SAP2000 application of Post-tension Prestress Design
Full description
For ACI 318-08 Herein describes in detail the various aspects of the post-tensioned concrete design procedure with the American code ACI 318-08 [ACI 2008].Full description
Ribbed slabs are used for long spans with relatively light loads. They are constructed in one of the following ways as described in clause 30 of IS: 456-2000 1. As a series of concrete ribs with t...
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POST TENSIONED FLOORS
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POST TENSIONED FLOORS ■ History of PrePre-stressed Concrete
1886 First Patent for Prestressed Concrete (P.H. Jackson of San Francisco)
1928 Modern Development of Prestressed Concrete (E. Freyssinet of France )
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POST TENSIONED FLOORS ■ Idea of PrePre-stressed Concrete
POST TENSIONED FLOORS ■ Idea of PrePre-stressed Concrete
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POST TENSIONED FLOORS ■ Idea of PrePre-stressed Concrete
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POST TENSIONED FLOORS ■ Types of Prestressed Concrete
POST TENSIONED FLOORS ■ PRE TENSIONING
Steel tendons are stressed prior to concrete placement, usually at a precast plant remote from the construction site
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POST TENSIONED FLOORS ■ POST TENSIONING
Steel tendons are stressed after the concrete has been placed and gained sufficient strength at the construction site
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POST TENSIONED FLOORS ■ STRANDS / TENDONS
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POST TENSIONED FLOORS ■ BONDED TENDONS Can be more costly due to duct placement & grouting Force transmitted by anchors and bond to concrete Greater total force can be applied Strain compatibility with concrete Openings less difficult Minimizes need for nonnon-prestressed reinforcement More complex stressing equipment required
POST TENSIONED FLOORS ■ UNBONDED TENDONS Economical Greater layout flexibility Force transmitted solely by the anchors Total force limited by anchor spacing Replaceable Simple stressing equipment
POST TENSIONED FLOORS ■ Main Applications of Prestressed Concrete
POST TENSIONED FLOORS ■ Main Applications of Prestressed Concrete
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POST TENSIONED FLOORS ■ Main Applications of Prestressed Concrete
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POST TENSIONED FLOORS ■ Main Advantages of Post Tension Floors ■ Functional Large Cantilevers Long economical spans Flexibility in Column Layout Effective use of high strength materials P/T slab is typically 30% 30% thinner
POST TENSIONED FLOORS ■ Main Advantages of Post Tension Floors ■ Economical Effective use of high strength materials P/T slab is typically 30% 30% thinner Forms are removed in 2-3 days not in 21 to 28 day as for rebar Concrete Significantly reduces amount of reinforcement Typically need about 50% 50% of crew
POST TENSIONED FLOORS ■ Main Advantages of Post Tension Floors ■ Technical Deflection and vibration control Long--term creep problems are virtually Long eliminated by load balancing Moment of inertia approaches Igross Crack control and waterwater-tightness
POST TENSIONED FLOORS ■ PT Slab Vs RC Slab ■ Slab Thickness Typical Span/Depth Floor System P/T
RC
One way slabs
48
21-28
Two-way slabs
45
33
Two-way slabs with drop panels
50
36
(minimum drop panel L/6 each way)
POST TENSIONED FLOORS ■ PT Slab Vs RC Slab ■ Span – Cost Relation
POST TENSIONED FLOORS ■ PT Slab Vs RC Slab ■Cost Breakdown
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POST TENSIONED FLOORS ■ Common Systems of PT Slabs Solid Slab
One Way Slabs Two Way Slabs
Flat Slabs
Flat Plates Flat Slab With Drop
Waffle Slabs
One Way Joists Two Way Waffle Slab
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POST TENSIONED FLOORS ■ One Way Systems
POST TENSIONED FLOORS ■ Two Way Systems
POST TENSIONED FLOORS ■ Banded & Waffle Slabs
POST TENSIONED FLOORS ■ Construction of PT Slabs ■ Shuttering
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POST TENSIONED FLOORS ■ Construction of PT Slabs ■ Laying of Bottom Reinforcement
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POST TENSIONED FLOORS ■ Construction of PT Slabs ■ Laying of Tendons & Laying of Top Reinforcement
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POST TENSIONED FLOORS ■ Construction of PT Slabs ■ Concrete Casting
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POST TENSIONED FLOORS ■ Construction of PT Slabs ■ Stressing of Strands
POST TENSIONED FLOORS ■ Construction of PT Slabs ■ Stressing of Strands INITIAL
FINAL
At 25 25% % of Concrete Strength
At 70 70% % of Concrete Strength
About 24 hours from casting
About 72 hours from casting
Shutter is removed and props are installed
Anchors are finished and shutter can be removed
POST TENSIONED FLOORS ■ Construction of PT Slabs ■ Measurement of Elongation
POST TENSIONED FLOORS ■ Construction of PT Slabs ■ Stressing Report Measured elongation needs to be within 7% of calculated value Per code, for tendons outside the 7%, the EOR shall ascertain and correct the “problem”. “problem”. What to do? DeDe-Tension / Re Re--Tension? Elongation records shall be sent to the engineer by next working day after stressing Elongations shall be approved or rejected within three working days after stressing
POST TENSIONED FLOORS ■ Construction of PT Slabs ■ Finishing of Tendon Ends
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POST TENSIONED FLOORS ■ Outlines of Design Procedure Selection of Initial Slab Thickness
Determination of the Amount of PrePre-Stressing Serviceability Calculations (Deflection, Vibration) Limit State Calculations (Stresses) Calculation of bonded reinforcement 12/10/2009
POST TENSIONED FLOORS ■ Selection of Initial Thickness
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POST TENSIONED FLOORS ■ Amount of Prestressing Minimum P/A =0.9
ACI
Maximum spacing of tendons is six times the slab thickness.
Prestressing always provide an uplift to overcome 6060-90 percent of dead loads
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POST TENSIONED FLOORS ■ Amount of Prestressing
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wb L2 P= 8h
POST TENSIONED FLOORS ■ Amount of Prestressing
wbx =
Ly
αLx + Ly 4
wbx Lx Px = 8hx
Py =
wbx Lx y
8hy
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2
2
4
4 4
wb
Lx wby = wb 4 4 αLy + Lx
POST TENSIONED FLOORS ■ Amount of Prestressing
α =1.0 for 4 edges continuous or discontinuous =1.0 for 2 adjacent edges discontinuous =2.0 for 1 long edge discontinuous =0.5 for 1 short edge discontinuous =2.5 for 2 long and 1 short edges discontinuous =0.4 for 2 short and 1 long edges discontinuous =5.0 for 2 long edges discontinuous =0.2 for 2 short edges discontinuous
