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David NashEngineering Tech 4
Outline
Background/ Stats Reasons for our action – FHWA Memoranda LADOTD Actions and Decisions (Bridge Maintenance)
Progress Report (Projects, Methodology)
Nat’l Background & Stats
590,000 highway bridges in the U.S. National Bridge Inventory
484,500 are over water 20,904 are scour critical 14,000 bridges in Louisiana database 4,127 bridges rated “U” 1,140 bridges rated “3”
SI&A Guide
•Provides guidance in evaluating and coding specific bridge data
•Item 113 – Scour Critical Bridge Rating
Item 113 ‐ Scour Critical Bridges
8 ‐ Bridge foundations determined to be stable for the assessed or calculated scour condition. Scour is determined to be above top of footing
6 ‐ Scour calculation/evaluation has not been made. (Use only to describe case where bridge has not yet been evaluated for scour potential)
3 ‐ Bridge is scour critical; bridge foundations determined to be unstable for assessed or calculated scour conditions
N ‐ Bridge not over waterway
U ‐ Bridge with "unknown" foundation that has not been evaluated for scour. Until risk can be determined, a plan of action should be developed and implemented to reduce the risk to users from abridge failure during and immediately after a flood event
T – Bridge over “tidal” waters that has not been evaluated for scour , but considered low risk.
Scour ?
Types of Scour Local Scour – removal of sediment from around bridge piers or abutments
Contraction Scour – removal of sediment from the bottom and sides of a creek, stream or river
Degradational Scour – removal of sediment from the river bottom by the natural flow of a river
•Schoharie Creek Bridge Collapse due to scour•FHWA Technical Advisory T5140.23
• Required states to evaluate existing bridges for scour• Required all DOT’s to develop Plans of Action for vulnerable to scour
•FHWA Memoranda• January 9, 2008 – Provided guidance and deadline (November 2010) for
eliminating the population of bridges having unknown foundations
• June 3, 2009 – Deemed it acceptable to have a bridge coded U for SI&A item 113 after November 2010 as long as a POA was developed
•Compliance
Schoharie Creek Bridge Final design accepted in January 1952 Bridge design called for five spans supported by concrete pier frames which consisted of tapered columns and tie beams
Columns were fixed into a lightly reinforced plinth, which rested on a shallow reinforced spread footing designed to be protected by dry riprap
Bridge opened to traffic in fall 1954
Schoharie Creek Bridge Cont’d
CLSymmetrical about
Plinth
Footing
Plinth Reinforcement
Column
Floor Beam atapprox. 20' o.c. Knee Brace
Cantilever FloorBeam Ends
Main GirderDeck
5'-0" wide X10'-0" deepTie Beam
7'-0" sq Column
Stringer at8'-6" o.c.
Bearing
27.75' 57' 27.75'
112.5'
Figure 1 - Pier Section ( after "Collapse," 1987 )
PIER 1 PIER 2 PIER 3 PIER 4
Flow
EastAbutment
SlopedEmbankment
RiprapSlopedEmbankment
West Abutment
NORTH
110' 120' 110' 100'SPAN 1100'
SPAN 2 SPAN 5SPAN 4SPAN 3
Figure 2 - Schematic plan of bridge ( after "Collapse," 1987 )
Observations
•6 inches of rainfall combined with snowmelt produced 50 year flood morning of April 5, 1987 causing collapse
•100‐yr. flood experienced 1 year after completion
Schoharie Creek Cont’d
Schoharie Creek Cont’d
Here in Louisiana…
1990’s ‐ Began assessing bridges for scour susceptibility.
Bridges without plans rated “U” ‐ Unknown Keeping an eye on the uncertain ones. 2002 ‐ 5,460 Unknown Foundations Explored the many methods being used and recommended by FHWA
For A Good Cause…
FHWA January 2008 Mandate
November 2010 – target date for eliminating bridges with unknown foundations from the NBIIf structures rated “scour critical” remain, there must be a POA created for them.
Louisiana Scour Bayou Anacoco Hickory Branch
Bogue Chitto Relief System
What Have WE Done ?
1990’s – LADOTD began assessing bridges for scour susceptibility.
Bridges without plans rated “U” ‐ Unknown Keeping an eye on the uncertain ones.
Research In 2003 LA DOTD was approached by FDH‐SE, Inc.
Specialized in Dispersive Wave methodology
Other methods available included;• Parallel Seismic• Cross bore‐hole logging• Sonic Echo
Pilot Project
107 Bridges were tested 11 “Control” structures
Distribution of Test Sites
Results Inconsistency in results for steel piles Some errors in timber piles, due to poor selection of timber piles
2.69% ‐ Average error for concrete piles Parallel Seismic testing would be used for steel piles
Intentions for Data Usage Bridge Scour analysis to be conducted Scour Susceptibility rated based on predicted scour and (3) tested pile lengths per bridge
Decrease number of bridges with unknown foundations
Construct/Implement Plans Of Action for all structures still rated “U”
Current Testing LA DOTD contracted FDH‐SE, Inc. to test 1248 concrete and timber piles (1156 bridges)
FDH and LADOTD agreed that parallel seismic NDT would be done for all steel piles
State Project No. 700‐99‐0499 (Phase 1 and 2 Analysis) 900 bridges
50% timber 42% concrete 8% steel
Unknown Foundations Elimination
Category A: composed of bridges that are actively scouring or have existing scour holes
Category B: composed of bridges that have had inactive scour in several years. Cat B also includes bridges with drift problems
Category C: composed of bridges that have had no measured scour over the last 20 years (also includes those built after 2000)
LADOTD Mandate Response Category definitions were developed based on Functional Classification (I26), ADT (I29),
and Detour Length (I19). Some target dates are below.
November 30, 2010 – All Bridges still coded “U” shall have a POA developed and implemented (4000 bridges)
November 30, 2010 – Obtain a final approved action plan for the elimination of “Unknown Foundations” from FHWA
December 31, 2011 – Completion of all bridge evaluations for bridges in Category A
December 31, 2013 – Completion of all bridge evaluations for bridges in Category B
December 31, 2014 – Completion of all bridge evaluations currently under contract and evaluation of bridges that have previous pile length testing completed. (1621 bridges)
December 31, 2015 – Completion of all bridge evaluations for bridges in Category C.
Sometimes, you have to look underground