Week 1 - Introduction to Prestressed Concrete Design
- 3. Objectives ◎ Explain what prestressed concrete is and why we prestress concrete. ◎ Differentiate Reinforced Concrete Design versus Prestressed Concrete Design 3
- 5. What is Prestress? Classic Example: A row of books 5
- 7. Introduction ◎ To understand how engineer’s design reinforced concrete structures, we have first to understand the design criteria – the goals of the structure. 7
- 8. Introduction ◎ The obvious goal that we all understand that it shouldn’t fall down. 8
- 9. Introduction ◎ Example: when a car drives on a bridge and the bridge doesn’t collapse, the structure is achieving its design criterion 9
- 11. Introduction ◎ The other important goal is to avoid deflection or movement under the load. 11
- 12. Introduction ◎ Most structural members deflect quite a bit and this can be bad news. 12
- 13. In RCD, deflection has an impact, what is it? 13
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- 15. Introduction ◎ Concrete is strong in compression, but weak in tension. Concrete can withstand a huge amount of compressive stress, but when you try to pull it apart it gives up easily. ◎ Its tensile strength varies from 8 to 14 percent of its compression strength. ◎ Due to such a low tensile capacity, flexural cracks develop at the early stages of loading. 15
- 16. Introduction ◎ The reinforcement within the concrete is usually made of steel, and which is much more elastic than concrete. ◎ Unlike steel, concrete is brittle – it doesn’t stretch, it cracks. ◎ That often means that in a concrete beam, the concrete has to crack first before the rebar can take any of the tensile stress. 16
- 17. How do we prevent such cracks? 17
- 18. Introduction ◎ In order to reduce or prevent such cracks from developing, a concentric or eccentric force is imposed in the longitudinal direction of the structural element. ◎ this force prevents the cracks from developing by eliminating or considerably reducing the tensile stresses at the critical midspan and support sections at service loads, thereby raising the bending, shear, and torsional capabilities of the section. ◎ Such an imposed longitudinal load force is called the prestressing force. 18
- 19. Introduction ◎ Prestressing force is a compressive force that prestresses the sections along the span of the structural element prior to the application of the transverse gravity dead and live loads or transient horizontal live loads. 19
- 20. Introduction ◎ The idea of prestressed concrete has been around since the latter decade of the 19th century, but its use was limited by the quantity of the materials. ◎ It took until the 1920s and 30s for the material development to progress to a level where prestressed concrete could be used with confidence. ◎ Freyssinet in France, Magnel in Belgium, and Hoyer in Germany were the primary developers. 20
- 21. Introduction ◎ Linear prestressing –prestressing force is applied longitudinally along or parallel to axis of the member. 21
- 22. Introduction ◎ The idea of prestressing has also been applied to many other forms, such as: Wagon wheels Riveting Barrels ◎ In these cases, heated materials is made to just fit an object. When the metal cools, it contracts, inducing prestress into the object. 22
- 23. Introduction ◎ Circular Prestressing – used in liquid containment tanks and pipes essentially follow the same basic principles as does linear prestressing. 23
- 24. Introduction ◎ Circular Prestressing – the circumferential hoop or “hugging” stress on the cylindrical or spherical structure, neutralizes the tensile stress at the outer fibers of the curvilinear surface caused by internal pressure. 24
- 26. RCD ◎ Reinforced concrete, is a composite material used in construction. The low tensile strength and ductility of concrete are fortified by the addition of reinforcing steel bars having higher tensile strength and ductility. ◎ During construction, steel bars are placed inside the formwork before concrete is poured. ◎ Concrete is then poured into the formwork and vibrated to remove air voids in the fresh concrete and ensure consolidation of aggregates within the concrete mixture. It is imperative that the concrete completely surrounds each bar to ensure a strong 26
- 27. RCD ◎ RC is widely used due to its workability, strength, and the availability of its raw materials. It is mainly used as main members of a particular structure such as columns, piers, piles, beams, slabs, and footings for buildings, houses, dams, bridges and other similar structures. ◎ Reinforced concrete is easily configured to unconventional shapes because it fills the container that it is supporting it. ◎ Reinforced concrete is also typically used in public works construction of highway paving and sidewalks. 27
- 28. PCD ◎ PC is concrete formed under stress. Reinforcement bars are placed in a form and stressed by stretching the bars at each end, inducing tension in the bar. Concrete is poured into the form and all around the bars while they are still being stretched. ◎ When they are released, the steel tries to return to its original length, and adds a compressive force to concrete laterally, giving it the strength to span longer distances than normal reinforced concrete. 28
- 29. PCD ◎ Prestressing is used to make composite beams and piers in large-scale construction such as highway overpasses and commercial buildings. ◎ It enables a concrete beam to support weight between piers on either side. Without such reinforcing, concrete’s lack of tensile strength would cause it to collapse without support in the middle 29