JKrajewski_ODOT214_Bridge of the Gods

OREGON DOT BRIDGE SEMINAR
MAY 13-14, 2014
BIRDGE OF THE GODS
STRINGER STRENGTHENING REPAIR
Joe Krajewski, PE - HNTB

OUTLINE
1. Introduction
2. AASHTO
3. Existing Stringer Capacity
4. Open Grid Deck Information
5. Strengthening Assumptions
6. References
7. Details
8. Summary
BRIDGE OF THE GODS – STRINGER STRENGTHENING
ODOT BRIDGE SEMINAR, MAY 13-14, 2014

1. INTRODUCTION
Sunday June 18, 1922
3700 people made the trek from Portland to Cascade Locks to attend a
dedication of the Oregon Main Pier.
The sky was clear blue.
A Native American stoop atop the new pier.
He raised his bow upward toward Washington and let fly an arrow as a
symbol of cooperation and hope for future prosperity as the Man-made
version of the Bridge of the Gods was heading towards completion.
For almost 90 years the Bridge has been a vital link for the region.
Connecting people that were separated long ago, as legend has it, by and
angry god called Tyheesaghalie.

1. INTRODUCTION
 Bridge Built from 1922 to 1926
 Named for the Natural Stone Bridge that once spanned the location
 Bridge Raised 44 feet between 1938 to 1940
 Bridge Re-Decked & Modified 1966 to 1968
 Owned by the Port of Cascade Locks
 Tolled Bridge
 Restricted Load Posting in Summer of 2013
 Posted Weight Limit = 8 Tons for all Vehicles down from 40 Tons
 Severe Economic Impact to the Area
 Fall 2013 - Design, Bid and Build Stringer Strengthening on Short
Schedule
 Goal to Restore Load Posting to 40 Tons by January 1, 2014

Elevation – Looking Up River
1. INTRODUCTION
1,131 ft (Open Grate Deck)
WASHINGTON
OREGON
48 Stringer Spans (22’-9” ft to 25’-3”)

AASHTO LRFD – Connections & Splices
 Section 6.13.1 - General
− Insofar as Practical, connections should be
• Symmetrical
• Not less than 2 Bolts
• Eccentric connections should be avoided
• Designed for combined effects of Shear & Moments.
 Section 6.13.2 – Slip Critical Connections
− Joints subject to
• Stress Reversal,
• Heavy Impact Loads,
• Severe Vibration,
• Joints with Welds & Bolts are transmitting load on a common faying
surface,
− Load Combination Service II

AASHTO LRFD – Decks and Deck Systems
 Section 9.8.2.2 – Open Grid Floors
− Open Grid Floors shall be connected to their supporting members by either
− Welding
− 3” Long Single-Sided Weld on Main Deck Grid Elements
− 1.5” Long Weld on each of Main Deck Grid Elements
− Mechanically Fastening
− “Long-term experience indicates high stresses may develop between deck &
supporting member resulting in local connection failures & separation of deck
regardless of the degree of composite action between Deck & Supporting Member”.
− Welding within Open Grid Floors should be considered a Category E Fatigue Detail.

Stringers – CB15X37.5 (Carnegie Beam) – ASTM A-7 (1923)
Unbraced Length of Compression
Flange
Fy = 33 ksi,
rt = 1.73 in. (eq. 6.10.8.2.3-9)
Lp = 4.27 ft (eq. 6.10.8.2.3-4)
Lp = 13.43 ft (eq. 6.10.8.2.3-5)
Lb = 25 ft
Fcr = 16.47 ksi (eq. 6.10.8.2.3-8) < ½ Fy
To Reach Full Fy which would
achieve 40 Ton Load Posting
Requires -
Lateral Bracing Every 13.43 ft.

Grid Deck – 5 in x 4 way Greulich Open Bridge Deck System

Strengthening Assumptions
 Deck Main Bars are Field Welded to each Stringer in accordance with AASHO 1966.
− These connections provide lateral bracing to the stringers, but how much bracing is
not entirely certain.
− In addition, the connections are a Class E Fatigue Detail.
− Also, relying on these connections for maintaining the posted load rating makes these
Connections a Primary Structural Member instead of a secondary member like they
were designed as.
− Finally, relying on the Main Bar to Stringer Welded Connections would mean that
Biannual Bridge Inspections would require that all these welds be closely inspected &
repairs performed immediately.
 According to Bridge Maintenance Records, annual maintenance includes weld break
repairs.
 Inspection Reports & Field Observation indicate that the quality of the welding varies
greatly.
 The Stringers are very sensitive to additional Dead Load.
 Strengthening Requirement is to increase the Load Posting by 500% (8 tons to 40 tons).

15C33.9
CB15x37.5
CB15x37.5
Strengthening Solution
 Suppose we make a stronger connection between 2 deck main bars and stringers at mid-
span of each stringer span.
 It would significantly reduce the number of connections designated as primary structural
members
 Provide reliable lateral bracing designed in accordance with AASHTO
 Reduce Bridge Inspection
time significantly.

5/8”
5/8”
5/8”

Single Plate Variation based on “As
Built” Conditions
Typical Installation

OPTION D

 In-Depth Discussion of Bracing of
Superstructure Elements
 Information can be applied to Columns
& Frames as well.
 Section 1.2 Lateral Torsional Buckling
Overall Stability Improved by providing
braces to reduce the Unsupported Length.

 In-Depth Discussion of Steel Grid Deck Design
 Includes Experimentation On Load Distribution & Fatigue
 Includes Recommendations for LRFD compatible design equations

Other References
 LIGHTWEIGHT BRIDGE & SIDEWALK DECKING – New Products, New Designs, New
Test Findings by Gene Gilmore – IKG Industries, 1987
 STEEL BRIDGE DESIGN HANDBOOK – Bridge Deck Design, FHWA-IF-12-052-Vol. 17,
November, 2012
 GUIDE TO STABILITY DESIGN CRITERIA for Metal Structures, Edited by Ron Ziemian,
Structural Stability Research Council, 2010
 STRUCTURAL ENGINEERS’ HANDBOOK – Data for the Design & Construction of
Steel Bridges & Buildings, Milo Ketchum, 1924
 SHAPE BOOK containing Profiles, Tables & Data, Carnegie Steel Company, 1917

 Structural Strengthening/Repair/Rehab is more difficult than New Design
− Creation of Hybrid Bridge Components whose Design Life is Uneven.
− We have to use the existing materials and dimensions as part of the design.
− There is no Design Code Specifically for Strengthening/Repair/Rehab.
− Existing Structures are very sensitive to additional Dead Load.
 Strengthening conservatism depends on how much improvement in capacity is
required.
− For the Stringers here the improvement was significant (8 to 40 tons)
− If the improvement was slight, say improving from 8 tons to 10 ton load
posting then one would be inclined to rely on all the deck to stringer welds
instead or perform an alternative analysis of stringer capacity.
 Finally, Strengthening details are ever evolving as they are installed –
there is always room for improvement.
Bridge of the Gods – Stringer Strengthening
Summary

Bridge of the Gods – Stringer Strengthening
QUESTIONS ?????
The Original Bridge of the Gods Design started in 1922.

JKrajewski_ODOT214_Bridge of the Gods

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