Week 1: Composite Structures Strength

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Questions and Answers

What is the primary characteristic of tensile strength?

Ability to resist temperature changes

Ability to withstand weight without deforming

Ability to withstand pushing forces without bending

Ability to withstand pulling forces without breaking (correct)

Which material is known for its high compressive strength?

Concrete (correct)

Brick

Steel

Glass

In composite structures, which material is primarily responsible for handling tensile forces?

Steel (correct)

Concrete

Wood

Aluminum

What advantage does composite construction provide in terms of weight?

Reduces weight by 20-40% Signup and view all the answers

What is a benefit of composite construction regarding construction speed?

Each floor can be constructed without waiting for the previous one to harden Signup and view all the answers

Why is concrete often employed in composite structures?

For its ability to resist compressive forces Signup and view all the answers

What role does steel play in a composite structure?

Managing tensile forces Signup and view all the answers

Which of the following is NOT a benefit of using composite construction?

Decreased building costs Signup and view all the answers

What is one reason composite construction has been popular for over a hundred years?

It prevents parts from slipping apart Signup and view all the answers

How does composite construction contribute to labor efficiency?

It lessens the need for specialized labor and allows permanent steel decking Signup and view all the answers

What is the primary purpose of pre-stressing concrete?

To improve its performance under load Signup and view all the answers

What characterizes bonded tendons in pre-stressed concrete?

They are embedded and act as one unit with the concrete Signup and view all the answers

Which of the following is NOT a common type of slab system?

Triangular Slab Signup and view all the answers

What is a key benefit of using unbonded tendons?

They allow for movement within the concrete Signup and view all the answers

Why is it crucial to account for losses in pre-stressing?

To ensure long-term structural stability Signup and view all the answers

What is the typical tensile strength of steel tendons used in pre-stressing?

Up to 1,870 MPa Signup and view all the answers

How do prestressed concrete structures generally perform compared to traditional concrete structures?

They deflect less and are more stable Signup and view all the answers

Which factor is NOT a consideration when choosing materials for pre-stressing?

Material color Signup and view all the answers

What kind of structure allows for easier calculation of forces?

Determinate Structures Signup and view all the answers

In what application is pre-stressing particularly useful?

Preventing cracking in water tanks Signup and view all the answers

What are the functions of concrete and steel in a composite floor system?

Concrete handles squeezing forces, and steel handles pulling forces. Signup and view all the answers

Which of the following is a characteristic of long-span flooring systems?

They must be strong and stiff to prevent bending and vibrations. Signup and view all the answers

What is the primary reason for incorporating pre-stressing into concrete structures?

To prevent cracking and control deflection. Signup and view all the answers

What distinguishes post-tensioned methods from pre-tensioned methods?

In post-tensioned methods, tendons are stretched after the concrete hardens. Signup and view all the answers

Which type of beam is characterized by holes that allow services like ducts and pipes to pass through?

Castellated Beams Signup and view all the answers

What is the purpose of encased steel columns in composite construction?

They provide better fire resistance and structural strength. Signup and view all the answers

What happens to the composite action during construction before the concrete hardens?

Steel supports the wet concrete until hardening occurs. Signup and view all the answers

Which aspect is crucial when calculating natural frequency in composite structures?

The effective mass and dynamic flexural rigidity. Signup and view all the answers

Which type of connection between beams and columns in composite frames is described as flexible during construction?

Semi-rigid connections Signup and view all the answers

Why is it important to address vibration characteristics in composite structures?

To maintain comfortable levels of vibration from activities like walking. Signup and view all the answers

What is the primary reason for adding steel bars to concrete structures?

To resist tensile forces effectively Signup and view all the answers

Which type of rebar is designed to provide better bonding with concrete?

Deformed bars Signup and view all the answers

What is the purpose of rebar cover in concrete reinforcement?

To protect the metal from corrosion Signup and view all the answers

In one-way slabs, how is the reinforcement oriented?

In one direction only Signup and view all the answers

What defines development length in concrete reinforcement?

The length of rebar embedded in concrete to transfer stresses Signup and view all the answers

What is the primary characteristic of two-way slabs?

Supported on four sides with reinforcement in both directions Signup and view all the answers

What is the recommended practice for placing reinforcement bars?

Reinforcement should be placed in layers, starting with bottom bars Signup and view all the answers

Why would a coastal structure require thicker rebar cover?

To resist potential saltwater corrosion Signup and view all the answers

What occurs during a lap splice in reinforcement?

Bars overlap to continue reinforcement beyond their length Signup and view all the answers

What happens to the effective weight distribution in slabs using mesh reinforcement?

It leads to uniform distribution of forces Signup and view all the answers

Study Notes

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.

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Explore the concepts of tensile and compressive strength in composite structures. Understand the benefits of combining materials like steel and concrete to create stronger and lighter buildings. This quiz will cover key principles and applications in composite construction.