pt slab deflections - post-tensioning institute convention/session... · pt slab deflections what...
TRANSCRIPT
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PT Slab
Deflections
What is acceptable?
By Douglas Sarkkinen, PE,SE
Senior Structural Engineer
Otak, Inc.
May 1, 2011
2011 PTI Annual Conference,
Kansas City, MO
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Types of Buildings
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Presentation Overview
• Deflection Problems
• Causes of Deflections
• Varying Levels of Expectation
• Lack of Specific Requirements
• Communication – Awareness
• Predicting Deflections
• Coordination of Tolerances
• Responsibilities
• Recommendations
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Deflection Problems
• Dissatisfied Owners
• Visual
• Damage of Finishes, Window leaks
• Uneven floors – file cabinets, desks
• Drainage
• Long term deflections – not live load
deflections.
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Damage of Finishes
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Deflection of Deck
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Causes of Deflection or Floor
Uneveness
• Dead Load
• Live Load
• Prestress
• Creep
• Shrinkage
• Cracking
• Concrete quality
• Construction control – geometry
• Reshoring at pour strips
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Expectations
• High End vs Low End
• Rental vs Own
• Equipment / Furniture
• Use, Interior, Exterior
• Architectural Finish Detailing
(Accommodating movement)
• Persnickety Personalities
• Lack of Knowledge / Understanding
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Serviceability Requirements
• ASCE 7-10
• C.2 Design for Long-Term Deflection
“Where required for acceptable building
performance, members and systems shall be
designed to accommodate long-term irreversible
deflections under sustained loads.”
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IBC
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ACI-318
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Communication - Awareness
• Owner
• Architect
• Structural Engineer
• Contractor
• Subcontractor (Forming, Placement, Finishing)
• Meeting minutes
• Drawings
• Memorandum
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Predicting Deflections
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Measured Deflections
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Measured Deflections
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Predicting Deflections
• Creep affect can vary substantially
• Watch out for “Aggressive” Designs
• Keep maximum P/A around 300 psi
• Cantilevered slabs are very sensitive to
tendon placement, P/A, and backspan
deflection
• Cracking under service levels can change
the moment of inertia
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Construction Tolerances
• ACI-117
• 4.3.1.2 Elevation of top surfaces of formed
slabs before removal of supporting shores has a
tolerance of +/- ¾ inch
• 4.4.1 The thickness tolerance for slabs less
than 12 inches thick are + 3/8 inch and – ¼ inch.
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Construction Tolerances
• ACI 117
• 4.5.6.1 The FL levelness tolerance shall not
apply to slabs placed on unshored form surfaces
and/or shored form surfaces after the removal of
shores. FL levelness tolerances shall not apply
to cambered or inclined surfaces and shall be
measured within 72 hours after slab concrete
placement.
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Cast-in-place tolerances
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Example
• Typical 30 foot bay with an 8 inch PT slab
• l/240 = 1-1/2 inches, then add +/- ¾ inch.
• Picture what the sill condition looks like at a full
height window in a condominium with a 6 inch
high sill.
• Revisit the perpendicular GWB wall in an office
building with this deflection.
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Responsibility
• Owner – 10%
• Architect – 10%
• Contractor – 10%
• Structural Engineer – 70%
• Contractor cannot predict
deflections
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Recommendations
• Have experienced PT Slab Designers
• Communicate early on with architect / owner
• Document discussions
• Follow up with contractor during construction
• Perform field measurements to confirm analysis
methods
• Beware of “aggressive” designs, high prestress,
thin or long cantilevers
• Coordinate slab edge – wall conditions/details
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Recommendations
• For buildings with decks, and for parking
garages that are sloped for drainage, review the
drainage and slopes to insure that resulting
long-term deflections still allow positive drainage
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Recommendations
• For typical buildings (30 foot +/- bays), target the
calculated long term deflection (creep factor is 3)
plus live load to:
3/4 inch for Parking Garages
½ inch for office space
3/8 inch for condominiums
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Summary
• Deflection Problems
• Causes of Deflections
• Varying Levels of Expectation
• Lack of Specific Requirements
• Communication – Awareness
• Predicting Deflections
• Coordination of Tolerances
• Responsibilities
• Recommendations
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What is acceptable?
• An acceptable level of long-term deflection is
that which is agreed upon prior to construction,
and then having the final slabs be within the
agreed upon range.