LRFR vs. LFR

August 2, 2011Bryan J. Silvis, P.E.Senior Structural EngineerStructure and Bridge Division

Virtis Opis User Group Meeting, August 2, 2011

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Presentation Overview

VDOT Load Rating Background

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

Additions Made to Case Study

Results

Recommendations to VDOT

Miscellaneous

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VDOT Load Rating Background

Decision made to move to LRFR for new structures designed in LRFD and existing inventory.

Move from Bars to Virtis:

• Procured three 3-year load rating contracts• Interstate, Primary and Secondary (Interstate 90% and

Primary 50% complete)• 4,400 structures complete (5,000 anticipated by October

when contract ends)• Preparing for another three 3-year contract to rate 5,000

more structures

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IBC Presentation

Methodology:

• Compare rating results using the Virtis LFR engine to those using the Virtis LRFR engine in Version 6.2.

• Rate AASHTO legal load vehicles (Type 3, 3-S2 and 3-3) at inventory in LFR and as legal loads in LRFR

• As-built plans used for all runs

• All load factors, distribution factors, etc. were calculated as the load rating method dictated.

• The lowest rated structural member would control the rating regardless of location (specific member or point on a member), limit state or mode.

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IBC Presentation

Assumptions:

• Rating results are correct• VDOT bridges similar to those of other states

9 Bridge Types (10 structures selected for each type):

• Type 1 - Simple span concrete slabs• Type 2 - Continuous concrete slabs• Type 3 - Simple span concrete T-beams• Type 4 - Simple span rolled beams• Type 5 - Continuous rolled beams• Type 6 - Simple span plate girders• Type 7 - Continuous plate girders• Type 8 - Prestressed Bulb-T’s• Type 9 - Simple span prestressed AASHTO I-beams

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Modifications to Material Presented

Concerns:

• Virtis LFR engine (PennDOT) verse Virtis LRFR engine (AASHTO)• States that post between inventory and operating (apples to apples)

Changes made for this presentation:

• Worked with 3 of the 10 original files for each type• Verified data and updated rating results for Version 6.3 (BRASS in 6.2)• Added LFR operating rating results for comparison to LRFR• Added rating results using the AASHTO LFR and BRASS LFR engines• Added 10th bridge type (voided slabs – investigated 6 structures and

reported 3)• Looked for similar (yellow), close (green) and differing (red) results. The

colors in handout are meant as a visual aid only.

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Simple Span Concrete Slabs

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Continuous Concrete Slabs

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Simple Span Concrete T-beams

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Simple Span Rolled Beams

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Continuous Rolled Beams

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Simple Span Plate Girders

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Continuous Plate Girders

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Prestressed Bulb T’s

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Simple Span Prestressed AASHTO I-beams

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Concrete Voided Slabs

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Summary

LFR Engine Comparison (AASHTO, Virtis, BRASS):

• Similar results between engines for Types 4, 6 and 7• Close results between engines for Types 1, 2, 9 and 10• Differing results between engines for Types 3, 5 and 8

LRFR/LFR Inventory Comparison (focus on AASHTO engine):

• Average ratios for concrete structures are around 1.2 (T-beams and voided slabs), 1.5 (simple span / continuous slabs and PSC I-beams) and 1.9 (Bulb-T’s)

• Average ratios for steel superstructures are around 1.3 (simple span rolled beams and plate girders), 1.1 (continuous rolled beams and plate girders)

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Summary

LRFR/LFR Operating Comparison (focus on AASHTO engine):

• Average ratios for concrete structures are around 0.7 (T-beams and voided slabs), 0.85 (simple span / continuous slabs and PSC I-beams) and 1.2 (Bulb-T’s)

• Average ratios for steel superstructures are around 0.8 (simple span rolled beams and plate girders), 0.65 (continuous rolled beams and plate girders)

LRFR/LFR Inventory/Operating Average Comp. (AASHTO engine):• Average ratios for steel superstructures are around 1.05 (simple

span rolled beams and plate girders), 0.88 (continuous rolled beams and plate girders)

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Recommendations made to VDOT

Move to Version 6.3 in earliest time frame possible.

Use AASHTO engines exclusively:

• Have confidence in AASHTO engines• Less work/confusion involved by focusing resources on one engine• Only exception is simple span rolled beams with cover plates where

recommend using the Virtis LFR engine until Version 6.4 release

For LFR rating using Version 6.2 until version change can be made:

• BRASS LFR engine for Types 2, 3, 4 and 5• Virtis LFR engine for Types 1, 6, 7, 9 and 10• Engine with the higher rating for Type 8

Recommendations are under review, but so far supported.

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Miscellaneous

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VDOT Load Rating Website

http://www.virginiadot.org/business/bridge_load_rating.asp

Instructional and Informational Memorandums:

• Current IIM-S&B-86, Load Rating and Posting of Structures• Current IIM-S&B-27, Bridge Safety Inspections

Frequently Asked Questions

Information on VDOT Database modifications, libraries, rating vehicles, and other miscellaneous topics

Complete Virtis examples for each bridge type are under development and will be added to the website when available.

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Questions??

Contact bryan.silvis@vdot.virginia.gov 804-371-2737