Steel Girder Design per AASHTO LRFD Specifications (Part 2)

This is the second of two companion papers discussing and illustrating the AASHTO LRFD Bridge Design Specifications for the design of steel girders subject to flexure and shear. In the first paper, notation was introduced that allows reformulation of the AASHTO design equations in a more convenient format and the design of steel I-girders in flexure was presented. The second paper addresses design of box girders for flexure and design of box and I-girders for shear. The design approach is illustrated by two detailed example problems.     

Single-Span Prestressed Girder Bridge: LRFD Design and Comparison

This report summarizes the comparative design of a single-span AASHTO Type III girder bridge under the AASHTO Standard Specification for Highway Bridges, 16th Edition, and the AASHTO LRFD Bridge Design Specification. The writers address the differences in design philosophy, calculation procedures, and the resulting design. Foundation design and related geotechnical considerations are not considered. The LRFD design was similar in most respects to the Standard Specification design. The significant differences were: (1) increased shear reinforcement; (2) increased reinforcement in the deck overhang; and (3) increased reinforcement in the wing wall. The comparisons would likely change if the bridge were designed purely according to LRFD Specifications rather than as a comparative design. Design procedures under the LRFD Specification tend to be more calculation-intensive. However, the added complexity of the LRFD Specification is counterbalanced by the consistency of the design philosophy and its ability to consider a variety of bridges.     

Steel Girder Stability during Bridge Erection: AASHTO LRFD Check on L/b Ratios

The erection of steel plate girders during the construction process of a steel bridge is a complex operation, which is often left to the contractor and/or the subcontractor to plan and execute. Rules of thumb have been developed through experience to check the lateral torsional buckling of the steel girder during erection using the maximum L/b (unbraced length/compressive flange width) ratio, below which no lateral torsional buckling would occur. Although the L/b ratio check has proven to be useful and convenient on-site, it is necessary to provide a more rational basis for the rules of thumb, and find the maximum L/b ratios by checking the lateral torsional buckling failure of girders under erection according to the latest AASHTO LRFD code. A series of parametric studies were conducted on cantilever and simply supported girders under self-weight as well as self-weight plus wind load, in order to: (1) check the rules of thumb on L/b ratios and (2) determine the effects of girder flange width, flange thickness, web depth, web thickness, and yield strength on the maximum L/b ratio and girder stability during erection. From the results, rules of thumb were modified for girders with common shapes, and it was obvious that (1) self-weight plus wind load controls the girder stability during erection in most cases and (2) flange width and web depth have the most effects on the maximum L/b ratio and girder stability during erection.     

Assessment of AASHTO LRFD Specifications for Hybrid HPS 690W Steel I-Girders

This paper details research conducted to determine the applicability of the 2nd and 3rd editions of the AASHTO LRFD Specifications to hybrid I-girders fabricated from high-performance steel (HPS) 690W (100?ksi) flanges and HPS 480W (70?ksi) webs. Specifically, the scope of this paper is to evaluate the applicability of the negative moment capacity prediction equations for noncomposite I-girders subjected to moment gradient. This evaluation is carried out using three-dimensional nonlinear finite-element analysis to determine the ultimate bending capacity of a comprehensive suite of representative hybrid girders. In addition, a design study was conducted to assess the economical feasibility of incorporating HPS 690W (100?ksi) in traditional bridge applications. This was accomplished by designing a series of I-girders with varying ratios of span length to girder depth (L/D ratios) for a representative three-span continuous bridge. Results of this study indicate that both the 2nd and 3rd editions of the specifications may be used to conservatively predict the negative bending capacity of hybrid HPS 690W (100?ksi) girders, however increased accuracy results from use of the 3rd edition of the AASHTO LRFD Specifications. Thus, it is concluded that the restriction placed on girders fabricated from steel with a nominal yield strength greater than 480?MPa (70?ksi) can be safely removed. Additionally, results of the design study demonstrate that significant weight saving can result from the use of hybrid HPS 100W girders in negative bending regions, and that hybrid HPS 690W/HPS 480W girders may be ideally suited to sites with superstructure depth restrictions.     

Relaxing the Stud Spacing Limit for Full-Depth Precast Concrete Deck Panels Supported on Steel Girders (Phase I)

The AASHTO LRFD Bridge Design Specifications state that the spacing between the shear connectors for steel girders should not exceed 610 mm (24 in.). This decision was made based on research conducted more than three decades ago. The goal of this research is to investigate the possibility of extending this limit to 1,220 mm (48 in.) for stud clusters used with full-depth precast concrete deck panels installed on steel girders. This paper presents the history of the 610 mm (24 in.) limit, various formulas developed to calculate fatigue and design capacity for stud clusters and concerns about extending the current LRFD limit. This paper also presents information on the first phase of the experimental investigation, which is conducted on push-off specimens to validate extending the limit to 1,220 mm (48 in.).     

New AASHTO Guide Manual for Load and Resistance Factor Rating of Highway Bridges

This paper introduces the American Association of State Highway Officials’ (AASHTO) new Guide Manual for Condition Evaluation and Load and Resistance Factor Rating of Highway Bridges that was completed in March 2000 under a National Cooperative Highway Research Program research project and adopted as a Guide Manual by the AASHTO Subcommittee on Bridges and Structures at the 2002 AASHTO Bridge Conference. The new Manual is a companion document to the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications in the same manner that the current Manual for Condition Evaluation of Bridges is to the AASHTO Standard Specifications. The new Manual is consistent with the LRFD Specifications in using a reliability based limit states philosophy and extends the provisions of the LRFD Specifications to the areas of inspection, load rating, posting and permit rules, fatigue evaluation, and load testing of existing bridges. This paper presents an overview of the manual; specifically, the new Load and Resistance Factor rating procedures are explained and the basis for their calibration is discussed.     

Field Evaluation of Decommissioned Noncomposite Steel Girder Bridge

Field tests conducted on a noncomposite steel girder bridge are described. Two separate 36.6 m (120 ft) units, each three-span continuous, were subjected to increasing static loads by means of a trailer and concrete barriers. Results show that the girders acted as partially composite sections in the positive moment region up to the onset of yield. Due to curb participation and the transverse location of the applied load, exterior girders exhibited a higher degree of partially composite action. In the negative moment region, partially composite action was evident only in the exterior girders. As a result of partially composite action and curb participation, the yield load was about 7% higher than predicted. Bearing restraint is shown not to have a significant impact on the behavior of the tested bridges. In addition, the stiffness of the interior girders, as measured under the constant weight of a dump truck, are shown to be virtually unaffected by the heavy trailer loads. More significant changes in girder stiffness were observed between different transverse load positions of the dump truck.     

Recommended Procedures for Simplified Inelastic Design of Steel I-Girder Bridges

This paper presents summary recommendations pertaining to new AASHTO procedures for simplified inelastic design of steel I-girder bridges. First, key developments are summarized that lead to the proposed inelastic design approach. The paper then outlines a set of equations that provide an improved characterization of the inelastic moment-rotation response for a wide range of I-beams and plate girders. Effective plastic moment predictions based on these equations are combined with the recently proposed design method, resulting in greater accuracy and simplicity of the proposed approach. The ease of use of the resulting procedure is illustrated by a design example.     

Design for Shear in Curved Composite Multiple Steel Box Girder Bridges

Modern highway bridges are often subject to tight geometric restrictions and, in many cases, must be built in curved alignment. These bridges may have a cross section in the form of a multiple steel box girder composite with a concrete deck slab. This type of cross section is one of the most suitable for resisting the torsional, distortional, and warping effects induced by the bridge’s curvature. Current design practice in North America does not specifically deal with shear distribution in horizontally curved composite multiple steel box girder bridges. In this paper an extensive parametric study, using an experimentally calibrated finite-element model, is presented, in which simply supported straight and curved prototype bridges are analyzed to determine their shear distribution characteristics under dead load and under AASHTO live loadings. The parameters considered in this study are span length, number of steel boxes, number of traffic lanes, bridge aspect ratio, degree of curvature, and number and stiffness of cross bracings and of top-chord systems. Results from tests on five box girder bridge models verify the finite-element model. Based on the results from the parametric study simple empirical formulas for maximum shears (reactions) are developed that are suitable for the design office. A comparison is made with AASHTO and CHBDC formulas for straight bridges. An illustrative example of the design is presented.     

Analytical and Experimental Investigation of Bridge Girder Shear Distribution Factors

Tests on twelve bridges (six along Interstate 55 and six along Interstate 70/270 in Illinois) were performed to determine the validity of certain provisions for calculating bearing forces in the load and resistance factor design (LRFD) and the load factor design bridge specifications. The bridges were selected to be typical of Illinois interstate highway bridges and maintain a range of parameters to study. These bridges were instrumented on their beam webs with three strain gauge rosettes installed on each beam to measure shear stresses caused by loads. Static tests and slow moving 8 km/h (5?mi/h) tests with a loaded truck in specified locations were performed. Dynamic tests at highway speeds were also completed. Finite-element models were developed and compared to the test results. The study shows that the LRFD specification procedures closely approximate the shear distribution factors determined by finite-element analysis and testing.     

Distribution of Wheel Loads on Continuous Steel Spread-Box Girder Bridges

Composite concrete-steel spread (multispine) box girder bridges remain one of the most common types constructed. Current design practices in North America recommend few analytical methods for the design of such bridges in simply supported construction. However, the effects of continuous construction have not been dealt with fully. In designing a continuous bridge, it is important to determine the maximum negative and positive stresses, maximum reactions, and shears in the bridge subjected to various loadings. This paper presents an extensive parametric study using a finite-element model in which 60 continuous bridge prototypes of various geometries, each subjected to various loading conditions, are analyzed for the distribution of flexural stresses, deflection, shears, and reactions. The parameters considered in the study are span length, number of spread boxes, and number of lanes. Distribution factors for maximum flexural stresses, deflection, shears, and reactions, suitable for design, are deduced for AASHTO truck loading. Results from tests on five box girder bridge models verify the finite-element model. A design example is presented to illustrate the use of the deduced formulas for the distribution factors.     

Comparative Study of Fatigue Provisions for the AASHTO Fatigue Guide Specifications and LRFR Manual for Evaluation

The recently developed Manual for condition evaluation and load and resistance factor rating (LRFR) of highway bridges in 2003 provides an alternative procedure for practicing engineers to evaluate the fatigue life of steel bridge structures. Although the evaluation manual maintains several aspects used in the AASHTO fatigue guide specification in 1990, it also utilizes formulas and values specified in the AASHTO LRFD bridge design specifications in 1998. A comparative study of the fatigue lives provided by the procedures in the Evaluation manual and the Guide specifications was performed using a life prediction of 14 steel bridges with different structural configurations and various fatigue details. It has been shown that longer predicted fatigue lives are typically obtained when using the Evaluation manual. The ratio of the finite evaluation fatigue lives for the two procedures was found to be in a range of 0.99–2.14.     

Monitoring Steel Girder Stability for Safer Bridge Erection

The collapse of the State Route 69 Bridge over the Tennessee River near Clifton, Tennessee, is an example of how instability and lateral torsional buckling failure of a single steel bridge girder during erection might cause collapse of the whole steel superstructure. Close attention should be given to the stability of steel plate girders during erection when the lateral support provided to the compression flange might temporarily not be present. Rules of thumb in use today have been adopted by contractors/subcontractors to check the stability of cantilever or simply supported girders under erection using the L/b ratio, where L is the unbraced length and b is the compression flange width. For each girder section, a maximum L/b ratio exists beyond which lateral torsional buckling failure would occur under girder self-weight. Parametric studies were conducted following the latest AASHTO LRFD code in order to indentify the maximum L/b ratio for various girder sections and check the rules of thumb, as well as determine the dominating section parameters on girder stability under erection. Advanced nonlinear finite-element analyses were also conducted on a girder section for both the cantilever and the simply supported case in order to further understand the behavior of girder instability due to lateral torsional buckling under the self-weight, as well as to develop a trial-and-error methodology for identifying the maximum L/b ratio using computer analysis. At the same time, the effect of lateral bracing location on the cantilever free end has been investigated, and it turned out that bracing the top tension flange would be more effective to prevent lateral torsional buckling than bracing the bottom compression flange.     

Effect of Live-Load Deflections on Steel Bridge Performance

This paper presents an evaluation of the influence of AASHTO live-load deflection criteria on the performance of steel I-girder bridges. Background information is provided regarding previous research studies focused at understanding the role and suitability of live-load deflection limits on steel bridge design. Further, the results of an extensive survey of state transportation departments regarding the use of these limitations is provided. The results of a computer analysis package developed to evaluate the variability of these survey results are also presented. Last, a series of analyses of existing steel bridges conducted to examine the effects of the live-load deflection limits on typical and damaged bridges to determine the role that these limits play in overall superstructure performance is provided.     

Seismic Behavior of Steel Girder Bridge Superstructures

Recent earthquakes exposed the vulnerabilities of steel plate girder bridges when subjected to ground shaking. This paper discusses the behavior of steel plate girder bridges during recent earthquakes such as Petrolia, Northridge, and Kobe. The paper also discusses the recent experimental and analytical investigations that were conducted on steel plate girder bridges and their components. Results of these investigations showed the importance of shear connectors in distributing and transferring the lateral forces to the end and intermediate cross frames. Also, these investigations showed the potential of using end cross frames as ductile elements that can be used to dissipate the earthquake input energy. The paper also gives an update on specifications and guidelines for the seismic design of steel plate girder bridges in the United States.     

Influence of Secondary Elements and Deck Cracking on the Lateral Load Distribution of Steel Girder Bridges

The AASHTO LRFD load distribution factor equation was developed based on elastic finite element analysis considering only primary members, i.e., the effects of secondary elements such as lateral bracing and parapets were not considered. Meanwhile, many bridges have been identified as having significant cracking in the concrete deck. Even though deck cracking is a well-known phenomenon, the significance of pre-existing cracks on the live load distribution has not yet been assessed. The purpose of this research is to investigate the effect of secondary elements and deck cracking on the lateral load distribution of girder bridges. First, secondary elements such as diaphragms and parapets were modeled using the finite element method, and the calculated load distribution factors were compared with the code-specified values. Second, the effects of typical deck cracking and crack types that have a major effect on load distribution were identified through a number of nonlinear finite element analyses. It was established that the presence of secondary elements may produce load distribution factors up to 40% lower than the AASHTO LRFD values. Longitudinal cracking was found to increase the load distribution factor by up to 17% when compared to the LRFD value while the transverse cracking was found to not significantly influence the transverse distribution of moment.     

Implementation of a Long-Term Bridge Weigh-In-Motion System for a Steel Girder Bridge in the Interstate Highway System

This technical paper discusses the implementation of a long-term bridge weigh-in-motion system for use in determining gross vehicle weights of trucks crossing steel girder bridges. The system uses strain data to determine truck weights using an existing structural health monitoring system installed on a interstate highway bridge. The applied system has the advantage of not using any axle detectors in the roadway; and instead all analyses are performed using strain gauges attached directly to the steel girders, providing for a long-term monitoring system with minimal maintenance. Long-term data has been used to demonstrate that this method can be readily applied to gain important information on the quantity and weights of the trucks crossing the highway bridge.     

Rotation Compatibility Approach to Moment Redistribution for Design and Rating of Steel I-Girder Bridges

Recent research has culminated in the development of moment redistribution design and rating procedures based on a “rotation compatibility” procedure. The key aspects of the rotation compatibility method are presented herein along with the resulting series of simple equations that may be used for both design and rating of straight continuous-span steel I-girders. This procedure has several advantages over the previous moment redistribution procedures. Most significantly, the rotation compatibility method provides a rational basis for removing the current restrictions on girder geometries permissible for use with moment redistribution provisions. Thus, sections that are more slender and/or have greater unbraced lengths, compared to previous inelastic procedures, may be considered. This is particularly beneficial for incorporating inelastic methods into rating specifications because many existing bridges have geometries such that they have previously been outside the scope of applicability of inelastic procedures. A second key advantage of the rotation compatibility procedure is that maximum allowable redistribution moments are specifically computed, which justifies the use of higher levels of moment redistribution and consequently greater design economy in some cases.     

Construction of a Horizontally Curved Steel I-Girder Bridge. Part II: Inconsistent Detailing

The erection of horizontally curved steel I-girder bridges tends to be more complex than the erection of straight steel I-girder bridges. The erection of a curved steel I-girder bridge can be further complicated when the cross-frame members and girders are detailed inconsistently in an effort to force bridge components into some desirable geometric condition. Inconsistent detailing involves the intentional specification of cross-frame members that are either too long or too short to align with girder connector plates properly so as to force the girders into a given position, resulting in connection misalignments that must be resolved by applying external forces to the bridge components. The current research investigates the erection of a recently constructed horizontally curved steel I-girder bridge and highlights the fact that practice of inconsistent detailing can lead to very formidable and costly fit-up problems in the field; especially when girder sizes are large.