AASHTO LRFD Specifications Research Articles

Investigation of seismic behavior of horizontally curved bridges in comparison with equivalent straight bridges accordıng to the AASHTO LRFD specifications

Investigation of seismic behavior of horizontally curved bridges in comparison with equivalent straight bridges accordıng to the AASHTO LRFD specifications

Development of load-resistance-factor-design-based prestressed concrete tub girders in Colorado

This paper presents a new tub girder series for Colorado, conforming to the articles of the American Association of State Highway and Transportation Officials’ AASHTO LRFD Bridge Design Specifications. Although the B618 tub girders developed by the Colorado Department of Transportation in the 1990s have been successfully used for decades, the need for an upgrade is essential to satisfy the requirements of contemporary bridge design and construction. After examining the performance of tub girders selected from six transportation agencies in the United States, an optimization algorithm was employed to generate efficient prototype sections. Then, detailed investigations were conducted to examine various practical aspects concerning the serviceability and ultimate limit states of AASHTO LRFD specifications. Parametric analysis with five bridge superstructures that accommodate up to four traffic lanes demonstrated the applicability of the prototype girders. A simplified version of the prototype sections was also delineated for regional precasters. Furthermore, a comparative study was carried out to evaluate the geometric stability and production costs of the prototype, simplified, and existing B618 girders.

Experimental and analytical assessment of LTB resistance of built-up steel I-section members

Analytical and experimental research investigations conducted subsequent to the development of the unified AISC 360–05 Section F4 and F5 and AASHTO [3] LRFD provisions for built-up I-section members have identified several areas where improvements are needed. However, the experimental data is relatively scarce within the design space for inelastic lateral-torsional buckling (LTB) of these member types. This study presents the results of eight experimental tests of doubly symmetric built-up I-section members and a large number of full-nonlinear shell finite element analysis (FEA) simulations validated against the experimental results. The study was conducted to supplement existing data and to evaluate three recommended changes to the AISC 360 Specification: (1) reduction of the ordinate at the transition between the elastic and inelastic LTB equations to ML = 0.5RpgMyc for the base uniform bending case, where Rpg is the web bend-buckling strength reduction factor and Myc is the yield moment to the compression flange; (2) use of the more general equation in AISC 360–16 Table B4–1 Case 16 for the compact-web limit, λpw; and (3) use of a variable crw factor from the current AASHTO LRFD Specifications for the calculation of the noncompact-web limit, λrw. Comparisons were also made to the predictions from the first generation of Eurocode 3. Results from the study support the recommended changes to the AISC 360 Specification and confirm the significant impact of flange lateral bending from the amplification of initial compression flange sweep for members with LTB slenderness close to the noncompact bracing limit associated with ML.

Shear performance of high-strength friction-grip bolted shear connector in prefabricated steel–UHPC composite beams: Finite element modelling and parametric study

As a competitive alternative for accelerated bridge construction (ABC), prefabricated steel–ultra-high-performance concrete (UHPC) composite beams containing high-strength friction-grip bolt (HSFGB) shear connectors offer numerous advantages, including reduced on-site construction time and ease of replacing/removing deteriorated components. However, the failure mechanism of HSFGBs in UHPC remains unclear due to the lack of internal stress analysis, which hinders the design of these innovative composite beams. To clarify the shear performance of HSFGBs in prefabricated steel–UHPC composite beams, an effective finite element model (FEM) considering the non-linearities of materials and geometry was developed through ABAQUS. Based on the experimentally verified model, the internal stress transfer mechanisms of HSFGBs and the failure mechanism of precast UHPC were revealed. According to the extension parametric analysis results, a stronger HSFGB presented better shear performances in terms of ultimate shear strength, initial shear stiffness and slip capacity. Adopting oversized holes with appropriate bolt-to-hole clearance can improve the constructional efficiency without considerable strength and stiffness reduction. HSFGBs with low bolt pretension exhibited unfavorable initial shear stiffness, while smaller slip capacity in high bolt pretension conditions. A smaller ductility was observed as the steel beam tensile strength and slab concrete strength increased. Additionally, the ACI 318–19, Eurocode 3, and AASHTO LRFD specifications underestimated the shear strength of HSFGBs, whereas the Eurocode 4 presented acceptable predictions in determining the ultimate shear capacity of HSFGBs in prefabricated steel–UHPC composite beams.

Experimental background behind new AASHTO LRFD specifications for partially partially debonded strands

To develop a unified approach for the design of partially debonded strands in prestressed concrete highway bridge girders, a coordinated analytical and experimental investigation was conducted. This investigation examined amounts of partial debonding, distribution of partially debonded strands within the cross section, debonded lengths, locations and staggering of termination of debonded strands, confinement of debonded regions and their termination points, and the impact of adding nonprestressed reinforcement in the region of partial debonding. The results from testing full-scale I- and U-shaped girders indicate that partially debonding strands does not result in deleterious performance if adequate reinforcement is provided to resist the longitudinal tension due to bending and shear. Crack patterns and angles were not noticeably affected by the amount of debonding. Regardless of the debonding ratio, the maximum measured crack widths at the capacities predicted by the eighth edition of the American Association of State Highway and Transportation Officials’ AASHTO LRFD Bridge Design Specifications remained small. The requirements for debonded strands were revised significantly in the ninth edition of the AASTHO LRFD specifications based on the presented research.

High strength concrete prestressed bridges: Exposure conditions, mechanical properties, modified design equations, parametric study, and structural reliability

The majority of the current requirements for prestressed concrete were established based on the mechanical properties of normal strength concrete. However, with the more common use of High Strength Concretes (HSC) for bridges and structures, some of these requirements become conservative and require re-evaluation. The main objective of this research study was to modify the allowable stresses for HSC prestressed members and evaluate their impact on both structural design and reliability. An experimental program was developed and a statistical analysis was conducted to determine the effects of various field exposure conditions on the mechanical properties of HSC. Accordingly, property relationships and prediction formulas were proposed and compared to the current codes and standards. A parametric study was conducted to evaluate the economic impact of the modified stress limit formula on the design of typical HSC prestressed bridges, in terms of the maximum span lengths that can be achieved. Results revealed that the proposed modified design equation resulted in longer maximum spans in comparison with the current standards specifications, which would lead to cost reduction. Results also revealed that the majority of the prediction formulas used by codes and standards and suggested by various researchers overestimated the results obtained from this study and only the AASHTO LRFD Specifications and ACI 318 formulas provided conservative estimates. Furthermore, the safety of the proposed modified stress limit was evaluated by applying structural reliability theory and utilizing Monte-Carlo simulations..

Proposed lump-sum formulas for long-term prestress losses

The current American Association of State Highway and Transportation Officials’ AASHTO LRFD Bridge Design Specifications’ approximate formula for estimating long-term prestress losses is the outcome of the research work presented in National Cooperative Highway Research Program report 496. It is produced by simplifying the detailed method and taking into account the variability of concrete properties and the interaction between the precast concrete girder and cast-in-place deck. This paper presents two detailed parametric studies based on the average con­ditions for the design and construction of commonly used bridge girders. Three spans and, consequently, three levels of prestressing for each section have been considered. The first study establishes the creep multiplier Nc, whereas the second study evaluates the shrinkage multiplier Ns. Both multipliers are used in the lump-sum formulas for estimat­ing long-term prestress losses for different bridge girders. The multipliers produced by these studies are compared with that of the current AASHTO LRFD specifications’ approximate method, and new lump-sum formulas for long-term prestress losses are proposed.

Shear behavior of short headed studs in Steel-ECC composite structure

Engineering cementitious composite (ECC) is a promising candidate as the overlay material of orthotropic steel bridge deck (OSBD), to improve the durability and extend the service life. The mechanical performance of short headed stud connectors in steel-ECC composite structure has been limited studied. This study investigated the shear behavior of short headed studs embedded in ECC through experimental push-out tests and validated finite element (FE) analysis. The experimental and analytical results were analyzed accordingly in terms of shear strength, shear stiffness and failure mode. The push-out testing results showed that the existence of natural interfacial bond was beneficial for improving the initial stiffness of the specimen. The FE simulation results agreed well with the experimental ones. Base on the validated FE model, a parametric analysis was then conducted to investigate the influence of the stud diameter, stud height, stud strength, corrosion of stud, and compressive strength of ECC on the shear capacity of short headed studs. The analysis indicated that the diameter and tensile strength of headed stud had a significant improvement on the shear strength of headed studs, while the stud height and compressive strength of ECC exhibited the opposite trend. The corrosion of stud drastically reduced the shear performance of headed studs in terms of shear strength and ductility. It was also observed that AASHTO LRFD specification overestimated the shear strength of short headed studs embedded in ECC, while the Eurocode 4 and Chinese code were relatively conservative.

Analytical Solutions for Girder Distribution Factor in Steel-Concrete Composite Girders with the Effect of Parapets

The existing studies have shown that parapets have great influence on the girder distribution factor (GDF) of bridges. However, there is no method in the design guide to estimate the GDF considering the effect of parapets. This research aims to develop a simplified method for estimating the GDF by considering the effect of parapets. First, a simply supported steel-concrete composite girder bridge was tested to investigate the effect of parapets on the GDF. Then, finite-element (FE) model was established and verified by the field test data of strain and deflection. In addition, error study showed that the bending stiffness of the bridge was increased by about 92% and 19.1%, respectively, due to the effects of parapet and continuous layer. As the effect of the continuous layer on each girder was relatively uniform, the simplified method was optimized only considering the effect of the parapet. Finally, the effect of the parapet on the GDF was compared and discussed. Considering the effect of the parapet, the GDF of the exterior girder calculated by the simplified method and FE analysis decreased by about 26.92% and 23.53%, respectively, and the adjacent interior girder decreased by about 15.22% and 12.77%, respectively. Comparing the GDF calculated by the AASHTO LRFD specifications, the GDF calculated by the simplified method decreased by about 30.77% in the exterior girder and 41.30% in the interior girder, respectively. The results indicate that the method of calculating the GDF without considering the effect of the parapet in AASHTO LRFD specifications is conservative. The GDF calculated by the simplified method was basically close to the field test results, meaning that the proposed simplified method considering the effect of the parapet was relatively accurate.

Effect of temperature loading on the performance of a PC bridge in Oklahoma: Reliability analysis

Spatial and temporal temperature variations in bridge structures due to changes in the surrounding climate produce movements which, if restrained, may induce stresses in the structure. Temperature effects have always been considered to be one of the critical issues that affect the performance of prestressed concrete (PC) bridges. In order to (1) develop an efficient temperature loading model for predicting bridge temperature-induced response; and (2) develop guidelines to incorporate the effects of temperature actions in reliability-based condition evaluation, numerical and analytical studies were conducted on a PC girder bridge in Oklahoma, USA. This paper focuses on the structural reliability analysis on the temperature loading action in accordance with AASHTO LRFD specification. A three-dimensional (3D) finite element method (FEM) model was established to estimate the movement and stress under temperature loading, and compared with monitoring counterparts. Then, the validated model was utilized for probabilistic analyses to assess the temperature load effects. Based on the probabilistic temperature loading model proposed in the first paper, loads and resistances statistics from published literature, reliability analyses of AASHTO LRFD limit states were performed by Monte Carlo simulation (MCS) and Rackwitz-Fiessler procedure to determine the safety level in terms of reliability index.

Study on the additional support length requirements of single-span bridges due to skew using a simplified method

Skew bridges with seat-type abutments are frequently unseated in earthquakes due to large transverse displacements at their acute corners. It is believed these large displacements are due to in-plane rotation of the superstructure. Lack of detailed guidelines for modeling of skew bridges, many current design codes give empirical expressions rather than theoretical solutions for the additional support length required in skew bridges to prevent unseating. In this paper, a parametric study has been carried out to study the influence of skew angle, aspect ratio and fundamental periods of bridges on the additional support length requirements of single-span bridges due to skew using a shake table experiment validated Simplified Method, which is capable of simulating gap closure based on response spectrum analysis. This method is developed based on the premise that the obtuse corner of the superstructure engages the adjacent back wall during lateral loading and rotates about this corner until the loading reverses direction. A design response spectrum specified in AASHTO LRFD Specifications was employed to represent the design-level earthquakes. The results show the additional length required to prevent unseating due to skew increases with the skew angle in an approximately linear manner when the angle is less than a critical value and decreases for angles above this value. This critical skew angle increases with the aspect ratio approximately in a linear manner and shows negligible dependence on the fundamental periods of the bridges, and combination of span length and width. In addition, the critical skew angle varies between 58° and 66°, when the aspect ratio is varied from 3.0 to 5.0. The results also show that the empirical formulas for minimum support length requirements of skew bridges in current codes and specifications can not accurately reflect the influence of skew.

Effect of Curvature on the Dynamic Response of Composite Steel I-Girder Bridges

Elaborated three-dimensional finite element models are generated to study the effect of the radius of curvature on the free vibration and dynamic response of the composite steel I-girder bridges. The bridge is modelled by using ANSYS 15.0 program with solid and shell elements to represent concrete and steel members, respectively. AASHTO LRFD HL-93 truck is idealized as 3D model consisting of five lumped masses connected by rigid beams and supported by spring-dampers. The profiles of road surface roughness are generated by MATLAB developed program depend on the power spectral density (PSD). The models used are capable to take all bridge and vehicle characteristics into consideration. The dynamic responses of the horizontally curved bridge are investigated under conditions of various vehicular loading positions. The dynamic behaviour is presented in terms of Impact Factors (IM). The results show that the bridge natural frequencies are significantly affected by the radius of curvature. The relationship between the IM and the radius of curvature is governed by the position of vehicle on transverse sections of the bridge. AASHTO LRFD specification tends to underestimate the values of IM.

Optimal design of longitudinal stiffeners of unsymmetric plate girders subjected to pure bending

This paper aims at investigating the optimum positions and the required flexural rigidity (γrq) of a single and two longitudinal web stiffeners of unsymmetric plate girders subjected to pure bending by using gradient-based optimization algorithms. The optimization procedure is performed into two steps: The first step is to maximize the bend-buckling coefficient kb generated from eigenvalue buckling analyses, and then the second step aims to minimize the flexural rigidity of the stiffeners (γ). This procedure is implemented using the Abaqus2Matlab toolbox which allows for the transfer of data between Matlab and Abaqus and vice versa. Based on this research, the optimum locations of the stiffeners are recommended for the unsymmetric plate girders under pure bending. Additionally, equations are suggested to determine γrq of the longitudinal web stiffeners of the unsymmetric plate girders. These proposed equations are more realistic in the practical designs than those recommended by AASHTO LRFD specifications and several works in the literature in which γrq of the longitudinal web stiffeners is determined based on the simply supported plates but not the unsymmetric plate girders under bending.

Experimental study on skew inverted-T bent caps with minimum traditional and skew transverse reinforcing

In this paper, the shear behavior of six full-scale Inverted-T Bent Caps (ITBC) specimens with skew angles of 30°, 45°, and 60° are reported to study the effect of skew angle and rebar arrangement on structural performance. The transverse reinforcement in skewed ITBCs is designed as per the traditional methods the TxDOT Bridge Design Manual LRFD, which conform to the AASHTO LRFD Bridge Design Specifications. The traditional design of flaring the transverse rebar in skew ITBCs brings notable difficulty in manufacturing. An alternative way is to apply uniform skewed reinforcing, which is compared to traditional design in this research. The minimum ratio of transverse reinforcement required by AASHTO LRFD specifications was adopted in this research to investigate the most unfavorable design. The load–displacement curves, the internal force diagrams, the strength and displacement capacity, and the strain measuring results are reported in detail. The specimens with a skew angle of 30° or 45° fail in shear-critical mode, while the specimens with a skew angle of 60° fail in torsion-critical mode. The experimental results show that replacing the traditional reinforcement with the skew reinforcement does not reduce the capacity of ITBCs. The cracking behavior of ITBCs with skew rebar are better than the ITBCs with traditional rebar. Based on the extensive experimental study on ITBCs, general design guidelines for the design and construction of the skewed reinforcing in ITBCs are provided.

Saint-Venant torsion constant of modern precast concrete bridge girders

Many bridge owners have developed new precast, prestressed concrete bridge girder sections that are optimized for high-performance concrete and pretensioning strands with diameters greater than 0.5 in. (12.7 mm). Girder sections have been developed for increased span capacities, while others fill a need in shorter span ranges. Accurate geometric properties are essential for design. Common properties, including cross-sectional area, location of centroid, and major axis moment of inertia, are generally easy to compute and are readily available in standard design references. Computation of the torsion constant is a different matter. This paper presents the methods and results of a study to determine the torsion constant for many of the modern precast, prestressed concrete bridge girders used in the United States and compares the results with values from the approximate methods of the AASHTO LRFD specifications.

Parametric study of deteriorating precast concrete double-tee girder bridges using computational models

This paper aims to study the effects of different parameters of precast concrete double-tee (DT) girder bridges on the live-load distribution factors (LLDFs) using 3D computational models. The models were created for two DT girder bridges (including 584 mm deep and 762 mm deep bridges) tested with a rating truck using solid and shell elements in CSi Bridge. From the field tests, LLDFs were initially calculated. The models were calibrated with the field LLDFs and then different parameters, encompassing span length, concrete strength, deck width, diaphragm usage, and width to length ratio, were adjusted individually to explore the variation in LLDFs. Each of the parameters was investigated in this study for DT girder bridges. The AASHTO LRFD interior LLDFs were generally accurate for the DT girder bridges with significant joint damage. It was demonstrated that the deck width, concrete strength, and diaphragm location had very little influence on the LLDFs, whereas the span length and width to length ratio had significantly affected the LLDFs. Strictly speaking, the exterior and interior LLDFs decreased as the span length increased. The pattern for the interior LLDFs is only in agreement with the AASHTO LRFD specifications, while the pattern for the exterior LLDFs is not consistent with the AASHTO LRFD LLDFs calculated using the lever rule without consideration of span length. As a final point, as the width to length ratio increased, the LLDFs increased.

Revisited Live Load for Simple-Span Bridges

Abstract Twenty-five years ago, a bridge live-load model was developed along with the calibration of the AASHTO LRFD Specifications. The statistical parameters of truck loads were based on the Onta...

Fatigue load testing of full-scale dynamic message signs

This paper summarizes results acquired from the fatigue testings of two full-scale dynamic message signs (DMSs) fabricated by a local manufacturer in the United States. One DMS with adhesive joints and one DMS with welded joints were tested with fatigue loading. For the testing, each of the DMSs was loaded up to 500,000 cycles with a force ranging between 0.249 kN and 0.818 kN equivalent to the design natural wind gust pressure based on a yearly mean speed of 18.02 km/h according to the AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals. Strain, displacement, and load were recorded during each test to analyze the structural adequacy of tested DMSs. During the testing, all the stress ranges observed in each of the tested DMSs were considerably below the threshold of the DMS panel, which was determined by generating its S-N curves. The maximum deflection range was found to be highest near the center for both adhesive and welded DMSs when encountered with the fatigue loads. It is recommended that additional stiffeners be installed to reduce the deflection range of the DMS at the center. It was also found that any damage was not observed in both the adhesive and welded DMSs due to the fatigue loading.

PARAPET STIFFNESS EFFECT ON LOAD CARRYING CAPACITY OF MULTI-LANE CONCRETE SLAB BRIDGES

Bridge specifications do not consider the effect of parapet stiffness in the analysis and design of reinforced concrete slab bridges. This paper performs a parametric investigation using finite element analysis (FEA) to study the effects of parapet stiffness on live load-carrying capacity of two-span, three-and four-lane concrete slab bridges. This study analyzed 96 highway bridge cases with varied parameters such as span-length, bridge width, and parapet stiffness within practical ranges. Reinforced concrete parapets or railings, built integrally with the bridge deck, were placed on one and/or both sides of bridge deck. The longitudinal bending moments calculated using the FEA results were compared with reference bridge cases without parapets, as well as AASHTO Standard and LRFD specifications. The FEA results presented in this paper showed that the presence of concrete parapets reduces the negative bending moments by 15% to 60% and the positive bending moments by 10% to 45%. The reduction in longitudinal bending moments can mean an increase in the load-carrying capacity of such bridges depending on the parapet stiffness. This investigation can assist engineers in modeling the actual bridge geometry more accurately for estimating the load-carrying capacity of existing concrete bridges. Hence, new bridges can be designed by considering the presence of concrete parapets. Parapets can be used as an alternative for strengthening existing one and two-span reinforced concrete slab bridges.

Lateral Load Distribution for Hollow Slab Bridge: Field Test Investigation

Distribution factors (DFs) for one typical cross-section as specified in the AASHTO LRFD specification can be varied when the bridge parameters such as span length, loading lanes and skew are changed. The diversity between design and actual DFs may be varied as the bridge parameters changed. To study this diversity, this paper presents an evaluation of lateral load DFs for prefabricated hollow slab bridges. The response of the bridge was recorded during the field test. This field test was divided into two stages: a concentrated force loading test on the prefabricated girder that settled on the bridge supports before the girders were connected transversely and a vehicle loading test after the girders were connected transversely. The instruments used to record the response of the bridge were strain gauges and dial indicators. The measured data in the multi-stages of the field test could be used to calibrate the support condition of the bridge and transverse connection between adjacent girders in the finite element model (FEM) using beam and plate elements. From the FEM, DFs for this hollow slab bridge were determined and compared with the DFs in the AASHTO LRFD specification. A parametric study using the calibrated FEM was then used to investigate the effect of various parameters including span length, skew and bridge deck thickness on the DFs. It was found that AASHTO LRFD specification is conservative compared with the analysis in the FEM, while this conservatism decreased as the span length and skew of the hollow slab bridge increased.