Definition

It is any concrete structure, whether reinforced or otherwise, deliberately precompressed before service for higher load-carrying strength & less deflections

How?

• Using high-strength steel tendons to apply initial compressive stresses to counteract undesirable tensile stresses in concrete

Methods

• Pretensioning, for fresh concrete
• Posttensioning, for hardened concrete

Using

• Chemical prestressing
• Electro-thermal prestessing
• Mechanical prestressing

Relative advantages

• Inherently superior to reinforced concrete
• Tends to be economical for long spans & heavy loads
• May be designed for crack-free structures for liquid retaining
• Lower weight
• Less deflection
• Deemed to be pretested, thus of better quality assurance

Design standards

• BS 8110 Part 1 Section 4

Serviceability classification

• Class 1: no flexural tensile stresses
• Class 2: flexural tensile stresses but no visible cracking
• Class 3: flexural tensile stresses but surface width of cracks less than 0.1~0.2mm
• Class 4: Reinforced concrete

Topics

• Design principles
• Elastic analysis of sections for flexure
• Preliminary design for flexure
• Prestress losses
• Service load deflections
• Analysis for ultimate strength in bending
• Design for shear
• Design of end blocks
• Composite construction
• Design of continuous beams

How to begin concrete design?

Sequential & iterative decision making process:

• Project Planning
• Stage I: General
• Stage II: Detailed

Project Planning

• Where
• When

• Why, the structure is to be built

Taking proper account of various activities:

• Design
• Construction
• Operations
• Maintenance
• Repair
• Demolition
• Replacement

Stage I: General

General Decisions:

• Type of structure
• Structural layout
• Geometry
• Construction materials

Stage II: Detailed

Detailed Design:

• Estimation of loads
• Analysis of structure
• Proportion of various components
• Adjustments where necessary

Design Objectives

• Adequate strength
• Serviceability under working loads
• Economy

Strength

• Consider ALL possible load combinations, taking the worst only
• Consider ALL possible ways for failure to occur
• Design & proportion accordingly - ductility

Economy

• Design cost
• Material cost
• Construction cost
• Repair
• Maintenance
• Insurance
• Demolition

Methods of Design

• Working Stress Method
• Ultimate Strength Design
• Limit State Design
• Probabilistic Design

Limit State

When a structure is rendered unfit for use for its designated function by one or more causes, it is said to have entered a Limit State.

Ultimate Limit State

• Stability or collapse
• Strength
• Stiffness
• Robustness
• Special hazard

Serviceability Limit State

• Deflection
• Cracking
• Vibration
• Fatigue
• Durability
• Fire resistance

Others

• Lightning
• Special requirements specific to the structure

Purpose of Design

• Ensure the structure being designed will not reach a limit state in its expected working life & conditions
• Some limit states are inapplicable
• Some can be satisfied with proper detailing
• Usual approach: Pick most critical limit state, satisfy it & check the rest