Coding & SI&A (Cont’d) Item 64F/66 – Operating/Inventory Rating Can be metric tons or rating factor LRFR Rating MUST be rating factor Must correspond with Item 63/65 Metric tons, not English. Rating Factor of 1.0 = 32.4 mTons (not 36) Minimum allowable is 2.7 mTons (or 3 english ton equivalent) If less, close the bridge, and record 0.00 If temporarily supported, must be coded for un-supported case (item 103 = T) Record 99.9 if live load is negligible
Coding & SI&A (Cont’d) Item 64MB – Michigan Operating Rating Can be English tons or rating factor Rating factor is preferred Must correspond to item 64MA Based on the lowest rating factor of the 28 Michigan Legal trucks Note: this is a change from past practice. When rating factor is less than 1.0, bridge must be posted Minimum allowable is 3 tons If less, close the bridge, and record 0.00 If temporarily supported, must be coded for un-supported case (item 103 = T) Record 99.9 if live load is negligible
Coding & SI&A (Cont’d) Item 64MC – Michigan Operating Truck Truck with lowest rating factor of 28 legal trucks (Chapter 2 of BAG)
Coding & SI&A (Cont’d) Item 70 – Bridge Posting If 64MB is > 1.0, Item 70 = 5 Reflects Item 64MB NOT actual Posting (Item 141)
Coding & SI&A (Cont’d) Item 141 – Posted Loading Leave blank if structure not posted Bridge must be posted if item 64MB < 1.0 4 Options for Posting – Includes 6 digits Minimum posting is 3 Tons Gross 081216 1002NN NN05NN 10NNNN
Coding & SI&A (Cont’d) Item 193A&C – Overload Rating Class A, B, C, or Restricted Optional for local agency use
Summary & Assumption Sheets Per AASHTO MBE 188.8.131.52: “ A general statement of the results of the analysis with note of which members were found to be weak, and any other modifying factors that were assumed in the analysis, should be given.” Calculations must include: Analysis methodology Assumptions Factors that affect the rating (condition, unique loads) Results – to include controlling members MDOT MBIS/MBRS Summary and Assumption Sheets Meet these requirements
Summary & Assumption Sheets As of July, 2012 MBIS eliminated access to load rating values in the SI&A screen
Coding Inconsistencies/FHWA Audit 2009 FHWA Audit, Final Report March 2010 US DOT OIG audit 2006 FHWA division Offices directed to perform in-depth reviews of state load rating and posting practices Findings – Conditional Compliance Current practices “are generally in compliance with NBIS and AASHTO requirements”. Many MDOT bridges in the database “may not be load rated in compliance with NBIS” and “as many as 2,900 bridge load ratings must be revised”. Many local agency bridges in the database may not be in compliance with NBIS “as many as 4,100 or more”.
Prioritization List Tier 1 – No Rating – Due 12/31/2012 Nulls in load rating values Item 63 or 65 = 5 (no rating) Item 64f = 66 Tier 2 – Poor Condition – Due 12/31/2014 Deck, superstructure, substructure OR culvert ratings equal to 4 or less AND Deterioration indicator in MBIS equals “No”.
Prioritization List (Cont’d) Tier 3 – Other Irregularities – Due 12/31/2016 Built after 1993 AND ASR (Item 63 or 65 equal 2 or 7) Built after 2010 AND not LRFR (Item 63 or 65 not equal to 3 or 8) NHS bridge AND ASR (Item 63 or 65 equal 2 or 7) Fed Operating is greater than 3X Fed Inventory (Item 64F > 3*Item 66)
Load Rating Bridge Advisories BA-2010-03August 2010 Load Rating Compliance with NBIS BA-2010-06October 2010 Licensing and Use of AASHTOWARE Virtis Software BA-2011-02 March 2011 Local Agency Load Rating Prioritization and Coding BA-2012-01July 2012 MBIS Update & Coding Revisions
Load Rating Troubleshooting If an analysis yields low rating results AND the results are not consistent with field observations (consider whether bridge has ever seen full legal load)……. Review model & verify field conditions of structure Attempt to adjust distribution factors Guide Specifications for Distribution of Loads for Highway Bridges, 1994 Calculate lever rule for cases where code equations may not apply Other Specs/Virtis Control Options Different methodology (LFR→LRFR, etc) Plastic Moment Capacity (if applicable) 1979 Shear specs (if applicable) Material Sampling (ASTM Standard) 10% increase in steel yield 20% increase in concrete compressive Advanced Modeling Virtis 2d/3d Other FEM software Load Testing
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