Connections for Concrete-Filled Steel Tubes in Bridge Applications

Dawn E. Lehman and Charles W. Roederand Muzi Zhao (Graduate Student)

University of Washington

> Authors have jointly researched CFST for nearly 20 years

> Past research described here funded by –• National Science Foundation• US Army• California Dept of Transportation• Washington State Dept of Transportation

> Current research on RC pier to CFST pile connections funded by—• FIU Accelerated Bridge Construction UTC for nonlinear analysis• Pacific Earthquake Engineering Research Center (PEER) for

experimental work

Acknowledgements

> Brad Cameron> Mark Gaines> Ryan Thody> Angela Kingsley> Travis Williams> Kenneth ONeill

> Eric Bishop> Arni Gunnerson> Jie Chen > Max Stephens> Jiho Moon> Lisa Berg

> Ashley Heid> Todd Maki> Muzi Zhao> Xin Zhang> Spencer Lindsley

Many graduate students worked on this sequence of CFT projects

There may be others I have missed. But theydid most of the work I take credit for today.

Why concrete filled steel tubes (CFST)?

This presentation will make statements and recommendations based upon extensive prior experimental and analytical research. The experimental research shown in this presentation are large scale tests, but not full scale tests. Full scale tests are possible but extremely expensive and research agencies do not want to pay the costs. However, the behavior observed in these approximately half scale tests likely reflects the behavior of full scale specimens.

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Extensive Testing of Members

62005 - 2007

B6KJM31TTBYW

Simple, filled-tube push-through tests

t = ¼” d = 19 ½”

strain gagelocation typ.

rigid basesupport

air gap toallow slip

concrete filllength = 60”

appliedload

welded wiregage locationtyp.

Applied Load2.4m lb. UTM

Instrumented20 in. Dia. Tube

Tube Concrete

Conventional

LowShrinkage

Conventional

LowShrinkage

StraightSeam

SpiralWeld

4-Test Matrix

Extensive Testing of Specific Behaviors - Bond Tests

1990’s with additionalTests 2012 - 2014

Extensive Testing of Components or Specific Behaviors – Shear Resistance of CFST

82014 - 2017

Extensive Testing of Connections and Subassemblages

92007 - Present

Application of CFSTqCFST can be used as piers or columns in

highway bridgesqCFST can be used as piles or for drilled

shafts in deep foundationsqFocus of this presentation

qOverview of CFSTqCFST for Bridge Piers

qConnections of CFST Pier Columns to Pier Caps or Foundations

qCFST Connections for Deep FoundationsqPractical applications10

CFST is a Composite Solution

q Improved seismic performance, blast and collapse resistance

q Reduced size relative to RCq Eliminates labor and cost for formwork and

reinforcementq Tube confines and reinforces concrete fill q Concrete restrains against buckling of the tubeq Integration with precast superstructure. Lighter

columns for placement.

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Composite Action Requires Shear Stress Transfer Between Steel and Concrete

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This Shear Stress

> Normally developed by friction between the steel tube and concrete fill

> With significant bending this natural friction is more than enough to assure composite behavior in CFST, because the concrete fill and steel tube have different stiffness characteristics which induce binding and subsequent friction forces.

> With no bending moment and large axial force, mechanical transfer may be required.– Shear studs are not effective and should not be used.– A stiff single rib to prevent slip between the steel tube and concrete

fill will prevent slip and assure that friction will occur. 13

Internal Rib

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Bridges Collapsed in 1995 Kobe Earthquake

Some were Rebuilt with CFST Piers

Application of CFST for ABC of Bridge Piers

> CFST can be used as piles or pier columns in highway bridges> Inelastic deformation can be isolated to CFST component for

use in seismic regions – Surrounding concrete components remain essentially elastic for large

reversed cyclic displacement demands

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2012 to Present

Accelerated Construction Sequence

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Accelerated Construction Sequence

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Accelerated Construction Sequence

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Accelerated Construction Sequence

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Accelerated Construction Sequence

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Accelerated Construction Sequence

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Accelerated Construction Sequence

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Accelerated Construction Sequence

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Accelerated Construction Sequence

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Application of CFST to ABC of Deep Foundations

> CFST columns fulfill ABC objectives while providing excellent structural performance in extreme events (e.g., earthquakes)

> Direct advantages of CFST for ABC– Can be integrated with precast

components– Length and diameters can

easily vary– Smaller diameters than RC

columns– Lighter than precast columns

Photo Courtesy of MDT

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CFST for ABC of Bridge Piers

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2010 - 2014

CFST Column to Cap Beam Connections

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o Fully restrained (Full Strength) CFST moment connection

o Tube embedded in foundation concrete

o Annular ring used to transfer overturning forces

Embedded Ring Connection

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Test Apparatus – Long.

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Embedded Ring Connection

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Response of Embedded Ring Connectionwith Adequate Embedment

buckling visible initiation of tearing

� Failure mode and loss of lateral load initiated by ductile tearing of steel tube

� Buckling does not impact performance

� Theoretical plastic moment capacity achieved

� Axial load capacity maintained after tearing

①

② ③

④

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Performance of Embedded Ring Connection

~3.5%-4% Drift Tube Buckling Develops

Tube Buckling Increases

~7%-8% DriftInitiation of Tearing

Final State

Welded Dowel Connection

D (in) t (in) D/t P/Po Reinforcing Steel Fy (ksi) f’c (ksi) ρL(%) (fy/f’c)ρL

25.75 0.25 103 0.10 A706 Grade 60 68 7.7 3 0.335

Performance of Welded Dowel Connection

� Achieved symmetric drifts of up to 9% with no degradation

� Theoretical plastic moment capacity of CFST achieved due to similar mechanical reinforcing ratio

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Performance of Welded Dowel Connection

3.5% Drift 7.5% Drift

9% Drift

slightrocking

Increasedrocking

Groutdamage

Exposed Spiral

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Key Considerations of the Reinforced Concrete (RC) Connection

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Performance of RC Connection

north barfracture northwest and

northeastbar fracture

south barfracture

southwest andsoutheastbar fracture

qFailure mode fracture of reinforcing bars (~8% drift)

qStrength limited by reinforcing ratio and moment arm

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Performance of RC Connection7.5% Drift

12% Drift

Final State

Groutdamage

Groutdamage

Deformedspiral

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Hysteretic Comparison

Reinforced. Concrete

Embedded CFST,D/t = 96

Welded Dowel

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Comparison of CFST and RC Pier Performance – Same diameter, similar

aspect ratio

• Well outside allowable AASHTO limits

• Significantly stronger but larger reinforcement ratio

• First significant loss of resistance at 8% drift

• Satisfies AASHTO seismic• Significant loss of

resistance at 6% drift• 2.2% total reinforcement

(shear + flexure)

CFST RCPier

CFST vs. RC Damage Comparison at same Scale

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3% 5% Cycle 2

Conc

rete

Fille

d Tu

beRe

info

rced

Conc

rete

3% Drift 5% Drift

Comparison of RC and CFST

CFST at 10% Drift

CFST at 8.9% Drift

CFST for ABC and Seismic Design of Pile and Drilled Shaft Foundations

2014 - Present

> Steel tube is stiffer and stronger than RC member of same size

> No internal reinforcement or formwork is required> Field work rapidly accomplished> Avoid costly placement of slender Rebar cage> Eliminate or reduce concerns of voids in concrete due to

interference with internal reinforcement

Concrete Filled Steel Tubes (CFST) are ideal for seismic design and ABC of piles and drilled shafts

However, a reliable direct connection between bridge pier and CFST pile or shaft is required

Nonlinear FEM with the LSDyna is performed to study this direct connection

> Different force and moment transfer mechanisms > Effect of supplemental ribs inside the tube > Connection performance due to effects of:

– rib location, – tube diameter, – reinforcing bar diameter,– embedment depth

> Damage to concrete fill and plastic deformation documented

Parameter study evaluated

Effect of CFST diameter and embedment Length with same RC column

Note that the RC column develops smaller resistance with smaller CFST than the larger CFST particularly with shallower embedment depths

Concrete damage with different CFST diameter, same RC column, and AASHTO embedment length

Note that less concrete damage with larger CFST diameter

Smaller CFST Larger Diameter CFST

Addition of a rib increases resistance and reduces deterioration

Effect of Rib Size Investigated

Effect of Rib location Investigated

Regardless of reinforcing bar size, the most reliable resistance and least deterioration In resistance occurs with rib 50 mm below top of CFST

> Analysis suggests that the direct connection can develop the full resistance and achieve the inelastic deformation required of seismic design without excessive deterioration

> Resistance and deterioration capacity depend upon the relative diameters of the CFST tube and RC column and the embedment length of rebar

> Supplemental rib increase resistance and reduce deterioration

> Optimal rib is 50 mm at 50 mm below top for sizes investigated

Summary of Analysis

Experimental program is in progress to verify the results and potentially improve mode with PEER funding.

>Two tests completed–Both have same pier column– Two different CFST tube diameters with no

rib>Data analysis is not complete for these

specimens but preliminary results are available

Experimental Program

Specimen 1 – 20” RC Pier and 30” CFST

Specimen 1

Specimen 2 – 20” RC pier column – 48” CFST

Specimen 2

Comparison of Normalized Moment-Drift Response

Specimen 1 – 30” Pile Specimen 2 – 48” Pile

Smaller diameter CFST has greater deteriorationAs noted in the analytical study

1. It is not possible to precisely place a pile or drilled shaft. As a result, the pier column many not be concentric to the pile or shaft. How does this affect the behavior of the connection?

Initial Study is in Progress

Further Issues

Practical Implications

Recent changes to the LRFD Design Specifications (AASHTO Articles 6.9.6 and 6.12.2.3.3)

make it practical to take advantage of the benefits of

CFST.

Some recent applications of CFST in pile and drilled shaft bridge designs.

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CFST piles for Alaskan Way Viaduct (2010)

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CFST Shaft for Puyallip River Bridge (2015)

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Mukilteo Ferry Terminal (2018)

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Benefits and Cost Savings

> CFST provides performance that is at least as good as RC piles and shafts

> CFST saves labor in tying rebar, placing rebar, and building formwork and shoring

> The concrete fill enhances the performance of the steel by delaying local buckling

> CFST develops more strength and stiffness with smaller member sizes that RC construction

> Concrete fill will not have voids caused by closely spaced rebar in RC construction

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Cost Savings

Researchers are not very good at predicting construction costs. However, they can predict strength, stiffness, and the member sizes needed to develop the required strength and stiffness. They can also predict the total material weights required by these designs.

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Laguna de Santa Rosa Bridge

Laguna de Santa Rosa BridgeOriginal RC Pier

CFST Pier

Concrete Strength (ksi) 6.0 6.0

Steel Strength (ksi) 60.0 50.0

Diameter (in.) 48 44

Tube Thickness (in.) - 0.5

Concrete Area (in.2) 1810 1452

Steel Area (in.2) 73 68

Weight/ft of Pier (kips) 2.1 1.7

Total Pier Weight (kips) 34 27

Difference in Pier Weight

20.5% Reduction

Reduction in Weight of Steel

7%

Reduction in Weight of Concrete

19%

Ebey Slough Pier 2 – Recent WSDOT Drilled Shaft

• 6 ft diameter steel tube 1” wall thickness not considered in strength and stiffness

• 111 ft reinforcing cage with #6 spiral and 28 #14 bars

• 194 ft total shaft length

Replace with CFST Member• Develop the required strength and stiffness with 60

inch diameter tube with 1” wall thickness and no internal reinforcement.

• Develops required strength using 75% of wall thickness to facilitate handling and provide corrosion allowance

• Eliminate– 11.85 tons steel rebar– 1.95 tons of spiral reinforcing– 12.4 tons less steel in tube– 31% reduction in concrete – eliminate 62 cubic yds– 30% less soil to remove

Replace with CFST• Cost savings due to materials are fairly

obvious• Additional savings are expected in labor cost– Avoid tying and placing spindly rebar cages– Concrete placed more quickly since not working

around rebar– Smaller drilled shaft– Reduced concerns about voids in concrete

We recognize that other factors affecting bridge design than required

strength and stiffness of members. Other factors affect the cost of a

bridge than cost of materials. However, there are significant potential savings in materials and labor combined with accelerated construction with CFST.

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What are the cost benefits of avoiding tying and placing a rebar cage and placing concrete around this

cage without voids in concrete? What is the risk of injury to workmen when this is done?

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