1. Characteristics of Steel Structures
1. Steel structures have a relatively light self-weight.
2. The reliability of steel structure work is comparatively high.
3. Steel exhibits good resistance to vibration and impact.
4. The manufacturing of steel structures benefits from a high level of industrialization.
5. Steel structures can be assembled quickly and with high precision.
6. They are easy to fabricate into sealed structures.
7. Steel structures are susceptible to corrosion.
8. Steel has poor fire resistance.
2. Grades and Properties of Commonly Used Steel for Steel Structures
1. Carbon structural steels: Q195, Q215, Q235, among others.
2. Low alloy high-strength structural steels.
3. High-quality carbon structural steels and alloy structural steels.
4. Special purpose steels.
3. Principles for Material Selection of Steel Structures
Material selection for steel structures aims to ensure sufficient load-bearing capacity and to prevent brittle failure under specified conditions. This selection is based on a comprehensive evaluation of factors such as the importance of the structure, load characteristics, structural form, stress state, connection method, steel thickness, and working environment.
The four types of steel materials recommended in the “Code for Design of Steel Structures” GB50017-2003 are considered optimal and preferred when conditions allow. Other steel types are permitted provided they meet the code’s requirements.
4. Key Steel Structure Technologies
High-rise Steel Structure Technology
Depending on height and design requirements, frame, frame-support, cylindrical, and mega-frame structures are used. Components may consist of steel, reinforced concrete, or steel tube concrete. Steel components are lightweight and ductile, fabricated from welded or rolled steel, making them suitable for ultra-high-rise buildings. Reinforced concrete components offer high stiffness and fire resistance, ideal for medium to high-rise buildings or base structures. Steel-reinforced concrete is easy to construct but mainly used for columns.
Space Steel Structure Technology
Spatial steel structures are lightweight, highly rigid, visually appealing, and allow fast construction. Popular types in China include spherical node flat plate trusses, multi-layer variable cross-section trusses, and grid shells with steel pipe members. These structures provide high spatial stiffness with low steel consumption and benefit from comprehensive CAD design, construction, and inspection processes. Besides grid structures, large-span suspension and cable membrane structures are also employed.
Light Steel Structure Technology
This technology uses lightweight colored steel plates for wall and roof enclosures. The system comprises large-section thin-walled H-shaped steel wall beams and roof purlins welded or rolled from steel plates 5mm or thicker, flexible round steel supports, and high-strength bolt connections. Column spacing ranges from 6 to 9 meters, spans reach over 30 meters, and heights can exceed ten meters, allowing installation of light cranes. Steel usage averages 20-30 kg/m². Standardized design processes and specialized manufacturers ensure high product quality, fast installation, light weight, low investment, and year-round construction, making it ideal for light industrial plants.
Steel-Concrete Composite Structure Technology
Steel-concrete composite structures, combining steel beams and columns with concrete components, have seen increasing application. They merge the advantages of steel and concrete, offering high overall strength, rigidity, and excellent seismic performance. Using outsourced concrete enhances fire and corrosion resistance. Composite components typically reduce steel usage by 15-20%. Composite floors and steel-reinforced concrete elements require minimal or no formwork support, enabling fast and convenient construction. These structures suit multi-story or high-rise buildings with heavy loads and industrial building columns and floors.
High-Strength Bolt Connection and Welding Technology
High-strength bolts transmit stress via friction and consist of bolts, nuts, and washers. Their advantages include simple construction, easy disassembly, high bearing capacity, excellent fatigue resistance, self-locking capabilities, and enhanced safety. They have largely replaced riveting and partial welding in steel structure production and installation. Thick steel plates are welded in workshops using automatic multi-wire arc submerged welding, while box column partitions employ methods like melting nozzle electroslag welding. On-site, semi-automatic welding, gas-shielded flux-cored wire, and self-protection flux-cored wire technologies are employed.
Steel Structure Protection Technology
Protection of steel structures involves fireproofing, corrosion resistance, and rust prevention. Generally, anti-corrosion treatment is unnecessary after fireproof coating, except in environments with corrosive gases. In China, common fireproof coatings include TN series and MC-10, comprising alkyd enamel, chlorinated rubber paint, fluororubber, and chlorosulfonated coatings. Appropriate coatings and thicknesses must be selected based on steel type, fire resistance requirements, and environmental conditions during construction.
5. Goals and Measures for Steel Structures
Steel structure engineering covers a broad technical scope and presents challenges. Its promotion and application must adhere to national and industry standards. Construction authorities should emphasize professional development in steel structure engineering, organize training for quality inspection teams, and document best practices and new technologies. Educational institutions, design departments, and construction companies should accelerate training of steel structure technicians and advance mature CAD technologies. Academic organizations should support steel structure technology development through domestic and international exchanges and training, aiming to enhance overall design, production, construction, and installation capabilities and achieve recognition in the near future.
6. Connection Methods for Steel Structures
Steel structures are connected mainly by three methods: welding, bolting, and riveting.
Weld Connection
This connection is formed by locally melting the welding rod and workpieces using arc heat, then cooling to form a solid weld seam, joining components into a unified whole.
Advantages: Welding does not weaken component cross-sections, saves steel, features a simple structure, is easy to manufacture, offers high stiffness and good sealing, supports automation under certain conditions, and achieves high production efficiency.
Disadvantages: The heat-affected zone near the weld may cause brittleness; uneven heating and cooling can introduce residual stresses and deformation impacting load capacity, stiffness, and performance; high stiffness can cause local cracks to propagate, especially at low temperatures where brittle fracture is likely; welded joints have lower plasticity and toughness, and weld defects can reduce fatigue strength.
Bolted Connection
Bolt connections use bolts as fasteners to join components into a single unit. They are categorized into ordinary and high-strength bolt connections.
Advantages: Simple construction and installation, particularly suitable for on-site assembly; easily disassembled, ideal for temporary or modular structures.
Disadvantages: Requires precise drilling and alignment of holes, increasing manufacturing effort; bolt holes weaken cross-sections; often requires overlapping parts or auxiliary connection plates, complicating construction and increasing steel usage.
Rivet Connection
Riveting involves heating a rivet, inserting it into connecting holes, and forming a head on the opposite end using a rivet gun to achieve a tight joint.
Advantages: Reliable force transmission, good plasticity and toughness, easy quality control, suitable for heavy, load-bearing structures.
Disadvantages: Complex process, labor- and material-intensive, resulting in high labor intensity; largely replaced by welding and high-strength bolt connections.
7. Welding Connection
Welding Methods
Arc welding is the most common method for steel structure welding, including manual, automatic or semi-automatic arc welding, and gas-shielded welding.
Manual arc welding is widely used due to simple equipment and flexible operation, but has lower efficiency and quality variability depending on welder skill.
Automatic welding offers stable quality with fewer defects, good plasticity, and impact toughness, suited for long, straight welds. Semi-automatic welding allows flexibility for curved or irregular welds. Both require welding wire and flux compatible with base metal, complying with standards.
Gas-shielded welding uses inert or CO2 gas to protect the arc, isolating molten metal from air, resulting in concentrated heat, fast welding, deep penetration, higher weld strength, good plasticity, and corrosion resistance, ideal for thick steel plates.
Weld Types
Weld seams are classified by the relative position of connected components: butt joint, lap joint, T-joint, and corner joint. Two basic weld types are butt welds and fillet welds. Selection depends on stress conditions, manufacturing, installation, and welding factors.
Weld Seam Construction
1. Butt Weld: Provides direct, smooth force transmission without significant stress concentrations, suitable for static and dynamic loads. Requires high quality and precise gaps; typically used in factory settings.
2. Fillet Weld: Classified by direction relative to force (side fillet parallel, front fillet perpendicular, oblique fillet intersecting, and circumferential). Cross-section shapes include ordinary (1:1 ratio resembling an isosceles right triangle), flat slope (1:1.5 ratio for smooth stress transfer), and deep penetration (1:1 ratio). For dynamic loads, flat slope and deep penetration types are preferred to reduce stress concentration.
8. Bolt Connection
Construction of Ordinary Bolted Connections
1. Forms and Specifications
Common bolts have large hexagonal heads, designated by an “M” followed by nominal diameter in millimeters (e.g., M18, M20, M22, M24). Bolts are classified by performance grades such as 4.6 or 8.8, where the first number indicates minimum tensile strength in hundreds of MPa (e.g., 4 = 400 MPa), and the second number represents the yield strength ratio.
Bolt machining accuracy divides them into three levels: A, B, and C.
B-grade (refined) bolts made from 8.8 grade steel are precisely machined with smooth surfaces and Class I holes (accurate drilled or expanded holes). They offer close contact, minimal deformation, and good stress performance, suitable for high shear and tensile force connections but are labor-intensive and costly.
C-grade (rough) bolts made from 4.6 or 4.8 steel grades have rougher machining with Class II holes (punched or drilled with larger hole diameters). They permit larger deformations under shear but maintain good tensile performance and low cost, typically used in static or indirectly dynamic load connections.
2. Bolt Arrangement
Bolt patterns should be simple, uniform, compact, and meet stress requirements for easy installation. Arrangements can be parallel or staggered, with staggered offering a more compact layout.
Force Characteristics of Ordinary Bolt Connections
1. Shear bolt connection
2. Tensile bolt connection
3. Combined pull-shear bolt connection
Stress Characteristics of High-Strength Bolts
High-strength bolt connections are divided into friction-type and bearing-type based on design and stress:
- Friction-type: Shear loads are resisted by friction between plates up to a limit state. When relative slip occurs, the connection fails.
- Bearing-type: After overcoming friction, plates slide relative to each other, allowing external forces to increase until ultimate failure of bolt shear or hole wall bearing occurs.
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