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

Types of Buildings

Presentation Overview

• Deflection Problems

• Causes of Deflections

• Varying Levels of Expectation

• Lack of Specific Requirements

• Communication – Awareness

• Predicting Deflections

• Coordination of Tolerances

• Responsibilities

• Recommendations

Deflection Problems

• Dissatisfied Owners

• Visual

• Damage of Finishes, Window leaks

• Uneven floors – file cabinets, desks

• Drainage

• Long term deflections – not live load

deflections.

Damage of Finishes

Deflection of Deck

Causes of Deflection or Floor

Uneveness

• Dead Load

• Live Load

• Prestress

• Creep

• Shrinkage

• Cracking

• Concrete quality

• Construction control – geometry

• Reshoring at pour strips

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

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.”

IBC

ACI-318

Communication - Awareness

• Owner

• Architect

• Structural Engineer

• Contractor

• Subcontractor (Forming, Placement, Finishing)

• Meeting minutes

• Drawings

• Memorandum

Predicting Deflections

Measured Deflections

Measured Deflections

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

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.

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.

Cast-in-place tolerances

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.

Responsibility

• Owner – 10%

• Architect – 10%

• Contractor – 10%

• Structural Engineer – 70%

• Contractor cannot predict

deflections

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

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

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

Summary

• Deflection Problems

• Causes of Deflections

• Varying Levels of Expectation

• Lack of Specific Requirements

• Communication – Awareness

• Predicting Deflections

• Coordination of Tolerances

• Responsibilities

• Recommendations

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.