 U.S. Department of Transportation Federal Highway Administration |
MEMORANDUM |
| Subject: | ACTION: Revision of Coding Guide, Item 113 - Scour Critical Bridges | Date: | April 27, 2001 |
| From: | /s/ Original signed by: James D. Cooper, Director of Bridge Technology |
Reply to Attn. of: | HIBT-30 |
| To: | Directors of Field Services, Division Administrators, Federal Lands Highway Division Engineers |
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This memorandum implements the revisions to Item 60 - Substructure and Item 113 - Scour Critical Bridges, of the Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation's Bridges (Coding Guide). These revisions have been developed in coordination with the National Bridge Inventory Steering Committee and my staff. The revisions represent new guidance for coding bridges over waterways for the observed and assessed scour condition and for scour and stream instability countermeasures for the protection of bridge foundations. Please add these revisions to the 1995 edition of the Coding Guide.
Attachments A and B show a mark-up version of the revisions to Items 60 and 113, respectively. The final version of Items 60 and 113 are shown in Attachments C and D, respectively. The revision to Item 60 is intended to make the coding of Items 60 and 113 consistent when a rating factor of 2 or below is determined for Item 113. The revisions to Item 113 are intended to expand the description of Codes 1, 2, 4, 5, 7, and 8. In addition, the revisions are intended to encourage bridge owners to develop a plan of action for each scour critical bridge as recommended in the FHWA Technical Advisory T 5140.23 titled "Evaluating Scour at Bridges" and for bridges coded "7" and "U" (unknown foundations).
We believe that these revisions will help to improve communication between the bridge inspector and the engineer and to enhance coding of bridges for the scour and/or stream instability condition. If you have any questions, please contact Mr. Jorge E. Pagán-Ortiz of my staff at (202) 366-4604.
Attachments
Attachment A
| Item 60 - Substructure | 1 digit |
This item describes the physical condition of piers, abutments, piles, fenders, footings, or other components. Rate and code the condition in accordance with the previously described general condition ratings. Code N for all culverts.
All substructure elements should be inspected for visible signs of distress including evidence of cracking, section loss, settlement, misalignment, scour, collision damage, and corrosion. The rating factor given by Item 113 - Scour Critical Bridges, may have a significant effect on Item 60 if scour has substantially affected the overall condition of the substructure to Item 60 should be consistent with the one given to Item 113 whenever a rating factor of 2 or below is determined for Item 113 - Scour Critical Bridges.
The substructure condition rating shall be made independent of the deck and superstructure.
Integral-abutment wingwalls to the first construction or expansion joint shall be included in the evaluation. For non-integral superstructure and substructure units, the substructure shall be considered as the portion below the bearings. For structures where the substructure and superstructure are integral, the substructure shall be considered as the portion below the superstructure.
Attachment B
| Item 113 - Scour Critical Bridges | 1 digit |
Use a single-digit code as indicated below to identify the current status of the bridge regarding its vulnerability to scour. Scour analyses Evaluations shall be made by hydraulic/geotechnical/structural engineers. Details Guidance on conducting a scour analysis evaluation are is included in the FHWA Technical Advisory T 5140.23 titled, "Evaluating Scour at Bridges."1 Detailed engineering guidance is provided in the Hydraulic Engineering Circular 18 titled "Evaluating Scour at Bridges."2 Whenever a rating factor of 4 2 or below is determined for this item, the rating factor for Item 60 -- Substructure and other affected items (i.e., load ratings, superstructure rating) may need to should be revised to reflect be consistent with the severity of actual observed scour and resultant damage to the bridge. A plan of action should be developed for each scour critical bridge (see FHWA Technical Advisory T 5140.23, HEC 18 and HEC 233). A scour critical bridge is one with abutment or pier foundations which are rated as unstable due to (1) observed scour at the bridge site (rating factor of 2, 1, or 0) or (2) a scour potential as determined from a scour evaluation study (rating factor of 3). It is assumed that the coding of this item has been based on an engineering evaluation, which includes consultation of the NBIS field inspection findings.
| Code | Description |
|---|---|
| N | Bridge not over waterway. |
| U | Bridge with "unknown" foundation that has not been evaluated for scour. |
| T | Bridge over "tidal" waters that has not been evaluated for scour, but considered low risk. Bridge will be monitored with regular inspection cycle and with appropriate underwater inspections until an evaluation is performed ("Unknown" foundations in "tidal" waters should be coded U.) |
| 9 | Bridge foundations (including piles) on dry land well above flood water elevations. |
| 8 | Bridge foundations determined to be stable for the assessed or calculated scour condition |
| 7 | Countermeasures have been installed to |
| 6 | Scour calculation/evaluation has not been made. (Use only to describe case where bridge has not yet been evaluated for scour potential.) |
| 5 | Bridge foundations determined to be stable for assessed or calculated scour condition |
| 4 | Bridge foundations determined to be stable for assessed or calculated scour conditions; field review indicates action is required to protect exposed foundations |
| 3 | Bridge is scour critical; bridge foundations determined to be unstable for assessed or calculated scour conditions:
- Scour within limits of footing or piles. (Example B) - Scour below spread-footing base or pile tips. (Example C) |
| 2 | Bridge is scour critical; field review indicates that extensive scour has occurred at bridge foundations, which are determined to be unstable by: - a comparison of calculated scour and observed scour during the bridge inspection, or - an engineering evaluation of the observed scour condition reported by the bridge inspector in Item 60. |
| 1 | Bridge is scour critical; field review indicates that failure of piers/abutments is imminent. Bridge is closed to traffic. Failure is imminent based on:
- a comparison of calculated and observed scour during the bridge inspection, or - an engineering evaluation of the observed scour condition reported by the bridge inspector in Item 60. |
| 0 | Bridge is scour critical. Bridge has failed and is closed to traffic. |
1 FHWA Technical Advisory T 5140.23, Evaluating Scour at Bridges, dated October 28, 1991.
2 HEC 18, Evaluating Scour at Bridges, Fourth Edition.
3 HEC 23, Bridge Scour and Stream Instability Countermeasures, Second Edition.
4 FHWA Memorandum "Scourability of Rock Formations," dated July 19, 1991.
Attachment C
| Item 60 - Substructure | 1 digit |
This item describes the physical condition of piers, abutments, piles, fenders, footings, or other components. Rate and code the condition in accordance with the previously described general condition ratings. Code N for all culverts.
All substructure elements should be inspected for visible signs of distress including evidence of cracking, section loss, settlement, misalignment, scour, collision damage, and corrosion. The rating factor given to Item 60 should be consistent with the one given to Item 113 whenever a rating factor of 2 or below is determined for Item 113 - Scour Critical Bridges.
The substructure condition rating shall be made independent of the deck and superstructure.
Integral-abutment wingwalls to the first construction or expansion joint shall be included in the evaluation. For non-integral superstructure and substructure units, the substructure shall be considered as the portion below the bearings. For structures where the substructure and superstructure are integral, the substructure shall be considered as the portion below the superstructure.
Attachment D
| Item 113 - Scour Critical Bridges | 1 digit |
Use a single-digit code as indicated below to identify the current status of the bridge regarding its vulnerability to scour. Evaluations shall be made by hydraulic/geotechnical/structural engineers. Guidance on conducting a scour evaluation is included in the FHWA Technical Advisory T 5140.23 titled, "Evaluating Scour at Bridges."1 Detailed engineering guidance is provided in the Hydraulic Engineering Circular 18 titled "Evaluating Scour at Bridges."2 Whenever a rating factor of 2 or below is determined for this item, the rating factor for Item 60 -- Substructure and other affected items (i.e., load ratings, superstructure rating) should be revised to be consistent with the severity of observed scour and resultant damage to the bridge. A plan of action should be developed for each scour critical bridge (see FHWA Technical Advisory T 5140.23, HEC 18 and HEC 233). A scour critical bridge is one with abutment or pier foundation rated as unstable due to (1) observed scour at the bridge site (rating factor of 2, 1, or 0) or (2) a scour potential as determined from a scour evaluation study (rating factor of 3). It is assumed that the coding of this item has been based on an engineering evaluation, which includes consultation of the NBIS field inspection findings.
| Code | Description |
|---|---|
| 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 a bridge failure during and immediately after a flood event (see HEC 23). |
| T | Bridge over "tidal" waters that has not been evaluated for scour, but considered low risk. Bridge will be monitored with regular inspection cycle and with appropriate underwater inspections until an evaluation is performed ("Unknown" foundations in "tidal" waters should be coded U.) |
| 9 | Bridge foundations (including piles) on dry land well above flood water elevations. |
| 8 | Bridge foundations determined to be stable for the assessed or calculated scour condition. Scour is determined to be above top of footing (Example A) by assessment (i.e., bridge foundations are on rock formations that have been determined to resist scour within the service life of the bridge4), by calculation or by installation of properly designed countermeasures (see HEC 23). |
| 7 | Countermeasures have been installed to mitigate an existing problem with scour and to reduce the risk of bridge failure during a flood event. Instructions contained in a plan of action have been implemented to reduce the risk to users from a bridge failure during or immediately after a flood event. |
| 6 | Scour calculation/evaluation has not been made. (Use only to describe case where bridge has not yet been evaluated for scour potential.) |
| 5 | Bridge foundations determined to be stable for assessed or calculated scour condition. Scour is determined to be within the limits of footing or piles (Example B) by assessment (i.e., bridge foundations are on rock formations that have been determined to resist scour within the service life of the bridge), by calculations or by installation of properly designed countermeasures (see HEC 23). |
| 4 | Bridge foundations determined to be stable for assessed or calculated scour conditions; field review indicates action is required to protect exposed foundations (see HEC 23). |
| 3 | Bridge is scour critical; bridge foundations determined to be unstable for assessed or calculated scour conditions:
- Scour within limits of footing or piles. (Example B) - Scour below spread-footing base or pile tips. (Example C) |
| 2 | Bridge is scour critical; field review indicates that extensive scour has occurred at bridge foundations, which are determined to be unstable by:
- a comparison of calculated scour and observed scour during the bridge inspection, or - an engineering evaluation of the observed scour condition reported by the bridge inspector in Item 60. |
| 1 | Bridge is scour critical; field review indicates that failure of piers/abutments is imminent. Bridge is closed to traffic. Failure is imminent based on:
- a comparison of calculated and observed scour during the bridge inspection, or - an engineering evaluation of the observed scour condition reported by the bridge inspector in Item 60. |
| 0 | Bridge is scour critical. Bridge has failed and is closed to traffic. |
1 FHWA Technical Advisory T 5140.23, Evaluating Scour at Bridges, dated October 28, 1991.
2 HEC 18, Evaluating Scour at Bridges, Fourth Edition.
3 HEC 23, Bridge Scour and Stream Instability Countermeasures, Second Edition.
4 FHWA Memorandum "Scourability of Rock Formations," dated July 19, 1991.