AASHTO Load And Resistance Factor Design Bridge Design Specifications Research Articles

Load effects and associated forces for bridges subjected to light rail transit

This paper presents a numerical study concerning light rail (LR) bridges, with emphasis on the characterisation of load effects and the interaction between LR loadings and supporting structures. The predicted bridge responses were employed to appraise the provisions of the Association of State Highway and Transportation Officials (Aashto) load and resistance factor design (LRFD) bridge design specifications (BDS) and the American Railway Engineering and Maintenance-of-Way Association manual. Because existing methods to determine live load distributions and skew correction factors are inadequate for LR bridges, new design equations are proposed. A single multiple presence factor of 1·0 is recommended for LR bridges, regardless of loaded tracks. The centrifugal force of curved bridges decays as the radius of curvature increases due to a change in centrifugal acceleration. Regarding the longitudinal force resulting from train braking, multipliers are suggested for design provisions. Temperature-induced loading is distributed to the bridge girders. Rail break is expected up to 75 mm within the range of thermal loading specified in Aashto LRFD BDS, which leads to an increase in dynamic load allowance. The force transfer from superstructure to substructure is marginally influenced by bearing arrangement. A statistical test was conducted to comparatively assess the equivalency of the bridge responses related to the above-mentioned design parameters.

Reliability-Based Design (RBD) For Everyone: (No Monte Carlo Simulation Required!)

In North America, the design of earth structures for transportation applications is most often carried out using load and resistance factor design (LRFD). In the U.S., the AASHTO LRFD Bridge Design Specifications are followed. In Canada, the primary code is the Canadian Highway Bridge Design Code. The premise behind LRFD is that when a limit state design equation is used with prescribed load and resistance factors, a minimum margin of safety, expressed as a reliability index b, or probability of failure, is assumed to be assured. Unless the design is at the point where the limit-state equation is satisfied, the true margin of safety in probabilistic terms is unknown. Furthermore, designers may have a choice of load and resistance models to use in a limit-state design equation. Because these models will have different accuracies, using different design models with the same load and resistance factors will result in different true margins of safety in probabilistic terms.

Building steel bridges in New York, Virginia, Pennsylvania and Maryland: Progressive application of LRFD to steel bridge erection engineering

This paper tells the story of how a Bridge Construction Engineer and a Civil/Structural Engineering Professor learned from each other through the observation of bridge erection projects, and concurrent participation with industry organizations during development of the AASHTO LRFD Bridge Design Specifications (3rd–5th editions). Illustrations from several highway bridge projects in four states are used to explain a successful transitional journey from Service Load/Allowable Stress (SLD/ASD) to Load and Resistance Factor Design (LRFD) in steel bridge superstructure erection. Stability analysis and construction practice included the following: evaluation of controlling strength and serviceability criteria, steel bridge erection load (γi)) factors, reconciliation of state-assigned construction wind loadings to the AASHTO Design Guide for Bridge Temporary Works and ASCE 7 (Minimum Loads for Buildings and other Structures), computation of lateral flange bending stress (fl)) utilized on straight, curved, haunched multi-girder bridges, as well as cross-box girder (straddle bent) piers during erection; member resistance during construction under local flange/web and lateral torsional buckling of single girders, girder slenderness (λi), L/b ratio rules of thumb and controlling bracing lengths during critical stages in the construction sequence, bearing point development, bridge system slenderness and the securing of partially constructed bridges (during storm events). Bridge erectors often exercise individual preferences in applying construction means and methods toward achieving specified geometry within alignment tolerances, so the interface between LRFD designed bridge members, ASD rigging components and ASD/Load Factor Design (LFD) temporary foundation components is also discussed.

Bond and Anchorage of High-Strength Reinforcing Steel

Recent revisions to the AASHTO load and resistance factor design (LRFD) construction specifications permit the specification of ASTM A1035 reinforcing steel. ASTM A1035 reinforcing bars are low-carbon, chromium steel bars characterized by a high tensile strength and a stress–strain relationship having no yield plateau. The AASHTO LRFD bridge design specifications limit the yield strength of reinforcing steel that may be used in design to 75 ksi (520 MPa) for most applications. Therefore, although ASTM A1035 may be specified, the higher yield strength cannot be used in design. The objective of this work is to evaluate existing AASHTO LRFD bridge design specifications in reference to the use of high-strength reinforcement represented by ASTM A1035. Issues of bar splice development and hooked-bar anchorages of ASTM A1035 reinforcing steel are discussed. Four spliced-beam straight-bar development tests and 18 hooked-anchorage pullout tests were performed as proof tests of the current AASHTO requirements. The study clearly demonstrates that the current AASHTO and American Concrete Institute requirements for both straight-bar tension development and hooked-anchorage tension development may be extended to develop bar stresses of at least 125 ksi in up to 10-ksi concrete. The results of this study and previous work clearly indicate that confining reinforcement should always be used when ASTM A1035 reinforcing steel is developed, spliced, or anchored.

Distribution Factors for Short-Haul Vehicular Loads on Prestressed Concrete Open Box Beam (U-Beam) Bridges

The design and analysis of a bridge system is a complicated task. In order to simplify the design process, it is necessary to break the system into smaller, less complex, and more manageable subsystems. The AASHTO load-and-resistance factor design (LRFD) bridge design specifications provide the design methods and procedures to design the bridge components explicitly and on an individual basis. In particular, the AASHTO LRFD bridge design specifications employs the beam-line method or distribution factor method to reduce the spatial dimensionality of bridge structures into line elements. The distribution factor is used to modify the load effects determined from the beam-line analysis to account for the effects at other locations in the structure. The published vehicular design loads adopted by the AASHTO LRFD bridge design specifications are considered to be an “average design truck.” The average design truck loads exclude vehicles that are above the legal weight limits for the United States, but are regularly allowed to operate on the U.S. highway systems. These loads are short-haul vehicles such as solid waste trucks, aerial rescue fire trucks and concrete mixers. The most common type of bridge in the United States is the slab-girder bridge system. The principal function of the slab-girder bridge is to provide a roadway surface and transmit the applied loads of the roadway to the girder system supporting the roadway. This research examines the loading effects of the short-haul vehicular loads on a typical prestressed concrete open box beam (U-beam) bridge. This study consists of analytically determining the distribution factors of special vehicles such as, a concrete mixer, a solid waste truck, and an aerial fire rescue truck, and comparing the results to distribution factors recommended in the AASHTO LRFD bridge design specifications.

Comparison of Standard Load and Load and Resistance Factor Bridge Design Specifications for Buried Concrete Structures

For the past 30 years, it has been common practice to use AASHTO standard design specifications for underground precast concrete structures. Today, the bridge engineering profession is making the transition from the standard AASHTO load factor design bridge specifications to the load and resistance factor design (LRFD) specifications. The FHWA has mandated that all culverts, retaining walls, and other standard structures designed after October 2010 be designed according to the AASHTO LRFD bridge design specifications for a project to receive federal funding. This paper compares related provisions from both specifications used in designing underground concrete structures, such as underground utility structures, drainage inlets, three-sided structures, and box culverts. Although many provisions of these two codes are the same, important differences could have a significant impact on the amount of reinforcement, member geometry, and cost to produce buried reinforced concrete structures. Many AASHTO LRFD code provisions that differ from the standard specifications include terminology, load factors, implementation of load modifiers, load combinations, multiple presence factors, design vehicle live loads, distribution of live load to slabs and earth fill, live load impact, and live load surcharge. This study demonstrates that greater reliability and a more uniform factor of safety are obtained when the LRFD specifications are used.

Resistance Factors for Drilled Shafts in Weak Rock Based on O-Cell Test Data

Load and resistance factor design (LRFD) has been mandatory for all FHWA-funded bridges since October 2007. The resistance factors included in the current AASHTO specifications for foundation design are not all calibrated by using field data. A calibration of resistance factors for side resistance of drilled shafts in weak rock is based on the statistical data collected from 19 O-cell tests in the midwestern United States. The field test data were used to determine the measured resistance, and the in situ rock properties and the dimensions of drilled shafts were used to calculate the predicted resistance by using the FHWA method. The Monte Carlo method was selected to perform the calibration. On the basis of the normally distributed loads and log normal distributed resistance from the test data, side resistance factors were determined at a target reliability index of 3.0. The calibrated resistance factors were compared with those in the current AASHTO LRFD Bridge Design Specifications.

Calibration of resistance factors for reliability based design of externally-bonded FRP composites

This paper describes a methodology for the calibration of preliminary resistance factors for the design of externally-bonded fiber reinforced polymer (FRP) composite renewal strategies for reinforced concrete structures using the load and resistance factor design (LRFD) approach. Reinforced concrete T-beam bridge girders rehabilitated flexurally with field-manufactured, carbon fiber reinforced epoxy composites are the specific example case studied. The calibration process is briefly described and then followed in a step-by-step manner to arrive at preliminary values for resistance factors. Continued deterioration of the existing structure due to loss of steel reinforcement is considered through use of a simple corrosion model. The work described herein is largely consistent with existing specifications for new design of bridges, including the use of the load models used to calibrate the AASHTO LRFD Bridge Design Specifications. Resistance factors are calibrated to meet three possible target reliability levels, β = 2.5, 3.0, and 3.5, and are found to depend on the amount of steel lost relative to the amount needed to carry the design load. Degradation of the FRP layer due to environmental exposure over time is considered as well, and a summary of the proposed design procedure is presented.

Guideline for Interpreting AASHTO Specifications to Design or Evaluate Buried Structures with Comprehensive Solution Methods

Design engineers and consultants are required to satisfy the criteria set forth in the AASHTO LRFD Bridge Design Specifications when designing buried culverts within the national transportation infrastructure. Adherence to the specifications is especially difficult when an engineer uses a comprehensive solution method such as a finite element program to predict the deformations and internal forces of the culvert-soil system. Part of the difficulty is in knowing how to apply factored loads as well as to determine their values. A more difficult problem is to assess whether the culvert is structurally safe according to the AASHTO LRFD specifications. Said another way, the design criteria and the corresponding factored capacities as written in the specifications are less than well defined. A succinct interpretation is presented of the AASHTO specification for load and resistance factor design (LRFD) for buried structures with regard to factored loads and design criteria. Tabularized values are provided for load factors and load modifiers to quantify dead loads, earth loads, and live loads, which are required for a LRFD analysis to produce the factored demands. Also presented are tabularized descriptions of the design criteria and the corresponding factored capacities, which are defined and quantified for corrugated metal, reinforced concrete, and plastic pipe materials. For some culverts, the design criteria have been generalized to provide sufficient information that is compatible with comprehensive analysis methods. Although intended especially for design engineers who use comprehensive analysis tools, the information presented is also useful and educational for engineers who use the simplified design solutions in the AASHTO LRFD specifications.

Live Load Distribution Widths for Reinforced Concrete Box Sections

AASHTO introduced the AASHTO LRFD Bridge Design Specifications in 1994. These load and resistance factor design (LRFD) specifications included new provisions for distributing live loads to reinforced concrete bridge decks that also apply to the design of reinforced concrete box sections with depths of fill less than 2 ft. Designs according to the LRFD specifications can be much more conservative than designs according to the AASHTO Standard Specifications for Highway Bridges and are inconsistent at a span of 15 ft. A finite element study sponsored by the Pennsylvania Department of Transportation investigated these issues and determined appropriate live load distributions for box sections subjected to live loads; the results are presented. The computer models were three dimensional for analysis of the distribution of live load effects along the length (flow direction). Box section models had spans of 8 ft, 16 ft, and 24 ft, and all had an 8-ft rise. The models had different slab and wall thicknesses, diffe...

System Factors for Highway Bridge Superstructures

Although traditional methods of bridge structural design proportion each member separately, the members do not behave independently but interact to form a single structural system. To account for overall system safety during the design process, the current AASHTO load and resistance factor design (LRFD) specifications (AASHTO LRFD Bridge Design Specifications, 2002) provide a load modifier factor, η, which explicitly accounts for the effects of ductility, redundancy, and operational importance. In the absence of precise information, the values used for the load modifiers are assigned subjectively. The results of a research project funded by NCHRP that led to the development of objective measures of redundancy and a method that uses system factors, Φs, during the design and load capacity evaluation process to account for the system's reserve strength are summarized. A set of proposed system factors was calibrated to provide bridges with adequate levels of overall safety and system reliability. Nonredundant...

Vessel Impact Risk Assessment by Texas Department of Transportation

On September 15, 2001, a four-barge tow struck and collapsed Bent 32 of the Queen Isabella Causeway Bridge (Park Road 100 bridge crossing the Gulf Intracoastal Waterway) in Texas. In the aftermath of the collapse, the Bridge Division of the Texas Department of Transportation conducted a vessel impact risk analysis for all bridges crossing waterways with documented barge or cargo vessel traffic. To identify all bridge crossings over navigable waterways, Bridge Division engineers queried the Bridge Inspection Database and confirmed the resulting list with district personnel. Investigating engineers screened the bridges in steps on the basis of exposure of structural elements to vessel traffic, bridge type, and level of protective system already in place. They then evaluated bridges requiring analysis in accordance with the load and resistance factor design (LRFD) vessel collision provisions of the AASHTO LRFD Bridge Design Specifications. The engineers then scored bridges on the basis of vulnerability to ve...

NCHRP Project 12-56, Application of Load and Resistance Factor Design Specifications to High-Strength Structural Concrete: Shear Provisions

The load and resistance factor design (LRFD) specifications in the current AASHTO LRFD Bridge Design Specifications limit the compressive cylinder strength to 10 ksi in relationships for calculating shear design strength, whereas concrete compressive strengths close to twice this level are commercially available in some parts of the United States. To overcome this limitation, allowing the same-size structural sections to span longer distances or carry higher loads, NCHRP has funded Project 12-56, Application of the LRFD Bridge Design Specifications to High-Strength Structural Concrete: Shear Provisions. In the experimental component of this project, 12 prestressed concrete bridge girders 52 ft long and 6 ft deep are being subjected to a uniformly distributed load until failure in shear. The primary variables in this study are concrete strength (ranging from 10 to 18 ksi), the maximum shear design stress (700 to 2,500 psi), strand anchorage details (straight, unbonded, and draped), and end reinforcement de...

Simplified Live Load Distribution Factor Equations for Bridge Design

Live load distribution factors have been used in bridge design for decades as a simple method with which to estimate live load effects. Live load distribution is important for the design of new bridges as well as for the evaluation of existing bridges. AASHTO's Standard Specifications for Highway Bridges and the AASHTO LRFD Bridge Design Specifications contain the most common methods in use for computing live load distribution factors. The load and resistance factor design (LRFD) equations were developed under NCHRP Project 12-26 and reflect the wide variation in modern bridge design. The limited ranges of applicability and complexity have been viewed as weaknesses of these equations since their adoption into the LRFD specifications. When bridges fall outside these ranges, the specifications mandate that a refined analysis be used. The design community would welcome a simple and reliable procedure for live load distribution. Research was done to develop simplified live load distribution factor equations f...