Post-Tensioned Concrete Fundamentals

After the concrete hardened and bonded to the rods

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Unformatted text preview: tresses and then the concrete was placed around the steel rods. After the concrete hardened and bonded to the rods, the ends of the steel rods were released and their tension was thus transferred into the concrete in compression by bond stress between the concrete and the steel rods. This is known as pre-tensioned concrete since the steel is tensioned before the concrete is placed. Refer to the following two figures. www.SunCam.com Copyright 2010 John P. Miller Page 4 of 49 Fundamentals of Post‐Tensioned Concrete Design for Buildings – Part One A SunCam online continuing education course Fixed Buttress Bonded Tendon Tensile Force Pre-Tensioned Beam Before Force Transfer Concrete Compressive Force Pre-Tensioned Beam After Force Transfer Because it is only practical to build buttresses in the shop to resist pre-tensioning forces, most pre-tensioned members are manufactured at a pre-casting facility and shipped to the jobsite. These building components are said to be precast. The fundamentals learned in this course can be applied to the design of precast concrete members. Some common types of pre-cast building elements are shown below. Pre‐stressed Reinforcement Bridge Girder www.SunCam.com Double Tee Copyright 2010 John P. Miller Page 5 of 49 Fundamentals of Post‐Tensioned Concrete Design for Buildings – Part One A SunCam online continuing education course Hollow Core Plank Box Culvert Pre-stressed concrete is the general term used to describe concrete members that have stresses induced in them before the application of any design loads. Pre-stressed concrete includes both pre-tensioned and post-tensioned concrete. Post-tensioned concrete is widely used in bridges, shell structures, water tanks, and folded plates. However, the primary focus of this course covers applications of posttensioning that are typically used in buildings and parking structures, specifically oneway slabs, two way slabs, and continuous beams. There are two general types of pre-stressed reinforcement in use today; bonded and unbonded. Bonded reinforcing typically consists of high-strength steel wires twisted into a 7-wire strand. These so-called mono-strands are commonly used in pre-tensioned concrete members, although there are other types not as common. Bonded monostrands are first tensioned, and when concrete is cast around these mono-strands, there is excellent mechanical bond between the concrete and the steel throughout the strand's entire embedded length. After the concrete has cured a sufficient amount, the strands at both ends of the member are released and the force is transferred to the concrete through bond stress. Pre-tensioned strands generally can only be straight between two points (i.e., not draped), unless they are placed inside special hollow tubes that have been cast into the concrete member. www.SunCam.com Copyright 2010 John P. Miller Page 6 of 49 Fundamentals of Post‐Tensioned Concrete Design for Buildings – Part One A SunCam online continuing education course Unbonded reinforcing usually consists of a 7-wire strand coated with grease and encapsulated in a plastic sheathing over its entire length to prevent bond to the concrete. This is called a tendon. Tendons are draped in a specific profile and secured within the concrete formwork before concrete is placed. After the concrete has been placed and has cured sufficiently, one or both ends of the unbonded tendon are tensioned by using a hydraulic jack to physically stretch the tendon. The stretched tendon is then locked off against the end of the concrete member thereby transferring the tendon force to the concrete through bearing of the cast-in anchorage. Common anchorage hardware is shown below. Unbonded Mono-Strand Anchorage Bonded Multi-Strand Anchorage When very large post-tensioning forces require numerous strands, hollow metal tubes, or ducts, are cast into the concrete member in a specific profile. The duct contains uncoated, unstressed strands and has multi-strand anchorages at both ends. After the concrete has been placed in the formwork and cured sufficiently, the strands are tensioned one by one from one or both ends of the member. Once the appropriate amount of tension has been applied to all strands, they are locked off and the remaining air space in the duct is replaced by grout which is pumped in under high pressure. The grout ensures and effective bond between the strands and the concrete and also provides corrosion protection. Since we will be dealing only with post-tensioned concrete structures in this course, that is, structures that are first cast-in-place and then post-tensioned in place, we will only cover unbonded tendon applications. www.SunCam.com Copyright 2010 John P. Miller Page 7 of 49 Fundamentals of Post‐Tensioned Concrete Design for Buildings – Part One A SunCam online continuing education course The Load Balancing Method The load balancing method is the most widely used technique to design post-tensioned concrete beams and slabs. This method will be used exclusively in this course. In the...
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