HC-Lecture20-Post-tensioning-Design

HC-Lecture20-Post-tensioning-Design - Heavy Construction...

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Unformatted text preview: Heavy Construction Heavy Lecture #20 PostPost-tensioned Concrete: Design Principles courtesy of Carlos Palomino (Suncoast Post-tension) Post- SUNCOAST POST-TENSION • Suncoast was founded in 1983, and today we are the largest supplier of fabricated, un-bonded post-tensioning materials and services in the United States for buildings and foundations. • We provide the specialty fabricated materials, equipment, engineering support, and technical field services necessary to meet any post-tensioned concrete structural job requirements. • Suncoast currently operates 16 branch offices in the United States. SCPT has the presence and capabilities to serve projects in North, Central, and South America. SAMPLES OF POST-TENSIONED CONCRETE CONCRETE STRUCTURES This structure is a cylinder, square in plan. The rear rib is supported by columns, and the front rib is essentially an arch supported at the abutments, and at the crown, laterally, by prestressing cables running across the top of the shell. Pre-stressed arches at the Moscone Center - San Francisco – 1981. Demonstrated effective use of prestressing to meet strict architectural guidelines (spans of 275 ft) THRUST RESISTED BY PRESTRESSED CABLES Arch under gravity loads Thrust forces at base Prestressed cables added at foundation level The Normandy Bridge - 1995. Built on the river Seine, near Le Havre (FR). At the inauguration date, it was the world’s record of cable-stayed bridges with a 856-m span and a total length of 2141-m. The two towers are the world bridge' s highest ones in prestressed concrete. Also prestressed concrete main span deck was utilized. Falconhead Golf Course, Golf Green Cantilevered Slab (Bee Cave, TX). Bonded, encapsulated post-tensioned slab (thickness: 32" at cantilever support. 18" at cantilever tip). The main cantilevered slab spans 20 feet and is completed by a tip-cantilever which adds an additional 8-foot span to the cantilevered slab Pike at Rainbow Harbor Parking Building – Long Beach California shows long cantilevered spiral ramps built using post-tensioned concrete Calgary Olympic Oval-1988. First 400m covered speed skating oval. Roof consists of a series of post-tensioned concrete arches ! " # $ !" !" # PT PROJECTS • Largest developments in the U.S.A. are using posttensioned concrete slabs in commercial buildings. • High Rise Commercial (Condo units) • Office Buildings • Began in 1950 with pre-cast lift-slabs • PT cast in place slabs started in 1960’s $ % & ' ( ) & AXIS CONDO & $ AXIS CONDO Courtesy of Brokers Cervera Real Estate ' *++ *++ ), AXIS CONDO . Courtesy of Florida East Coast Realty Behavior of concrete member Concrete beam or slab Span between supports Loaded member Deflection issue Effect of the loading Original member length Top length shortens ∆top Top face feels compression ∆bot Bottom length increased Bottom face feels tension Response of member Top face feels compression Bottom face feels tension •Concrete weak in tension •Starts cracking soon If member is not reinforced with steel Cracks will continue to open until…… If member is not reinforced with steel Collapse is imminent Now if member has rebar Bottom reinforcement added Bottom face feels tension •Cracks will still develop •Cracks will be smaller and “controlled” •Collapse is prevented NOW IF THE MEMBER HAS POST-TENSIONING Top face feels compression Bottom face feels tension EFFECT OF THE LOADS NOW IF THE MEMBER HAS POST-TENSIONING Deflected shape Possible TENSION at top face Bottom face feels COMPRESSION EFFECT OF THE POST-TENSIONING OPPOSITE EFFECT FROM THE LOADS Cracks, deflections, loadings, behavior is controlled by active reinforcement called post-tensioning. Wload P Wprec T P T Forces in the member after tendons are stressed and anchored a Reduced load Wload P P a Final effective forces in the member after tendons are stressed and anchored ADVANTAGES • In summary, designer of a posttensioned structure has a great flexibility in choosing the amount and location of the prestressing steel to best resist the anticipated loads and deflection. • Active reinforcement POST-TENSIONED CONCRETE Main Advantages: – – – – – – Reduction of slab thickness (material savings). Thinner slabs mean savings in time and labor. Reduction of deflections. Reduction of building height (cladding, mechanical, etc.) Reduction of building weight (foundation savings). Because prestressing steel has higher strength than mild steel, PT slabs are built with approximately 60 to 70% less steel (more space for MEP installations). – Reduction of cracking due to permanent pre-compression, better corrosion resistant – Architectural freedom of larger spans and irregular slab geometries – Reduction in construction time since early stripping of formwork is permitted % % % ( % % % % & ') * , +- ) / . 1 & ' 2) ' “ +& 0 * ') * ” ( , 4 &&5 & 3 4 67 & 8 7 % / 9 ) 0 +" $ ! %: %: 7 & & %, + " .) ; •“1 ” %( “< %5 ' & / * ” 0 - ) ) ' ) &) ) ' - & ' " # %& 7 %' ) % %6 * * %& 7 %' %& 7 ,> & = * $ % . . " & & 4 # ?. ?. $ ?. ?. ??. 6 7 * #. ?. ? #. * ) & & & * > 2 … & & %6 > * * %- ’ %" ’ '$ ( )+ *, 7 +/ ,# /2 @ 0 / 20 4 (6 > * & * < 6 ** 0 . "’ A * * = / ** )2 0 % %4 $ @ B & 1 6 2 1+ .. . ;1 .! + . C1 .. " "- / # M Demand M Resisting Results of structural analysis EQUILIBRIUM EQUATIONS STRAIN COMPATIBILITY MATERIAL STRENGTH vDemand vProvided $ $ ! $ - 4 2 & - * 2 2 @ / 2 2 * & ' , & 2 22 * * - - / 4 2 & 2 ' 0 12 8 General framing layout of pan joist system. - - / & ' 0 12 Mat foundations - - / Slabs on grade & ' 0 12 TYPES OF UNBONDED P.T. SYSTEMS STANDARD SYSTEM: Usually utilized in non-aggressive environments. Tendon is coated with corrosion inhibiting grease within a plastic sheathing by the process of extrusion. ENCAPSULATED SYSTEM: System that provides watertight connection at all stressing and dead ends and has the anchor cavity of the anchorage covered by a watertight cap. Provides better corrosion resistance. Tendon is still coated with corrosion inhibiting grease within a plastic sheathing by the process of extrusion. Note: Even though codes do not strictly mandate for one or another, we recommend encapsulated, especially for parking structures (exposure condition, wide temperature variations) TYPES OF UNBONDED P.T. SYSTEMS What does the code define as aggressive environment? One in which structures are subject to direct or indirect applications of deicing chemicals, seawater, brackish water, or spray from these sources; structures in the immediate vicinity of seacoasts, and structures where anchorage areas are in direct contact with soils. With the exception of those building located near the coast, nearly all enclosed buildings (office, condos, apartments) are considered to be non-aggressive environments. As per PTI recommendations for Construction and Maintenance of Concrete Parking Structures, and also as per ACI-362-Guide for Design of Durable Parking Structures, “Aggressive environments” are defined as areas within 5 miles of the Atlantic Ocean (Coastal Chloride Zone I) and the Encapsulated System is recommended. ...
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This note was uploaded on 09/15/2011 for the course CCE 4001 taught by Professor Staff during the Fall '08 term at FIU.

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