Week 1: Composite Structures Strength

Week 1: Tensile and Compressive Strength in Composite Structures

• Tensile Strength: Maximum pulling force a material can withstand without breaking. Steel has high tensile strength, making it good for cables and beams. In composite structures, steel handles the stretching forces.
• Compressive Strength: Maximum pushing force a material can withstand without crushing. Concrete has high compressive strength, used for foundations and columns. In composite beams, concrete resists the squeezing forces.
• Composite Construction: Combining steel's tensile strength with concrete's compressive strength to make stronger and lighter structures. This method is efficient, resilient, and prevents parts from separating.
• Benefits of Composite Construction: Strength optimization, weight reduction (20-40% lighter), faster construction, labor efficiency, and improved fire resistance (up to 2 hours).
• Composite Floor Systems: Steel beams or joists with a concrete floor slab ("T-beam" shape). This design handles tension and compression, and allows space for utilities.
• Floor Slab Construction Methods: Flat-soffit, precast planks, precast slabs, and metal steel deck methods.
• Composite Structure Construction Sequence: Erect steel, install deck/reinforcement, pour concrete, and achieve composite action (steel & concrete share the load).
• Long Span Flooring Systems: Suitable for large spaces and must be strong to withstand bending, vibrations, and more.
• Beam Types: Castellated (with openings for utilities), fabricated tapered, haunched, parallel beam, composite trusses, and stub girder systems. Each type caters to different span lengths, loads, and utility requirements.
• Composite Columns: Combining steel and concrete to provide strong, quick-to-construct supports. Methods include encased steel columns and concrete-filled steel columns.
• Stress Distribution: Concrete takes compressive stress, steel takes tensile stress. Design considerations include local buckling of steel elements.
• Composite Connections: Between beams and columns. Design flexibility and cost savings by using combined strength properties during construction and after concrete hardens.
• Vibration Characteristics: Longer, lighter structures can vibrate. Engineers use natural frequency calculations to control vibration levels from activities. Two main vibration modes are secondary (joist) and primary (girder). Dunkerly’s Approximation estimates the main frequency.
• Design Considerations: Factors including bending, vibrations, fire safety, comfort, and space for services in composite designs.

Week 2: Pre-Stressing in Concrete

• Pre-Stressing Definition: Applying forces to a structure before it's loaded to increase strength and handle more weight without failure.
• Pre-Stressing Objectives: Prevent cracking, control deflection, use stronger materials.
• Basic Pre-Stressing Concept: High-tensile steel wires (tendons) are stretched and placed in the concrete. This compresses the concrete to prevent cracking. Tension is in the steel, and compression in the concrete.
• Terminology: Strands (bundles of wires), tendons (individual wire or strand bundle), cable (group of tendons).
• Types of Pre-Stressing: Pre-tensioned (steel stretched before concrete pouring) and post-tensioned (steel stretched after concrete hardens). Post-tensioning is more common.
• Types of Tendons: Bonded (glued to concrete) and unbonded (in plastic sheaths).
• Load Balancing: Tendons pull upward to balance the weight of the slab/member, preventing sagging or bending under load.
• Material Properties: Concrete is strong in compression but needs high tensile strength. Steel tendons have extremely high tensile strength capabilities.
• Pre-Stress Losses: Decreases in prestress over time, like friction and concrete shrinkage, which engineers account for.
• Slab Systems: One-way (supported on two sides, reinforcement in one direction) and two-way (supported on four sides, reinforcement in two directions) slabs. Also, flat slabs and flat plates, which have no beams and create flat ceilings.
• Key Considerations: Accounting for concrete shrinkage (long-term effects) and losses to maintain strength over time; using stronger materials.
• Serviceability Improvements: Reduced cracking, less deflection, thinner sections, and allowing for longer spans.

Week 3: Detailing in Reinforced Concrete Members

• Concrete Composition: Sand, cement, aggregate, water, and admixtures (additives).
• Concrete Procurement and Pouring: Performance specifications (strength, shrinkage, and workability) for concrete mix design. Concrete trucks and placement, vibrators to remove air pockets.
• Concrete Finishing and Curing: Different finishes (trowel, broom, burnished) and curing methods to harden concrete over time. Strength testing (7 & 28 days).
• Reinforcement Basics: Concrete is strong in compression but weak in tension. Steel reinforcing bars (rebars) are added to resist tensile forces.
• Types of Reinforcement: Deformed bars (ridged surface for better bond with concrete) and mesh (grid of small bars).
• Reinforcement Placement: Rebar cover (distance between concrete surface and reinforcement), placement order.
• Slab Types: One-way and two-way slabs – one-way supported on two sides, two-way supported on four sides.
• Anchorage and Development Length: Length of rebar embedded into concrete to properly transfer stresses and prevent pulling out.
• Lap Splices: Using overlapped reinforcing bars to create additional length.
• Openings in Slabs: Construction and detail considerations for reinforcement around openings.