Kenneth O’Neill Experimental Investigation of Circular Concrete Filled Steel Tube Geometry on Seismic Performance

Outline I.Design II.Prior Research III.30” Diameter Test and Results IV.20” Diameter Tests and Results V.Comparisons VI.Fragility Curves VII.Conclusions and Future Work

What is a Concrete Filled Tube? Flexural resistance of steel is maximized by placing it at the perimeter of the cross section Steel tube prevents spalling and confines the concrete Concrete infill delays local buckling of the steel tube and provides compression strength Steel tube replaces formwork and reinforcing steel

DESIGN

AISC Flexural Resistance Methods Plastic Stress Distribution Method (PSDM) Steel Stress – uniform stress equal to Fy (Plastic Stress) Concrete Stress – uniform 0.95f’c  Enforce equilibrium on the cross section Roeder, Lehman & Bishop, 2010

D/t Limits (too small) ACI EquationAISC Equation For 50 ksi steel:

Footing Shear Stress Demand Put the next three slides together

Footing Shear Stress Demand

(1) (2) (3) (4)

Footing Shear Stress Demand (1) (2) (3) (4)

Footing Shear Stress Demand (1) (2) (3) (5) (4) Assume β = 45°

Cantilevered Column Stiffness ACI Equation AISC Equation Proposed Equation

UW TEST PROGRAM ON CFT

Test Parameters (fill this in) Connection type Anchorage Steel Strength Tube Type Etc…

UW Test Program

UW Test Program (Cont.)

20” Diameter Specimen Design Monolithic ConnectionGrouted-Recessed Connection

Test Setup

Embedment Length Embedment has greatest effect on overall specimen performance Shorter embedment leads to more footing damage Little/no effect on stiffness Impact:

Axial Load Ratio Increased Specimen Stiffness Local buckling damage state occurs at lower drift ratio value with increasing axial load Tearing occurs at higher drift ratio value with increasing axial load Does not affect strength Impact:

Monolithic vs. Grouted Connection In specimens that did not develop Mp, reduced damage to footing Does not affect strength DEFORMABILITY?? Anchorage Impact:

Steel Tube Material Properties Affect onset of steel tube local buckling Cyclic strain ductility of steel tube affects steel tube tearing Affects strength Impact:

Asymmetric Loading Pattern Affects drift ratio of when damage states occur but does not affect maximum capacity Impact:

LARGE DIAMETER TEST

30” Diameter Specimen

30” Diameter Specimen (Cont.)

Load Protocol

30” Diameter Results

Force-Displacement Response Yield – % Drift Ratio Buckling – % Drift Ratio Max – % Drift Ratio Tearing – % Drift Ratio

Moment-Drift Ratio Response Yield – k-in ~ 0.8% Drift Ratio Buckling – k ~ 2.2% Drift Ratio Max – k-in ~ 6.3% Drift Ratio Tearing – k-in ~ 7.1% Drift Ratio

Buckled Shape Cycle 16, -2.6% Drift Ratio to North

Buckled Shape Cycle 18, -3.9% Drift Ratio to North

Buckled Shape Cycle 20, -5.3% Drift Ratio to North

Buckled Shape Cycle 22, -6.6% Drift Ratio to North

Buckled Shape Cycle 16, 2.2% Drift Ratio to South

Buckled Shape Cycle 18, 3.6% Drift Ratio to South

Buckled Shape Cycle 20, 4.9% Drift Ratio to South

Buckled Shape Cycle 22, 6.3% Drift Ratio to South

Residual Buckling 2” Above Footing Surface 4” Above Footing Surface

Segmental Rotation indicate buckling and tearing

Lee Spec Response this is only helpful if compare Yield – 0.8% Drift Ratio Buckling – 3.2% Drift Ratio Max – 3.4% Drift Ratio Tearing – 7.4% Drift Ratio

Preloaded 20” Diameter Specimens Tested by Arni Gunnarsson Preloaded with 360 kips for 126 days (Creep Test) Same dimensions and rebar as Kingsley, Williams, Chronister and Lee Difference in Concrete Mix (SCM and SCC)

Force-Displacement Response Yield – % Drift Ratio Buckling – % Drift Ratio Max – % Drift Ratio Tearing – % Drift Ratio

Moment Envelope Comparison

EVALUATION OF PERFORMANCE AND DESIGN EXPRESSIONS

Embedment Length

Moment Capacity

Fragility Curves Initial BucklingSteel Tube Tearing Loss of 20% of Peak Moment Capacity

Theoretical Fragility Curves

Conclusions Similar normalized moment capacity for 30 inch diameter specimen with 20 inch diameter specimens Buckling did not occur sooner in 30 inch diameter specimen than in comparable specimens Tearing occurred slightly earlier and reduction in resistance after tearing was more rapid No significant difference in stiffness

Future Work Shear strength of CFT CFT with 50 ksi yield stress steel tube with short embedment length so Mp not developed Column to bent cap connection test CFT test incorporating internal rebar

Acknowledgements Dawn Lehman Charles Roeder Jeff Berman Students Joanna

Questions?