ODOT BRIDGE LOAD RATING. A basic how and why overview. What is a Bridge?. Federal Highway Administration (FHWA) Definition: A Bridge is ANY structure that… 1.) is erected over a depression or obstruction (water, highway, railway, etc.) AND
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ODOT BRIDGE LOAD RATING A basic how and why overview
What is a Bridge? • Federal Highway Administration (FHWA) Definition: A Bridge is ANY structure that… 1.) is erected over a depression or obstruction (water, highway, railway, etc.) AND 2.) has a track or passageway for carrying traffic or moving loads AND 3.) has an opening greater than 20’-0”(measured along centerline of roadway between undercopings of abutments or spring lines of arches, or extreme ends of openings for multiple boxes; it may also include multiple pipes, where the clear distance between openings is less than half of the smaller contiguous opening)
What is a Bridge? (cont.) • Ohio Department of Transportation (ODOT) Definition (based on the Ohio Revised Code): A Bridge is ANY structure that… 1.) is on, above, or below a highway AND 2.) Is greater than or equal to10’-0” (measured the same way as in the FHWA definition)
There are Two Primary Types of Loads used in Load Ratings? • Permanent or Dead Loads • Structure self weight • Superimposed dead loads (barriers, overlays, utilities, etc.) • Transient or Live Loads • Vehicular loads • Impact loads
Three Types of Live Loads in Ohio • Design or inventory loads • HS20-44, HS25, HL-93 • Ohio Legal Loads • 2F1, 3F1, 4F1, 5C1 • Permit Loads & Superloads Note: The Feds only require highway loads to be used for load rating. Consequently, even though pedestrian and railroad structures fall under the definition of “Bridge”, and they are inspected and inventoried, they are not load rated
AASHTO HS 25 Truck (current Ohio design load) HS 25 40k 40k 10k Varies (14’-30’) 14’
AASHTO HS 20-44 Truck (still to be used for Ohio load ratings) HS 20-44 8k 32k 32k Varies (14’-30’) 14’
Ohio Legal Loads (2F1) 2F1 2F1 10k 20k 10’
Ohio Legal Loads (3F1) 3F1 12k 17k 17k 10’ 4’
Ohio Legal Loads (4F1) 4F1 12k 14k 14k 14k 4’ 4’ 10’
Ohio Legal Loads (5C1) 5C1 12k 17k 17k 17k 17k 31’ 12’ 4’ 4’
What Is Bridge Load Rating? The safe live load carrying capacity of a highway structure is called its load rating. It is usually expressed as a (rating) factor (RF) of a defined vehicle OR as a gross tonnage for a defined vehicle axle configuration
Basic Equation for Calculating the Rating Factor (RF) A load rating can be expressed in terms of a “rating factor” for a particular vehicle. How to calculate the rating factor (RF) A1 = Factor for dead loads A2 = Factor for live load C = Capacity of the bridge D = Dead load effect I = Impact factor L = Live load effect C – A1 * D RF = A2 * L*(1 + I)
Three Different Load Rating Methods • Allowable Stress (ASD) • Also known as Working Stress (WSD) • Used for ODOT steel trusses and timber structures • Load Factor (LFD) • ODOT Preferred • FHWA Preferred • Load & Resistance Factor Rating (LRFR) • NCHRP Project 12-46
Two Types of Ohio Load Ratings Inventory Rating (Design Level) 1.) HS20-44 Operating Rating (Service Level) 2.) HS20-44 3.) 2F1 4.) 3F1 5.) 4F1 6.) 5C1 Every bridge in Ohio has six different load rating factors calculated.
Factors for LFD Load Rating Ref: AASHTO Manual for Condition Evaluation of Bridges 2000
What components of a bridge is ODOT interested in Load Rating • All primary superstructure components of a bridge shall be load rated. • Unless specified in a scope, this excludes the following: Decks (unless bridge is a deck slab type) Bearings Substructures Field splices Railing / Parapets
When Should a Load Rating be Revised? The load rating of a bridge should be revised when: • there is a change in the dead load on the structure • there is a physical change in any structural member of the bridge. • there is a change in the proposed live loading • A different method of analysis is required
Why do we rate structures? (1) The Silver Bridge Collapse The Dec. 15, 1967, collapse of the Silver Bridge at Point Pleasant, Ohio killed 46 people. The failure of the 39 year old eye bar suspension bridge across the Ohio River prompted US Congress to pass National Bridge Inspection Standards (NBIS) in 1968.
Why do we rate structures? (2) Why do we rate structures? (2) • Required by Federal government (NBIS) • To monitor safety of structures over time • To help determine when rehabilitation or replacement is needed
Why do we rate structures? (3) • To determine if a bridge needs to be posted for a load restriction as required by the Ohio Revised Code • To have a consistent summary of load carrying capacities of all state bridges • To assist Office of Permits in their processing of Permit and Superloads
A Permit* load equals… any vehicle or combination of loads having a gross weight in excess of 40 tons (or 80 kips). * = Permits are also required for over-SIZED vehicles. But, for the purposes of load rating, we are referring to permits that are required due to over-weight only.
A Superload equals… any vehicle or combination of loads having a gross weight in excess of 60 tons (or 120 kips).
Vehicle Categories based on weight 1.50 * 40 Gross weight of heaviest Ohio Legal Load (5C1)
Computer programs to assist in load rating ODOT bridges • AASHTO BARS-PC (SHALL be used where possible) (http://www.dot.state.oh.us/srg/download.htm) • BRASS (SHALL be used for buried structures) (http://www.dot.state.wy.us/generic/brass/index.html)
Additional Computer programs to assist in load rating ODOT bridges • DESCUS I (www.opti-mate.com) • STAADIII/Pro (www.reiusa.com) • GT STRUDL (Finite Element based) (www.gtstrudl.gatech.edu) • SAP 90 / SAP 2000 (Finite Element based) (www.csiberkeley.com)
Why is BARS desirable to ODOT? • Can run Batch routines (useful for fast analysis of superload routes) • Fast, free, and doesn’t require a lot of resources • Tested and used over the last 20+ years • Provides output in rating format • Supports both Mainframe and PC computers • Custom vehicles can be easily defined • Useful for reviewing new designs
BARS on the Internet • The program and instruction manuals can be downloaded for free at… http://www.dot.state.oh.us/srg/download.htm • An introductory tutorial can be downloaded at… http://www.dot.state.oh.us/srg/bars-pc.htm
ODOT Customization of BARS • Custom.std file at… http://www.dot.state.oh.us/srg/download.htm • Ohio adopted custom allowable stresses • ODOT BDM Tables 904 & 905 • Ohio legal vehicles
BDM Section 900 (1) • BURIED STRUCTURES • ALL bridges (> 10’) with fill > 2’-0” shall be considered “buried” and shall be load rated EXCLUDING… • Circular Steel pipes • Circular Plastic pipes • Circular Concrete pipes • Buried Metal Boxes • Buried Metal Frames • Junction Chambers • Manholes • Inlets
BDM Section 900 (2) • BURIED STRUCTURES (cont.) • All buried structures that are a part of new construction, replacement, or rehabilitation projects shall be load rated as follows… • CIP Concrete bridges shall be loadrated by the designer using BRASS-Culvert. • Precast concrete frames, arches, Conspans, and Bebo type structures shall be load rated by the manufacturer. • Precast boxes will be load rated by OSE using BRASS-Culvert.
BDM Section 900 (3) • NON-BURIED STRUCTURES • ALL bridges (> 10’) with fill < 2’-0” (or no fill at all) shall be considered “non-buried” and shall be load rated • BARS-PC SHALL BE USED when possible • BRASS-Culvert SHALL BE USED for concrete boxes and three-sided culverts • OSE MUST be contacted if any other software is going to be used
BDM Section 900 (4) • MISCELLANEOUS INFORMATION • Only load rate spans or portions of buried structure that will experience live load • Load ratings of new or rehabilitation bridge projects shall be based on final design plans and shall show results that meet or exceed the design loading • No FWS shall be included in load ratings
BDM Section 900 (5) • WHEN DO YOU SUBMIT YOUR ANALYSIS? • MAJOR OR MINOR PLAN DEVELOPMENT (OR DESIGN BUILD) PROCESS • Include load rating report with STAGE 2 submittal ** • MINIMAL PLAN DEVELOPMENT (OR DESIGN BUILD) PROCESS • Include load rating report with STAGE 3 submittal ** **Revise and resubmit load rating to District Project Manager if design plans change after Stage 2 (or Stage 3) and prior to contract sale
BDM Section 900 (7) • WHEN DO YOU SUBMIT YOUR ANALYSIS? • VALUE ENGINEERING CHANGE PROPOSAL (VECP) • Include load rating analysis with the Final VECP submission to the District Construction Engineer
BDM Section 900 (8) • WHAT DO YOU PUT IN THE REPORT? • A full project description • Printouts of analysis software input/output • Explanation of how material properties were determined (for existing structures) • All hand calculations • A table summarizing the following rating factors for each live load truck… • Inventory & Operating ratings for each main bridge member analyzed (Beam 1, Beam 2, Interior, Exterior, etc.) • Overall ratings of each structure unit (mainline, ramps, etc.) • Overall ratings of entire bridge
BDM Section 900 (9) 36*1.38 = 49.8 • WHAT DO YOU PUT IN THE REPORT? 20*1.38=27.7 Smallest * 100= 308
BDM Section 900 (11) • WHAT DO YOU SUBMIT TO ODOT? • 2 -Printed copies of load rating report (signed and sealed by an Ohio PE) • 1 -Electronic copy of load rating report • 1 –Electronic copy of input data files • BARS - LISTA.LIS, RATE2.LIS, SUMMARY.LIS, and FLEX.LIS files • BRASS - Files with the following extensions: DAT, CUS, and XML
AASHTO Load Rating References • AASHTO (2002), Standard Specifications for • Highway Bridges, 17th ed., • AASHTO (2000), Manual for Condition • Evaluation of Bridges, 2nd Edition. (Section 6) • AASHTO (1995), Bridge Analysis and Rating • System, BARS-PC, Release 5.5, Mod 3.3, • Users Manuals I and II,
AASHTO Load Rating References (cont.) • AASHTO (1989), Guide Specifications for • Strength Evaluation of Existing Steel and • Concrete Bridges • AASHTO (1978), Guide Specifications for Fracture Critical Non-Redundant Steel Bridge Members
ODOT Bridge Load Rating References • ODOT (2004), Bridge Design Manual – Section 900 http://www.dot.state.oh.us/se/BDM/BDM2004/bdm2004.htm • ODOT Structural Rating Group’s http://www.dot.state.oh.us/srg/
Appendix 1 –Rating Equations Reference: BARS Manual 1