Amir Botros, Sami Rizkalla, Paul Zia, Gregory Lucier · Amir Botros, Sami Rizkalla, Paul Zia, Gregory Lucier Department of Civil, Constructio n and Enviromental Engineering Precast

Amir Botros, Sami Rizkalla, Paul Zia, Gregory LucierDepartment of Civil, Construction and Enviromental Engineering

Precast double tees with thin stems are a widely used inparking structures and other buildings. The end supports aredapped such that the bottom of the double tee is level with thebottom of the inverted tee or ledger beam on which it issupported.

The dapped connection detailis important in parkingstructures because the overallstructural depth and floor-to-floor height need not beincreased where the double teeis supported by ledge beam.

• Problems Associated with Dapped End Beams1. Cracking Problems

2. High Bearing Stresses 3. Constructability / Tolerances

1. Develop Standard and Effective Reinforcement Details forDapped Ends.

2. Develop Rational Methodologies for Proportioning KeyReinforcement for Dapped Ends.

P P

FE Model For Dapped End Beam

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90

100

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Vert

ical

Dap

Rea

ctio

n, k

ips.

Displacement under Load, in.

L3B Specimen

Experimental FE

Measured Dap Reaction: 67 kips FE prediction: 65 kips

Prediction/ Measured = 97 %

Cracks’ width > 0.4 mm (0.016 in) Red zones have principal compressive strains greater than 0.002 value

Re-entrant corner cracks at

Crack pattern at Bond Failure Cracks Diagonal Tension Cracks

Nib Region Cracks

Parametric Study

5Reinforcement

Schemes

15Parameters Studied

forEach Scheme

C-Shaped Vertical L

Inclined L Z-Shaped

Custom Welded Wire Mesh

1. Prestressing Level2. Amount of Hanger Reinforcement, Ash3. Shear Friction Reinforcement, Ah4. Vertical Shear Reinforcement, Av5. Flexural Reinforcement, As6. Nib Prestressing7. Length of Hanger Steel Tail, Ash’8. Concrete Compressive Strength, fc’9. Shear Reinforcement in Full Section10. Nib Depth11. Friction of Bearing Pads12. Held Back Flange13. Flange Width14. De-bonded Length of Strands15. Corner Angle Construction Details

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Vert

ical

Dap

Rea

ctio

n, k

ips

Vertical Displacement under Load, in.

Vertical L ShapeC Shape

Vertical Z Shape

Inclined Z Shape

Inclined L Shape

Welded Wire Fabric

• Inclined L Most efficient• C Shaped Poorest performance

0

0.005

0.01

0.015

0.02

0.025

C Shape V L Shape I L Shape V Z Shape I Z Shape WWF

Crack width(in)

Crack Width at Re-entrant Corner

0.0077

0.0148

0.0118

0.0230

0.01180.0135

Minimum

Comparison between Reinforcement Schemes

Typical failure of analyzed dapped end beams occurred due to diagonal tension cracking in the full depth section.

Inclined schemes performed better than others in regards of strength and crack control.

C-shape scheme showed poorest performanceThe effect of various parameters showed same trend for all reinforcement schemes.Several parameters were found to have significant effect on the ultimate strength of dapped end and in controlling cracking such as:

Nib PrestressingConcrete StrengthShear reinforcement in full section

7’ 3’’3’ 0’’ 22’ 7’’7’ 2’’

P P

Inclined link support

Pinsupport

40’ 0’’

30’’15’’

• Ten Beams 40’ long (20 dapped end tests)• One Full Scale 60’ long Beam

Test Setup

• Completing the Experimental Tests• Select the two most Efficient Reinforcement Schemes in regards of

Strength and Crack control at Service Load Level.• Develop Rational Design Approach for proportioning the key

Reinforcement of the Selected Schemes.

Crack Pattern at Service Load Level Crack Pattern after Failure

Literature and Previous Tests

Industry Survey

ATENA Verification

Parametric Study Using ATENA

Proposing Experimental Program

Lap Splice Tests I

Phase 1 Report

Design of Full Scale Beam Specimens

Lap Splice Tests II

Experimental program

Phase 1 (Completed) Phase 2 (In progress)

Re-entrant Corner Cracks Shear Cracks Splitting Cracks

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100

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Vert

ical

Dap

Rea

ctio

n (k

ips)

Vertical Displacement, in.

No Strands in Nib1 Strand in Nib2 Strands in Nib3 Strands in Nib

+ 25%

1 Strand in Nib 3 Strands in Nib

Effect of Nib Prestressing

Crack pattern at service load level M4B Specimen

Major DT cracking at Failure M4B Specimen

Parameters Selected for Testing:

• Nib Prestressing• Concrete Strength, fc’• Nib Depth• Length of Hanger Steel Tail, Ash’• Shear Reinforcement in Full Section• Flange Held Back

1- Six Reinforcement Schemes

2- Experimentally verify influence of the following six parameters:

The authors would like to acknowledge the Precast/Prestressed Concrete Insitute forfunding this research project in addition to providing continuous advise and guidethrough all phases of the research.