Frequently Asked Questions (FAQ) About Bridge Rating and Superload Analysis
1. What is structure or bridge rating?
In a nutshell, the safe live load carrying capacity of a highway structure is called its load rating. The rating of each bridge on the Bridge Inventory is determined in three different ways: Inventory load rating for HS20 or HL93 loading, Operating Rating for HS20 or HL93 loading and posting load rating for Ohio legal loads. There are other ratings of a bridge, such as appraisal rating also known as General Appraisal (GA) that is done at the time of the inspection of the structure.
2. Where can I find more information about load rating?
There are several useful resources available on Structural Rating. A few references are given here:
A. AASHTO, LRFD Bridge Design Specifications, current Edition and all subsequent Interims
B. AASHTO, The Manual for Bridge Evaluation, current Edition and all subsequent Interims
C. AASHTO, Standard Specifications for Highway Bridges, 17th Edition
D. AASHTO, Guide Specifications for Fatigue Evaluation of Existing Steel Bridges, current Edition and all subsequent Interims
E. AASHTO, Guide Specifications for Strength Evaluation of Existing Steel and Concrete Bridges
F. AASHTO, Manual for Maintenance Inspection of Bridges, current Edition and all subsequent Interims
G. AASHTO, Guide Specifications for Fracture Critical Non-Redundant Steel Bridge Members, current Edition and all subsequent Interims
H. AASHTO Bridge Rating and AASHTO Bridge Design Software, developed by Baker Corp, Moon Township, Pittsburgh,
I. WYDOT, BRASS-Culvert software developed by the Wyoming Department of Transportation
J. Duncan, J.M., 1979, “Design Studies For Aluminum Structural Plate Box Culverts,” Kaiser Aluminum and Chemical Sales, Inc.
K. FHWA, 1995, “Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation’s Bridges, Report No. FHWA-PD-96-001, December 1995
L. NCSPA, “Load Rating & Structural Evaluation of In-Service Corrugated Steel Structures,” & Design Data Sheet No. 19, National Corrugated Steel Pipe Association (NCSPA, 202-452-1700)
There are several reasons to do the bridge rating:
o Federal Highway Administration (FHWA) requires us to do load ratings of all the structures of length greater than 20 feet to be in compliance with National Bridge Inspection Standards (NBIS).
o For the safety of general public and traffic using highway structures, the load rating is performed.
o OHIO Revised Code (ORC), Section 5591.42, requires us to post warning signs where the safe load carrying capacity of a structure is ascertained and found to be less than the load limits prescribed in ORC Sections 5577.01 through 5577.12. Generally, a load rating analysis of a structure can provide vital information about the load carrying capacity of a bridge to an engineer who decides whether a bridge needs to be posted for safe load limits lower than the state legal loads or not.
o Different design vehicles have been used in the past for the design of bridges (e.g., S-15, CF-130, CF-400, CF-2000, H-15, HS20-44, MS 18, HL-93, etc.). Bridges age, deteriorate or become structurally deficient during the course of their life. To have a consistent summary of the load carrying capacities of all the bridges in the state of Ohio, all bridges are rated using a standard set of vehicles, called Ohio Legal Loads.
4. What is the definition of a bridge?
In Ohio, a structure including supports erected over a depression or an obstruction such as water, highway, bikeway or railway; and having a track or passageway to carry traffic or other moving loads and having an opening measured along the centerline of the roadway of 10-ft or more between under-copings 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 that of the smaller contiguous opening.
According to the Federal Highway Administration (FHWA), a structure including supports erected over a depression or an obstruction such as water, highway, or railway; and having a track or passageway for carrying traffic or other moving loads and having an opening measured along the centerline of the roadway of more than 20 feet between under-copings 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 that of the smaller contiguous opening.
In Ohio, a Superload is any highway vehicular load with the total gross load equal to or more than 120,000 pounds (60 tons or 54, 431 kg).
6. When should a bridge’s rating be revised?
The load rating of a bridge should be revised:
· When there is a change in physical condition of a structural member of the bridge
· When there is an addition of a new beam or girder
· A new deck is added or the existing deck width is changed
· Rusting or damage to a beam, girder or other structural element that has resulted in section loss
· When there is a structural damage to steel, like a hit by a vehicle, excessive deflection or elongation under temperature or highway loads
· When there is structural damage to concrete, like spalling or salt related damage
· When there is a change in the dead load on the superstructure, like addition/removal of wearing surface, sidewalks, parapets, railings, etc.
· When there is a request to evaluate the rating of a structure for a different truck
· When there is a method of analysis proposed different from the one previously used for load rating
· Under special circumstances, when re-analysis of the structure is required
7. What computer programs are being used in Ohio to rate the bridges?
A. Our preferred software is AASHTO Bridge Rating and Design (BrR & BrD)
C. Bentley LARS
D. Other programs for special types of structures & analyses
8. Is there any technical help or support available for structure rating programs?
Limited technical support is available from the Office of Structural Engineering, Structural Rating Group. Please e-mail your questions to firstname.lastname@example.org.
9. Do we load rate culverts?
We have started load rating culverts. Reinforced concrete box and 3-sided structures can be load rated using AASHTO BrR or BRASS-CULVERT program. For corrugated metal pipe structures, we are using ODOT’s in-house developed spreadsheets. Please download them from the ODOT FTP site.
10. When should a bridge be closed?
When safe load carrying capacity of a bridge drops below 3 tons, the bridge shall be closed for all the highway traffic immediately, till the replacement or rehabilitation work is completed to increase the safe load carrying capacity to 3 tons or more.
Disclaimer: All these answers and interpretations are strictly my own. They do not represent my employer’s policies. Amjad Waheed, PE