Not needed Substructure unit foundation (including piles, caissons, etc.) is well above flood water elevation for 500-year frequency flood or is not subject to stream/tidal flow

In document Bridge Safety Inspection Manual (Page 81-94)

Type 24 month NBI Insp

9 Not needed Substructure unit foundation (including piles, caissons, etc.) is well above flood water elevation for 500-year frequency flood or is not subject to stream/tidal flow

8 5 Years

Substructure unit foundation has been determined to be STABLE by:

 Calculated scour conditions - Calculated scour is above the top of footing. (This includes new bridges with properly designed counter-measures), or

 A culvert with an integral bottom and no scour problems, or

 The Scour Critical Bridge Indicator (SCBI) from PA Observed Scour Assessment for Bridges (OSAB) would rate the substructure unit = 8.

7 5 Years

Countermeasures have been installed and remain in place to correct a previous problem with scour at this substructure unit. Countermeasures have been verified through scour assessment. Unit is no longer considered to be scour critical by calculated scour method or PA OSAB (SCBI = 7).

Integrity of substructure unit foundation against scour has not been determined. Neither scour calculations nor the SCBI rating from PA OSAB for this substructure unit foundation has been made.

5 2 Years

Substructure unit foundation was determined to be STABLE by :

 Calculated scour is at or above bottom of footings, or

 For deep foundation, calculated scour is between footing and pile/caisson tip and analysis of substructure unit using calculated scour depth indicates foundation is stable, or

 PA OSAB would score substructure unit SCBI = 5

4

Substructure unit foundation was determined to be STABLE by:

 Calculated scour conditions and analysis results are same as SCBI = 5 except that field conditions indicate action may be needed to protect exposed foundation from additional scour and erosion.

 PA OSAB would score substructure unit SCBI = 4

3

Substructure unit foundation is SCOUR CRITICAL because its foundation determined to be UNSTABLE by:

 Analysis of substructure unit foundation considering scour indicates instability.

Scour may be within limits of spread footings or of pile tips and be unstable. Scour below spread footing or deep foundation tips cannot be deemed stable. Or

 PA OSAB would score substructure unit SCBI = 3

NOTE: Action is recommended to protect the substructure unit foundation from further scour.

Substructure unit foundation is SCOUR CRITICAL. Field conditions indicate that extensive scour has occurred at the substructure unit and it is considered UNSTABLE.

NOTE: Immediate action is required to protect the substructure unit foundation from further scour.

Substructure unit foundation is SCOUR CRITICAL. Bridge is closed to vehicular traffic.

NOTE: Additional underwater inspections may be needed to monitor stability so actions may be taken to prevent uncontrolled collapse (especially if bridge remains open to pedestrians).

2.6.3 Assessment for Bridge Scour

2.6.3.1 GENERAL REQUIREMENTS FOR SCOUR ASSESSMENTS

INTRODUCTION: One of the more effective ways of preventing the loss of a bridge due to scour failure is to identify those bridges most likely to be vulnerable to scour. With this determination, called a scour assessment, the bridge inspectors and owners can concentrate inspection/monitoring efforts and remedial actions to mitigate conditions at bridges with critical vulnerability. To this end, scour assessments become an important part of bridge safety inspections in PA. Moreover, such assessments are required by the NBIS because they are deemed to be a key part of a comprehensive underwater inspection program.

PURPOSE OF SCOUR ASSESSMENTS: The main purpose of the scour assessment of an existing bridge is to determine whether the bridge is vulnerable to scour. A scour critical bridge is one whose foundation(s) has been determined to be unstable for the predicted scour conditions. The BMS2 item named 4A08 Scour Critical Bridge Indicator (also NBI Item 113) is used to record a single digit coding for the bridge’s vulnerability against scour. See page 3-35 of BMS2 Coding Manual Pub. 100A for 4A08 coding instructions.

DESCRIPTION: The results of the scour assessment are to be used in conjunction with information from regular and underwater bridge inspections to ensure that current stream (and bridge) conditions are used to evaluate the ongoing vulnerability of the bridge. Changes in stream and streambed conditions (e.g., scour depth/location, aggradation, degradation, debris, installation of countermeasures) discovered during inspection can dramatically affect the vulnerability of a substructure unit foundation and must be considered. Accordingly, the inspection information and the scour assessment must be used together for the evaluation of the overall safety of the bridge.

The inspection information is needed to validate the input parameters and results of the scour assessment. The scour assessment results are used to determine if scour poses a threat to the bridge. These scour assessments must be updated as part of the Routine Inspections for NBIS bridges.

The two acceptable methods of performing scour assessments in PA are:

1. Theoretical Scour Calculations

2. PA’s Observed Scour Assessment for Bridges methodology

NBIS BRIDGE REQUIREMENTS: For all NBIS bridges over water that are subject to stream/tidal flow and whose foundations are below the water surface elevation of the 500-year flood:

1. A Scour Assessment for current conditions must be in the inspection file

2. BMS2 Item 4A08 Scour Critical Bridge Indicator must be determined at each Regular and Underwater Inspection

REQUIREMENTS FOR 8’-20’ BRIDGES: Scour assessments are not required for non-NBIS bridges.

However, the Districts and other bridge owners are urged to apply them for any bridges at risk due to scour.

2.6.3.2 SCOUR ASSESSMENT USING THEORETICAL SCOUR CALCULATIONS

A scour assessment of a bridge using the theoretical scour calculations is a method based on hydrologic and hydraulic analyses of the stream and bridge opening. Use DM4 PP Chapter 7 and FHWA Technical Advisory

“Evaluating Scour at Bridges” (T 5140.23 October 1991) for guidance on the methodology. In good design practice, the bottom elevations of foundations are established considering the calculated scour depth. These design scour computations may be used for the scour assessment and should remain in the bridge inspection file.

If existing scour at the bridge is deeper than the calculated scour, the theoretical scour analysis is not correctly modeling the real conditions and the scour assessment should be re-analyzed. Any significant change in site conditions should also warrant re-visiting the scour calculations.

For the scour assessment, the following guidance is provided for checking the resultant calculated depth of the theoretical scour to the substructure unit foundation:

1. For spread footing foundations:

 If the calculated scour is above the bottom of footings - Not scour critical

 If the calculated scour is below the bottom of footings founded on soil or erodible rock – Scour critical

2. For deep foundations (piles or caissons):

 If the calculated scour is above the bottom of footings - Not scour critical

 If the calculated scour is below the bottom of footing and above the bottom of pile/caisson – a structural analysis of the foundation unit is needed to determine its stability. If not stable – Scour Critical

 If the calculated scour is below the bottom of pile/caisson – Unstable and Scour Critical 2.6.3.3 SCOUR ASSESSMENT USING PA OBSERVED SCOUR ASSESSMENT FOR BRIDGES

The Department developed an alternative method of scour assessment based upon the observance of geomorphic, hydrologic, and hydraulic features at the bridge site. This multi-disciplinary assessment, which has been approved by the FHWA, is seen as a cost–effective approach to meeting the NBIS requirements for evaluating existing bridges without theoretical scour computations. The Department developed this observed assessment method under an agreement with the United States Geological Survey (USGS).

Under the same agreement, the USGS performed the initial observed scour assessment of some 13,600 Department and local bridges over water. These initial observed scour assessment reports were reviewed for acceptance by the Department’s District Bridge Units. All data for these initial observed scour assessments were captured for the Department in a Microsoft Access® database by USGS. The initial reports were distributed to the Districts in electronic format.

The methodology for PA Observed Scour Assessment for Bridges (PA OSAB) is outlined in the USGS Open-File Report 00-64 titled Procedures for Scour Assessment at Bridges in Pennsylvania. This report is available at: ftp://ftp.dot.state.pa.us/public/Bureaus/design/bqad/Pubs/Procedures-for-Bridge-Scour-Assessments.pdf.

The PA OSAB uses an algorithm in a Department software program named “SCBI/SAR Calculator” to determine the value for BMS2 Item 4A08 Scour Critical Bridge Indicator. The SCBI/SAR Calculator Manual, which is located at ftp://ftp.dot.state.pa.us/public/Bureaus/design/bqad/Pubs/Scour-Calculator-Manual.pdf, provides guidance on the use of the calculator. If the 4A08 SCBI value from the PA OSAB is based on conditions valid at the time of inspection, it should be used in the inspection as the value for BMS2. The “SCBI/SAR Calculator” can be accessed by the inspectors through BMS2 Web and iForms. Inspectors can re-calculate the SCBI value by selecting the “Calculate” button in the Form G of iForms or the Underwater Inspection Screen for BMS2 Web after updating the appropriate IU and IN fields as part of the inspection. Item 4A08 shall not have a value of “6” or “U”

for NBIS bridges > 20’. In addition, a data item called Scour Assessment Rating (SAR) was developed for the PA OSAB to provide bridge owners with another measure of the threat for hydraulic failure of the bridge. The SAR ranges from 0 (extremely vulnerable) to 100 (scour resistant). The SAR analysis also provides a list of potential scour-related deficiencies at the bridge.

PA OSAB report is to be reviewed for changes in site condition during inspections. Because this method is based on field observations of features, maintenance of the pertinent data through Regular and Underwater Inspections is vital to the accuracy of its evaluation of the bridge’s safety. The Scour Calculator can be used to compute correct values for 4A08 and the SAR for new conditions. All changes are to be documented in the inspection report.

Table 1 in the SCBI/SAR Calculator Manual identifies the data components needed to run the calculator.

The Manual also provides situations that may arise for which the calculator can be used to update the SCBI. The most common situations include:

Review of bridge records identify previously undetermined or unknown PennDOT historical data or revised USGS field data needed to calculate the SCBI Code and SAR.

Site conditions change and the original SCBI Code and SAR values no longer reflect current site conditions.

(H&H Report) do not need a scour assessment or revision to the SCBI unless there is clear evidence that the above situations are applicable (e.g., scour countermeasure failure).

2.6.3.4 EVALUATION OF SCOUR COUNTERMEASURES

The Department coding instructions for BMS2 Item 4A08 SCBI indicate that the scour countermeasures are to be properly designed or verified through analysis before they can be considered as effective against scour.

2.6.4 Scour Plans of Action

In accordance with the Title 23 of the Code of Federal Regulations (CFR), Section 650.313(e)(3), a Scour Plan of Action (Scour POA) must be prepared for each National Bridge Inventory (NBI) Scour Critical bridge.

Scour POA’s document and describe the approaches used to monitor known and potential deficiencies and to address critical findings for bridges identified to be Scour Critical. Scour Critical bridges are defined as those bridges with an SCBI code (BMS2 item 4A08) of 3 or less, as well as those bridges with an Observed Scour rating (BMS2 item IN03) of 4 or less. Bridges are to be placed into one of four categories based on the descriptions below, with categories A through C addressing Scour Critical bridges and requiring a Scour POA.

Category A bridges include all bridges with a BMS2 Item 4A08 SCBI code of 2 or BMS2 Item IN03 Observed Scour rating of 3 or less.

Category B bridges include all bridges with a BMS2 Item 4A08 SCBI code of 3 and a BMS2 Item IN03 Observed Scour rating of 4.

Category C bridges include all bridges with a BMS2 Item 4A08 SCBI code of 3 and a BMS2 Item IN03 Observed Scour rating of 5 through 7.

Category D bridges include all bridges with a BMS2 Item 4A08 SCBI code of 4 or 7. These bridges are not considered to be Scour Critical and do not require a Scour POA. However, bridges in this category will need to be inspected after significant flooding events.

The Scour POA’s for Scour Critical bridges include a monitoring program and post-flood inspection plan based on the above categories. In order for a bridge to not be categorized A through D, a hydrologic study and hydraulic analysis is required to ensure properly designed scour countermeasures are present at the bridge site.

A sample Scour POA form is available in Appendix IP 02-A. These Scour POA’s can be generated from a standard Crystal Report titled “Scour Plan of Action” that is available on Crystal Enterprise. Most information is pre-populated on these forms from BMS2. Other attachments (i.e. plan view scour sketch, cross sections, photos, etc.) may be added and a hardcopy is to be filed in the Bridge Inspection file. It is also recommended that an electronic file be stored in BMS2 under the EDMS Documents link.

The Federal Highway Administration requires that Scour Plans of Action be developed for each Scour Critical NBI bridge by local bridge owners and other bridge owners as well. The procedures detailed here can be used or modified to better meet the owner’s needs as approved by the Department. The local owners’ procedures shall be approved by, and a copy kept on file, by the District.

2.6.4.1 SCOUR CRITICAL BRIDGE MONITORING RESOURCES

Information is available to the Districts to prepare for and respond to flooding events. The following resources are available for use to comply with the monitoring requirements of Publication 23 Ch 9.15:

 Scour Critical Bridge Lists: The Districts and Counties will maintain lists of Scour Critical bridges that are to be monitored during significant flood events. These lists can be generated from a standard Crystal Report titled “Scour Critical Bridge Category List” that is available on Crystal Enterprise and will be updated on a monthly basis by the District. Districts and counties must use these lists as the primary source to determine which bridges are to be monitored.

 Scour Critical Bridge Maps: GIS maps showing the locations of all Scour Critical bridges in categories A, B and C as well as non-Scour Critical bridges in Category D will be updated by Central Office and provided to District bridge units on a monthly basis. Districts and counties can use these

maps to locate the bridges to be monitored. See Publication 23 Ch 9.15 for the directory of map locations.

Field Manual: A field manual titled “Scour Critical Bridge Monitoring Field Manual” is available in Appendix IP 02-A. Bridge monitoring personnel are to be familiar with the contents of this manual in order to have the basic knowledge necessary to perform bridge flood monitoring effectively.

Weather Forecasts: National Weather Service forecasts and Accuweather alerts provided to the County Maintenance offices or Engineering Districts contain essential information to assist with preparations for the start of flood monitoring of bridges. In some months, Accuweather alerts are issued when 2 inches or more of rain is forecasted and/or when Flood Warnings are issued by the National Weather Service. Department personnel should anticipate the need for monitoring when these alerts are received.

2.6.4.2 MONITORING

Monitoring Scour Critical Bridges shall be in accordance with Publication 23 Ch 9.15.

Bridges are to be monitored in order of precedence with Category A bridges having the highest priority.

Category A bridges on the Interstate must be monitored without exception (BPN = 1). Using the information presented on the Scour Critical Bridge Lists and the Scour POA’s, an order of precedence can be further defined using information such as:

 Scour Critical bridge Categories A through C

 Business Plan Network (BPN), BMS2 Item 6A19

 Traffic Volumes, BMS2 Item 5C10

 Overtopping frequency of bridge and/or roadway approach, BMS2 Item 1A06

 Bridge configuration (length, # spans, support types, redundancy, etc.)

The recommended minimum frequency for monitoring Category A bridges is once every four (4) hours from the onset of monitoring. The recommended minimum frequencies for monitoring other bridges are once every 12 hours for Category B bridges and once every 24 hours for Category C bridges. Note that each scour critical bridge being monitored requires a minimum of two (2) visits. Discontinue monitoring per Publication 23 Ch 9.15.

Notes: 1) High water events and flash flooding can be localized. Conversely, flood events can be widespread and may not occur for a day or two after rain has begun. In either case, flood monitoring of bridges is to be performed when Bridge Closure/Outage occurs (See Publication 23 Ch 9.15).

2) Except for Category A bridges on the Interstate, the above monitoring procedures may be waived when a Declaration of Emergency has been authorized.

3) Category A bridges must be closed at such time as the water level reaches the low chord of the bridge (i.e. pressure flow condition). If it is deemed necessary to keep a bridge open, (e.g., for emergency vehicle passage or emergency evacuations), then the bridge must be continuously monitored (i.e. placed under a 24/7 watch) until flood waters subside and a post-flood damage inspection has been completed.

2.6.4.3 INSPECTION

Post-flood damage inspections are required for Scour Critical Bridges Categories A through C, and Category D bridges in areas or watersheds where significant flooding events have been reported by the counties or bridge closures have occurred. Depending on the size of the area affected by flooding, numerous flood damage inspections may need to be completed in a short time frame. Statewide bridge inspection open-end contracts may be a source for accomplishing these inspections.

Once flood waters have subsided, Scour Critical bridges are to be inspected for flood damage as follows:

 Category A: Inspect all bridges for flood damage where a significant flooding event was experienced.

Bridges in this category that have been closed due to approach roadway or bridge overtopping or

 Category B and C: Inspect all bridges for flood damage where a significant flooding event was experienced or if the bridge has been closed due to either approach roadway or bridge overtopping or pressure flow.

 Category D: Inspect for flood damage after each significant flooding event or if the bridge has been closed due to overtopping or pressure flow.

 For Categories A through C, refer to the Scour POA’s for additional guidance.

All Category A bridges which have been closed as a precautionary measure are to be inspected first so that bridges can be re-opened as quickly as possible. It is recommended that the inspection of the remaining bridges be prioritized in order of Category (Category A, B, C, D) since these represent the level of vulnerability to scour.

Scour Critical Bridges that have been closed due to the flood event require District Bridge Engineer approval prior to re-opening.

2.6.4.4 REPORTING

Bridge flood monitoring personnel are to complete a Bridge Flood Monitoring Log for each Category A bridge monitored. Entries are to be made on the log each time the bridge is visited. See Appendix IP 02-A for a sample log sheet.

Bridge closures are to be reported by the counties using the Road Condition Reporting System (RCRS).

This system reports all active bridge and roadway closures. Bridges that have been closed as a precaution, such as those that have gone into pressure flow or have overtopped, are to be reported to RCRS with a “Cause” description of “Bridge Precaution”. Those bridges closed due to scour failure are to be reported in RCRS with a “Cause”

description of “Bridge Outage”.

 During flood events:

o Per Publication 23 Ch 9.15, County Managers are to report to the District Bridge Engineer by phone or email on a daily basis.

o District Bridge Engineers are to report all counties where active flood monitoring is occurring to

o District Bridge Engineers are to report all counties where active flood monitoring is occurring to

In document Bridge Safety Inspection Manual (Page 81-94)