What Is Prestressed Concrete Design
Janita Locklin
This document provides an introduction to prestressed concrete, including:1. The basic principles of prestressing concrete by applying compressive stresses that counteract tensile stresses from loads. This allows for smaller member sizes. 2. The main advantages are smaller sections, reduced deflections, increased spans, and improved durability due to reduced cracking. 3. The two main methods are pre-tensioning, where strands are stressed before casting, and post-tensioning, where strands are tensioned after casting through ducts. 4. Uses include precast beams, slabs, piles, tanks, and bridges constructed with either precast or post-tensioned segments.Read less
So, what exactly is prestressed concrete?? To explain that, we first need to talk about conventional reinforced concrete as a comparison. Normally, all the stresses of the weight on a reinforced concrete structure are held by the steel reinforcements. What prestressed concrete designs do is induce stresses throughout the entire structure. The end result is a product that better handles vibrations and shocks than conventional concrete. In addition, this makes it possible to form longer, thinner structures that can still handle those heavier loads.
When using pre-tensioning, steel is stretched out before the placing of the concrete. This entails steel tendons set between two abutments, then getting stretched to roughly 80% of their strength. The concrete then gets poured into molds around them and cured. When the concrete is cured and at the right strength, the steel is released. The steel will try to get back to its original length, creating the tensile stress that becomes a compressive strength in the concrete.
With post-tensioning, there is also generally an additional step that needs to be done, generally related to the ducts. This is either bonded construction or unbonded construction. Bonded construction involves filling the space between the tendon and duct with cement grout. This is generally done to help the steel minimize corrosion, but also generally increases the overall strength of the structure as well. This particular grout is made with cement, water, and sometimes admixture, with no sand.
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This course explains the basic concepts and methods of prestressed concrete design. Attendees will work through the design of a simple prestressed concrete rectangular beam of a building. Both straight strand and harped-strand design will be covered in the example, exposing participants to realistic design conditions. The course is based on ACI 318-14, ASCE-7 (2010), and IBC (2015).
Prerequisite: CE 638. Prestressed concrete design and analysis for gravity and lateral loading. Design of reinforced and prestressed structural elements. Safety and economy. Connection design for earthquake and wind loadings. Design projects using professional practice standards, including latest codes. 3 hours of lecture. (Design units: 3.)
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Prestressed concrete is a structural material that uses steel to create predetermined engineering stresses within concrete members to counteract the stresses that will occur when they are subject to loading. This combines the high strength compressive properties of concrete with the high tensile strength of steel.
In ordinary reinforced concrete, stresses are carried by the steel reinforcement, whereas prestressed concrete supports the load by induced stresses throughout the entire structural element. This makes it more resistant to shock and vibration than ordinary concrete, and able to form long, thin structures with much smaller sectional areas.
Prestressed concrete was patented by San Franciscan engineer P.H Jackson in 1886, although it did not emerge as an accepted building material until 50 years later when a shortage of steel, coupled with technological advancements, made prestressed concrete the building material of choice during European post-war reconstruction.
Prestressed concrete is commonly used for floor beams, piles and railways sleepers, as well as structures such as bridges, water tanks, roofs and runways. Generally, prestressed concrete is not necessary for columns and walls, however, it can be used economically for tall columns and high retaining walls with high bending stresses.
As a general rule, traditional reinforced concrete is the most economic method for a span of up to 6 m. Prestressed concrete is more economical when spans are over 9 m. Between 6 and 9 m, the two options must be considered according to the particular requirements as to which is the most suitable option.
Wire is made by cold-drawing a high carbon steel rod through a series of reducing dies. The wire diameter typically ranges from 3-7 mm and may be round, crimped or indented to give it better bond strength. Another form of tendon is strand which consists of a straight core wire around which is wound in helixes around further wires to give formats such as 7 wire (6 over 1) and 19 wire (9 over 9 over 1). Similar to wire tendons, strand can be used individually or in groups to form cables.
Pre-tensioning prestressed concrete involves the stressing of wires or cables by anchoring them at the end of a metal form, which may be up to 120 m in length. Hydraulic jacks stress the wire as required, often adding 10% to accommodate creep and other pre-stress losses that may be incurred. Side moulds are then fixed and the concrete placed around the tensioned wires. The concrete hardens and shrinks, gripping the steel along its length, transferring the tension from the jacks to exert a compressive force in the concrete.
Once the concrete has reached the desired strength, the tensioned wires are released from the jacks. A typical concrete strength of 28 N/mm2 can be achieved by 24-hour steam curing, as well as using additives.
Post-tensioning prestressed concrete follows the reverse method to pre-tensioning, that is, the concrete member is cast and the prestressing occurs after the concrete is hardened. This method is often used where stressing is to be carried out on site after casting an insitu component or where a series of precast concrete units are to be joined together to form the required member.
The wires, cables or bars may be positioned in the unit before concreting, but bonding to the concrete is prevented by using a flexible duct or rubber sheath which is deflated and removed when the concrete has hardened.
Stressing is carried out after the concrete has been cured by means of hydraulic jacks operating from one or both ends of the member. Due to the high local stresses at the anchorage positions it is common for a helical (spiral) reinforcement to be included in the design. When the required stress has been reached, the wire or cables are anchored to maintain the prestress. The ends of the unit are sealed with cement mortar to prevent corrosion due to any entrapped moisture and to assist in stress distribution.
Anchorages used in post-tensioning depend on whether the tendons are to be stressed individually or as a group. Most systems use a form of split cone wedges or jaws which act against a form of bearing or pressure plate.
There are many different post-tensioning systems. For example, the Freyssinet system enables the stressing strands to be tensioned simultaneously using centre hole tensioning jacks, anchored by tapered jaws. This is suitable for pre-stressing elements up to 50 m in length.
The Macalloy system on the other hand, involves applying stress to the concrete by means of a solid bar, usually with a diameter of 25-75 mm. The bar is anchored at each end by a special nut which bears against an end plate to distribute the load.
Prestressed concrete is used in a wide range of building and civil structures where its improved performance can allow for longer spans, reduced structural thicknesses, and material savings compared with simple reinforced concrete. Typical applications include high-rise buildings, residential concrete slabs, foundation systems, bridge and dam structures, silos and tanks, industrial pavements and nuclear containment structures.[6]
First used in the late nineteenth century,[1] prestressed concrete has developed beyond pre-tensioning to include post-tensioning, which occurs after the concrete is cast. Tensioning systems may be classed as either monostrand, where each tendon's strand or wire is stressed individually, or multi-strand, where all strands or wires in a tendon are stressed simultaneously.[5] Tendons may be located either within the concrete volume (internal prestressing) or wholly outside of it (external prestressing). While pre-tensioned concrete uses tendons directly bonded to the concrete, post-tensioned concrete can use either bonded or unbonded tendons.
Pre-tensioning is a common prefabrication technique, where the resulting concrete element is manufactured off-site from the final structure location and transported to site once cured. It requires strong, stable end-anchorage points between which the tendons are stretched. These anchorages form the ends of a "casting bed" which may be many times the length of the concrete element being fabricated. This allows multiple elements to be constructed end-to-end in the one pre-tensioning operation, allowing significant productivity benefits and economies of scale to be realized.[5][7]
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