LRFR Ratings on Existing Bridge Inventories A Case Study

Transcription

1 LRFR Ratings on Existing Bridge Inventories A Case Study Daniel Whittemore, PE, LEED AP AI Engineers, Inc. Prasad Nallapaneni, PE, Virginia Department of Transportation

2 VDOT Load Rating for Existing Bridges AI was retained by the Virginia Department of Transportation to perform LRFR Load Ratings using AASHTOWARE s Virtis Program in In past 3 years, AI has rated over 1,500 bridges throughout the Commonwealth PS I Beam 16% PS Bulb 16% Bridges by Type Conc T Beam 14% Conc. Slab 15% Steel Plate 16% Rolled Steel 23%

3 Bridge Data Set In came to our attention that we had, with 1,500 bridges to choose from, a large dataset to analyze VDOT gave AI permission to work with the data to draw conclusions for other bridge owners.

4 LFR Versus LRFR Using Virtis Software, bridges are easily able to be switched from LFR ratings to LRFR ratings from same data input Virtis software version 6.2 has introduced a new analysis engine of their own designing The Virtis Engine It was decided to use the data set to draw conclusions between LFR and LRFR directly applicable for bridge owners

5 AASHTO Legal Loads

6 Why AASHTO Legal Loads? Apples to apples comparison live loads are identical Widely understood by all AASHTO aware bridge owners Directly related to posting, which for a bridge owner is: Money Time Safety Inconvenience Limited Scope / $

7 Bridge Types Selected 10 Each Category 1. Concrete Slab- Simple Spans 2. Concrete Slab- Continuous Spans 3. Concrete T-Beams Simple Spans 4. Rolled Shape Simple span 5. Rolled Shape Continuous Spans 6. Plate Girder Simple Span 7. Plate Girder Continuous Spans 8. Prestressed Bulbs Tees 9. Prestressed AASHTO-I Beams (Simple Spans)

8 Project Assumptions Virtis Engine is accurate and complete Virginia bridges are similar to yours As-Inspected conditions were ignored All loads factors, distribution factors, etc. were calculated as the load rating method dictated The lowest rated structural member controlled the load rating for each method even if they were located on different structural members or resulted from different limit states.

9 Type 1 - Concrete Simple Spans LRFR/LFR Structure Span Length Year Built Type 3 Type 3-3 Type 3S AVERAGE STD DEV

10 Type 2 Concrete Continuous Slabs LRFR/LFR Structure Max Span Length Year Built Type 3 Type 3-3 Type 3S AVERAGE STD DEV

11 Type 3 Concrete T Beams, Simple Slabs LRFR/LFR Structure Max Span Length Year Built Type 3 Type 3-3 Type 3S AVERAGE STD DEV

12 Type 4 Rolled Shapes, Simple Spans LRFR/LFR Structure Max Span Length Year Built Type 3 Type 3-3 Type 3S AVERAGE STD DEV

13 Type 5 Rolled Shapes, Continuous Spans LRFR/LFR Structure Max Span Length Year Built Type 3 Type 3-3 Type 3S AVERAGE STD DEV

14 Type 6 Plate Girder, Simple Spans LRFR/LFR Structure Max Span Length Year Built Type 3 Type 3-3 Type 3S AVERAGE STD DEV

15 Type 7 Plate Girder, Continuous Spans LRFR/LFR Structure Max Span Length Year Built Type 3 Type 3-3 Type 3S AVERAGE STD DEV

16 Type 8 Prestressed Bulb Tees LRFR/LFR Structure Max Span Length Year Built Type 3 Type 3-3 Type 3S AVERAGE STD DEV AVERAGE STD DEV For the prestressed bulb tee bridges, given the large deviation on structure 8.6, average LRFR/LFR ratios results are given with and without this structure.

17 Type 9 Prestressed AASHTO I-Beams, Simple LRFR/LFR Structure Max Span Length Year Built Type 3 Type 3-3 Type 3S AVERAGE STD DEV

18 Analysis by Span Length 200 Span Length - LRFR/LFR Span Leng gth (ft) Type 3 Type 3-3 Type 3S LRFR / LFR

19 Analysis by Year Built 2020 Year Built - LRFR/LFR Year Bu uilt Type 3 Type 3-3 Type 3S LRFR / LFR

20 A word about Concrete Check you assumptions: As-Inspected conditions were ignored The lowest rated lowest rated structural member controlled the load rating for each method 2011 Manual for Bridge Evaluation (MBE) section 6A.5.8 Evaluation for Shear states that for legal loads, in service concrete bridges need not be checked for shear unless evidence of shear distress is evident. Concrete bridges switched from shear to moment controlling the rating

21 Analysis Conclusions For most standard bridge types, materials, ages within 100 years, and span lengths, at the legal load levels, rating bridges with LRFR results in significant additional load carrying capacity 15% Average with steel As much as 60%+ with concrete, based on condition per 6A.5.8 Useful for bridge owners with borderline structures, or critical infrastructure links that posting would impact There is little correlation with the results and span length or structure age

22 Analysis Conclusions (cont.) There are no conclusions drawn from this analysis regarding design or permit loads using LRFR or LFR method Future Study NCHRP 12-78

23 Questions?