ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/297000
Concrete segmental construction has provided a durable and economical solution for many bridges and, as a result, more than 250 of these types of bridges have been built since the early 1970s in the United States. Concrete segmental construction has been used successfully in the construction of interchanges with complex geometric constraints and long-span bridges across navigational waterways. Many concrete segmental bridges provide critical links in the U.S. highway system. Consequently, the economic impact resulting from unforeseen closure of one of these bridges due to functional or safety concerns will be significant. Therefore, it is important to load rate these bridges to ensure the safety of the structure and the traveling public. Furthermore, load rating will safeguard the bridge from premature deterioration due to unintended overloads. Bridge load rating and posting are also mandated by Federal Regulation 23 CFR 650 Subpart C: National Bridge Inspection Standards (NBIS). The AASHTO Manual for Bridge Evaluation (MBE) further defines the methology and procedures for load rating and posting, including provisions for segmental bridges. A c c o r d i n g t o t h e F H WA P o l i c y Memorandum for Bridge Load Ratings for the National Bridge Inventory dated October 30, 2006, new bridges and totally replaced bridges designed after October 1, 2010, must be load rated with the load and resistance factor rating (LRFR) method. A questionnaire was sent to Federal Highway Administration (FHWA) Division Bridge Engineers in September 2011 to collect information about the status of the national implementation of the LRFR method. The data collected was used to develop recommendations and services to aid FHWA Division Bridge Engineers in the oversight of load rating, posting, and permitting programs and practices using the LRFR method. The responses to the questionnaire demonstrated that 23% (12) of the states had started to use the LRFR method to rate segmental bridges. To f u r t h e r s u p p o r t t h e n a t i o n a l implementation of LRFR load rating of segmental bridges, an informational webinar was conducted on January 19, 2012, by FHWA. More than 150 individuals from across the nation participated in this webinar. LRFR Methodology Limit States Since the major concern for bridge load rating is determining the vehicular live load capacity of the structure under its permanent load condition, other transient loads (wind, ice, earthquake, and the like) are generally not required to be included in the analysis. Table 6A.4.2.2-1 of the MBE further defines the limit states that should be considered when load rating different bridge types. Loads Load rating should consider live loads in the presence of all permanent loads applied to the structure and other loads that may affect the live load carrying capacity of the structure. Live loads should include the design notional load (HL-93), legal vehicles, or permit vehicles, depending on the purpose of the rating. Legal loads are the vehicles legally allowed to use bridges in the United States or in a specific state. The Bridge Formula in Section 658, Title 23 of the Code of Federal Regulations defines the limits on configuration and axle weight for a vehicle that can legally operate on an interstate highway without special permission (such as a state-issued permit). MBE includes the configuration and axle weight of some common vehicle types to be considered during load rating such as the Routine Commercial Vehicles Type 3, 3S2, and 3-3, and Specialized Hauling Vehicles SU4, SU5, SU6, and SU7. Most states also have state-specific legal loads that also need to be considered. Permit load rating should be conducted based on the actual configuration and axle weight of a permit vehicle or vehicle group. Dynamic load allowance should also be included in a load rating analysis. LRFR allows the use of a reduced dynamic load allowance for legal and permit load rating based on the riding surface condition. Structural Reliability In the calibration of LRFR/load and r e s i s t a n c e f a c t o r d e s i g n ( L R F D ) , Percentage of states that have used the LRFR method to load rate concrete segmental bridges. All drawings: Federal Highway Administration. Unknown = 5 10% No = 35 67% Yes = 12 23% Load and Resistance Factor Rating of Concrete Segmental Bridges by Lubin Gao, Joey Hartmann, Reggie Holt, and Thomas Saad, Federal Highway Administration AASHTO Manual for Bridge Evaluation provisions and special considerations 30 | ASPIRE , Spring 2013 LRFR_Spr13.indd 30 4/1/13 11:43 AM