1. What are the advantages of steel structures over concrete structures?
Steel structures are generally lighter and stronger than concrete structures. They can be prefabricated and assembled quickly, reducing construction time. Steel has high tensile strength and ductility, which makes it suitable for long spans, high-rise buildings, and earthquake-resistant structures. Unlike concrete, which is brittle, steel can undergo significant deformation before failure, providing warning before collapse. In addition, steel structures can be dismantled and reused, giving them an edge in terms of sustainability.
2. What are the common types of loads considered in the design of steel structures?
The primary loads considered in steel design are dead loads, which include the self-weight of the structure and permanent fixtures; live loads, which are variable loads such as people, furniture, and equipment; wind loads, which exert lateral pressure on frames and cladding; earthquake loads, which produce dynamic effects due to ground motion; and snow loads, which apply vertical pressure on roofs. Additionally, temperature effects and impact loads may also be considered in special cases.
3. What are limit states in steel design?
Limit states are conditions beyond which a structure no longer satisfies its intended function. In steel design, we consider two categories: the ultimate limit state, which ensures safety against collapse or failure, and the serviceability limit state, which ensures that deflections, vibrations, and local damage do not impair usability or comfort. The design philosophy is to ensure that structures remain safe under ultimate conditions and functional under service conditions.
4. What is the difference between working stress method and limit state method?
In the working stress method, structures are designed so that stresses under service loads remain within permissible values. It relies on a global factor of safety applied to material strength. In contrast, the limit state method applies partial safety factors to both loads and material properties. It is more rational because it considers uncertainties in loads, resistance, and structural behavior. Modern steel codes like IS 800:2007 and Eurocode primarily adopt the limit state method.
5. Explain the concept of slenderness ratio in steel design.
The slenderness ratio is the ratio of the effective length of a member to its radius of gyration. It is a measure of the member’s susceptibility to buckling. A higher slenderness ratio indicates a more slender member, which is more prone to instability under axial compression. Codes prescribe maximum slenderness ratios for different members, such as struts, compression members in trusses, and columns, to ensure safety against buckling.
6. What is buckling in steel columns?
Buckling is a sudden lateral deflection of a compression member due to instability when the critical load is reached. Unlike crushing, which is a material failure, buckling is a structural instability phenomenon. The critical load depends on the slenderness ratio, end conditions, and elastic modulus of the steel. Euler’s formula is used for long, slender columns, while empirical curves are used for intermediate and short columns.
7. What are the different types of structural steel sections used in design?
Structural steel sections come in different shapes to optimize strength and economy. Common rolled sections include I-sections, channels, angles, tees, and hollow circular or rectangular sections. Plates are used in built-up sections, while box sections are preferred in cases where torsional rigidity is required. Each section type has specific applications, such as I-beams for bending members, angles for trusses, and hollow sections for lightweight but strong frames.
8. What is lateral-torsional buckling in beams?
Lateral-torsional buckling occurs when a beam subjected to bending about its major axis tends to deflect laterally and twist simultaneously. This instability is common in slender, unbraced beams with compression flanges not restrained against lateral movement. The buckling strength depends on section properties, effective length, and bracing conditions. Providing lateral bracing or choosing compact sections helps prevent this mode of failure.
9. What are bolted connections, and why are they used?
Bolted connections are joints where steel members are fastened together using high-strength bolts. They are widely used due to ease of installation, disassembly, and inspection. Bolted connections may be designed as bearing type, where load transfer occurs through bolt shank bearing against the hole, or friction type (slip-critical), where load transfer occurs through friction between contact surfaces. Modern design often prefers high-strength friction grip bolts for better performance under dynamic loading.
10. What is the difference between riveted, bolted, and welded connections?
Riveted connections are traditional but largely obsolete due to labor-intensive installation. Bolted connections are faster, easier to replace, and widely used today. Welded connections provide rigid, continuous joints with good strength and stiffness, making them suitable for permanent structures. However, welding requires skilled labor, controlled conditions, and careful inspection. Bolted joints are more common in field assembly, while welding is often used in fabrication shops.
11. What are plastic and elastic design methods in steel structures?
In elastic design, members are designed such that stresses remain within elastic limits under applied loads. Plastic design, on the other hand, utilizes the ability of steel to undergo plastic deformation beyond yield, allowing redistribution of moments. Plastic design is economical for continuous beams and frames, as it provides higher load-carrying capacity. However, it requires ductile detailing and careful consideration of stability and serviceability.
12. How is the effective length of a steel column determined?
The effective length of a column depends on its end conditions, which influence its buckling behavior. For example, a column with both ends pinned has an effective length equal to its actual length, while a column with both ends fixed has an effective length of half its length. Codes provide effective length factors for various end conditions, which are multiplied by the actual length to obtain the effective length used in buckling calculations.
13. What is a plate girder, and where is it used?
A plate girder is a built-up beam consisting of a web plate and flange plates, connected usually by welding or bolting. It is used where rolled sections are insufficient to carry large spans or heavy loads, such as in bridges, industrial buildings, and cranes. The web resists shear, while the flanges resist bending moments. Stiffeners are often provided to prevent buckling of the web under shear or compression.
14. Explain shear lag in steel design.
Shear lag is a phenomenon where not all parts of a cross-section are equally effective in resisting tensile force, especially in wide-flange or angle members connected only along one leg. This leads to a non-uniform stress distribution, reducing the effective area available for tension. Codes provide reduction factors to account for shear lag effects, ensuring safe design of tension members.
15. What are the different types of loads on a steel roof truss?
A steel roof truss is subjected to dead load, which includes the weight of the truss itself, roofing sheets, and purlins; live load, which is due to maintenance or access; wind load, which applies uplift or lateral forces; and sometimes snow load, depending on the geographical location. Proper load combinations are considered as per design codes to ensure stability and safety.
16. What is a moment connection in steel structures?
A moment connection, also known as a rigid connection, is a joint that resists rotation and transfers bending moments in addition to shear forces. Such connections are typically made using full penetration welds or flange plates bolted to column flanges. They are essential in rigid frames where continuity of bending resistance is required, such as in portal frames and high-rise buildings.
17. Explain the concept of torsional rigidity in hollow steel sections.
Hollow steel sections, such as circular or rectangular tubes, provide higher torsional rigidity compared to open sections like channels or I-sections. This is because the closed geometry offers resistance against twisting, distributing shear flow more effectively around the perimeter. For this reason, hollow sections are used in applications involving torsion, such as transmission towers, poles, and space frames.
18. What is the difference between compact, non-compact, and slender sections?
Sections are classified based on their local buckling behavior. Compact sections can reach their full plastic moment capacity without local buckling. Non-compact sections can attain yield stress in extreme fibers but may buckle locally before developing plastic capacity. Slender sections buckle locally before reaching yield stress, leading to reduced strength. These classifications help in determining the strength and ductility of members.
19. What are load combinations in steel design?
Load combinations account for the fact that different loads may act simultaneously. For example, dead load always acts, while live load, wind, and seismic loads may act together or separately. Codes such as IS 800 or AISC provide load combination factors like 1.5(DL + LL) or 1.2(DL + LL + WL). These ensure that structures are designed for realistic worst-case scenarios without being overly conservative.
20. Why is corrosion protection important in steel structures?
Steel is prone to corrosion when exposed to moisture, chemicals, or industrial pollutants. Corrosion reduces cross-sectional area and weakens the member, leading to premature failure. Protection methods include painting, galvanizing, and use of weathering steel. In addition, proper detailing for drainage and avoiding water traps are essential. Maintenance and periodic inspections ensure long-term durability of steel structures.
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