THE VALUE OF RESEARCH

Joseph A. Yura

Professor Emeritus

Univ. of Texas at Austin

The problem with research is …..

IT COSTS MONEY

DIVERTS FUNDS FROM OTHER PROJECTS

NOT COST EFFECTIVE

FACULTY SHOULD BE TEACHING, NOT

CONDUCTING RESEARCH

MAKES SPECIFICATIONS MORE COMPLEX

PROFESSORS AND STUDENTS ARE NOT

FAMILIAR WITH DESIGN

RECOMMENDATIONS ARE NOT PRACTICAL

PROJECTS TAKE TOO LONG

THE PUBLIC MAY THINK WE HAVE A

PROBLEM

BENEFITS OF RESEARCH

SOLVE A PROBLEM

DEVELOP ALTERNATIVE SOLUTIONS

ADVANCE THE PROFESSION

EVALUATE NEW CONCEPTS

EXPAND EXISTING KNOWLEDGE

UNINTENDED EFFECTS

TRAINING

Apollo 13 – “Houston, we have a problem”

Elastomeric Pad

Walking

States with Bearing

Pad Slipping

No

Slipping

45% Slipping

55% 28

23

SLIP SURVEY - 1992

Excluding States

w/ Mounting Plates

No Slipping

Slipping

63%

14 24

37%

Slipping Cases per State

3 8

23

17

0

5

10

15

20

25

None 1 to 5 6 to 10 Over 10

Bridges per State with Slipping

Number of States

Cleaned

Surfaces

0

30

60

90

120

150

0 200 400 600

Days After Resetting

Wax on Concrete Surfaces

Dis

pla

cem

en

t (m

m)

FIELD STUDY

CONCLUSIONS FROM SLIP STUDY

SLIP CAUSED BY PARAFFIN WAX

WAX WAS ADDED TO NATURAL RUBBER PADS TO

PASS THE REQUIRED OZONE TEST

TAPERED PADS DID NOT CAUSE THE PROBLEM

FIX - REPLACE NR PADS WITH NEOPRENE BUT

SURFACES MUST BE CLEANED

IMPACT

Based on a NCHRP project, the ozone test has

been eliminated so there is no need to add wax

BENEFITS OF RESEARCH

SOLVE A PROBLEM

DEVELOP ALTERNATIVE SOLUTIONS

ADVANCE THE PROFESSION

EVALUATE NEW CONCEPTS

EXPAND EXISTING KNOWLEDGE

UNINTENDED EFFECTS

TRAINING

RECOMMENDED CONNECTION DETAILS

BENEFITS OF RESEARCH

SOLVE A PROBLEM

DEVELOP ALTERNATIVE SOLUTIONS

ADVANCE THE PROFESSION

EVALUATE NEW CONCEPTS

EXPAND EXISTING KNOWLEDGE

UNINTENDED EFFECTS

TRAINING

Lean-On Bracing

in Frames

Lean-On Bracing

in Beams

LEAN - ON BRACING

Advantages of Lean-On Bracing System

• Reduce brace forces due to truck loading in

completed bridge

– Reduction in forces due both to lean-on system and

from moving first intermediate cross-frame line off

support location

• With fewer braces:

– Economic benefit both at time of erection and in

ongoing maintenance (fewer cross-frames to inspect)

Implementation of Lean-On System

• Three bridges with severe skew were constructed in Lubbock, TX:

1) Two-span (150.5', 139.0'); six 54" deep girders; 59.6º skew

2) Two-span (182.5', 171.7'); nine 66" deep girders; 53.7º

skew (Two Bridges)

Conventional layout:

Total of 128 intermediate cross-frames L4×4×3/8

(larger cross-frames at support - L5×5×3/4)

Lean-on layout:

Total of 35 intermediate cross-frames L5×5×3/4

Bracing Layout for Lubbock 9 Girder Bridge A

A

B

B

C

C

D

D

128 intermediate cross-frames in conventional layout

35 intermediate cross-frames in final lean-on layout

(all cross-frames in final layout 5x5x3/4)

X → added to control differential deflection

X → added for stability during girder erection

Section A-A

Final Bracing Layout for 9 Girder Bridge

Section B-B

Section C-C

BENEFITS OF RESEARCH

SOLVE A PROBLEM

DEVELOP ALTERNATIVE SOLUTIONS

ADVANCE THE PROFESSION

EVALUATE NEW CONCEPTS

EXPAND EXISTING KNOWLEDGE

UNINTENDED EFFECTS

TRAINING

SHOULD OPINION TRUMP

RESEARCH RESULTS?

1992 AASHTO COMMENTARY

p. 624

Tapered layers in reinforced bearings are expressly

prohibited because they cause large shear strains

and bearings made from them fail prematurely because

of delamination or rupture of the reinforcement.

An Experimental Study of Flat and Tapered

Elastomeric Bridge Bearings

Horizontal force/deflection

Shear stiffness

Compressive stiffness

Rotational stiffness

Compression failure

Shear fatigue

Compression fatigue

Compressive creep

Tapered Bearing Test Specimens

• Taper: 4.17, 5.5%

• Shear Modulus(psi): Nominal 0.7 MPa - 100 psi

Nominal 1.4 MPa - 200 psi

• Elastomer thickness (mid-height): 44.5 mm - 1.75 inches

• 3 and 6 shims -- 2.66 mm - 12 gage A570 steel

Radially Spaced Shims Parallel Shims

229mm (9") 229mm (9")

a

a a a

b

b

b

b

a

a a a

a a a

c

Tapered

Bearing Test

Conclusions 1995

• Tapered bearings performed well

- up to 6 % slope (max tested) with no problems

- slope mismatch to 0.01 radians OK

- use harder elastomers for steeper slopes

- modify Dc for tapered bearings

- parallel shim orientation

• Shear strains over 3.0 possible without damage

Commentary.

Tapered layers cause larger shear strains and bearings

made with them fail prematurely due to delamination or

rupture of the reinforcement. All internal layers should be

the same thickness because the strength and stiffness of the

bearing in resisting compressive load are controlled by the

thickest layer.

CURRENT BRIDGE SPECIFICATION

Tapered elastomer layers shall not be used. All

internal layers of elastomer shall be of the same thickness.

The problem with research is …..

IT COSTS MONEY

DIVERTS FUNDS FROM OTHER PROJECTS

NOT COST EFFECTIVE

FACULTY SHOULD BE TEACHING, NOT

CONDUCTING RESEARCH

MAKES SPECIFICATIONS MORE COMPLEX

PROFESSORS AND STUDENTS ARE NOT

FAMILIAR WITH DESIGN

RECOMMENDATIONS ARE NOT PRACTICAL

PROJECTS TAKE TOO LONG

THE PUBLIC MIGHT THINK WE HAVE A

PROBLEM

IT CAUSES HAIR LOSS

Test Program

• Horizontal force/deflection

• Shear stiffness

• Compressive stiffness

• Rotational stiffness

• Compression failure

• Shear fatigue

• Compression fatigue

• Compressive creep

Non-Uniform

Loading

Rotation

Capacity

OBJECTIVE

DEVELOP A STATE OF THE ART DESIGN GUIDE

FOR STEEL TRAPEZOIDAL GIRDERS

PROVIDE A ONE-DAY TRAINING SESSION

FOR TXDOT DESIGN PERSONNEL

Internal K-Brace External Diaphragm

Strut

GUIDE OUTLINE

General Behavior of Trapezoidal Girders

Tub Girder Properties

Structural Analysis

Pouring Sequence

Diaphragms and Cross Frames

Top Flange Lateral Systems

Bearings and Bearing Stiffener Details

Field Splices

GENERAL BEHAVIOR OF TRAPEZOIDAL GIRDERS

Recently expanded this task because of the failure of the

Marcy, NY pedestrian bridge that failed by global lateral-

torsional buckling

UTrAp 2.0:

Analysis of Steel Box Girders During Construction

UTrAp 1.0 - EXPANDED

•elastic buckling analysis of trapezoidal girders under

construction loading

•stability evaluation not readily available from other

sources

•accurately predicts failure of Marcy pedestrian bridge Formula developed to predict lateral buckling

MARCY PEDESTRIAN BRIDGE COLLAPSE

• October 10, 2002: bridge buckled during concrete deck pour

• One fatality, nine serious injuries

• Eight lawsuits filed seeking $332 million in damages

Failure of Marcy Pedestrian Bridge

Characteristics :

1. Straight

2. Single trapezoidal box girder

3. Open-section: No top lateral bracing system

a. Open section (no top lateral bracing system Very flexible)

b. Pseudo-closed section (with top lateral bracing system 10-100 times torsionally stiffer than open-section

GLOBAL LATERAL TORSIONAL BUCKLING

Buckled Shape

• Clearly predicts lateral

torsional buckling, the

actual failure mode of

the bridge

• Predicts failure when the

concrete deck has

reached 68 feet – actual

failure occurred at just

over 85 feet.

Typical runtime – 5 minutes or less

SOLE PLATE AND ROCKER REPLACED WITH ROLLED WIDE - FLANGE SECTION

ANCHOR BOLTS REMOVED

RA

RB

TM

PTRUCK LIVE LOAD

d d

C

ARMT

2d= +

P

RMT

2dB = -

P

2

2

Center of Rotation

y

= y

DIAPHRAGMS AND K-FRAMES

Place internal K-frames at every other panel point of the top

lateral truss. Closer spacing often results in larger forces in the

braces due to in-plane bending.

Distortional normal stresses will be less than 5% of the bending

normal stress if internal K-frames have a maximum spacing of 1/5

the span but not less than 30 ft.

The function of the external K-frames can be categorized as

supporting constructability. Using just 2 or 3 external K-frames

per span will often result in good control of the relative twist

between adjacent boxes.

The K-frames should be biased towards the midspan region of the

girders to control twist. Hand methods are very conservative in

predicting forces in external K-frames

The research presented in this paper showed that a

rolled wide-flange section can be used as a bridge

support bearing. A simple interaction equation is

used to design the bearing for static strength and the

fatigue tests showed that a rolled wide-flange can be

classified as a category A detail when subject to out-

of-plane shear distortion. Also described in a

qualitative manner was the behavior of the flange as

it acts as a bearing plate to distribute the web load to

the concrete. The flange acts more as a beam on an

elastic foundation than as a rigid plate. The primary

advantages of the wide-flange section compared with

the rocker detail are its simplicity and cost-

effectiveness.

Draft AASHTO specification developed for top lateral systems

- Stability and torsional loads loads

- Diagonal systems and PMDF

- Strength and stiffness provisions

PMDF alone can be used to stabilize the compression

flanges of straight girders during construction

PMDF have insufficient strength to support the torsional

forces in a typical curved trapezoidal box girder

Top diagonal systems are needed for torsional loads

during construction

TOP FLANGE LATERAL SYSTEMS