CE 159: Prestressed Concrete Design Introduction

• CE 159 covers Prestressed Concrete Design
• The instructor for this course is Dexter John S. Martinez

Introduction to Prestressed Concrete

• Prestressed Concrete combines concrete's high compressive strength with steel's high tensile strength
• Development of prestressed concrete, general principles, classification, types, stages of loading, comparison, partial prestressing, and design codes will be touched on,

Why Use Prestressed Concrete?

• Prestressing addresses concrete's weakness in tension and strength in compression
• Enhances the bending, shear, and torsional capacity of flexural members
• Prevents or minimizes concrete defects like cracks
• Results in elastic behavior of the concrete section
• Allows maximizing the depth of flexural members under full load

History of Prestressed Concrete

• Prestressed concrete originated in 1888 with P.H. Jackson's patent
• Eugène Freyssinet recognized the need for prestressed concrete as a structural material
• Gustave Magnel wrote the first design book and designed the Walnut Lane Bridge in Philadelphia
• Ulrich Finsterwalder invented free-cantilever construction for prestressed concrete bridges
• Paul Abeles and H. von Emperger studied prestressed concrete and developed "partial prestressing"
• The Roebling family developed stress-relieved wire and strand
• In 1957, T.Y. Lin introduced Load Balancing at the First Prestressed Concrete World Conference

Principle of Prestressing

• Prestressing applies a compressive force to reinforced concrete.
• It reduces tensile stress below the cracking stress, preventing cracking
• Concrete can be treated as an elastic material with internal prestressing and external forces that counteract each other

Classification and Types of Prestressing

• Pre-tensioning: typically used in factories, involves tensioning tendons before concrete is cast
• Post-tensioning: typically a site operation, applying tension after the concrete has hardened

Pre-tensioning Summary

• Anchoring tendons against abutments.
• Placing jacks.
• Applying tension to the tendon.
• Casting concrete.
• Curing of concrete.
• Cutting tendons.

Advantages of Pre-Tensioning

• Suitable for bulk precast members
• Offers long-term durability.
• Reduces the risk of steel corrosion
• Offers high load and shock resistance.
• Effectively uses both concrete and steel strength.

Disadvantages of Pre-Tensioning

• Requires high-quality, strong concrete
• Requires high-tensile steel, significantly more expensive than ordinary steel
• Needs complicated equipment and skilled supervision
• Can be uneconomical for short spans and light loads

Post-Tensioning Explained

• Post-tensioned concrete subjects the mix to compressive stress after pouring and setting

Post-Tensioning Stages Summary:

• Placement of tendons or conduits
• Concrete casting
• Curing of the concrete
• Jack placement
• Applying tension to the tendons
• Cutting of the tendons

Advantages of Post-Tensioning

• Reduces structural dimensions and material usage
• Enables construction of long spans with few columns
• Low maintenance
• Extends the element's functional life
• Allows design flexibility for complex geometries
• Can be adapted to different construction situations

Disadvantages of Post-Tensioning

• Specialized labor and equipment required
• More complex design calculations
• Susceptible to corrosion
• Higher costs due to anchor requirements
• Requires caution during installation

Loading Stages of Prestressed Concrete

• Initial: divided into during tensioning steel and transfer of prestress to concrete
• Intermediate: includes loads during member transportation
• Final: divided into at service and at ultimate load during extreme events

Reinforced vs Prestressed Concrete

Partial Prestressing

• Partial prestressed concrete (PPC) allows some tension and cracking under full-service load
• Should ensure sufficient ultimate strength and is used to control camber and deflection, increase ductility and save costs

Design Codes for Prestressed Concrete

• Uses the NSCP 2015
• Must consider Flexure, Shear and Torsion during design