Current Design Procedures Research Articles

Comparison of Estimates of Travel Time Losses on High-Capacity Roads

This paper compares two approaches to estimating travel time losses on freeways that are based on a whole-year analysis. Both approaches are intended to improve the design process for freeways by accounting for the annual traffic demand profile as well as short time variance of demand and capacity. One approach estimates breakdown probability and delay-related time losses by using a probabilistic model. The other approach uses Monte Carlo simulation to estimate travel time losses at freeway bottlenecks on the basis of a queuing model with stochastic demand and capacity. Both approaches are based on annual hourly demand profiles. The approaches are compared for demand profiles and deliver similar but not equal results. Travel time losses for both approaches increase exponentially with traffic volume, with the simulation-based approach consistently generating higher time losses for higher traffic volumes. Both approaches are a substantial improvement on current design procedures, are easy to apply, and are recommended for adoption in traffic analysis guidelines.

Lateral Torsional Buckling of Thin-Walled Cold-Formed Steel Beams with Web Holes - Finite Element Analysis

This paper deals with the problems of analysis and design of thin-walled steel beams with circular web holes. The analysis based on the finite element method modeling is being performed on the case of perforated Sigma beams used in the storage systems and expedient flooring structures. The Sigma beams are generally used as floor girders which are subjected to the bending. In some cases the beams are not laterally restraint and the effect of lateral torsional buckling may occur. In the current engineering practice several design procedures are used to determine the bending resistance with respect to lateral torsional buckling. In this paper the results of FEM analysis are compared with the bending resistances calculated according the Eurocodedesign procedure.

Assessment of current design procedures for conical tanks under seismic loading

This study is motivated by the fact that no design provisions currently exist specifically for conical tanks under seismic excitations. The design of liquid storage structures is governed by the American Water Works Association (AWWA), American Petroleum Institute (API), or Eurocode 8. An approximate method based on replacing the conical tank with an equivalent cylinder is given in these design codes. The state of stresses in conical tanks is different than that of cylindrical tanks. A previously established equivalent mechanical model is used to determine the response of a number of conical tanks under horizontal excitations. The models incorporate the different components of the hydrodynamic pressure and account for flexibility of the tank walls. Using an assortment of seismic hazard areas, the maximum base shear force and overturning moment are evaluated for a number of conical tanks. Those are compared to the corresponding values predicted by the design codes using the equivalent cylinder approach. The results reveal that this approximate approach is not adequate for designing conical tanks to resist seismic excitations.

CHARACTERIZATION OF EFFECTIVE TIRE CONTACT AREA FOR VARIOUS TREAD PATTERNS

The effective tire-pavement contact area affects the relative damage of asphalt pavement and should be incorporated in both mechanistic and empirical response analyses of pavements. A new machine called ROTOCOM Wheel Tracker (RCWT) was designed and fabricated to capture the effective tire contact area apart from slab compacting, and conducting simulative laboratory wheel tracking tests. The main focus of this paper is laboratory measurement of effective tire contact areas for various tread patterns. Seven tire treads were selected for the footprint image analyses at five tire loads and four tire inflation pressures. An image processing MATLAB-based program was coded to calculate the contact areas of the 280 imprints obtained from both sides of the RCWT. Factorial analysis indicated significant effects of tire tread, tire load, and inflation pressure on the resulting contact area. Comparison between effective and traditional contact areas indicated that the current pavement design procedure with traditional circular contact area extremely overestimates the actual tire-pavement contact area up to 92%.

Performance-based seismic design of integral abutment bridges

Integral abutment bridges (IAB) are experiencing increasing diffusion in the short to mid-range lengths, where they offer some advantages over traditional girder bridges with non-monolithic connection at the abutments. One challenging problem with their analysis and design is that consideration of the interaction between foundation soil, structure and backfill is unavoidable, also for the deck design. Further, the end of the construction is only one of the conditions that need to be verified during design. Cyclic deformations, such as those occurring during ground shaking, typically lead to an increase in stresses in the abutments and connections, due to progressive compaction (ratcheting) of the backfill soil. This problem is magnified when the bridge is comprised between two embankments, whose response may amplify the input motion and drive the deformation of the bridge. Performance-based design aims at superseding current design procedures by explicitly checking that the target performances set out are achieved, and not overly exceeded. Such a design paradigm naturally calls, on the one hand, for improved accuracy in response determination and more refined analyses, and, on the other, for taking into account the uncertainties entering into the problem by means of an explicitly probabilistic approach. With this objective in mind, the paper presents an inelastic dynamic model for the seismic analysis and design of IABs. The model, that features a balanced compromise between the setup and evaluation effort on one hand, and accuracy on the other, has been developed for implementation in typical commercial analysis packages. It builds on 1D site-response analysis and on inelastic Winkler-like modeling, to reproduce the main physical aspects of the seismic response of IABs. One example application to a highway overpass in Italy illustrates the model and the relevance of a fully probabilistic approach to performance-based design. The application offers also important insight into the choice of an efficient intensity measure for this type of structure.

Behavior and design modification of RBS moment connections with composite beams

Reduced beam section (RBS) connections are innovative connections for steel beam composite frames. The current design procedure for RBS connections is based on bare steel beams. For RBS connections with composite beams, the potential of fracture at the beam bottom flange increases because the strains at the corresponding location increase. This paper focuses on the behavior of RBS connections with composite beams. A mathematical model of the RBS connections with composite beams is derived first. Then an amplification factor is proposed for the design of the RBS connections with composite beams. Parametric analyses based on the mathematical model are conducted to examine the influence of the slab dimension, reinforcement and the beam dimension on the amplification factor. The simplified design formula is proposed for the amplification factor based on a large number of theoretical data from the parametric analysis. Finally, the mathematical models are verified by the finite element (FE) models. It is found that the proposed design formula can give satisfactory results for considering the slab effects in the design of the RBS connections with composite beams.

Discrete Element Modeling of the Resilient Behavior of Subgrade Sandy Soils

Subgrade soils are very important materials to support highways. Current AASHTO pavement design procedures recommend the resilient modulus (Mr) of subgrade soils or pavement design and analysis. Laboratory repeated triaxial loading tests and field falling weight deflectometer (FWD) are two common ways to characterize the resilient modulus (Mr) of subgrade soils. The primary objective of this study is to conduct numerical simulations of the resilient behavior of subgrade sandy soils under repeated triaxial loading conditions using a commercial discrete elements code, e.g., particle flow code in two dimensions (PFC2D). Stress-controlled loading scheme is designed using a commercial DEM code, Particle Flow Code (PFC2D). One run with a confining stress of 41.4kPa and deviator stress of 12.4kPa is conducted for simulating the biaxial test of sandy soils under cyclic loading. Simulation results show that confining stress can be effectively applied to the testing sample. However, the error of axial stress is quite large. In addition, preliminary results of resilient behavior of sandy soils under cyclic loading are consistent with the classic results in real laboratories.

Tests and design approach for plain channels in local and coupled local-flexural buckling based on eurocode 3

Thin-walled plain channels are often subjected to coupled local and global flexural buckling. The objective of this paper is to verify and discuss the current design procedure of Eurocode 3 for plain channels in coupled instabilities. This paper deals with experimental investigations on plain channel stub columns subjected to local buckling only and long columns subjected to coupled local-flexural buckling. The test series consists of cold-formed and welded plain channels made of steel grade S355 and S460. Based on this experimental results modifications of the method of effective width (and thickness) as stated by Eurocode 3 Part 1-3 are discussed. Previously developed modified buckling coefficients, which take into account the rigidity of the connection of web and flange, are verified by comparative calculations. Additionally an approach for an approximation of effective stiffness for coupled instabilities is discussed as well.

Experimental Behavior of Steel Fixed Bearings and Implications for Seismic Bridge Response

Steel fixed bearings are commonplace structural elements for transmitting loads from superstructures to substructures, and they have typically occupied a role of elastic force transfer elements within the overall scheme of an earthquake resisting system (ERS). Recent revisions to design and guide specifications have acknowledged the possibility of bearings acting as fuses, but there is little research available to characterize bearing behavior for such design roles or the associated bridge response to be expected when bearings have fused. One design approach, adopted by the Illinois DOT (IDOT), applies capacity design principles and permits the bearings and superstructure to slide on the substructure. The intent of this design approach is to capture some of the beneficial aspects of conventional isolated systems, such as period elongation, reduction of force demands, and protection of substructures from large inelastic displacement demands, without incurring the additional design provisions and fabrication costs to satisfy the requirements for seismic isolation systems. To achieve this quasi-isolated bridge response, steel fixed bearings are used as fusing elements, where the steel pintles or anchor rods rupture, and the fixed bearing plates become free to slide on the supporting pier cap. A properly proportioned bearing will fuse prior to superstructure/substructure elements experiencing inelastic demands. The University of Illinois has been collaborating with IDOT to investigate the behavior of quasi-isolated bridge systems and to calibrate and refine IDOT’s ERS design and construction methodology. The research is composed of experimental testing to characterize fundamental bearing behavior, coupled with nonlinear global bridge modeling to evaluate limit state progression and estimate maximum displacement demands of the superstructure relative to the substructure. The cyclic response of full-scale steel low-profile fixed bearings demonstrates predictable sliding behavior, but based on current design procedures, these bearings are often overdesigned for use as fuses in quasi-isolated bridges. Consequently, a bridge, which in other respects may exhibit satisfactory quasi-isolated response, might also incur significant damage to the substructure unit where fixed bearings are provided. A parametric study of global bridge response demonstrates that the anchorage of fixed bearings to substructures could be reduced to limit the damage to the supporting substructure unit while incurring only a nominal increase in superstructure displacement demands.

Comparative study on the properties of WMA mixture using foamed admixture and free water system

Warm mix asphalt (WMA) technologies that use a foaming effect are one of the most commonly used WMA technologies due to their cost-effectiveness. The concept behind the foamed WMA is that the water turns to steam and is dispersed throughout the asphalt, which provides corresponding temporary reductions in viscosity while requiring no extra additives. The purpose of this paper is to evaluate both WMA techniques using foamed admixture and free water systems. For foamed admixture, ADVERA® WMA was selected; whereas for free water systems, WMA was foamed by injecting a small amount of water that was produced under a laboratory setup. Subsequently, for the asphalt mixture preparation, the mixture design used was based on specifications for a local asphalt mixture used in Michigan, USA. The nominal maximum aggregate size is 9.5mm and the designed traffic level is less than 3 million ESALs based on the current Superpave™ asphalt mixture design procedure. The control mixture and WMA mixture were evaluated through the asphalt pavement analyzer rutting test, flow number test, indirect tensile strength ratio, and the four point beam fatigue test. From the results, it was found the fatigue life of most of the samples prepared with ADVERA® WMA were higher than the control hot mix asphalt (HMA). However, foamed WMA has a significantly lower aging factor compared to control HMA which results in higher rutting potential.

High-Efficiency Multiphase DC–DC Converter for Fuel-Cell-Powered Truck Auxiliary Power Unit

This paper presents the design and control of a high-efficiency multiphase dc-dc converter for a truck auxiliary power unit (APU) fed by a fuel-cell stack. The power-stage design methodology and the operation of the designed power stage used to achieve high efficiency are described in detail. To operate the designed converter as a truck APU, the mode transfer control scheme, where smooth transitions of the voltage and current control modes are necessary, is proposed, and the current controller design procedure is described. A 5-kW four-phase dc-dc converter prototype is built with the proposed power-stage configuration, and the proposed control strategy is applied to control the dc-dc converter. The efficiency of the converter is more than 99.5% at near 10% load and above 98% from 10% to 100% load range. Simulations and experimental results are provided to verify the feasibilities of dc-dc converter operation and the proposed control strategy in the truck APU application.

Development of rainfall hazard envelope for unsaturated infinite slope

Rainfall during summer season is one of the major causes of slope failure in Korea, especially in unsaturated weathered granite soils. The current slope design procedure for the rainy season is to just raise the ground-water level, which is not adequate for the unsaturated soil slope. Previous rainfall hazard envelopes only tend to use rainfall intensity and duration as the main variables, not considering mechanical and hydraulic characteristics of unsaturated soils. The general equation for Factor of Safety (FS) in infinite unsaturated slopes is derived to consider the rainfall infiltration. The three important factors governing the equation are as follows: rainfall characteristics such as intensity and duration to determine a wetting front depth using an infiltration model, shear strength theory for unsaturated soil, and variation of suction profile during rainfall. The minimal FS is calculated from the FS profile with depth for a given rainfall intensity and duration, and a rainfall hazard envelope considering soil infiltration characteristics is built for a given slope angle. The procedure to develop a hazard envelope for various slope angles is applied to three unsaturated weathered granite soils in Korea.

Horizontal dynamic forces generated by swaying and jumping

Modern assembly structures must be designed to withstand the dynamic forces generated by their human occupants; the characterization of these forces has become an important research topic. This paper focuses on the measurement and modelling of horizontal dynamic forces, which have previously received much less attention than vertical forces. Horizontal forces resulting from movements such as swaying and jumping were recorded using a tri-axial force plate and characterized using three properties: the impulse shape, the mean of the split impulse and the frequency spectrum. The fundamental force due to swaying occurred at the swaying frequency. However, the fundamental force due to jumping did not always occur at the jumping frequency, indicating that the current design procedure of assuming a horizontal load model similar to the vertical load model is not always appropriate. For some subjects the position of the centre of mass was simultaneously recorded using a motion capture system to establish a relationship between its velocity and the side-to-side force. Finally, a new, semi-empirical analytical model for representing horizontal forces in design calculations was developed. Results from the model show excellent agreement with measurements from the laboratory tests.

Design of slip resistant lap joints with long open slotted holes

Current design procedures for slip resistant connections according to Eurocodes and American specifications are reviewed. Although failure of a slip resistant connection is defined at different levels of slip, 0.15mm and 0.5mm respectively, the calculation of the resistance is similar. Most of the research is performed on bolts in normal clearance holes. A testing program was conceived to evaluate the influence of long open slotted holes on the behavior of slip resistant lap joints with tension control bolts because of possible use of such connections in towers for wind turbines. In comparison to specimens with normal clearance holes it is found that the friction coefficient is about 4% lower. This reduction is lower than suggested by the current correction factors. Since none of the design procedures takes the reduction in bolt forces by time into account, the loss of pretension force is experimentally studied and an approximation is proposed. A second testing program was carried out with friction standard specimens in order to determine the slip factor for different surface treatments and steel grades in range between S275 and S690. The achieved slip factors for different surface preparations are in accordance with the classification of friction surfaces in EN 1090-2:2008 and differences obtained are addressed to variations in exposure to weather conditions. The steel grade does not have a significant influence.

Combined Effect of Three-Dimensional Contact Load and Thermal Gradients on the Cracking Performance of Heavy-Duty Asphalt Pavements

Top-down cracking is well recognized as a major distress for heavy-duty asphalt pavements, yet the mechanism is not fully understood. Studies have indicated that nonuniform tire–pavement contact stress may play a significant role in the initiation of top-down cracking, and thermal stress caused by the temperature gradient may contribute to the initiation and development of top-down cracking. These relevant factors were evaluated with a sophisticated three-dimensional viscoelastic finite element model that was developed for studying the cracking performance of heavy-duty flexible pavements under combined three-dimensional tire–pavement contact stress and thermal stress induced by the temperature gradient. It was found that the stress distribution in asphalt concrete layers continuously changed as the pavement was loaded because of the rheological behavior of the asphalt concrete. Significant maximum principal tensile stresses are present at the tire edge at the asphalt concrete surface, which may result in an accumulation of dissipated creep strain energy over time (or with a cumulative number of loads) and may eventually lead to crack initiation. Additionally, near-surface pavement responses associated with top-down cracking under three-dimensional tire–pavement contact pressure are found to be more severe than those from uniform contact pressure. In a comparison with pavements without thermal effect, the negative temperature gradient increases the potential for top-down cracking, and the positive temperature gradient increases the potential for bottom-up cracking. The study provides insight into crack mechanisms, which may improve current heavy-duty asphalt pavement design procedures.

Development of Full-Scale Reflective Cracking Test at the FAA National Airport Pavement Test Facility

Asphalt concrete overlays must be designed and constructed to withstand reflective cracking. However, the current FAA overlay design procedure (Advisory Circular 150/5320-6E) does not address this common form of premature failure. This paper presents a research project that studied one of the accepted causes of reflection cracks: horizontal movements concentrated at joints in the existing portland cement concrete pavement. A prototype, the temperature-effect simulation system, was developed at the FAA National Airport Pavement Test Facility so that joint opening and closing caused by temperature changes could be simulated mechanically. A test pavement representing a typical airport overlay structure was constructed to support full-scale tests. Both theoretical and experimental studies were performed to determine key test parameters, critical pavement temperature, joint opening, loading rate, and the bonding condition at the interface of asphalt concrete and portland cement concrete. Preliminary full-scale tests demonstrated that the simulation system could not only generate forces that create precise and repeatable joint openings but also effectively control the pavement temperature. Instrumentation data revealed that once bottom-up reflection cracks were initiated, crack development could quickly progress. When the crack length reached a certain level, the crack propagation rate slowed considerably. Information and data presented in this paper will be of immediate assistance to general aviation airports that carry small and light aircraft and experience significant temperature variations.

Resistance Model of Lightweight Concrete Members

The objective of this study is to develop a resistance model for lightweight concrete (LWC) beams and slabs and compare it with normalweight concrete (NWC) beams and slabs using new material and component test data. The focus is on the development of statistical parameters for bending moment capacity and shear capacity. Resistance parameters can then serve as a basis for the determination of rational resistance factors for LWC. Load carrying capacity (resistance) is considered a product of three random variables representing the uncertainty in material properties, dimensions, and geometry (fabrication factor) and analytical model (professional factor). In general, it was observed that the compressive strength of LWC is similar to that of NWC and, in some cases, is even better. However, the lab tests of beams and slabs show that the current shear design procedure is less conservative for LWC compared to NWC. Therefore, the resistance factor for LWC has to be revised.

Evaluation of Environmental Susceptibility of Bituminous Mastic Viscosity as a Function of Mineral and Biomass Fillers

Bituminous mastics influence many other important asphalt mixture properties in addition to their own allowance for the load transfer in the aggregate skeleton. The influence of bituminous mastics extends to the overall stability of a mixture, air void distribution, bitumen draindown during transport, a mixture's workability during the laying process, and the overall in-time performance of the pavement. To understand the properties of asphalt mixtures and their resistance to environmentally induced failure mechanisms, it is paramount to study not only bitumen and the asphalt mixture but also the mastic itself. Current asphalt design procedures do not take mastic behavior into account, however; this omission leads to a significant flaw in the ability to design and predict asphalt concrete response. This paper presents the results of an ongoing research project to enhance the understanding of the mastic phase as well as to develop a new test protocol to characterize mastics. A description is given of the measurements of mastic viscosity for different types of mastics in which the bitumen source is kept as a constant but with varying fillers as well as concentrations. Environmental susceptibility was investigated by subjecting the samples to aging and moisture conditioning. Biomass fillers were included in some of the mastics, in addition to some of the traditional fillers, to show their impact on the viscosity under varying conditions. Results showed that the developed test protocol was able to identify clearly the impact of filler properties on the mastic viscosity. A critical filler concentration was identified beyond which the viscosity behavior became nonlinear. The results also showed that moisture and aging had significant effects on the viscosity of mastics.

Pounding of San Francisco-Type Soft-Story Corner Buildings

We investigated 1920s-era four-story wood-frame corner buildings common to San Francisco, many of which were damaged in the city's Marina District during the 1989 Loma Prieta earthquake. Such buildings can have relatively weak and flexible first stories and are referred to as soft-story buildings. We calibrated two building computer models to simulate actual earthquake response. We then performed computer analysis to assess collapse performance under 22 hypothetical pounding situations of both as-built and retrofitted soft-story buildings. For this building type, we found that a typical pounding situation increased the collapse rate by 14%, and for retrofitted versions, pounding slightly decreased the collapse rate. However, there were factors that appear to significantly increase the rate, especially in combination: negligible building separations and multiple adjacent buildings having low effective damping and large mass. Based on the results, we outline an approximate way to account for pounding within the context of current design procedures. An upper bound on the pounding effect was found to be equivalent to about a 30% increase in demand.

A probabilistic performance‐based approach for mitigating the seismic pounding risk between adjacent buildings

ABSTRACTExisting design procedures for determining the separation distance between adjacent buildings subjected to seismic pounding risk are based on approximations of the buildings' peak relative displacement. These procedures are characterized by unknown safety levels and thus are not suitable for use within a performance‐based earthquake engineering framework.This paper introduces an innovative reliability‐based methodology for the design of the separation distance between adjacent buildings. The proposed methodology, which is naturally integrated into modern performance‐based design procedures, provides the value of the separation distance corresponding to a target probability of pounding during the design life of the buildings. It recasts the inverse reliability problem of the determination of the design separation distance as a zero‐finding problem and involves the use of analytical techniques in order to evaluate the statistics of the dynamic response of the buildings. Both uncertainty in the seismic intensity and record‐to‐record variability are taken into account.The proposed methodology is applied to several different buildings modeled as linear elastic single‐degree‐of‐freedom (SDOF) and multi‐degree‐of‐freedom (MDOF) systems, as well as SDOF nonlinear hysteretic systems. The design separation distances obtained are compared with the corresponding estimates that are based on several response combination rules suggested in the seismic design codes and in the literature. In contrast to current seismic code design procedures, the newly proposed methodology provides consistent safety levels for different building properties and different seismic hazard conditions. Copyright © 2012 John Wiley & Sons, Ltd.