PT Slab Deflections. What is acceptable? By Douglas Sarkkinen, PE,SE Senior Structural Engineer Otak, Inc PTI Annual Conference, Kansas City, MO

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1 PT Slab Deflections What is acceptable? By Douglas Sarkkinen, PE,SE Senior Structural Engineer Otak, Inc PTI Annual Conference, Kansas City, MO May 1, 2011

2 Types of Buildings

3 Presentation Overview Deflection Problems Causes of Deflections Varying Levels of Expectation Lack of Specific Requirements Communication Awareness Predicting Deflections Coordination of Tolerances Responsibilities Recommendations

4 Deflection Problems Dissatisfied Owners Visual Damage of Finishes, Window leaks Uneven floors file cabinets, desks Drainage Long term deflections not live load deflections.

5 Damage of Finishes

6 Deflection of Deck

7 Causes of Deflection or Floor Uneveness Dead Load Live Load Prestress Creep Shrinkage Cracking Concrete quality Construction control geometry Reshoring at pour strips

8 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

9 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.

10 IBC

11 ACI-318

12 Communication - Awareness Owner Architect Structural Engineer Contractor Subcontractor (Forming, Placement, Finishing) Meeting minutes Drawings Memorandum

13 Predicting Deflections

14 Measured Deflections

15 Measured Deflections

16 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

17 Construction Tolerances ACI Elevation of top surfaces of formed slabs before removal of supporting shores has a tolerance of +/- ¾ inch The thickness tolerance for slabs less than 12 inches thick are + 3/8 inch and ¼ inch.

18 Construction Tolerances ACI The F L levelness tolerance shall not apply to slabs placed on unshored form surfaces and/or shored form surfaces after the removal of shores. F L levelness tolerances shall not apply to cambered or inclined surfaces and shall be measured within 72 hours after slab concrete placement.

19 Cast-in-place tolerances

20 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.

21 Responsibility Owner 10% Architect 10% Contractor 10% Structural Engineer 70% Contractor cannot predict deflections

22 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

23 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

24 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

25 Summary Deflection Problems Causes of Deflections Varying Levels of Expectation Lack of Specific Requirements Communication Awareness Predicting Deflections Coordination of Tolerances Responsibilities Recommendations

26 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.

Structural Option April 7 th, 2010

Structural Option April 7 th, 2010 Gravity System (Depth Topic I) Post Tensioned Slab A new floor system was designed in an attempt to create a more consistent flooring system throughout the entire building. This new design consists of