تصميم الهياكل الفولاذية

Course Notes: Steel Structures

• Course Title: Steel Structures
• Department: Architecture Engineering
• Instructor: Dr. Hossein Parvizi Sani
• Institution: Zanjan Branch, Islamic Azad University

Chapter 1: Introduction to Steel and Steel Structures

• Advantages of Steel Structures:

  • High strength: Steel's strength-to-weight ratio is better than concrete, crucial for large spans, high-rise buildings, and buildings on unstable ground.
  • Predictable elastic properties: Steel's homogenous nature provides accurate estimations of its mechanical behavior, closely following Hooke's law over a wide range of stresses.
  • Ductility: Metals can sustain concentrated stresses (a cause of failure), dynamic loads, and impacts, while concrete is brittle and performs poorly under these forces.
  • Uniform Properties: Steel is manufactured in factories with strict quality control, so its properties are uniform, unlike concrete. This uniformity allows for smaller safety factors, resulting in cost savings.
  • Durability: Steel provides good durability. With proper maintenance, steel structures can last for many years.
  • Material Continuity: Steel parts are generally continuous and homogeneous based on their materials, while concrete parts degrade gradually in earthquakes, with the concrete cover around rebars deteriorating. The cracks appearing in the concrete cover weaken the section and may lead to building failure after aftershocks.
  • Low weight: The average weight of a steel frame is significantly lower than that of a concrete structure.
  • Space occupation: In comparable buildings of equal height and dimensions, the steel structure occupies a smaller area than a concrete structure; the dead space in concrete buildings is generally larger.
  • Possibility of reinforcement and structural development: Weak parts of a steel structure (due to design errors, changes in regulations, construction, etc.) can be reinforced by adding new parts, which is not easily done in a concrete frame.
  • Easy construction and installation: The fabrication of steel components in factories and their on-site installation are feasible in any weather with appropriate measures. Concrete buildings have more constraints in this regard.
  • Construction speed: The installation speed of steel components is much faster than that of concrete components.
  • Material waste: The amount of material waste is typically lower in steel structures than in the preparation and use of concrete since the steel sections are made in a factory.

• Disadvantages of Steel Structures:

  • Poor heat resistance: Steel's strength decreases with temperature increase. If the temperature of the steel frame reaches 600 degrees Celsius, the building's stability will be threatened.
  • Corrosion: Steel structures are susceptible to corrosion from environmental factors, reducing their useful dimensions and increasing maintenance costs.
  • Tendency of compression members to buckle: Given the numerous steel components, their small dimensions can lead to a high degree of buckling.
  • Poor welding: The use of bolts and the use of factory components is the most economical and technically sound method. However, this is not possible for standard buildings. Using welding to connect parts is a major disadvantage in steel frames due to workers' low skill levels, old equipment, poor oversight by inspectors, high cost of welding tests, and more.
  • Higher cost: Steel structures cost more than concrete structures.

• Types of Steels:

  • Plain Carbon Steels (Fe-C)
  • Alloy Steels (Fe-C + Alloy Elements)
  • Classification of Plain Carbon Steels:
    • Low Carbon Steel (%C < 0.25-0.3)
    • Medium Carbon Steel (0.25-0.3%C < 0.65-0.7)
    • High Carbon Steel (%C > 0.65-0.7)
  • Effect of Various Alloys on Steel Properties:
    • Carbon (C): Crucial element determining hardenability in steel; higher carbon content leads to stronger steel. If carbon level above 0.3% is welded and cooled quickly, a brittle region will form near the weld. Excessive carbon from the welding gases can result in welds that are so hard they easily crack.
    • Manganese (Mn): Improves hardenability and tensile strength. High manganese content (above 0.60%) reduces weldability.
    • Silicon (Si): Improves quality and tensile strength. High silicon content, especially combined with high carbon, can lead to cracks.
    • Sulfur (S): Often added to improve machinability, but its level should be kept low(0.035% max 0.05%) in other types of steel, as high sulfur content increases crack potential.
    • Phosphorus (P): Considered an impurity and kept low. In welding, high phosphorus can lead to brittle welds.
    • Other Elements (Ni, Cr, V,...): Modify weldability. Usually require preheating (preheat) and postheating (posheat) to avoid hard and brittle regions in the welds.

• Stress-Strain Curve of Steel

  • Graphical representation of stress vs. strain for steel. Key points include the elastic region, the yield point, the ultimate tensile strength, and the fracture point.

• Steel Properties: (e.g., Modulus of Elasticity, Poisson's Ratio, Density, Coefficient of Thermal Expansion)

• Types of Steel Sections: (e.g., I-sections IPE, IPB, INP; L-sections (angles); U-sections (channels); T-sections; box sections; pipes/tubes)

• Basic Design Concepts: (e.g., different types of loading, limit state design, safety factors, load combinations)

Chapter 2: Structural Loads

• Dead Loads: Weights of permanent building elements (beams, columns, walls, floors, roofs, stairs, finishes). Tables provide unit weights for materials, calculation examples.

• Live Loads: Non-permanent loads during use (e.g., people, furniture, snow, wind, or earthquake). Tables give live load values for various building uses.

• Roof Loads: Values for different types of roofs.

• Wind Loads: Values based on location and building geometry, tables provide examples.

• Earthquake Loads: Calculation methods, site classification.

  • Basic design acceleration (A): Values from seismic hazard maps.
  • Response factor (B): Building response to ground motion; influenced by building height and type, soil characteristics.
  • Importance factor (I): Classifies buildings into importance categories (high, medium).

Chapter 3: Common Roof Systems

• Concrete Joist-Block Systems:
  • Advantages (e.g., cost, simplicity)
  • Disadvantages (e.g., limitations on span length)
  • Details, specifications (reinforcement details)
• Steel Joist Systems: (e.g., "Krimit" style).
  • Advantages
  • Disadvantages
• Composite Systems:
  • Combination of steel and concrete for beams and slabs
  • Advantages (e.g., reduced frame height, increased span length)
  • Disadvantages (e.g., cost increase)
• Steel Deck Systems: -Use of profiled steel sheets, filled with concrete -Advantages (e.g., reduced weight, speed of construction) -Disadvantages (e.g., need for additional work)
• Cantilevered and Prestressed Systems:
  • Advantages (e.g., reduced height, spans without columns, reduced weight, flexibility in routing services, control over cracks, etc.)
  • Prestressed (e.g., pre-stressing cables. Different methods)
• Hollow Balloon Slabs:
• Roofix Composite Slab Systems:
• Pre-Stressed Hollow Slab Systems:
• Cast-in-Place Slab Systems:

Chapter 4: Seismic Considerations

• Structural System Types:

  • Bearing Wall Systems (loads carried by walls)
  • Frame Systems (loads carried mainly by columns and beams, with shear walls/braces to resist lateral forces)
  • Moment-Resisting Frame Systems (loads carried by frames that also resists lateral forces)
  • Dual/Combination Systems (combining elements from various systems, such as beams and columns, or bracing and walls)
  • Cantilever Column Systems (lateral forces resisted by cantilever-like columns).
  • Other Structural Systems (structural systems other than those listed must be validated by international codes.

• Architectural Considerations:

  • Separation of adjacent structures to prevent damage
  • Symmetry in building plans (avoiding large changes in floor spans to minimize mass changes, etc.).

• General Recommendations for Lateral Load Resisting Systems:

  • Placement of elements around the exterior of buildings to increase lever-arm effect for resisting torsion).
  • Proper connection of all vertical load-carrying elements on the various floors).
  • Ensuring adequate resistance and ductility in all building components.

• Building Irregularities:

  • Torsional Irregularity
  • Plan Irregularity
  • Diaphragm Irregularity
  • Vertical Irregularity (soft story, weak story)

Chapter 5: Connections

• Welding:

  • Definition of welding
  • Types of welding (e.g., manual, automatic)
  • Welding processes (e.g., Gas Metal Welding (MIG/MAG))
  • Types of welds
  • Preparation of edges for groove welds
  • Welding Positions (e.g., flat, vertical, overhead)

• Bolting:

  • Types of connections (friction, bearing)
  • Bolt grades (e.g. high-strength bolts).
  • Strength of fasteners
  • Bolt holes

• Types of Joints (Simple):

  • Beam-to-flange
  • Beam-to-seat
  • Reinforced seat connections

• Types of Resistant Joints:

• Detailing of Beam Splices:

• Detailing of Column Splices:

• Connection Details (Base Plates):

Chapter 6 & 7: Design of Tension and Compression Members

• Tension Members: Calculation of effective cross-sections for tension members (gross & net).
• Compression Members: Buckling in compression members (classification into slender and non-slender). Design criteria. Methods of calculation.

These notes provide a very general overview of the contents covered. Actual details and formulas might need to be consulted from the original notes.