Design of Steel Structures (EGCV3210) Chapter 1

Sultanate of Oman University of Technology and Applied Sciences Muscat Department of Engineering Section of Civil and Architectural Engineering Design of Steel Structures (EGCV321...

Sultanate of Oman University of Technology and Applied Sciences Muscat Department of Engineering Section of Civil and Architectural Engineering Design of Steel Structures (EGCV3210) Chapter 1 An Introduction to the Design of Steel Structures Semester 1 AY 2024-2025 Dr Hussain Al-Lawati ENGG_CC_PPT Notes_01Sep2020_V01 EGCV3210: Design of Steel Structures 1 Learning outcome Students will have a basic understanding of steel structures after finishing this chapter, which will aid them in understanding the more in-depth material in the upcoming chapters. Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 2 Structures Content Steel structures’ brief Advantageous and disadvantageous of steel Mechanical properties of steel Examples of steel cross-section Types of steel cross-section Steel cross-section classification Loads’ types Design and analysis methods in steel structures Steel’s design limit states Modes of failure in steel elements Copyright Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 3 Structures Steel structures’ brief Steel structures are buildings or frameworks that use steel as the primary material for their construction. Iron first came into use as a structural material when the cast-iron bridge, an arch of 30.4 m span, was built by A Darby in 1779 over the River Severn, near Coalbrookdale in England. Fig. 1 The Iron Bridge, UK Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 4 Structures Steel structures’ brief Steel structures are commonly used for a variety of buildings and infrastructure, including high-rise buildings, bridges, industrial facilities, sports stadiums and arenas, and towers and transmission lines. Fig. 2 Forth Bridge, UK Fig. 3 U. S. Steel Tower, Fig. 4 Beijing National Stadium, USA China Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 5 Structures Steel structures’ brief Steel structures are typically designed using steel beams, columns, and connectors to create a framework that supports various loads. Fig. 5 Steel beams Fig. 6 Steel columns Fig. 7 Steel connections Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 6 Structures Advantageous and disadvantageous of steel Table 1 Some steel’s advantageous and disadvantageous Advantageous Disadvantageous High strength Corrosion Light weight Fireproofing costs Ease of fabrication Susceptibility to buckling Uniformity Fatigue Elasticity Fire resistant Ductility Aesthetics Toughness High maintenance costs Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 7 Structures Mechanical properties of steel The stress-strain characteristics of structural steel have a fundamental influence on the whole design process. If a steel specimen as shown in Fig. 8 is tested in tension, the characteristics shown in Fig. 9 will be observed. Fig. 8 Steel tensile specimen Fig. 9 Typical stress-strain curve Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 8 Structures Mechanical properties of steel Table 2 describes each section of the curve shown in Fig. 9. More commonly used steel’s grades – according to BS 5950 – are S275, S355, and S460. Table 2 Steel’s stress-strain behavior. Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 9 Structures Mechanical properties of steel By looking at Fig. 9, different zones can be described as follows: If steel is loaded within the elastic zone (0-a), the strain will return to zero when the load is removed and there will not be any permanent deformation. The point ‘a’ is the yield point, at which the stress is the yield strength. Along the plateau (b-c) the strain increases while the stress remains constant (plastic zone). At the end of the plateau there is a zone (c-d) of strain hardening and an increase in stress to the ultimate tensile strength (point d), after which failure occurs. Structural steel can carry large loads even with deformation and is, therefore, a ductile material. Comparing to other famous construction materials, such as concrete, steel is better in tension. This is because concrete is a brittle material, which is strong in compression (Fig. 10). Since steel is a ductile material, its strength in tension and compression is about the same (Fig. 11). If steel has low elastic limits, then it does not constitute suitable structural material. Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 10 Structures Mechanical properties of steel Fig. 10 Steel vs concrete load-deformation curve Fig. 11 Idealized stress-strain curve for steel Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 11 Structures Mechanical properties of steel Modulus of elasticity (or Young’s modulus), E: Is the measure of the stiffness of a material. In other words, it is a measure of how easily any material can be bent or stretched. Also, it is the slope of the linear part of the stress–strain curve for a material under tension or compression (e.g., see Fig. 9, zone 0-a). 𝑎𝑥𝑖𝑎𝑙 𝑠𝑡𝑟𝑒𝑠𝑠 𝐸 = (in the elastic range). 𝑎𝑥𝑖𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛 The modulus of elasticity of structural steel is about 210000 N/mm2. Elastic shear modulus, G: Is the measure of the rigidity of the body, given by the ratio of shear stress to shear strain. In other words, is a measure of the ability of a material to resist transverse deformations (e.g., due to twisting). 𝑠ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑠𝑠 𝐸 𝐺 = = 2(1+ν) (in the elastic range), where ν is the Poisson’s ratio (=0.3 for steel). 𝑠ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑎𝑖𝑛 The elastic shear modulus of structural steel is about 81000 N/mm2. Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 12 Structures Examples of steel cross-section Fig. 12 Some famous steel cross-sections Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 13 Structures Types of steel cross-section Steel can be formed by rolling process, which uses a series of rollers to alter the shape, improve the uniformity, and/or enhance the mechanical properties of steel. Rolled steel can be categorized into two types, hot rolled steel and cold rolled steel. The main difference between hot rolled steel and cold rolled steel is the temperature at which they are processed. Hot and cold rolled steels offer different benefits to each other as shown in Table 3. Table 3 Hot and cold steels benefits Hot rolled steel benefits Cold rolled steel benefits Lower cost Greater strength Better workability Better surface finishes Little to no internal stresses Higher precision Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 14 Structures Types of steel cross-section Hot and cold rolled steels are used in different applications, as provided in Table 4. Table 4 Hot and cold steels applications Hot rolled steel Cold rolled steel Agricultural equipment Aerospace structures Automobile parts Home appliances Construction materials Metal furniture Railroad equipment Strips, rods, bars, and sheets Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 15 Structures Steel cross-section classification Fig. 13 Moment-rotation behavior for different section classes Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 16 Structures Steel cross-section classification The four classes of cross-section given in the BS 5950 code are (Fig. 13): 1. Class 1 Plastic Sections in which all elements subject to compression are relatively stocky (small width to thickness ratios) and which can develop the full plastic moment capacity with sufficient rotation capacity to allow redistribution of moments within the structure. 2. Class 2 Compact Sections in which the elements in compression are less stocky, but which can develop the full plastic moment capacity. However, local buckling of the section will prevent development of a plastic hinge with sufficient rotation capacity to allow redistribution of moments. 3. Class 3 Semi-compact Sections in which the design strength py can be attained at the extreme fibers but local buckling will prevent the development of the full plastic moment capacity. The moment capacity of Class 3 semi-compact sections will be between the plastic moment and the elastic moment capacity. 4. Class 4 Slender Sections in which elements subject to compression are slender and in which local buckling will prevent the stress in the section from reaching the design strength, based on gross section properties and elastic stress distribution. The moment capacity of a Class 4 slender section is less than the elastic moment capacity of the gross section. Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 17 Structures Loads’ types Main loads that should be considered when designing steel structures are: 1. Dead load: The weight of the structure itself and the weight of all loads permanently on it constitute its dead load. 2. Live load: All loads other than dead load on a structure are called live load. They include all movable weights, such as occupants, machinery and vehicles. Materials in storage, removable partitions and furnishings must also be included. Allowance must also be made for the effects of loads such as rain, snow, ice, wind, water, fluids and soil. 3. Dynamic load: Loads that change magnitude rapidly or are applied suddenly. 4. Temperature shrinkage, settlement, and improper fit: Shrinkage and settlement may be created in structures because of relative movements between various points in the structure. Improper fit happens when a member of improper size is forced into place during construction. Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 18 Structures Design and analysis methods in steel structures The BS 5950 standard describes three basic design methods that are recognized for use with structural steelwork. The methods are simple, continuous, and semi-continuous. In simple design, pin joints analysis is used. In continuous design, elastic, plastic, or elastic- plastic analysis is used. In semi-continuous design, elastic or elastic-plastic analysis is used. Pin joints analysis: the distribution of forces may be determined assuming that members intersecting at a joint are pin connected. Elastic analysis: relevant to the linear elastic region. The stress is proportional to the strain, that is, obeys the general Hooke’s law, and the slope is Young’s modulus. In this region, the material undergoes only elastic deformation with a maximum stress defined as yield strength. Plastic analysis: relevant to the nonlinear zone, which initiates just after the yield strength is reached. In this zone, the material undergoes permanent plastic deformation. Check Fig. 14 for the clarification. Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 19 Structures Design and analysis methods in steel structures Fig. 14 Elastic vs plastic regions Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 20 Structures Steel’s design limit states BS 5950 considers two classes of limit states, which are: The ultimate limit state (ULS): the point beyond which the structure would be unsafe. The serviceability limit state (SLS): the point beyond which the specified service criteria are no longer met. The principal limit states covered in BS 5950 are shown in Table 5. Table 5 Limit states in BS 5950. Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 21 Structures Steel’s design limit states Ultimate limit state (ULS): It is necessary to verify that there is an adequate factor of safety against this limiting condition. For steel design the load factors γf given in BS 5950 (Table 6) are applied to the specified loads. Table 6 Partial factors for loads. Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 22 Structures Steel’s design limit states In buildings not subject to loads from travelling cranes, the following load combinations should be checked: ✓Load combination 1: Dead load and imposed load (gravity loads) plus notional horizontal forces ✓Load combination 2: Dead load and wind load ✓Load combination 3: Dead load, imposed load and wind load. Serviceability limit state (SLS): For the SLS, only deflection will be considered in this course. Deflections are usually calculated under unfactored imposed load only. Table 7 gives limits on deflections that are normally regarded as acceptable. Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 23 Structures Steel’s design limit states Table 7 Suggested deflection limits Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 24 Structures Modes of failure in steel elements Steel elements, such beams and columns may fail because of (see Fig. 15): Bending (https://www.youtube.com/watch?v=6ZiTzN30rww) Local buckling (https://www.youtube.com/watch?v=QimlY0XCa08) Shear (https://www.youtube.com/watch?v=VcKuh42AxDs) Shear buckling (https://www.youtube.com/watch?v=Yl1JnlZfoYc) Web bearing and buckling (https://www.youtube.com/watch?v=cM1mVXSFnq0) Lateral torsional buckling (https://www.youtube.com/watch?v=OoORi_2Vkcg) Deflection (https://www.youtube.com/watch?v=6ZiTzN30rww). Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 25 Structures Modes of failure in steel elements Fig. 15 Examples of steel elements’ modes of failures (left to right): web buckling, local buckling, shear buckling, lateral torsional buckling, shear, and deflection Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 26 Structures Summary of Chapter 1 The slides provides an overview of steel structures, focusing on various aspects essential for understanding and applying design principles. It begins with an introduction to steel structures, highlighting their historical significance and common applications, such as in high-rise buildings and bridges. Key advantages of steel include its high strength, lightweight nature, and ductility, while disadvantages encompass issues like corrosion and high maintenance costs. The mechanical properties of steel are crucial, particularly its stress-strain characteristics, which influence design decisions. The chapter discusses the different types of steel cross-sections and their classifications according to the BS 5950 standard, which outlines design limit states, including ultimate and serviceability limits. Various design methods are introduced, emphasizing the importance of understanding loads, including dead, live, and dynamic loads, as well as temperature effects. The slides emphasize the significance of understanding modes of failure in steel elements, such as buckling and shear, and the need for adherence to deflection limits in design. Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 27 Structures Copyright ©All rights reserved. This presentation and its content are protected by copyright laws. Unauthorized use or reproduction of any part of this presentation without prior written permission to Dr Hussain Al-Lawati ([email protected] and/or [email protected]) is strictly prohibited. Chapter 1 An Introduction to the Design of Steel EGCV3210: Design of Steel Structures 28 Structures