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Heavy ConstructionHeavy Construction
Lecture #20Lecture #20
PostPost--tensioned Concrete:tensioned Concrete:
Design PrinciplesDesign PrinciplesDesign PrinciplesDesign Principlesccourtesy ofourtesy of
Carlos Palomino (Suncoast PostCarlos Palomino (Suncoast Post--tension)tension)
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 STRUCTURESCONCRETE 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 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
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PT PROJECTS
• Largest developments in the U.S.A. are using post-tensioned concrete slabs in commercial buildings.
• High Rise Commercial • High Rise Commercial (Condo units)
• Office Buildings• Began in 1950 with pre-cast
lift-slabs• PT cast in place slabs
started in 1960’s
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AXIS CONDO
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AXIS CONDO
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Behavior of concrete member
Concrete beam or slab
Span between supports
Loaded member
Deflection issue
Effect of the loading
Top face feels compressionTop length shortens
∆∆∆∆top
∆∆∆∆bot
Original member length
Bottom face feels tension
Bottom length increased
∆∆∆∆bot
Response of member
Top face feels compression
Bottom face feels tension•Concrete weak in tension•Starts cracking soon
If member is notreinforced with steel
Cracks will continue to openuntil……
If member is notreinforced 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
Possible TENSION at top faceDeflected shape
EFFECT OF THE POST-TENSIONING
OPPOSITE EFFECT FROM THE LOADS
Bottom face feels COMPRESSION
Cracks, deflections, loadings, behavior is controlledby active reinforcement called post-tensioning.
Wpreca
P P
Wload
T T
Forces in the member after tendons are stressed and anchored
T
aP P
Wload
Reduced load
Final effective forces in the member after tendons are stressed and anchored
ADVANTAGES
• In summary, designer of a post-tensioned structure has a great flexibility in choosing the amount and location of 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).– 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
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MDemand MResisting
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vProvided
EQUILIBRIUM EQUATIONSSTRAIN COMPATIBILITYMATERIAL STRENGTH
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Slabs on grade
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:
TYPES OF UNBONDED P.T. SYSTEMS
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)
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.
TYPES OF UNBONDED P.T. SYSTEMS
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.