STEEL STRUCTURES

You can't build this without BIM and composite materials. This is how... Museum of the Future, Dubai, UAE, 2022. The 78-meter high building houses seven floors within a torus-shaped shell that sits atop a three-story podium. The exterior facade of the torus comprises 1,024 fire-retardant (FR) composite panels. Clad with stainless steel, each panel is a unique 3D shape and integrates moulded-in Arabic calligraphy. The flowing script forms poems that describe the vision for Dubai’s future. They also serve as the building’s windows, casting daylight through the column-free interior and creating a dramatic effect at night via 14 kilometres of integrated LED lighting. The parametric design was used to develop the steel diagrid — a framework of 2,400 diagonally intersecting steel members — to which composite concrete floor slabs and 17,000 square meters of composite cladding are attached, the latter using 1 million square meters of multiaxial glass and carbon fibre-reinforced epoxy prepreg. Lots of diagrid structures involve them being used as facade elements or roof structures. But for this Museum, the diagrid itself was the primary structure. One of the key challenges was how the diagrid integrates into the floor plates inside, taking into account all of the irregular geometry. There was definitely an intuitive process.

••• Steelwork drawings ••• Steelwork drawings show the detail for fabrication and arrangement of the structure for erection. They are also used for taking off the materials list and preparing the bill of quantities and estimates of cost. It is essential that drawings are presented correctly and are carefully checked for accuracy. Drawing is an essential part of the design process. The designer must ensure that the detail is such that the structure acts in the way he or she has idealized it for design. He or she must also ensure that all the detailed construction is possible, will not lead to failure and can be painted, inspected and properly maintained. Steelwork drawings may be classified into: • General arrangement – Showing the function and arrangement of the structure • Making plans – Showing the location of separate numbered members for erection • Detail drawings – Giving details of separate members for fabrication. Many consulting engineering practices carry out the overall analysis and design only, preparing arrangement drawings showing the member sections required. Special joint types must be carefully specified to achieve the designer’s assumptions in practice. The fabricator then prepares the detail drawings for joints and shop fabrication. This enables the firm to use details and processes with which they are familiar and have the necessary equipment. Computer software is increasingly used to produce arrangement and detail drawings and take off quantities.

• CONCEPTUAL DESIGN Conceptual design in the structural engineering sense is the function of choosing a suitable form or system or framing arrangement to bring the architectural solution into being. The building layout, limits and parameters have often been determined solely by the architect. In such cases, the structural engineer may not be able to select the optimum structural solution. Ideally, conceptual design should result from a team effort, where architect, structural engineer and service engineers contribute to the final solution. Modern architectural practices take this multidisciplinary approach. The architectural decisions are based on functional, aesthetic, environmental and economic considerations. Any of these factors may control in a given case. For example, for an industrial plant it is the functional requirement, whereas for an exhibition building it is the aesthetic aspect. Financial control is always of paramount importance and cost over-runs lead to many legal and other problems. • INNOVATION Novelty and innovation are always desirable and we seem to strive after these goals. Architects, engineers and builders always push existing forms of construction to the limits possible with materials available and within the state of knowledge at the time. Structural failures determine when limits are reached and so modifications are made and new ideas developed. Often it is not a new solution that is required, but the correct choice and use of a well-proven existing structural system that gives the best answer. The engineer continually seeks new and improved methods of analysis, design and construction, and the materials scientist continually seeks to improve material properties and protection systems through research and development. These advances lead to safer and more economical structures. Much of recent structural research has centred on the use of computers in all aspects of the work from architectural and structural modelling and design for construction and building finishing control. The following are instances of recent structural engineering innovation: 1. Analysis – Elastic matrix and finite element analysis, second order analysis, cable net analysis, plastic analysis; 2. Design – Plastic design, limit state design, computer-aided design, structural optimization and neural network systems; 3. Construction – Space decks, geodesic domes, tension structures, box girder bridges, high-rise tube buildings, etc.

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: 1. Conceptual design and planning. This involves selecting the most economical structural form and materials to be used. Preliminary designs are often necessary to enable comparisons to be made. 2. Detailed design for a given type and arrangement of structure, which includes: • Idealization of the structure for analysis and design • Estimation of loading • Analysis for the various load cases and combinations of loads and identification of the most severe design actions • Design of the foundations, structural frames, elements and connections • Preparation of the final arrangement and detail drawings. • • The materials list, bill of quantities and specification covering welding, fabrication, erection, corrosion protection and fire protection may then be prepared. Finally, the estimates and tender documents can be finalized for submission to contractors. • • The structural designer uses his or her knowledge of structural mechanics and design, materials, geotechnics and codes of practice and combines this with his or her practical experience to produce a satisfactory design. He or she takes advice from specialists, makes use of codes, design aids, handbooks and computer software to help him or her in making decisions and to carry out complex analysis and design calculations.

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.

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 5. Stressed skin structures, where the cladding stabilizes the structure. Combinations with concrete are structurally important in many buildings. Illustrations of some of the types of framed steel structures are shown in Figure. Braced and rigid frame and truss roof and space deck construction are shown in the figure for comparison. 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.

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.

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 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 microstructure, 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.

Cantilever Canopy