Prestressed Concrete Quiz

Construction Systems

• Alternative building construction systems are covered in this presentation.

Prestressed Concrete

• Definition (1): Prestress is a method of applying pre-compression to control stresses resulting from external loads below the neutral axis. This method aims to exceed the permissible limits of plain concrete, overcoming its weakness in tension.
• Definition (2): Prestressed concrete builds in compressive stresses during construction to counter the tensile stresses during use. A combination of steel and concrete leverages the strength of each material.
• Definition (3): Applying forces to bend and compress a concrete element to counteract bending from structural load. The force involves tensioning the steel component (high tensile strands, wires, or bars).
• Principles: Pre-stressing uses tendons (high tensile steel cable or rods) to provide clamping load and compressive stress, balancing the tensile stress caused by bending loads. This reduces tensile stress to below the cracking stress, preventing cracks. Concrete can then be treated as an elastic material. Concrete has two compressive forces: internal pre-stressing force and external force.
• Materials:
  • Cement: Ordinary Portland cement, Portland slag cement, rapid hardening Portland cement, and high-strength ordinary Portland cement.
  • Concrete: High-strength concrete with high compressive and comparatively high tensile strength than ordinary concrete. Composed of gravel, crushed stones, sand, and cement. Minimum grade of M20 is used.
  • Steel: High tensile steel, tendons and strands. Tensile strength of around 2000MPa is required, adhering to IS: 1343-1980 prestressed concrete design.
• Forms of Pre-stressing Steel: Includes wire, strands (two, three or seven wires wound together), tendons (group of strands or wires wound together), and bars (single steel bar, larger than a wire).
• Pre-stressing Methods: Mechanical, hydraulic, electrical, and chemical devices are used.
• Mechanical Devices: Weights (with or without lever transmission), geared transmission, pulley blocks, screw jacks, and wire-winding machines. Employed for mass-produced components.
• Hydraulic Devices: Simplest means of producing large prestressing force, used extensively as tensioning devices.
• Electrical Devices: Involves electrically heating wires before placement. Often referred to as thermo-prestressing.
• Chemical Devices: Use expanding cements, controlling the degree of expansion by varying curing conditions.
• Pre-tensioning: Tendons are tensioned before concrete placement; concrete is introduced by bond between steel and concrete.
• Post-tensioning: Tendons are tensioned against hardened concrete; a duct is positioned in the concrete structure.

Reinforced Concrete

• Overview: Concrete is strong in compression but weak in tension. Steel is strong in tension. Reinforced concrete uses concrete to resist compression and steel to resist tension. Tensile strength of concrete is neglected. Reinforced concrete beams allow cracking under service load. In ordinary reinforced concrete, the beam supports a load by developing compressive stresses at the top, but since the concrete cannot resist tension at the bottom, it cracks. Reinforcing steel bars are placed within this tension zone to resist tension and control cracking.

Composite Construction

• Definition (1): Two or more different materials bonded securely to act as one structural unit. This interaction is known as composite action.
• Benefits: Speed of construction, performance, value.
• Materials: Reinforced concrete, masonry, composite wood (plywood, laminated veneer lumber, LVL, Parallel strand lumber (PSL), laminated strand lumber (LSL), oriented strand board (OSB), particleboard, and fiberboard), reinforced plastics (FRP/fiber-reinforced polymer), ceramic matrix composites, metal matrix composites, hybrid composites.
• Composite Beams: Combination of concrete and steel, similar to a T-beam. Shear connectors transfer forces between the materials.
• Composite Columns: Combination of structural steel and concrete.
• Composite Slabs: Reinforced concrete cast on top of profiled steel decking, which serves as formwork during construction and external reinforcement once complete.
• Composite Connections: Includes shear connectors (essential for composite construction) to provide longitudinal shear resistance. Examples include shear studs, shear bolt/pin, oscillating perfobond strip, continuous perfobond strip, welded T-section/T-rib connectors, and waveform strips.

Cable Structure

• Definition (1): A flexible structural component offering zero resistance to shear and bending, subjected to tensile forces.
• Structural Cables: Used in engineering structures as support and to transmit load between points, forming the main load-carrying element.
• Loading Mechanism: High tensile strength of steel combined with the efficiency of simple tension makes steel cable ideal for spanning large distances.
• Cable Sag: The triangular shape of a cable under load is characterized by sag (vertical distance between supports and lowest point). Without sag, horizontal forces cannot balance vertical load. Sagging cable's pull on supports is divided into a downward force equal to half the load and a horizontal inward pull/thrust. Cable deformation under load takes the funicular shape.
• Geometric Funicular Forms: As loads increase, funicular polygon approaches a geometrical curve—the parabola. Evenly distributed loads create a catenary shape.
• Classification of Cable Structures: Suspension, cable-stayed, radial, harp, fan, star-shaped, and classification by pylon shape (A, H, Y).
• Suspension-type Cable: Composed of stiffening girders/trusses, main suspension cables, main towers, and anchorages. The main load-carrying member is the main cable made of high-strength steel. It can only carry straight bridges across a water body, such as the Akashi Kaikyō Bridge. The longest suspension bridge is the 1915 Çanakkale Bridge which is 2023 meters in length.
• Cable-stayed Type: Supported by inclined stayed-cables from towers. It has fewer components than suspension bridges and can handle curved bridges.
• Cable Materials: Constructed from structural ropes, strands (assemblies of wires around a central wire in layered forms), or parallel wire strands.
• Types of Cable Materials: Parallel bars, Parallel wires, Parallel strands, Helical/Locked coil strands, Ropes, Standards, Strand Compacted, and Swage Compacted.
• Cable Construction (Cable-stayed type) stages: Erection of piers and support spans, central tower erection, temporary stay cable installation, extension of central span and temporary cable removal, and completion of central span and removal of temporary cables.

Membrane Structure

• Definition (1): A flexible surface (skin, fabric) that transmits only tensile stress, relying on double curvature for stability under load.
• History: Developed from tent-making craftsmanship, using modern materials and techniques. Pioneered by Fred Severud, Frei Otto, and Walter Bird.
• Design: Employing computers for design and fabrication, employing skilled and experienced fabrication teams.
• Structural Materials: Coated or woven synthetic fabrics, such as PTFE fiberglass, ETFE film, PVC, and ePTFE.
• Properties of Membrane Materials: High inherent strength, resistance to environmental pollution, service life, fire safety, thermal properties, acoustics, lighting, color fastness, and special shading materials.
• Styles and Shapes: Conical, hypar (anticlastic), parallel arch (barrel vault), cable net structures. Examples: White Rhino II, National Aquatics Center of China, Shanghai Expo axis, Tokyo Dome, Millennium Dome.
• Design Considerations: Shape determination, structural analysis, cutting patterns, and cost considerations.
• Environmental Issues: Low environmental footprint, PVC is intrinsically recyclable, energy-efficient, and low consumer of non-renewable resources.
• Special Construction Features: How membranes can span large distances, keep tension maintained, how roofs are designed to resist wind loads, avoid flat planar surfaces, and use pre-tensioned cables.