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.     

Evaluation of Live-Load Lateral Flange Bending Distribution for a Horizontally Curved I-Girder Bridge

This paper focuses on levels of live-load lateral bending moment (bimoment) distribution in a horizontally curved steel I-girder bridge. Work centered primarily on the examination of (1) data from field testing of an in-service horizontally curved steel I-girder bridge and (2) results from a three-dimensional numerical model. Experimental data sets were used for calibration of the numerical model and the calibrated model was then used to examine the accuracy of lateral bending distribution factor equations presented in the 1993 Edition of the (AASHTO) Guide Specifications for Horizontally Curved Bridges. It is of interest to examine these equations for potential use in preliminary design even though they have been eliminated during recent AASHTO specification modifications that addressed curved bridge analysis, the 2005 Interims to the AASHTO LRFD Bridge Design Specifications. In addition, they were developed using idealized computer models and small-scale laboratory testing with very few field tests of in-service full-scale curved steel bridges conducted to support or refute their use. Results from such experimental and numerical studies are presented and discussed herein.     

AASHTO-LRFD Live Load Distribution for Beam-and-Slab Bridges: Limitations and Applicability

This paper presents a comparison between the live load distribution factors of simple span slab-on-girders concrete bridges based on the current AASHTO-LRFD and finite-element analysis. In this comparison, the range of applicability limits specified by the current AASHTO-LRFD is fully covered and investigated in terms of span length, slab thickness, girder spacing and longitudinal stiffness. All the AASHTO-PCI concrete girders (Types I–VI) are considered to cover the complete range of longitudinal stiffness specified in the AASHTO-LRFD. Several finite-elements linear elastic models were investigated to obtain the most accurate method to represent the bridge superstructure. The bridge deck was modeled as four-node quadrilateral shell elements, whereas the girders were modeled using two-node space frame elements. The live load used in the analysis is the vehicular load plus the standard lane load as specified by AASHTO-LRFD. The live load is positioned at the longitudinal location that produced the extreme effect, and then it is moved transversely across the bridge width in order to investigate all possibilities of one-lane, two-lane and three-lane design loads. A total of 886 bridge superstructure models were built and analyzed using the computer program SAP2000 to perform this comparison. The results of this study are presented in terms of figures to be practically useful to bridge engineers. This study showed that the AASHTO-LRFD may significantly overestimate the live load distribution factors compared to the finite-element analysis.     

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.     

Calibrating Resistance Factors of Single Bored Piles Based on Incomplete Load Test Results

This paper addresses a problem often encountered in calibrating resistance factors for the ultimate capacities of piles based on load test databases. In practice, many pile load tests are not conducted to failure but only to a multiple (e.g., 2) of the design load. This leads to a difficult situation of incomplete information: for these test results, the ultimate bearing capacities of the test piles are unknown. How can these test results still be used to calibrate resistance factors of piles? A full probabilistic framework is proposed in this research to resolve this problem. A local pile test database of Taipei (Taiwan) is presented for demonstration. The analysis results show that the inclusion of the incomplete pile load test data enhances the stability of the calibrated resistance factors. For a target reliability index of 3, the calibrated resistance factor is in the range of 0.4–0.66 for two design models adopted in the current Taiwan design code. Moreover, it is found that the safety factor adopted in the Taiwan design code corresponds to a reliability index larger than 3.5, which is reasonably conservative.     

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.     

Truck Loads and Bridge Capacity Evaluation in China

Over the past 2–3 decades, the economic development in China has natured the establishment of a highway network with a large number of bridges. However, there are still no nationwide specification provisions for assessing their safe load carrying capacity. The research work reported herein focuses on developing reliability based requirements for this purpose. In this study, weigh-in-motion data for more than 7.3 million trucks were gathered from highways in three provinces of China, continuously over 1–16 months in 2006 and 2007. The data were processed and projected to model the live-load spectrum over 3-year and 100-year periods, respectively. The former is the required bridge inspection interval and the latter the bridge design life span, according to current Chinese maintenance and design specifications. The proposed projection method is shown to be more reliable compared with those reported. The resulting load spectra are used to assess the structural reliability of typical Chinese highway bridges at the component level. Based on the accordingly selected target reliability index, the live-load factors for bridge evaluation are developed in this study, proposed to be included in the Chinese national specifications.     

Importance of Lower-Bound Capacities in the Design of Deep Foundations

There is generally a physical limit to the smallest possible capacity for a deep foundation. However, a lower bound on the capacity has rarely been accounted for in performing reliability analyses and developing reliability-based design codes. The objectives of this paper are to investigate the effect of having a lower-bound capacity on the reliability of a geotechnical engineering system and to propose a load and resistance factor design (LRFD) checking format that includes information on the lower-bound capacity in design. It is concluded that a lower-bound capacity can cause a significant increase in the calculated reliability for a geotechnical design even if it is an uncertain estimate. Two alternative LRFD formats that incorporate lower-bound capacities and that would not require substantive changes to existing codes are proposed. Real-world examples dealing with the design of onshore and offshore foundations indicate that the incorporation of a lower-bound capacity into design is expected to provide a more realistic quantification of reliability for decision-making purposes and therefore a more rational and efficient basis for design.     

Uncertainty Analysis of Micropile Pullout Based upon Load Test Results

Micropiles are applied in foundation rehabilitation projects to enhance the pullout capacity of the existing foundation system and minimize the vertical deflection of the structures. Consequently, the methods to calculate the pullout load–displacement behavior are important for the design of micropiles used for such rehabilitation projects. Production pullout load tests were performed on approximately 120 micropiles used for seismic retrofitting of the 580/980/24 freeway interchange in Oakland, Calif. These micropiles were required to satisfy the serviceability design capacity at a maximum deflection of 1.27?cm (0.5?in.), in addition to the ultimate design capacity. Because the existing structures varied in height and loading, micropiles of different pullout capacities were designed. In this paper, the measured data are using a closed-form “t–z” method to develop probability distribution functions for the model parameters. As the back-calculated model parameters are assumed to be random variables, the Monte Carlo simulation process is employed to develop a series of load–displacement curves. A method for determining the probability of micropile failure at the service limit state is developed using the results of the simulations. The method is utilized to obtain resistance factors that can be applied to LRFD based service limit- state design.     

Resistance Factors for Use in Shallow Foundation LRFD

In shallow foundation design, the key improvements offered by LRFD over the traditional working stress design (WSD) are the ability to provide a more consistent level of reliability between different designs and the possibility of accounting for load and resistance uncertainties separately. In the development of LRFD, a framework for the objective, logical assessment of resistance factors is needed. Additionally, in order for LRFD to fulfill its promise for designs with more consistent reliability, the methods used to execute a design must be consistent with the methods assumed in the development of the LRFD factors. In this paper, a methodology for the estimation of soil parameters for use in design equations is proposed that should allow for more statistical consistency in design inputs than is possible in traditional methods. Resistance factors for ultimate bearing capacity are computed using reliability analysis for shallow foundations both in sand and in clay, with input parameters obtained from both the cone penetration test and the standard penetration test, and for both ASCE-7 2000 and AASHTO 1998 load factors. Resistance factor values are dependent upon the values of load factors used. Thus, a method to adjust the resistance factors to account for code-specified load factors is also presented.     

Effects of State Legal Loads on Bridge Rating Results Using the LRFR Procedure

The main objective of this research was to study the effects of different specified trucks on bridge rating with the load and resistance and factor rating (LRFR) procedure. Twelve specified trucks were selected for this study, which include one AASHTO design truck, three AASHTO legal trucks, and eight state legal trucks. These rating trucks were applied on 16 selected Tennessee Dept. of Transportation bridges to obtain the LRFR ratings. The selected bridges covered four commonly used bridge types, including prestressed I-beam bridges; prestressed box beam bridges; cast-in-place T-beam bridges; and steel I-beam bridges. The research results revealed that (1) LRFR AASHTO legal load ratings factors were enveloped by the LRFR HL-93 truck ratings factors, thereby confirming the validity of the LRFR tiered approach with regard to AASHTO legal loads; (2) the lighter state legal trucks were enveloped by the HL-93 loads, whereas the heavier state trucks with closer axle spacing typically resulted in load ratings that governed over the HL-93 loads; and (3) the bridges with both high average daily truck traffic and short spans were more likely to be governed by state legal load ratings instead of HL-93 load ratings.     

Multiple Resistance Factor Design for Shallow Transmission Line Structure Foundations

This paper presents the development of simplified reliability-based design (RBD) equations that are suitable for spread foundations subjected to uplift. Emphasis is placed on the loading and foundation characteristics relevant to the electric utility industry. A general reliability calibration procedure is used to derive robust resistance/deformation factors for load and resistance factor design (LRFD) and multiple resistance factor design (MRFD) formats. Two target reliability indices of 3.2 and 2.6 are proposed based on an extensive study of existing designs for ultimate and serviceability limit state, respectively. The main advantage of using these RBD factors is that a known level of reliability can be consistently achieved over a wide range of design conditions. Simple design calculations using the MRFD format are shown to demonstrate their ability to account for parametric uncertainties in a rational manner.     

Resistance Factors for Use in Load and Resistance Factor Design of Driven Pipe Piles in Sands

Load and resistance factor design of foundations is done in the offshore industry and is now being done in bridge projects in the United States. Common methods used to establish resistance factors include calibration to assumed factors of safety and reliability analysis using field load test databases. Reliability analyses are the preferred tools for this work but the needed probabilistic information regarding design method uncertainty is difficult to obtain. Furthermore, field load test databases, while relatively attractive for assessing design uncertainty, are not able to discriminate between uncertainties caused by soil variability, test methods, and model design relationships. In contrast to previous efforts, this paper illustrates an approach to uncertainty assessment that seeks to isolate the various sources of uncertainty. Using this approach, reliability analysis is used to develop resistance factors for the design of driven pipe piles in sand. The resistance factor results are used to highlight some of the differences between design methods that are exposed by the proposed uncertainty assessment technique. A brief design example is also given that illustrates the use of the resistance factors.     

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.     

Influence of Spatially Variable Side Friction on Single Drilled Shaft Resistance and LRFD Resistance Factors

Load and resistance factor design (LRFD) is a method that aims at meeting specified target reliabilities (probabilities of failure) of engineered systems. The present work focuses on ultimate side friction resistance for axial loads on single cylindrical drilled shaft foundations in the presence of spatially variable rock/soil strength. Core sample data are assumed to provide reliable information about local strength in terms of mean, coefficient of variation and spatial correlation structure (variogram) at a site. The geostatistical principle of support up-scaling is applied to quantify the reduction in variability between local strength and the average ultimate shaft side friction resistance without having to recur to lengthy stochastic finite difference/element simulations. Site and shaft specific LRFD resistance factors (Φ values) are given based on the assumption of lognormal load and resistance distributions and existing formulas recommended by the Federal Highway Administration. Results are efficiently represented in dimensionless charts for a wide range of target reliabilities, shaft dimensions, and geostatistical parameters including nested variograms of different types with geometric and/or zonal anisotropies. Field data of local rock strength is used to demonstrate the method and to evaluate the sensitivity of obtained resistance factors to potentially uncertain variogram parameters.     

Performance of Conservatories under Wind and Snow Loads

Wind and snow loads are the governing load cases for the design of conservatories. The performance of conservatories under these loads determines to a great extent the reliability and serviceability of these structures. This paper presents a study that evaluates the performance of several conservatory designs under wind and snow loads calculated according to the ASCE 7–05 standard. A full-scale model of one common conservatory design was constructed and tested under extreme wind and snow conditions. The model was able to withstand the applied loads without any signs of damage or partial failures. Measured deformations were used to calibrate the three-dimensional computer model developed to simulate the actual structure. Several other computer models were developed to structurally analyze different conservatory designs and estimate their vertical and lateral deflections. The design of critical sections in each model was checked using the load and resistance factor design of wood structures. The study concluded that the design of these conservatories is adequate and their performance is satisfactory under wind and snow loading conditions.     

Statistical Significance of Less Common Load Combinations

The purpose of this work is to provide a simple, rational basis for combining bridge loads in the extreme event limit state. Classic methods are used to evaluate the probabilities of traffic, seismic, and storm-related bridge loads occurring simultaneously and in various intensities. The loads are modeled using a Poisson distribution, which circumvents problems encountered when using normal- or log-normal distributions. The hazard level is evaluated directly using a negative exponential function in the time domain. An acceptable hazard level for combined events is subjectively based on that deemed acceptable for the strength limit state and extreme events occurring individually, as well as fiscal prudence. It is shown that (1) application of seismic loads to a structure already subjected to the combined effects of degradation, local pier scour, and contraction of the waterway is not justifiable; (2) live loads reduced from the anticipated 75-year maximum to a 2-week maximum are appropriate when designing a bridge in its 100-year “scoured-out” storm configuration; and (3) vehicular live loads are likely to be on a bridge during a seismic event, but other issues need to be considered.     

Full-Scale Field Load Testing of Storm-Water Storage Chamber Structures

One of the latest solutions available for storm-water storage purposes is the underground chamber structure. Recently, a research team at Ohio University conducted a series of field load tests on the underground chambers. Four chamber structures were placed side by side in an excavated area, backfilled with coarse granular soil, buried under a soil cover of 0.46?m (18?in.), instrumented with sensors, and subjected to a series of controlled live load tests at a field project site. The sensor readings and visual inspection results indicate that the chamber with the specified minimum soil cover had no problem in supporting a wheel load of 52.3?kN (11.8?kips) and an axle load of 109.0?kN (24.5?kips) in both the transverse and longitudinal directions. Maximum reduction in the rise dimension was only approximately 2.3% when the chambers were subjected to the live loads. The vertical soil pressure readings measured at the chamber crowns were within 15% of the values given by a live load spreading formula included in the AASHTO LRFD specifications.