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Steel Bridge Research at Ferguson Structural Engineering Lab

Investigators: Todd Helwig, Mike Engelhardt, Eric Williamson, Ozzie Bayrak, Tricia Clayton, John Tassoulas, and Lance Manuel

Sponsor: Texas Department of Transportation

Research Project Managers: Darrin Jensen and Wade Odell

• Improved Cross Frame Details • Strengthening Continuous Steel Girders with

Post-Installed Shear Connectors • Extending the Use of Elastomeric Bearings to

Higher Demand Applications • Partial Depth Prestressed Concrete Deck Panels

on Curved Bridges • Improved Tub Girder Details • Fatigue Resistance and Reliability of High Mast

Illumination Poles (HMIPs) with Pre-existing Cracks

Research Projects

2

Improved Cross Frame Details

Investigators: Todd Helwig, Michael Engelhardt, Karl Frank Graduate Research Assistants: Weihua Wang, Anthony

Battistini, and Sean Donahue Sponsor: Texas Department of Transportation

• To improve the fundamental behavior of cross frames by investigating both existing and alternative details

– Investigate new cross frame layouts, use of steel castings, for connections, etc.

– Understand current performance of cross frames in strength, stiffness, and fatigue (at member and system level)

– Develop recommendations for cross frame design and detailing

– Evaluate fatigue performance of various details in traditional and new systems

Research Objectives

4

Some of the more interesting findings in this study, had nothing to do with the initial focus of the research

Cross Frame Examples

5

The cross frames require significant handling during fabrication. Better details can improve the efficiency and fabrication economy

Background Single Diagonal Cross Frames

• Use increased compression capacity of diagonal to resist girder twist

• Results in same stiffness as tension-only system • Previous laboratory tests showed feasibility

6

F F F

F

RB RA S

hb

F F

M

M

Overview • While new cross frame details were studied, in

the process we found that we did not understand the stiffness behavior of the cross frame systems that are currently in practice.

• Most computer models have gross errors in the stiffness modeling of the braces.

• These errors can lead to unsafe conditions during construction, poor quality with respect to predictions of deformations during construction, and improper indications of fatigue problems.

7

Current Details Potential Details Considered

Single Angle X-Frame

Single Angle K-Frame Double Angle Z-Frame (Single Angle Struts)

Square Tube Z-Frame

Double Angle Z-Frame (Double Angle Struts)

Lab Tests: Full Scale Stiffness Cross Frame Specimens

Single Unequal Leg Angle X-Frame

• Tension-only System

• Tension-Compression System

• K Frame System

9

Background Brace Stiffness Analytical Formulas based upon truss formulations of the cross frame system.

𝛽𝑏𝑏 =𝐴𝑐𝐸𝑆2ℎ𝑏

2

𝐿𝑐3

𝛽𝑏𝑏 =𝐸𝑆2ℎ𝑏

2

2𝐿𝑐3𝐴𝑐

+ 𝑆3𝐴ℎ

𝛽𝑏𝑏 =2𝐸𝑆2ℎ𝑏

2

8𝐿𝑐3𝐴𝑐

+ 𝑆3𝐴ℎ

F F

F F

F F

F F

F F

F F

Z Frames

X Frames

K Frames

10

F F -F

-F

RB RA S

hb

F F

Lab Tests: Large Scale Stiffness Cross Frame Model

First tests carried out on full scale cross frames that allowed the direct measurement of the stiffness and strength of the braces.

F F

F

11

R

R R

Lab Tests: Large Scale Stiffness Test Setup

12

Lab Tests: Large Scale Stiffness Single Angle X Frame Test

2C

• Top and bottom struts are close to zero force member • In elastic range, the compression diagonal contributes as much as

the tension diagonal 13

-30

-20

-10

0

10

20

30

40

-30 -20 -10 0 10 20 30

Mem

ber F

orce

s, k

ips

Applied Force F, kips

Forces in Angle Members

Ftop

Fbot

-30

-20

-10

0

10

20

30

40

-30 -20 -10 0 10 20 30

Mem

ber F

orce

s, k

ips

Applied Force F, kips

Forces in Angle Members

Ftop

Fdiag 1

Fdiag 2

Fbot

Lab Tests: Large Scale Stiffness Single Angle X Frame Forces

14

y = -872105x - 0.2867R² = 0.9993

-2000

-1500

-1000

-500

0

500

1000

1500

2000

-0.002 -0.002 -0.001 -0.001 0.000 0.001 0.001 0.002 0.002 0.003

Mfr

ame,

kip-

in

θ, rad

β=872,000 kip-in/rad

Lab Tests: Large Scale Stiffness Single Angle X Frame Stiffness

15

Lab Tests: Large Scale Stiffness Comparative Stiffness Behavior

Type of Cross Frames Test Results

Single Angle X Frame 872,000

Single Angle K Frame 760,000

Unequal Leg Angle X Frame 1,054,000

Double Angle Z-frame (Single Struts)

597,000

Double Angle Z-frame (Double Struts)

1,182,000

Square Tube Z-frame 658,000

16

Lab Tests: Large Scale Stiffness Comparative Stiffness Behavior

Type of Cross Frames Test Results

Analytical Solution

Error %

Single Angle X Frame 872,000 1,579,000 82%

Single Angle K Frame 760,000 1,189,000 56%

Unequal Leg Angle X Frame 1,054,000 1,609,000 53%

Double Angle Z-frame (Single Struts)

597,000 907,000 52%

Double Angle Z-frame (Double Struts)

1,182,000 1,152,000 -2.5%

Square Tube Z-frame 658,000 649,000 -1%

17

Lab Tests: Large Scale Stiffness Comparative Stiffness Behavior

Type of Cross Frames Test Results

Analytical Solution

Error %

Line Element Solution

Error %

Single Angle X Frame 872,000 1,579,000 82% 1,572,000 81%

Single Angle K Frame 760,000 1,189,000 56% 1,180,000 55%

Unequal Leg Angle X Frame 1,054,000 1,609,000 53% 1,614,000 53%

Double Angle Z-frame (Single Struts)

597,000 907,000 52% 905,000 52%

Double Angle Z-frame (Double Struts)

1,182,000 1,152,000 -2.5% 1,152,000 -2.5%

Square Tube Z-frame 658,000 649,000 -1% 647,000 -2%

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Lab Tests: Large Scale Stiffness Comparative Stiffness Behavior

Type of Cross Frames Test Results

Analytical Solution

Error %

Line Element Solution

Error %

Shell Element Solution

Error %

Single Angle X Frame 872,000 1,579,000 82% 1,572,000 81% 867,000 -1%

Single Angle K Frame 760,000 1,189,000 56% 1,180,000 55% 781,000 3%

Unequal Leg Angle X Frame 1,054,000 1,609,000 53% 1,614,000 53% 1,065,000 1%

Double Angle Z-frame (Single Struts)

597,000 907,000 52% 905,000 52% 616,000 3%

Double Angle Z-frame (Double Struts)

1,182,000 1,152,000 -2.5% 1,152,000 -2.5% 1,164,000 -1.5%

Square Tube Z-frame 658,000 649,000 -1% 647,000 -2% 657,000 0%

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The reduction in the stiffness is due to the bending

caused by the eccentric connection

• Truss formulations and line element models overestimate the stiffness of cross frames with single angle members – Error largely due to eccentric connection of

single angle • Results from FEA shell element model have

good agreement with all test results – Use validated model to perform parametric

studies

20

Lab Tests: Large Scale Stiffness Observations

• Parametric studies were performed to find a correction value for single angle X and K frames:

R=βFEA / βanalytical

– R was found to be dependent upon S/hb, y, and t – R can be applied to the truss formulation – R can be applied to modify the member area in a

computer software model when cross frames are modeled using line elements

21

Computational Modeling Cross Frame Stiffness Reduction

0

500

1000

1500

2000

2500

3000

3500

4000

0 500 1000 1500 2000 2500 3000 3500 4000

β reg

-SX

βFEA-SX

1

1

βFEA [1000 kip-in/rad]

R∙β a

naly

tical

[100

0 ki

p-in

/rad

]

Computational Modeling X Cross Frame Reduction Factor

0

500

1000

1500

2000

2500

3000

3500

0 500 1000 1500 2000 2500 3000 3500

β ana

-SK

βFEA-SK

1

1

βFEA

R∙β a

naly

tical

βFEA [1000 kip-in/rad]

R∙β a

naly

tical

[100

0 ki

p-in

/rad

]

Computational Modeling K Cross Frame Reduction Factor

Type of Cross Frames Test Results

Analytical Solution

Error %

Single Angle X Frame 872,000 1,579,000 82%

Single Angle K Frame 760,000 1,189,000 56%

Unequal Leg Angle X Frame 1,054,000 1,609,000 53%

Design Recommendations Reduction Factor Verification

Type of Cross Frames Test Results

Analytical Solution

Error %

R*Analytical Solution

Error %

Single Angle X Frame 872,000 1,579,000 82% 860,000 -1.4%

Single Angle K Frame 760,000 1,189,000 56% 762,000 0.3%

Unequal Leg Angle X Frame 1,054,000 1,609,000 53% 1,018,000 -3.4%

Design Recommendations Reduction Factor Verification

• From a stability perspective, overestimating the stiffness of the cross frames will lead to larger forces induced in the cross frames compared to the computer prediction (unconservative).

• In the finished bridge, over estimating the stiffness of the cross frame will lead to over prediction of the cross frame forces from truck traffic, which can therefore lead to predictions of potential fatigue issues that are not actually a problem. (conservative – but potentially very costly).

26

Impact of Overestimating X-Frame Stiffness

• Eccentric connection of single angle reduces stiffness due to bending

• Using truss models or line element solutions can significantly over predict stiffness of single angle cross frames

• Applying reduction factor to analytical models can produce relatively accurate estimate of cross frame stiffness

• Concentric members show good agreement with analytical models and do not require reduction factor

• Extensive fatigue testing also showed angles are Category E’ and not E as in AASHTO. Backside welds on K-frames can be omitted, greatly simplifying handling during fabrication.

27

Lab Tests: Large Scale Stiffness Conclusions

Strengthening Continuous Steel Bridges with Post-Installed Shear Connectors

TxDOT Research Project 0-6719

FERGUSON STRUCTURAL ENGINEERING LABORATORY

Graduate Research Assistants: Kerry Kreitman and Amir Reza Ghiami Azad

Post-Installed Shear Connectors

Double-nut bolt

High-tension friction-grip bolt

Adhesive anchor

Conventional connectors such as wedge anchors experienced too much “slop” in shear behavior.

Post-Installed Shear Connectors

• While the original research study focused on non-composite simple span bridges, there are a number of continuous steel girder systems that are non-composite.

• Many of these bridges are load rated and therefore a simple method of strengthening that does not require closing the bridge are highly desireable

Composite ratio:

Composite Behavior

Non-composite beam Fully composite beam

Slip at interface

Negligible slip at interface

Steel and concrete act separately Steel and concrete act together Increased strength and stiffness

Shear connectors

Partially composite beam

Some slip at interface

Project 6719 Overview

• Strengthen continuous bridges – Focus on negative moment region over piers

Compression

Tension

Compression

Tension

Positive moment Positive moment Negative moment

Tension

Compression

Composite action is less efficient

Project 6719 Overview

• Strengthen continuous bridges – Focus on negative moment region over piers

• Considered two strengthening methods: 1. Install connectors along entire bridge 2. Install connectors in positive moment regions

only and allow for moment redistribution – Concern of repeated yielding

• Additional fatigue testing of connectors

Moment Redistribution

• Controlled by “shakedown” limit state – Formation of residual moments that counteract

applied loads – Future cycles resisted elastically (no significant

increase in deflection with cycles)

• Experimentally proven for steel, but not composite beams – Steel is ductile, concrete is not

Test Setup

6’-6”

W30x90

6.5”

(Design based on bridges from survey of non-composite bridges in Texas)

30% composite

Installing Connectors

Testing Procedure – First Specimen

1. Elastic testing of non-composite beam 2. Install connectors 3. Elastic testing of composite beam 4. Cycles of load up to first yield 5. Cycles of load up shakedown limit 6. Fatigue testing 7. Static ultimate strength testing (?)

Completed

Current

Inelastic Cyclic Loading

P1

16’

P2 P2

10’ 10’

Preliminary Test Results

• Elastic behavior similar to predictions

• Shakedown observed at up to 5% beyond the predicted shakedown limit – Deflections stabilized with cycles

• Promising technique for strengthening

Elastic Testing

-0.4

-0.2

0

0.2

0.4

0.6

0.80 8 16 24 32 40 48 56 64 72 80

Defle

ctio

n (in

)

Distance Along Beam (ft)

ANSYS, non-composite

ANSYS, composite, 440 k/in

ANSYS, composite, 800 k/in

ANSYS, composite, 1600 k/in

ANSYS, fully composite

Test, non-composite

Test, composite

P1 = 40 kips

Inelastic Testing

P1 (kips)

P2 (kips)

Cycles to Shakedown

Residual Deflection (in)

140 160 3 0.04

144 165 3 0.07

148 170 4 0.11

152 175 4 0.16

157 180 5 0.21

161 185 5 0.27

165 190 6 0.35

170 195 5 0.42

174 200 7 0.52

178 205 7 0.62

183 210 8 0.81

187 215 9 0.93

191 220 11 1.14

First yield predicted

Predicted shakedown

limit

(P1 = 0.87*P2)

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9

Incr

emen

tal P

eak

Defle

ctio

n (in

)

Cycle Number

P1=140kP1=161kP1=187k

Criteria: Shaken down when change in deflection < 0.01 in.

Colors in table coincide with bars in chart.

Inelastic Behavior

Inelastic Behavior

Finite Element Modeling

0

0.5

1

1.5

2

2.5

3

3.5

4

130 140 150 160 170 180 190 200 210 220 230

Defle

ctio

n un

der P

1 Loa

d (in

)

Applied P1 or P2 Load (kips)

P1

P2 P2

Not shaken down after 10 cycles

Test FEM prediction

Fatigue: Overall Approach

• Small-Scale Fatigue Tests Showed Conventional stress-based approach – Too conservative for partially composite beams – VQ/I is inaccurate (assumes no slip)

• Focus on developing a slip-based approach

– Compute slip demand – Compare to allowable slip limit

Fatigue Test 1 (South Span) Setup

• First test was carried out on span that had not been subjected

to shakedown. • P=50 kips (cycle from 2 to 52 kips) • Slip transducers and strain gages used along the length of the

beam. • Test Results - >2 Million cycles of load and no failure • A gap around the fasteners tends to form that greatly reduce

the fatigue-induced stress.

P

Strain Gages and Slip Transducers

• P1 = 2 to 77 kips both during fatigue testing and intermittent

static testing • Strain Gages and Slip transducers positioned around post-

installed connectors. • Test stopped at approximately 340,000 because the specimen

was tending towards a non-composite girder (for service levels of load). At ultimate we think the fasteners will engage to give composite section.

Slip Transducers

P1

Strain Gages and Slip Transducers

Fatigue Test 2 (North Span) Setup

Current Work

• Test current continuous beam to ultimate load.

• Construct another specimen – increasing total length by approximately 25%.

• Shakedown test • Fatigue tests

• Ultimate load test • Parametric FEA work • Project will complete in August

Questions

?