External PT to Increase Capacity of Existing Steel Bridges

External PT to Increase Capacity of Existing Steel Bridges

Clyde Ellis

Washington DC Office

May 7, 2013

•Approximately 600,000 U.S. highway bridges •69,223 deficient bridges•Nearly one-third of the nation’s highway bridges +50 years old•The average age of bridges in the U.S. is 42 years old

Database released in Feb 2011 by FHWA, Transportation for America (T4) analysis published Mar 2011

America’s Aging Infrastructure

~ $10B /YR

The maintenance backlog will only worsen as bridges age and costs rise.

Need to invest in our infrastructure again to enable our economy to grow, diversify and stay competitive

Bridge Repair Funding vs. Need

• Federal law requires bridge’s 20+ span to be inspected bi-annually

• Evaluate: Superstructure, Substructure, Deck

• Rating 0 to 9; 9 is best• If 1 of 3 components scores 4 or lower• “Structurally Deficient” requires

– Significant maintenance, rehab, replacement, restricted use

– Inspected annually– Consider load rating

Structurally Deficient Bridges

• Increased loads– Permanent loads (Restripping, addition of

safety median barrier, addition of deck overlay, increase in roadway width)

– Temporary loads (Overloaded trucks, heavier truck loads)

• Surface Transportation Assistance Act –increased truck sizes and weights as result of economic development

• Permit Design Live Load – allows substantial increase in live loads for special or key routes

Reasons for Low Rating

• Deterioration/Corrosion of primary load carrying members

– Deficient fatigue performance of details

– Lack of maintenance (exposure to de-icing salts or salt water)

– Wear/tear due to increasing truck traffic

• Human Factors– Design, Fabrication, Construction

errors

Reasons for Low Rating

1. Determine desired load rating2. Perform structural analysis to determine

deficient members • If overload in flexure: resulting from service load

stresses, fatigue stresses or ultimate capacity ?3. Identify most cost eff. technique to strengthen4. Develop cost estimate to strengthen vs. replace

• Rule of thumb: replace if strengthening is more than 45% - 54% of replacement cost

Strengthening vs. Replacement

• Considerations– construction cost – annual maintenance– safety aspects / service life

• Most frequent techniques– Replace deficient members– Increase the member

cross-section by adding steel cover plates

Bridge Strengthening

• Other techniques– Lightweight deck replacement– Composite action between deck

and supporting members– Supplemental supports to reduce

span length– Adding new girders– Converting single spans into

continuous span– External PT

Bridge Strengthening

First Prestressed Concrete Bridge (Germany); PT Bars

First Strengthening Steel Highway Bridge (MN)

First Strengthening Steel Truss Bridge (France)

Development of Ext. P-T Technology

1937

1964

1975

Ext PT Strengthening Applications Limited:•Lack of information on how to apply technique•No specific guidelines on this method of strengthening

• Introduction of externally applied load to a structural member with tendons that relieves longitudinal tension overstresses resulting from service load and fatigue stresses

PT Basic Concept

• Compressive force applied to tension flange to resist bending moment from applied loads

Q

Q fc 2fcfc

ft = fc

2Q 2fcfc 2fc

2ft = 2fc

ft

fc= ft

fr

e

Effect of Eccentric Tendons at Midspan

• Structurally very effective

– Negligible increase to dead load

– Reduces LL stresses and deflections

– Improved fatigue category details

– Low initial construction cost

• Ease of installation and overall speed of construction

• No disruption to traffic flow

• Ability to adjust the level of strengthening in future

Advantages of External PT

1. Determine moments for applied loads at mid span2. Calculate girder stresses in tension flange3. Calculate allowable stresses based on as-built material4. Assume PT eccentricity and calculate PT force

required to makeup deficiency5. Check member buckling, member yielding and tendon

yielding6. Repeat for other critical areas7. Determine PT anchor location and design anchor

bracket

Simple Span Girder – Design Process

Q

e

• Immediate Losses– Friction Losses– Seating Losses– Elastic Shortening

• Long Term Loss– Steel Relaxation

• Be cognizant of minimize clearances for mounting, tendon path and stressing

• Stage stressing – if PT is not concentric with member longitudinal axis

Add’l Design Considerations

• High Strength 7 wire strand• ASTM A416 • Ultimate Strength 270 ksi• Low Relaxation• Reeless coil• Common sizes

– 0.6”x 7, 12, 19

• Thread high strength bar• ASTM A722• Ultimate Strength 150 ksi• Common Sizes

– 1” to 1-3/4” dia.

• Straight lengths up to 60’

Prestressing Steel

External Tendon Cross-section

P-T Anchorages

• Built up section of steel plates• Mechanical locking device

– Wedges and wedge plate– Nut and washer

• Trumpet (strands only)

• Primarily used with strand tendons• Applicable if design requires P/A and uplift• Most common type of steel plate and bent pipe

PT Deviators

Minimum Tendon Radius

• HDPE pipe protective sheathing – Double corrosion protection for PT bars

• Fill duct voids with grout (alkaline environment)• Coated strand (Galvanized, G&W, epoxy)• Galvanized anchor components• Watertight connections• Anchorages details

– Weather proof enclosure– Fabricate with weep holes

Corrosion Protection

• Replaceable– Bolted connections– Tendons can be cut and replaced

• Inspectable– Magnetic flux leakage– Bore scope inspection

• Durable• Force verification

– Load cell, lift-off for ungrouted PT– Vibration analysis

• Ability to Increase Prestress Force– Future PT– Stress tendons to a higher force than

originally installed at

External PT Features

Strengthening Case Studies

Androscoggin River Bridge

• Part of Turnpike Ext. built in 1955; widened in 1995

• Max LL configuration accommodate 3 lanes; rates low

• Issue made worse with ongoing bottom flange corrosion

• 2 lane configuration could rate below 1 if not strengthened

Brunswick, ME

157 ft

Proposed Tendon Layout

G #2

G #1

FRAMING PLAN

Androscoggin River BridgeBrunswick, ME

End Anchorage Assembly

External PT Duct

Hialeah ExpresswayMiami, FL

• New loads from a wider bridge

• Retrofit straddle bent to upgrade structural capacity

• Steel plate girders strengthening with external P-T

PT tendons 1, 2, 3

Repair Details

Strengthening Completed

• External PT strengthening has a successful history 60+ years

• Longitudinal tendons can be used to strengthen steel girders, truss chords/diagonals and floor beams

• Ext PT has been used to relieve stresses, reduces deflections, improve fatigue details and add substantial live load capacity an existing bridge

Summary

• Anchorages and tendons can be prefabricated to reduce time onsite

• Anchorage/deviator details have been developed overtime with no reported issues

• Sufficient knowledge exists to develop a manual to assist engineers in applying PT principles

Summary

Thank You!