, hospitals, gaols; • exhibition—churches, theatres, museums, art galleries, leisure centres, sports stadia, etc.; • industrial—factories, warehouses, power stations, steelworks, aircraft hangers etc. Other important engineering structures are: • bridges—truss, girder, arch, cable suspended, suspension; • towers—water towers, pylons, lighting towers etc.; • special structures—offshore structures, carparks, radio telescopes, mine headframes etc. Each of the structures listed above can be constructed using a variety of materials, structural forms or systems. Materials are discussed first and then a general classification of structures is set out, followed by one of steel structures. Though the subject is steel structures, steel is not used in isolation from other materials. All steel structures must rest on concrete foundations and concrete shear walls are commonly used to stabilize multistorey buildings.

 1.2 STRUCTURAL MATERIALS—

TYPES AND USES From earliest times, naturally occurring materials such as timber, stone and fibres were used structurally. Then followed brickmaking, rope-making, glass and metalwork. From these early beginnings the modern materials manufacturing industries developed. The principal modern building materials are masonry, concrete (mass, reinforced and prestressed), structural steel in rolled and fabricated sections and timber. All materials listed have particular advantages in given situations, and construction of a particular building type can be in various materials, e.g. a multistorey building can be loadbearing masonry, concrete shear wall or frame or steel frame. One duty of the designer is to find the best solution which takes account of all requirements — economic, aesthetic and utilitarian. The principal uses, types of construction and advantages of the main structural materials are as follows. • Masonry—loadbearing walls or columns in compression and walls taking in-plane or transverse loads. Construction is very durable, fire resistant and aesthetically pleasing. Building height is moderate, say to 20 storeys. • Concrete—framed or shear wall construction in reinforced concrete is very durable and fire resistant and is used for the tallest buildings. Concrete, reinforced or prestressed, is used for floor construction in all buildings, and concrete foundations are required for all buildings. • Structural steel—loadbearing frames in buildings, where the main advantages are strength and speed of erection. Steel requires protection from corrosion and fire. Claddins and division walls of other materials and concrete foundations are required. Steel is used in conjunction with concrete in composite and combined frame and shear wall construction. Structural steels are alloys of iron, with carefully controlled amounts of carbon and various other metals such as manganese, chromium, aluminium, vanadium, molybdenum, neobium and copper. The carbon content is less than 0.25%, manganese less than 1.5% and the other elements are in trace amounts. The alloying elements control grain size and hence steel properties, giving high strengths, increased ductility and Table 1.1 Strengths of steels used in structures Steel type and use Yield stress (N/mm2 ) Grade 43—structural shapes 275 Grade 50—structural shapes 355 Quenched and self-tempering 500 Quenched tempered-plates 690 Alloy bars—tension members 1030 High carbon hard-drawn wire for cables 1700 fracture toughness. The inclusion of copper gives the corrosion resistant steel Cor-ten. High-carbon steel is used to manufacture hard drawn wires for cables and tendons. The production processes such as cooling rates, quenching and tempering, rolling and forming also have an important effect on the micro structure, giving small grain size which improves steel properties. The modern steels have much improved weldability. Sound full-strength welds free from defects in the thickest sections can be guaranteed. A comparison of the steels used in various forms in structures is given in Table 1.1. The properties of hot-rolled structural steels are given Chapter 2 (Table 2.3). Structural steels are hot-rolled into shapes such as universal beams and columns. The maximum size of universal column in the UK is 356×406 UC, 634 kg/m, with 77 mm-thick flanges. Trade-ARBED in Luxembourg roll a section 360×401 WTM, 1299 kg/m, with 140 mm-thick flanges. The heavy rolled columns are useful in high-rise buildings where large loads must be carried. Heavy built-up H, I and box sections made from plates and lattice members are needed for columns, transfer girders, crane and bridge girders, etc. At the other end of the scale, light weight cold-rolled purdins are used for roofing industrial buildings. Finally, wire, rope and high-strength alloy steel bars are required for cable-suspended and cable-girder roofs and suspended floors in multistorey buildings.

 1.3 TYPES OF STRUCTURES 1.3.1 General types of structures The structural engineer adopts a classification for structures based on the way the structure resists loads, as follows. 1. Gravity masonry structures—loadbearing walls resist loads transmitted to them by floor slabs. Stability depends on gravity loads. 2. Framed structures—a steel or concrete skeleton collects loads from plate elements and delivers them to the foundations. 3. Shell structures—a curved surface covers space and carries loads. 4. Tension structures—cables span between anchor structures carrying membranes. 5. Pneumatic structures—a membrane sealed to the ground is supported by internal air pressure. 2 STEEL STRUCTURES—STRUCTURAL ENGINEERING Examples of the above structures are shown in Figure 1.1 1.3.2 Steel structures Steel-framed structures .may be further classified into the following types: 1. single-storey, single- or multibay structures which may be of truss or stanchion frames or rigid frame of solid or lattice members; 2. multistorey, single- or multibay structures of braced or rigid frame construction—many spectacular systems have been developed; 3. space structures (space decks, domes, towers etc.)—space decks and domes (except the Schwedler dome) are redundant structures, while towers may be statically determinate space structures; 4. tension structures and cable-supported roof structures; Fig. 1.1 General types of structures. TYPES OF STRUCTURES 3 5. stressed skin structures, where the cladding stabilizes the structure. As noted above, combinations with concrete are structurally important in many buildings. Illustrations of some of the types of framed steel structures are shown in Figure 1.2. Braced and rigid frame and truss roof and space deck construction are shown in the figure for comparison. Only framed structures are dealt with in the book. Shell types, e.g. tanks, tension structures and stressed skin structures are not considered. For the framed structures the main elements are the beam, column, tie and lattice member. Beams and columns can be rolled or built-up I, H or box. Detailed designs including idealization, load estimation, analysis and section design are given for selected structures.

 1.4 FOUNDATIONS Foundations transfer the loads from the building structure to the ground. Building loads can be vertical or horizontal and cause overturning and the foundation must resist bearing and uplift loads. The correct choice and design of foundations is essential in steel design to ensure that assumptions made for frame design are achieved in practice. If movement of a foundation should occur and has not been allowed for in design, it can lead to structural failure and damage to finishes in a building. The type of foundation to be used depends on the ground conditions and the type of structure adopted The main types of foundations are set out and discussed briefly, as follows. 1. Direct bearing on rock or soil. The size must be sufficient to ensure that the safe bearing pressure is not exceeded. The amount of overall settlement may need to be limited in some cases, and for separate bases differential settlement can be important. A classification is as follows: • pad or spread footing used under individual columns; • special footings such as combined, balanced or tied bases and special shaped bases; • strip footings used under walls or a row of columns; • raft or mat foundations where a large slab in flat or rubbed construction supports the complete building; • basement or cellular raft foundations; this type may be in one or more storeys and form an underground extension to the building that often serves as a carpark. 2. Piled foundations, where piles either carry loads through soft soil to bear on rock below or by friction between piles and earth. Types of piles used vary from precast driven piles and cast-in-place piles to large deep cylinder piles. All of the above types of foundations can be supported on piles where the foundation forms the pile cap. Foundations are invariably constructed in concrete. Design is covered in specialist books. Some types of foundations for steelframed buildings are shown in Figure 1.3. Where appropriate, comments on foundation design are given in worked examples.

 1.5 STRUCTURAL ENGINEERING

 1.5.1 Scope of structural engineering Structural engineering covers the conception, planning, design, drawings and construction for all structures. Professional engineers from a number of disciplines are involved and work as a team on any given project under the overall control of the architect for a building structure. On engineering structures such as bridges or powerstations, an engineer is in charge. Lest it is thought that the structural engineer’s work is mechanical or routine in nature, it is useful to consider his/her position in building construction where the parties involved are: • the client (or owning organization), who has a need for a given building and will finance the project; • the architect, who produces proposals in the form of building plans and models (or a computer simulation) to meet the client’s requirements, who controls the project and who engages consultants to bring the proposals into being; • consultants (structural, mechanical, electrical, heating and ventilating etc.), who carry out the detail design, prepare working drawings and tender documents and supervise construction; 

4 STEEL STRUCTURES—STRUCTURAL ENGINEERING • contractors, who carry out fabrication and erection of the structural framework, floors, walls, finishes and installation of equipment and services. The structural engineer works as a member of a team and to operate successfully requires flair, sound knowledge and judgement, experience and the ability to exercise great care. His or her role may be summarized as planning, design preparation of drawings and tender documents and supervision of construction. He/she makes decisions about materials, structural form and design methods to be used. He/she recommends acceptance of tenders, inspects, supervises and approves fabrication and construction. He/she has an overall responsibility for safety and must ensure that the consequences of failure due to accidental causes are limited in extent. The designer’s work, which is covered partially in this book, is one part of the structural engineer’s work. 1.5.2 Structural designer’s work The aim of the structural designer is to produce the design and drawings for a safe and economical structure that fulfils its intended purpose. The steps in the design process are as follows