Hybrid External PT Saves Chicago Garage

By Don Kline, P.E

1 2 3

1 2 3

1 2 3

Topics Condition Survey of Garage Structural Analysis Construction

Topics Condition Survey of Garage Structural Analysis Construction

Project Overview

Chicago Garage Engineer: Pease Borst

& Associates Contractor: Structural

Preservation Systems Specialty Engineer:

Kline Engineering & Consulting

Project Overview

Chicago GarageBuilt in 19837” banded two-way

PT flat slab with 4” drops

½” unbonded PT with extruded sheath (non-encapsulated)

Condition Survey

Locate Existing PT Using GPRMap delaminations, spalls and

cracks Investigate Condition of PT

GPR Was Used to Locate Existing PT

Condition Survey

Condition Survey

Full Depth Cracks at Mid-Span

Condition Survey

Flexure Cracking at Top of Slab Near Column

Condition Survey

Slab Spalls and Exposed PT

Condition Survey

Slab Spalls and Exposed PT

Condition Survey

Ponding with Delaminations

Condition Survey

Water Leaking Through Slab

Condition Survey

Water Leaking Through Slab

Condition Survey

Spall at Supplemental Cable

Condition Survey

Trenched Supplemental Cable

Condition Survey

Supplemental and Uniform Tendons

Failed Supplemental Distributed Tendons

Condition Survey

Opened Pockets to Inspect Condition of Strands

Condition Survey

Corrosion

Testing of Strand

Condition Survey

Uniform Strand Remains Tensioned

Condition Survey

Inspection of Grease

Condition Survey

Tested Core Samples for Concrete Strength

Banded Tendons

Distributed Tendons

Supplemental Tendons

Condition Survey

Condition Survey Summary 100% of strands on banded line 5 failed 10% - 20% of strands failed at other

banded tendon lines 100% of distributed supplemental

strands failed Original distributed tendons showed

signs of moisture in sheath and pitting of steel

Significant cracking and spalling

Condition Survey

Installation of supplemental cables could have damaged bands at high points.

Supplemental Cable Trench

Topics Condition Survey of Garage Structural Analysis Construction

Structural AnalysisWidth = 28', Span=26', Slab = 7", Col.=18" dia, D.P.=6' sq. x 4"

Structural Analysis

Slab was designed for parking live load but not for parking + snow

Supplemental distributed tendons were likely added as a retrofit soon after original construction

Conclusions From Analysis

Structural Analysis

Repair PT using conventional methods

Demo and replace top level of garage Rehabilitate using hybrid external PT

system

Owner Presented 3 Options:

Structural Analysis

Distributed Direction: New External PT to replace distributed and supplemental PT

Banded Direction: Create new shallow continuous drops with unbonded PT below existing soffit

Modeled as a 2-way flat slab

Hybrid External PT Was Selected:

External Post-Tensioning

Structural Analysis

Structural Analysis

Replace Banded Tendons from Below

Structural Analysis

Repair E/F 4-6 and Band in Col Line 5

Structural Analysis

Topics Condition Survey of Garage Structural Analysis Construction

Construction

Install DowelsRoughen Surface

Construction

Encapsulated AnchorsMultistrand Anchors

Construction

Installed Formwork

Construction

Access Ports

Pumped SCC

Construction

Stripped Formwork

Construction

Installed Deviators

Monostrand with Intumescent

Coating

Construction

Stressed Tendons

Construction

Construction

Construction

Construction

Construction

Construction

Construction

Thank You!