AASHTO LRFD Bridge Design Specifications Research Articles

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.

The Risk of Failure Assessment in Bina Marga Standard Designed Prestressed Concrete Girder Bridges under B-WIM Load Measurement

The use of precast prestressed concrete girder bridges in Indonesia has been increasing rapidly due to their high quality, reliability, and faster construction on site. The girder components are typically designed for a specific bridge span and can be prefabricated. The Directorate General of Highways of the Ministry of PUPR (Bina Marga) has released a standard design for prestressed concrete girder bridges with a typical span of up to 40 m. This design is based on the bridge loading standard SNI 1725 2016, which determines the live traffic load through consensus due to limited data on actual traffic load measurement results. However, the Ministry of PUPR has been implementing actual traffic load measurements using weigh-in-motion (WIM) technology to directly measure the load of passing vehicles. In this study, a risk assessment of the failure risk of a standard Bina Marga bridge with a 40-m span prestressed concrete girder type was conducted based on B-WIM load measurements. The results of this assessment indicate that the standard Bina Marga bridge has a failure risk of 1.48 x 10-4, which is smaller than the acceptable risk of failure according to the AASHTO LRFD Bridge Design Specification as referenced in SNI 1725 2016.

Evaluation of minimum flexural reinforcement for precast prestressed concrete NEXT beam bridges

The minimum flexural reinforcement requirement has been used in the current bridge design specifications to protect the member from brittle failure after the formation of the first flexural cracks. Several variables have been reported to affect this requirement, such as concrete strength, amount of prestressing in the member, and type of cross section. Recently, the Precast/Prestressed Concrete Institute (PCI) developed a new type of beam section (NEXT beam) to accelerate bridge construction and enhance the sustainability of bridges. As a newly developed beam section, no research on the minimum flexural reinforcement has been reported for NEXT beam bridges. This paper aimed to examine the minimum flexural reinforcement requirements in the current AASHTO LRFD Bridge Design Specifications for NEXT beam bridges. A comprehensive parametric study was analytically conducted with various parameters, including bridge section, beam section, concrete strength, and span length. The results from this study showed that the current minimum flexural reinforcement requirements were met for all bridges examined herein; the concrete strength, beam cross section, and span length could affect the levels of safety against brittle failure after first flexural cracks for NEXT beam bridges.

Predicting fatigue life of shear connectors in steel‐concrete composite bridges using artificial intelligence techniques

AbstractFatigue limit states often govern the design of shear connectors in steel‐concrete composite bridges. AASHTO LRFD bridge design specifications provides a linear equation in a semi‐logarithmic S‐N curve for predicting the fatigue life of shear connectors. However, this equation can be too conservative in some cases, as supported by the available experimental data. In this paper, artificial intelligence (AI) was incorporated into the prediction of the fatigue life of shear connectors. Six different machine learning (ML) algorithms were considered for this purpose. The predictions of ML algorithms were compared both with the available experimental data and the equation provided by AASHTO. The results showed that the fatigue life predicted by ML methods is more accurate than that predicted by the current equation of AASHTO. The results of this study showed that AI can be a proper alternative to the existing methods for predicting the fatigue life of shear connectors.

Cost Optimization of Prestressed U-Shaped Simply Supported Girder Using Box Complex Method

The use of U-shaped girders has become increasingly popular in advanced projects such as metro rail systems due to their ability to provide greater vertical clearance beneath bridges. These girders, characterized by two webs and a bottom flange, contribute essential longitudinal stiffness and strength to the overall structure while effectively countering torsional forces in curved bridges. However, the design and construction of U-shaped girders present challenges, including their relatively higher self-weight compared to other girder types. Consequently, cost optimization has become a crucial focus in structural design studies. This research aims to develop an optimization model for prestressed U-shaped girders using the AASHTO LRFD bridge design specifications. The model is based on the Box complex method, with necessary modifications and improvements to achieve an optimal design. The objective is to minimize the total cost of materials, including concrete, steel reinforcement, and prestressing strands, while satisfying explicit and implicit design constraints. To facilitate the analysis, design, and optimization processes, a program is developed using Visual Studio 2010 and implemented in Visual Basic (VB.NET). The program incorporates separate subroutines for analysis, design, and optimization of the prestressed U-shaped girder, which are integrated to produce the desired results. When running the program, the optimization process required 229 iterations to converge to the optimal cost function value. The results demonstrate that the developed algorithm efficiently explores economically and structurally effective solutions, resulting in cost savings compared to the initial design. The convergence rate of the moment capacity constraint is identified as a key factor in achieving the optimal design. This research makes a significant contribution to the field of civil engineering by applying the classical Box complex method to the optimization of girders, an area where its utilization has been limited. Furthermore, this study specifically addresses the optimization of prestressed U-shaped girders in metro rail projects, where they serve as both the deck and support structure for train loading. By employing the Box complex method, this research aims to fill the research gap and provide valuable insights into the optimization of U-shaped girders. This approach offers a fresh perspective on designing these girders, considering their unique role in supporting metro rail loads. By leveraging the benefits of the Box complex method, researchers can explore new possibilities and uncover optimal design solutions for U-shaped girders in metro rail applications.

Shear Transfer Resistance with Different Interface Conditions: Evaluation of Design Provisions and Proposed Equation

The design provisions in current codes for shear resistance of concrete-to-concrete interfaces exhibit significant differences. In this study, the accuracy of design provisions for interface shear resistance was evaluated and compared. From the literature search, a database of 458 push-off test results of interface shear resistance was created to evaluate the shear transfer provisions from the ACI 318-19, PCI Design Handbook, AASHTO LRFD Bridge Design Specifications, CSA-S6, Eurocode 2, and Fib Model Code 2010. In addition, an equation was derived based on push-off test results collected from the literature to calculate the interface shear resistance for the monolithic uncracked interface. According to many analyses and evaluations of parameters affecting the interface shear resistance, the compressive strength of concrete played an important role, especially for the monolithic uncracked interface. Therefore, the compressive strength of concrete was included in the proposed equation to calculate the interface shear resistance in this study. It is expected that this equation can be applied more accurately than the existing design provisions when high-strength concrete is used. Statistical analyses were carried out for comparison with the existing design provisions to verify the applicability of the proposed equation. The results show that the proposed equation reasonably predicted the interface shear resistance for the monolithic uncracked interface. Appropriate conclusions were also drawn for the design provisions.

A Deep Learning-Informed Design Scheme for Shear Friction at Concrete-to-Concrete Interface: Recommendations for Inclusion in AASHTO LRFD Guidelines

Recent advancements in construction technology have led to high-strength concrete and steel. However, these developments have depreciated the accuracy of the design equations in current provisions, which were based on normal-grade materials. To fill such a research gap, this study presents a novel deep learning-based computation scheme that can replace the current design provisions by virtue of its superior accuracy and reliability. The proposed approach exploits Neural Additive Models (NAMs) in which geometric and material properties associated with a normalweight concrete-to-concrete shear interface are inputted to individual neural network blocks. The outputs of the individual blocks are linearly combined to produce the prediction for interfacial shear strength. This model provides a way to identify and quantify the individual contributions of the input parameters, thus enhancing the interpretability of the model predictions for shear strength at the normalweight concrete-to-concrete interface. The deep learning-informed design (LID) scheme improves the prediction accuracy of the shear strength equation in the existing AASHTO LRFD Bridge Design Specifications by over 32%.

Performance Case Study of Concrete Bridge-Rail Post on Deck Overhang

Concrete post-and-beam bridge rails are a common bridge-rail type in the U.S. However, direct investigations of their performance on bridge-deck overhangs are rare, and current specifications do not clearly address this rail type or the design of decks supporting them. In this paper, the results of a performance case study of concrete bridge-rail posts on deck overhangs are presented. This study included (1) bogie testing of a MASH TL-4 concrete post on an instrumented deck, (2) development and calibration of a corresponding LS-DYNA model, and (3) use of the calibrated model to further describe system behavior, evaluate the effect of common design alternatives on performance, and corroborate the system performance from a full-scale crash test. The results of this study indicated that current design methods of the AASHTO LRFD Bridge Design Specifications (BDS) resulted in a significant underestimation of lateral capacity, partially because their neglect of inertial resistance. In the bogie test of the concrete post, over 40% of the peak lateral resistance of the post was attributed to inertial effects. Similarly, the calibrated LS-DYNA model indicated that the full-scale system could withstand a simplified single-unit truck (SUT) loading consisting of a 150 kip pulse load while only sustaining minor damage, despite having a BDS-predicted capacity of 73 kips. Additional findings included a characterization of deck demands, performance effects of design alternatives, such as edge distance and slab thickness, and the influence of using straight versus hooked transverse deck bars.

Capacity of piles subject to downdrag: a comparison of North American bridge design codes and observations from a full-scale test pile program

Negative skin friction caused by ground settlement is an important consideration for deep foundations in limit states design. However, there are inconsistencies in the methodology whereby negative skin friction and associated drag force are considered in assessing the geotechnical capacity or geotechnical ultimate limit state (ULS) in various design codes. This includes two current North American bridge design codes, the Canadian Highway Bridge Design Code, and AASHTO LRFD Bridge Design Specifications. A test pile program was developed to observe effects of ground settlement on pile settlement, capacity, and drag force. Two instrumented steel H-piles were driven through a compressible clay layer to a hard end-bearing stratum, subjected to ground settlement by constructing a 1.5 m high embankment, followed by static load testing. A load-transfer model was calibrated from the test pile program observations. Test results and the calibrated model were used to compare geotechnical ULS requirements of the two bridge design codes. It is demonstrated that drag force did not detrimentally impact pile capacity. The results showed that for conditions of the test pile program, assessing the geotechnical ULS can be more conservative when adhering to the current AASHTO LRFD Bridge Design Specifications than the Canadian Highway Bridge Design Code.

Shear transfer strength of alkali-activated slag-based concrete

As a part of ongoing research to apply alkali-activated slag-based concrete in building structures, the shear transfer behavior of alkali-activated slag-based concrete was experimentally examined through the push-off tests of 32 initially uncracked reinforced planes. The test parameters were concrete type, concrete strength, shear reinforcement ratio, and shear plane size. The test results and comparative analysis showed that the shear strength of reinforced alkali-activated slag-based concrete (AAC) was comparable to Portland cement concrete (PCC) counterparts. However, AAC planes cracked earlier and displayed larger crack separation at the peak load, which is attributed to higher cracking prevalence and brittleness of AAC. Both the increase of concrete strength and lateral constraint stress promotes the development of shear strength in reinforced planes, and their effect is interactional. The shear strength decreased as the shear plane width and depth increased. The existing strength models from design codes and literatures for the shear transfer strength of PCC are still applicable to reinforced AAC planes. Among them, AASHTO LRFD Bridge Design Specifications and the nonlinear model proposed by Mau and Hsu gave better predictions of the measured shear strengths of AAC.

Noncomposite concrete deck bracing effects on continuous steel stringer lateral torsional buckling resistance

Louisiana’s bridges used two-girder or truss systems in the 1950 s and 1960 s, in which floorbeams are carried by main members and continuous stringers are supported by the floorbeams. The main members are either two edge (fascia) girders or trusses. Continuous span stringer bottom flanges are in compression in their negative moment regions, which could result in lateral torsional buckling (LTB). When these stringers are load rated using AASHTOWare Bridge RatingTM analysis software, lateral torsional buckling (LTB) resistance is calculated in accordance with the AASHTO LRFD Bridge Design Specifications, which the authors have determined may not adequately account for bracing effects provided by a noncomposite deck, and therefore could underestimate the flexural strength. Resulting ratings may be low enough to require restrictive, and possibly unnecessary, load postings or even bridge closures. This issue affects more than 20 significant bridges that are key parts of Louisiana’s highway system. To address the challenge of underestimated stringer LTB resistance and assess bracing effects provided by a concrete deck, experimental and analytical studies were performed on a two-span, continuous, steel stringers in a grillage system, which included three stringer lines, an interior transverse support (floorbeam), and transverse diaphragms at the end supports. The tests encompassed a variety of configurations, with and without a concrete deck. This paper describes laboratory testing setups and discusses experimental results, focusing on bracing effects on LTB resistance due to the presence of a noncomposite concrete deck. Finite element models using ANSYS were calibrated, validated using test results and used to simulate stringer behavior in Louisiana’s representative bridges. Finite element analyses of various tests showed generally comparable results to test data, and provided a feasible approach to load rate continuous stringers if a refined analysis is deemed necessary. The studies showed that stringer LTB resistance could possibly be tripled when noncomposite deck bracing effects are considered.

Interação entre torção, flexão e cortante em vigas de concreto armado segundo NBR 6118/2014 e AASHTO LRFD 2014

abstract: Reinforced concrete beams are subjected to bending, torsion and shear simultaneously. The interaction of combined loading is complex and demands a unified model for analysis and design. Using the models from NBR 6118/2014 (variable angle truss model with parallel chords) and AASHTO LRFD Bridge Design Specifications 2014 (Modified Compression Field Theory), an algorithm was created, converting the resistance problem into a constrained optimization problem and generating an interaction surface that also displays the active constraints for each degree of interaction. Applying it to three beams previously tested, the experimental data was plotted against the surface obtained by the standards. The procedure and its optimization approach were efficient and effective in predicting the beams’ resistance. Comparison between the interaction surfaces, empirical data and existing literature showed the procedure and the standards were consistent and its application simple and practical.

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.

Implication of bridge resilience design and lessons from negative examples

AbstractBridge resilience is a newly proposed bridge design criterion that involves robustness, redundancy and reparability targeting on the rapidity of functionality restoration after suffering extreme actions and long-term durability deterioration. It stipulates a lower probability of reaching the ultimate limit state or strength limit state, which have been only partly involved in bridge design codes around the world. In AASHTO LRFD Bridge Design Specifications and Eurocodes, there are some design principles related to bridge resilience. Yet, it is also necessary to give more requirements for structural ductility and collapse resistance when the actual load exceeds the load combination in the code. This paper focuses on the resilience-based principles for bridge design, and exposes some problematic bridge structural systems and details, such as bridges most likely to overturning, steel bridges with fracture critical members, arch bridges with suspended desk, Morandi cable-stayed bridges, poor details for seismic vulnerability, etc. Whereas overturning is one of the worst anti-resilience scenarios, the resilience design against bridge overturning is highlighted through a detailed discussion including the calculation methods of anti-overturning factor, overturning stability of curved bridges, reasonable disposition of supports, and anti-overturning countermeasures.

Simplified method for estimating restraint moment induced by vertical temperature gradient in continuous prestressed concrete bridges and verification using AASHTO BDS

Diurnal and seasonal temperature variations cause uniform temperature changes, vertical temperature gradients, and horizontal temperature gradients. Vertical temperature gradients occur when the bridge deck absorbs heat due to solar radiation more than the bottom parts of bridge girders. Consequently, the vertical temperature gradient causes redistribution of bearing reactions, and induces additional flexural stresses in continuous spans. Estimating the vertical temperature gradient effect on continuous concrete bridges requires knowledge of thermal analysis of statically indeterminate structures or advanced three-dimensional (3 D) finite element (FE) analysis. Therefore, this article focuses on developing a simplified analysis method for estimating vertical temperature variations effects on continuous prestressed concrete bridge girders, which is required for bridge design. Restraint moments induced by vertical temperature gradients are investigated using 3 D FE analysis. A total of 115 bridges with different configurations (number of spans, girder spacing, and girder type) were analyzed to estimate the positive thermal moments at support locations. Results from these analyses were then used to develop simplified equations for estimating the positive restraint moments due to vertical temperature gradients from AASHTO LRFD Bridge Design Specifications (BDS). The developed equations are validated using John James Audubon Bridge #2 field data, and for other AASHTO LRFD temperature zones.

Obliquely Loaded Welded Attachments Fatigue Categorization in Steel Bridges

The detail category for base metal at the toe of transverse stiffener-to-flange and transverse stiffener-to-web fillet welds is defined as Category C' in the current AASHTO LRFD Bridge Design Specifications (2020, 9th Ed.) and as Category C in the AREMA (2020) Manual for Railway Engineering and the AISC Steel Construction Manual (2017, 15th Ed.). These are often referred to as short attachments due to their very short length (< 2 in.) in the direction of the primary stress range. Sometimes it is necessary to place a stiffener or a connection plate at an angle different than perpendicular to the web, such as in skewed bridges. Increases in the effective length of the stiffener along the flange in the longitudinal direction are seen as the plate is rotated away from being perpendicular to the web. The other extreme occurs when the stiffener is rotated completely 90° and is perfectly parallel to the web and the longitudinal stress range. In this instance, this is identical to the long attachment and classified as Category E (length > 4 in.). The current specifications and manuals, on the other hand, do not have classification on how to address the potential effects on fatigue performance of angles in between these two extremes. This paper summarizes finite element analysis studies based on local stress and structural hot-spot stress approaches that were conducted to investigate and classify welded attachments placed at angles other than 0° (transverse) or 90° (longitudinal) for a variety of stiffener geometries and thicknesses. This study includes new classification for incorporating the findings into the AASHTO LRFD Bridge Design Specifications, AREMA Manual for Railway Engineering, and the AISC Steel Construction Manual.

Experimental investigation of headed studs in steel-ultra-high performance concrete (UHPC) composite sections

Featuring high compressive and tensile strengths, ultra-high performance concrete (UHPC) is an efficient and durable alternative for conventional concrete in steel–concrete composite sections. However, few experimental studies to date have involved large diameter (≥ 25 mm) headed studs, UHPC with compressive strengths larger than 130 MPa, and relatively low strength but cost-effective UHPC. This study investigates the shear performance of steel headed studs embedded in UHPC through pushout tests with 24 specimens. Specimens included headed studs with diameters of 16 mm, 19 mm, 22 mm, and 25 mm, and either UHPC with compressive strength of 152.5 MPa or a cost-effective UHPC with a strength of 85.7 MPa (named as HPC hereafter). The results showed that smaller-diameter specimens (16 or 19 mm) failed with stud shank fracture, while the larger-diameter specimens (22 or 25 mm) failed at welding lines. Compared to HPC, UHPC did not notably increase the performance of headed shear strength. The load response consisted of a linear stage, a softening stage, and a post-peak stage lasting up to a slip of 10 mm or greater, with high variability in post-peak response among specimens with identical parameters. A global database of 120 push-out test specimens from this study and references was established, covering concrete strengths from 23.5 MPa to 152 MPa. The formulae given by AASHTO LRFD Bridge Design Specification, Eurocode-4, and Chinese code GB50017-2003 overestimated the shear capacity of headed studs by 52 %, 5 % and 53 %, respectively. The equation for the slip modulus slightly overestimated the slip modulus of headed studs. In conclusion, this study broadened the ranges of concrete strength and diameter of headed shear studs, and revealed that the existing equations of predicting shear capacity and slip modulus need improvement.

A New Proposal for Live Load Distribution Factors of Bridges with Transverse Beams

Many bridge superstructures use transverse beams as load carrying components. In these systems, usually the transverse beams are connected to the main longitudinal girders or trusses on the two sides of the bridge. Such systems are commonly used in plate girder, box girder, cable-stayed and truss bridges.  The live load distribution factor (LLDF) for bridge superstructures with transverse beams in AASHTO-LRFD bridge design specification has remained unchanged for decades and is prescribed as a function of the distance between the transverse beams. However, for slab-beam superstructures in which longitudinal beams at close spacing carry the loads to the substructure, the LLDFs have gone through many changes throughout the years and in their current forms depend on many parameters such as concrete slab thickness, beam span, longitudinal beam stiffness as well as the distance between the longitudinal beams. This study investigates the factors affecting the LLDF for transverse beams and intends to obtain new equations similar to AASHTO’s longitudinal beam equations. For this purpose, 3D finite element models of different sample bridges were developed and critical parameters affecting the LLDF were identified and varied. Accordingly, the LLDFs for moment and shear forces of transverse beams were obtained through regression analyses. The proposed equations have less than 3.1% of average error for the cases considered.

Determination of Finite Element Modelling Errors for Box Culverts Using Field Load Tests

Buried culverts have been in service more than half a century and their age raises questions about their reliable serviceability for the new standard vehicles. The gross weight and axle spacing of these trucks play an important role on the performance of the buried culverts where the culvert is covered with shallow fill heights. Field live load tests were conducted by driving a loaded truck over some culverts at slow speed while recording data at the critical locations of these culvert. Finite element (FE) models were developed and calibrated using field load test results. Live load distribution formulations were adopted from AASHTO LRFD Bridge Design Specifications for different fill heights. Two- and three-dimensional FE models were modelled and loaded separately to compare the critical section forces. The results showed that the error using two different FE modelling approach yields more than 100% error for the maximum section forces when fill heights is between 60 cm and 120 cm. This study is aimed to highlighting the inherent error in numerical modelling of buried reinforced box culverts for design or rating purposes.

Unified approach for reinforcement development length using the partly cracked elastic stage for bond strength

Current structural design codes in the United States treat the development of straight bars/lap splices, standard hooks, and deformed headed bars with separate equations. The fib Model Code 2010, on the other hand, uses a more unified approach. Recent changes to the standard hook and headed bar equations in the ACI 318–19 Building Code Requirements for Structural Concrete have sparked conversation about the discrepancies between ACI 318–19 and the AASHTO LRFD Bridge Design Specifications within the bridge design community. The objective of this research was to develop and validate a unified approach for the design/evaluation of reinforcement anchorage. A procedure based on the partly cracked elastic stage of bond was proposed. The proposed procedure was validated with 1006 tests from the literature.