Strain Limits for Concrete Filled Steel Tubes in AASHTO Seismic Provisions. PIs: Mervyn Kowalsky and James Nau RA: Nicole King

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1 Strain Limits for Concrete Filled Steel Tubes in AASHTO Seismic Provisions PIs: Mervyn Kowalsky and James Nau RA: Nicole King

2 Project Overview Seismic behavior of reinforced concrete filled steel pipe piles

3 Concrete Filled Steel Tube Uses 1) Driven as a pile and filled with reinforced concrete. 2) Used as a permanent casing for a drilled shaft and filled with reinforced concrete. 3) Serves as a pile below the ground surface, and the above ground columns to support the cap beam. Omalley Bridge (Courtesy, AKDOT)

4 Goals 1) Study the effect of the diameter to thickness ratio on the pile columns performance limit states (strain limit states). 2) Determine the below ground plastic hinge length for the pile columns. 3) Confirm or improve current analysis and modeling assumptions in the design of the pilecolumns.

5 Methods 1) Large scale tests on reinforced concrete filled steel tubes subjected to reverse cyclic bending. Test Setup at Constructed Facilities Lab, NCSU 2) Finite Element Analysis of reinforced concrete filled steel tubes ABAQUS Finite Element Analysis Program

6 Outcomes 1) Design expressions relating D/t to strains at various limit states. 2) Design expressions for plastic hinge lengths for reinforced concrete filled steel pipes. 3) Modifications to moment curvature analysis tools (if necessary) to predict the forcedeformation response to cyclic loading. 4) The research findings will be summarized in a concise design manual appropriate for AKDOT use.

7 Impact of Diameter thickness ratio Study the effect of D/t on: Local pipe buckling Deformation capacity Strain limits associated with elastic, repairable and ultimate response D/t ratios from 24 to 180 will be tested.

8 Impact of Diameter thickness Past Research: ratio There have been multiple studies on concrete filled pipes without internal reinforcement. The majority of past studies were small scale tests (diameters ranging from 1.75 inches to inches). Elchalakani (2008) noticed a trend between D/t and ductility on a small scale but needed large scale tests to verify the findings. Large scale tests on reinforced concrete filled steel tubes were conducted at NCSU with a constant D/t = 48.

9 Impact of Diameter thickness Past Research: ratio

10 Impact of Diameter thickness Methods: ratio Large scale tests on pipe piles in two phases Phase 1: 5 piles All have 24 diameter, same rebar configuration, 30 foot span, 6 foot constant moment length D/t ratio ranging from 48 to 179 Geometric Properties Longitudinal Rebars Spiral Rebars Option Outer Wall Diameter Thickness [in] [in] Span [ft] Constant Moment Length [ft] D/t L/D Long. Pipe Ratio[%] Pipe Vol. Size Number Ratio [%] [#] Ratio [%] Size [#] Pitch (in) Volume Ratio [%] Material Sources: Consolidated Pipe Co and Naylor Spiral Weld Pipe

11 Impact of Diameter thickness ratio Phase 2: exact pile selection will depend on the results from phase 1 Will have a pile with a D/t ratio = 24 Option Outer Diameter [in] Wall Thickness [in] Span [ft] Geometric Properties Longitudinal Rebars Spiral Rebars Constant Long. Pipe Pipe Vol. Size Ratio Pitch Moment D/t L/D Number Size [#] Ratio [%] Ratio [%] [#] [%] (in) Length [ft] Volume Ratio [%] 1e

12 Pipe Suppliers Distributor (Options 1e,2,3,5,6,7,8) Consolidated Pipe and Supply, Inc. ASTM A500 Grade B Prices: Option 2 (18 OD x t) = $2,407; Option 3 (18 OD x 0.5 t) = $1,914; Option 6 (24 OD x t) = $2,566; (Option 7) 18 OD x 0.25 t = $1,102; (Option 8) 24 OD x t = $1,840 US each Manufacturers (Options 10,11,12, 13,14,18) Cannot manufacture wall thicknesses Cannot manufacture outer diameter US Steel Chicago Tube and Iron Brampton Plate & Structural Steel Maximum Diameter of 20 Rolling inches Southland Pipe Atlas Tube, JMC Steel Group Millerbend Company Maximum Diameter of 20 Minimum thickness = ¼ inches Pipe Industries MST Seamless Tube and Pipe Minimum thickness at an Maximum Outer Diameter OD = 24 is Alpha Pipe Company Northwest Pipe Company Could not fit it in to make it Maximum Outer Diameter cost efficient on such a low 16 quantity Skyline Steel Minimum thickness = ¼ Distributor (Options 10,11,12, 13,14,18) Texas Pipe Thinnest wall at 24 outer diameter is ¼ Can manufacture at least one of the piles Hodgson Custom Rolling 24 OD x thick x 420 long: $9,022 US each 24 OD x thick x 420 long: $7,992 US each Maximum roll width is 10 feet: must make in 4 sections with 3 girth welds Arntzen Corporation Custom Rolling 24 OD x thick x 30 long Ships loose as (3) 96 long and (1) 72 long: $2,842 US each Dixie Southern Custom Steel Fabrication 3/16 thickness with welded spool piece Spiral Pipe (Options 10,11,12, 13,14,18) Naylor Pipe Company (ASTM A139) Option 10 (24 OD x t): $840 US each Option 11 (24 OD x t): $1,160 US each

13 Impact of Diameter thickness Methods (continued): ABAQUS model ratio Include multiaxial stresses Run multiple models in order to decide which piles to test in phase two Analyze other configurations that will not be physically tested in the lab

14 Plastic Hinge Lengths Plastic hinges The below ground plastic hinge length varies depending on the stiffness of the soil. Soils with lower stiffness have larger plastic hinge lengths. The constant moment region will mimic the soil stiffness. The plastic hinge length correlates the material strains to the member deformations. Without knowing what the below ground plastic hinge length is, a fiber or finite element analysis is necessary in predicting the deformation capacity. Effect of Soil Stiffness on In ground Plastic Hinge Length.

15 Above Ground Plastic Hinge Length Plastic Hinge 1 and Plastic Hinge 2 : Lp=0.08L fydbl 0.044fydbl Plastic Hinge 3 and Plastic Hinge 4 : Lp is a function of the steel tube gap and strain penetration. Locations of plastic hinges on a double column bent.

16 Below Ground Plastic Hinge Length Locations of plastic hinges on a double column bent. Plastic Hinge 5: Lp=0.08L fydbl 0.044fydbl Plastic Hinge 6: The length and location of the plastic hinge is a function of the soil properties (Chai). Plastic Hinge 7: Lp is a function of the steel tube gap and strain penetration. Plastic Hinge 8: (currently assumed to be the same as plastic hinge 6) The plastic hinge length is unknown, possible factors that affect it are: Moment curvature response Stress Strain response Soil effects on the steel tube

17 Plastic Hinge Lengths Methods (continued): Use existing model for reinforced concrete (without steel casing) on test specimens to compare the actual to predicted displacements. If necessary, include the stress strain interaction to calculate the plastic hinge length. Correctly model the plastic hinge length in ABAQUS finite element analysis (would likely include soil springs).

18 Objectives: Analysis & Modeling Assumptions Determine the shape of the steel and concrete stressstrain curves for concrete filled steel tubes. Determine the appropriate level of f l (currently assumed to be 50% of fy). Determine accuracy of strain compatibility assumption along the pipe pile cross sections. Include the impact of reinforcing steel and steel casing in an analysis method.

19 Analysis & Modeling Assumptions Steel Stress Strain Curve Section A has lateral stress only (fl). Section B has both axial stress (fa) and lateral stress (fl) creating a multi axial stress state.

20 Analysis & Modeling Assumptions Steel Stress Strain Curve σ What? How do multi axial stresses in the tube alter the steel stress strain curve? Why? To obtain an accurate stress strain relationship of concrete confined by a steel tube. To be able to determine the limit states, material response and the plastic hinge length. Ɛ How? Analyze experimental stress strain curves Past work shows that the multi axial stresses have small effects on the stress strain curve.

21 Analysis & Modeling Assumptions Concrete Stress Strain Curve σ What? What is the confinement effectiveness of the steel tube on the concrete? What is the level of longitudinal stress in the hinge? MANDER Model assumes 100% fy, but when this is used to calculate the ultimate concrete strength it is unrealistically large. Model was not defined account for the large steel ratio. Ɛ Why? To obtain an accurate stress strain relationship of concrete confined by a steel tube. To be able to determine the limit states, material response and the plastic hinge length. How? Monitor the transverse strains in the pipe during testing.

22 Analysis & Modeling Assumptions Ɛ Strain Compatibility Strain Distribution along the pipe pile cross section. What? Is strain compatibility applicable? Why? Needed for section analysis. How? Use Optotrack to obtain strain distribution along the section. Past Research: Previous work at NCSU showed the strain was relatively linear.

23 Previous Research: Analysis & Modeling Assumptions Impact of confinement on axial behavior impacts flexural strength and deformation capacity. The hoop stress has been inferred from analysis results and has not been measured directly from the experiment. No research on the complex state of stress in the pipe and its effect on confinement.

24 Methods: Analysis & Modeling Assumptions Measure f l on full scale tests and incorporate it into the concrete model. Measure the longitudinal stresses and incorporate them into the steel model. Create a comprehensive model in ABAQUS finite element analysis.

25 Discussion?