AASHTO LRFD Specifications Research Articles

Transfer and Development Lengths and Prestress Losses in Ultra-High-Performance Concrete Beams

Results from a project that measured the transfer and development lengths and prestress losses of beams cast with ultra-high-performance concrete (UHPC) are presented. Seven beams measuring 6.5 × 12 × 18 ft were cast. The beams had compressive strengths that ranged from approximately 27 to 29 ksi. The results were compared with the calculated values derived from the AASHTO load and resistance factor design (AASHTO LRFD) bridge specifications, NCHRP Report 603, and the University of New South Wales design guidelines for prestressed UHPC beams. The research findings indicated that measured transfer and development lengths were shorter than those predicted with the use of the various design guidelines and recommendations developed for conventional concrete. However, measured transfer lengths correlated well with those predicted with NCHRP 603 (neglecting f'ci limits) and University of New South Wales recommendations and guidelines. The AASHTO LRFD specifications overestimated transfer length, development length, and prestress losses (refined method) by approximately 50%. The data also showed that the contribution of creep and shrinkage in prestress losses was minimal.

Residual Prestress Forces and Shear Capacity of Salvaged Prestressed Concrete Bridge Girders

Seven prestressed concrete bridge girders that had been in service for 42 years, and represented two span lengths and reinforcement designs, were tested to determine their effective prestress force and ultimate shear capacity. A cracking moment test was used to determine the effective prestress force in the girders. The measured effective prestress force was compared with calculated values according to the AASHTO LRFD prestress loss equations to investigate their adequacy. The AASHTO refined method was shown to provide the most accurate results to within 10% of the measured values. An ultimate shear test was also performed on two of the girders. An external load was applied near the support and increased until the girder failed in shear. The various procedures in the AASHTO LRFD specifications were compared with the measured results. The AASHTO simplified procedure predicted only 51% and 39% of the average measured shear capacity for the short and long span girders, respectively. The strut-and-tie models were found to estimate the shear capacity more accurately. The AASHTO refined method was shown to provide the most accurate results.

Revisit of AASHTO Effective Flange-Width Provisions for Box Girders

The nonuniform normal stress distribution along the flange of thin-walled flexural members, such as box girders, is recognized as shear lag effects. Effective flange width is widely used to consider shear lag effects in bridge design. The existing provisions for effective flange width in AASHTO LRFD Specifications can be unconservative compared with this study and other design regulations worldwide. An energy-based variation analysis was used in the study to evaluate the AASHTO provisions. With extensive parametric studies, equations were proposed to calculate the effective flange width for box girders. The new provisions consider the critical factors for the shear lag behavior: geometry of the girder, loading pattern, material properties, and the existence of longitudinal stiffeners.

Designing Stormwater Chambers to Meet AASHTO Specifications

Abstract Thermoplastic stormwater chambers have been successfully used on many projects to meet current federal regulations controlling discharges from newly developed sites. These structures are commonly located under parking lots or roadways and subjected to vehicular loads on a regular basis. Given this application, it is logical to design these structures for loads specified in the AASHTO LRFD Specifications for Highway Bridges, which provide design procedures for thermoplastic culvert pipe for use under roadways. However, experience has shown that many drainage engineers are not familiar with the AASHTO Specifications and have difficulty evaluating product manufacturer’s claims relative to AASHTO requirements. This paper presents the key aspects of the AASHTO specifications that should be applied to stormwater chamber design, such as determining applicable loads, load factors, design of profile sections, and consideration of time dependent properties. Also addressed are design matters for chambers not directly addressed by AASHTO. Of particular importance in developing unique structures is the validation of designs and chamber safety levels through full scale testing.

Creep and shrinkage behavior of high-strength concrete and minimum reinforcement ratio for bridge columns

This paper summarizes the findings of an extensive research program that examined the shrinkage and creep behavior of high-strength concrete (HSC) up to a strength of 18 ksi (124 MPa). Creep and shrinkage strains of 60 specimens were monitored for up to 2 years. The variables considered in this research were the concrete compressive strength, specimen size, curing type, age of concrete at loading, and loading stress level. Findings indicate that the current American Association of State Highway and Transportation Officials' (AASHTO) LRFD Bridge Design Specifications could be used to estimate the creep coefficient and shrinkage strain of HSC up to 15 ksi (103 MPa). However, the current AASHTO LRFD specifications do not provide appropriate predictions for concrete compressive strength greater than 15 ksi (103 MPa). A revised time-development correction factor is proposed to obtain better predictions for HSC up to 18 ksi (124 MPa). For HSC compression members, the current AASHTO LRFD specifications require an excessive amount of minimum longitudinal reinforcement to account for the long-term effects due to shrinkage and creep. Based on an analysis, a new relationship is proposed for the required minimum reinforcement ratio.

Inelastic buckling of torsionally braced I-girders under uniform bending, II: Experimental study

This paper presents a series of experimental test results on the inelastic buckling of torsionally braced I-girder system under uniform bending. In order to confirm the numerical study results on the design criteria for torsional diaphragm braces which are presented through the companion paper, a test program was performed. Totally four sets of test models that form 2-girder systems interconnected with I-shaped crossbeams were fabricated by using SM570-TMC steel. Pure bending test was conducted on each test model up to the ultimate bending capacity. Then, numerical analyses were performed for a comparative study. The bending resistance capacities of the test girders were evaluated by using the Eurocode 3 and the AASHTO LRFD specifications to compare with the test results. From the comparative studies, the torsional bracing requirements were experimentally proved for the inelastic buckling range approaching the compact unbraced length limit. Finally, the validity of the proposed design equations including the current design provisions is discussed.

Fatigue reliability analysis for the crack propagation compared with LRFD specification

In this study, a fatigue reliability is assessed taking into account the uncertainties in load and resistance of a steel specimen, in which fatigue crack propagates. The results of the deterministic fatigue crack propagation has been compared with AASHTO LRFD specification. A response surface method (RSM) combined with an advanced first order second moment method has been applied in order to assess the probability of the remaining life of the specimen under cyclic load as a function of crack length. For composing limit state functions, the stress ranges, stress intensity factor and the remaining life was selected as input random variables for the developed fatigue-reliability analysis program. The choice of Bayesian Belief Nets (BBNs) as a tool for assessing uncertainties in resistant component of a limit state function allows an extended probabilistic assessment for the resultant fatigue life of a target specimen, in terms of resistant components of stress range, stress intensity factor, and material property. Additionally, fragility curve models are proposed to compare the probabilistic fatigue life as describing the probability of a structure to suffer a given damage level when it is subject to a given fatigue life. The proposed integrated stochastic models can significantly contribute to the design and maintenance of infra-structures, demonstrated in the numerical example in terms of damage index with the probability of exceedance the considered damage levels.

Rapid Girder Selection and Cost Estimation for Simple-Made-Continuous Steel Bridge Design

Prestressed precast bridges currently dominate the short to medium bridge market in the state of Colorado. This can be attributed to a lack of competitive steel construction products combined with a strong presence of precast fabricators in the region. For steel girders there is a lack of readily available economical and innovative procedures to design and construct steel bridges, which has hindered the industry in certain areas of the country such as Colorado. A significant percent of the bridges throughout the United States are in need of repair and many in need of replacement. In many part of the country, steel bridges can provide an economic and rapid solution for new and replacement bridges. This paper summarizes the results of a study sponsored by the Colorado DOT to develop a rapid preliminary design and cost estimation tool for simple-made-continuous design and construction. In that method, steel girders are designed as simply supported for the noncomposite dead loads and continuous for composite dead loads and live loads, which can provide economy for spans up to 35 m (120 ft). The methodology summarized herein follows the logic of the AASHTO LRFD specifications. Design charts and tables were created to outline structural steel weight to span length and number of girders, which could be used to prepare initial bids during the material section process.

Shear Strengths of Prestressed Concrete Beams Part 2: Comparisons with ACI and AASHTO Provisions

In Part 1 of this paper, an equation for the shear strengths of prestressed concrete beams was developed. New formulas were also proposed for maximum shear strength and for the minimum stirrup requirement. Part 2 of this paper compares the proposed shear design method to the shear provisions in the ACI 318-08 and the 2007 AASHTO LRFD Specifications using 148 test beams. The proposed method is shown to be simpler and more accurate than the ACI and AASHTO specifications, despite the fact that the proposed method does not take into account two variables, prestress force and angle of failure crack, that are involved in the ACI and AASHTO shear provisions.

Finite element modeling of push-out tests for large stud shear connectors

An accurate nonlinear finite element model of the push-out specimen has been developed to investigate the capacity of large stud shear connectors embedded in a solid slab. The material nonlinearities of concrete, headed stud, steel beam and rebar were included in the finite element model. The damage and failure were included in the material model for the headed stud to accurately obtain the ultimate strength of the stud connector. The capacity and ductility of the connection, the load–slip behaviour and failure mode of the headed stud were predicted. The results obtained from the finite element analysis were verified against experimental results of other researches. An extensive parametric study was conducted to study the effect of the changes in stud diameter and concrete strength on the capacity and behaviour of the shear connection. The capacity and ductility of the shear connection obtained from the finite analysis were compared with those specified in EC4 and AASHTO LRFD. It is observed that the AASHTO LRFD specifications overestimated the capacity of the large stud shear connectors, whereas the design rules specified in Eurocode-4 were generally conservative for stud diameters of 22, 25 and 27 mm, and unconservative for diameter of 30 mm. The ductility of the large stud shear connectors is sufficient for practical application in composite bridges.

Elasticity modulus, shrinkage, and creep of high-strength concrete as adopted by AASHTO

This paper covers the experimental and theoretical components of National Cooperative Highway Research Program (NCHRP) research project Number 18-07, which is discussed in NCHRP Report 496. The components are related to modulus of elasticity, shrinkage, and creep of concrete. An experimental program was conducted at various bridge sites and at the University of Nebraska-Lincoln for specimens produced from raw materials and mixture proportions provided by 4 participating states: Nebraska, New Hampshire, Texas, and Washington. Previously reported measurements of material properties are also included. The experimental program was used to extend the pre-2005 AASHTO-LRFD specification prediction formulas to concrete with compressive strengths up to 15 ksi (104 MPa). For each material property, a summary of the experimental values is presented followed by a comparison with the values obtained from the pre-2005 AASHTO-LRFD specifications and the American Concrete Institute 209 committee report. The proposed formulas provide designers of prestressed concrete girders with more realistic estimates of long-term material properties, including effects of aggregate type and other significant factors. The use of these formulas with high-strength concrete (HSC) should result in more realistic camber predictions and lower prestress loss estimates. The background and recommendations for prediction of the modulus of elasticity, shrinkage, and creep of HSC are provided. To help with clarity, the notation and units employed in the pre-2005 AASHTO-LRFD specifications are adopted as much as possible. Stresses are expressed in units of ksi (MPa) rather than psi.

Transfer lengths of prestressed beams cast with self-consolidating concrete

Self-consolidating concrete (SCC) is a recent advancement in the concrete industry. SCC is not much different from conventional concrete; the constituent materials are the same, but SCC typically contains more fine aggregate and cementitious materials, and less coarse aggregate. These different proportions may affect the transfer length and flexural bond performance for SCC specimens compared with conventional concrete specimens. These differences may contribute to larger transfer lengths than those predicted by the American Concrete Institute's (ACI's) Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (ACI 318R-05) and the American Association of Highway and Transportation Officials,' AASHTO LRFD Bridge Design Specifications. This research program compares the measured transfer lengths for prestressed SCC beams with those for prestressed, conventional, high-strength concrete beams. Furthermore, the measured transfer lengths for the SCC prestressed beams were compared with those calculated using the ACI 318-05 and AASHTO LRFD specifications equations. Results from the research program show that there was little difference in the transfer lengths of the SCC beams and the conventional concrete beams. Furthermore, the measured transfer lengths of the SCC beams were about 60% of those predicted by ACI 318-05 and AASHTO LRFD specifications equations.

Quantification of intermediate diaphragm effects on load distributions of prestressed concrete girder bridges

Construction of intermediate diaphragms in prestressed concrete bridges impacts cost and schedule. The practice of providing intermediate diaphragms in prestressed concrete bridges is still controversial, and each state has a different practice. The American Association for State Highway and Transportation Officials' (AASHTO's) Standard Specifications for Highway Bridges specifies that intermediate diaphragms be used at the point of maximum positive moment for spans exceeding 12 m (40 ft), but clear reasons for this requirement are not given. While the AASHTO LRFD Bridge Design Specifications state that intermediate diaphragms can improve live-load distributions, this effect is not included in the calculation of load-distribution factors. Previous studies of intermediate diaphragms were limited to whether they increased or decreased the load-distribution factors. No significant work has been conducted to develop a formula to quantify the influence of intermediate diaphragms on load distributions. This study aims to quantify the intermediate diaphragm influence on load distributions and presents an approach to develop correction factors for load distributions. The correction factors are applied to the AASHTO LRFD specifications' load-distribution factors. This results in a revised load-distribution factor that accounts for the intermediate diaphragm influence on load distributions. The study illustrates how to apply these formulas in design. The presented information will help engineers examine the functions of intermediate diaphragms and develop their policies of intermediate diaphragm practice for prestressed concrete bridges.

Evaluation of Prestressed Concrete I-Girder Bridge Design Software with NCHRP Process 12-50

NCHRP Process 12-50 was used to evaluate and verify prestressed concrete bridge design software commonly used in Indiana. A test bed of 40 bridge structures was developed with input from practicing engineers and Indiana Department of Transportation (INDOT) specialists. The test bed included 20 simple-span and 20 multispan bridges. The primary parameters were bridge span, girder spacing, section type, strand pattern (straight or draped), and concrete strength. The remaining parameters were held constant with practical or consensus values provided by engineers or the INDOT bridge design manual. The section types included were AASHTO Types II, III, and IV and Indiana modified Bulb-Tee beams (54–78 in. deep). An indigenous computer program, PURDUE PSBD, was developed to implement the bridge design calculations specified in the AASHTO LRFD Bridge Design Specifications, which are currently endorsed by the INDOT bridge design manual. Both CONSPAN (the most common bridge design software in Indiana) and PURDUE PSBD generated calculation results for the complete test bed of bridges. The output from both computer programs was compared to identify assumptions and discrepancies from the AASHTO LRFD specifications. These comparisons indicated excellent agreement between the results from both programs for ( a) concrete stresses and deflections at service-level loads, ( b) nominal flexural strength, ( c) nominal shear strength, and ( d) initial camber and deflections. The test bed of bridge structures and the PURDUE PSBD program developed in this research are recommended for evaluating and verifying other bridge design software.

Design Recommendations for a FRP Bridge Deck Supported on Steel Superstructure

No appropriate provisions from either AASHTO Standard (2002) or AASHTO LRFD (2004) bridge design specifications are available for the design of fiber-reinforced polymer (FRP)-deck-on-steel-superstructure bridges. In this research, a parametric study using the finite-element method (FEM) is conducted to examine 2 design issues concerning the design of FRP-deck-on-steel-superstructure bridges, namely, deck relative deflection and load distribution factor (LDF). Results show that the strip method specified in AASHTO LRFD specification as an approximate method of analysis, can also be applied to FRP decks as a practical method. However, different strip width equations have to be determined by either FEM or experimental methods for different types of FRP decks. In this study, one such equation has been derived for the Strongwell deck. In addition, both FEM results and experimental measurements show that the AASHTO LDF equations for glued laminated timber decks on steel stringers provide good estimations of LDF for FRP-deck-on-steel-superstructure bridges. Finally, it is found that the lever rule can be used as an appropriately conservative design method to predict the LDF of FRP-deck-on-steel-superstructure bridges.

Ultimate Strength of Concrete Barrier by the Yield Line Theory

When the yield line theory is used to estimate the ultimate strength of a concrete barrier, it is of primary importance that the correct assumption is made for the failure mode of the barrier. In this study, a static test was performed on two full-scale concrete barrier specimens of Korean standard shape that simulate the actual behavior of a longitudinally continuous barrier. This was conducted in order to verify the failure mode presented in the AASHTO LRFD specification. The resulting shape of the yield lines differed from that presented in AASHTO when subjected to an equivalent crash load. Furthermore, the ultimate strengths of the specimens were lower than the theoretical prediction. The main causes of these differences can be attributed to the characteristics of the barrier shape and to a number of limitations associated with the classical yield line theory. Therefore, a revised failure mode with corresponding prediction equations of the strength were proposed based on the yield lines observed in the test. As a result, a strength that was more comparable to that of the test could be obtained. The proposed procedure can be used to establish more realistic test levels for barriers that have a similar shape.

Equivalent Wheel Load Approach for Slender Cable-Stayed Bridge Fatigue Assessment under Traffic and Wind: Feasibility Study

In the current AASHTO LRFD specifications, the fatigue design considers only one design truck per bridge with 15% dynamic allowance. While this empirical approach may be practical for regular short and medium span bridges, it may not be rational for long-span bridges (e.g., span length >152.4m or 500ft) that may carry many heavy trucks simultaneously. Some existent studies suggested that fatigue may not control the design for many small and medium bridges. However, little research on the fatigue performance of long-span bridges subjected to both wind and traffic has been reported and if fatigue could become a dominant issue for such a long-span bridge design is still not clear. Regardless if the current fatigue design specifications are sufficient or not, a real understanding of the traffic effects on bridge performance including fatigue is desirable since the one truck per bridge for fatigue design does not represent the actual traffic condition. As the first step toward the study of fatigue performance of long-span cable-stayed bridges under both busy traffic and wind, the equivalent dynamic wheel load approach is proposed in the current study to simplify the analysis procedure. Based on full interaction analyses of a single-vehicle–bridge–wind system, the dynamic wheel load of the vehicle acting on the bridge can be obtained for a given vehicle type, wind, and driving condition. As a result, the dimension of the coupled equations is independent of the number of vehicles, through which the analyses can be significantly simplified. Such simplification is the key step toward the future fatigue analysis of long-span bridges under a combined action of wind and actual traffic conditions.

Application of Orthotropic Thin Plate Theory to Filled Steel Grid Decks for Bridges

Although AASHTO LRFD specifications provide moment capacity equations as an approximate design method and recommend an orthotropic plate model as the refined method for the analysis of filled grid decks, no guidelines are provided for the determination of the flexural rigidities associated with the plate analysis. This technical note briefly reviews orthotropic thin plate theory, discusses the determination of the flexural rigidities using Huber’s assumption, and applies the theory to concrete-filled steel grid decks. The accuracy of the orthotropic plate analysis is assessed by comparing it to results of an earlier finite-element analysis.

Transverse Post-tensioning Design of Precast Concrete Multibeam Deck

Precast concrete multibeam deck systems are widely used to replace deteriorated bridge decks. The system's rapid erection substantially reduces traffic disruptions. The transverse precompression force provided by the posttensioning tendons is an important design consideration to ensure adequate load transfer between contiguous beams and to reduce potential differential cracking at shear key joint locations. This paper presents an analysis method, based on grillage analogy, for calculating the required transverse posttensioning stress for a deck built with precast concrete multibeams. The effects of skew--as well as other influencing factors, such as deck width, thickness, and span length--on the required posttensioning stress were studied. Results of the study indicate that the effect of skew is significant when the skew angle is greater than 30 deg. Results compare well with the AASHTO's LRFD specifications for right-angle decks; however, the required posttensioning stress is decreased with an increase in skew angle.

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 spec...