Ontario Highway Bridge Design Code Research Articles

Reliability-based geotechnical design in 2014 Canadian Highway Bridge Design Code

Canada has two national civil codes of practice that include geotechnical design provisions: the National Building Code of Canada and the Canadian Highway Bridge Design Code. For structural designs, both of these codes have been employing a load and resistance factor format embedded within a limit states design framework since the mid-1970s. Unfortunately, limit states design in geotechnical engineering has been lagging well behind that in structural engineering for the simple fact that the ground is by far the most variable (and hence uncertain) of engineering materials. Although the first implementation of a geotechnical limit states design code appeared in Denmark in 1956, it was not until 1979 that the concept began to appear in Canadian design codes, i.e., in the Ontario Highway Bridge Design Code, which later became the Canadian Highway Bridge Design Code (CHBDC). The geotechnical design provisions in the CHBDC have evolved significantly since their inception in 1979. This paper describes the latest advances appearing in the CHBDC along with the steps taken to calibrate its recent geotechnical resistance and consequence factors.

Discussion of “Effect of Deterioration on the Performance of Corrugated Steel Culverts” by Van Thien Mai, Neil A. Hoult, and Ian D. Moore

The authors have undertaken an interesting laboratory experimental research program to validate the results of numerical analyses studies done previously (El-Taher and Moore 2008; Mai et al. 2012) to evaluate by rational means the stability of buried culverts with varying degrees of deterioration. The overall intent of the rational approach is to identify quantitatively when a culvert structure has deteriorated to such an extent that rehabilitation of the structure is warranted. This approach could result in considerable savings in costly infrastructure repair or replacement if the remaining life of a structure could be evaluated without subjectivity. The vast majority of corrugated steel pipes (CSPs) along a highway system are often composed of circular pipes or culverts of 600–900 mm diameter used in small watercourses that cross the highway alignment and at entrances to residences adjacent to the highway whereas large-diameter structural plate CSPs (SPCSPs) or multiplates, as they are frequently called, are used typically as bridgesized culverts and underpass structures. According to the Ontario Highway Bridge Design Code (Ministry of Transportation Ontario 1992), such structures classified as bridge structures are 3 m or more in diameter, whereas in Alberta, culverts of 1.5 m in diameter or greater are classified as bridge structures. The authors’ experimental results have brought out some interesting findings regarding corrosion and stiffness of the structure and backfill or the lack thereof on the structural performance of the structures. However, one aspect that was not addressed that may be important is the documented history of pipe installation in relation to any defects of the structure noted prior to and during installation. This could be important to the continued use of the structure because it would tend to provide baseline information that can be used to determine reasons why the lack of structural stability had originated or what effect the initial defects have on the structure stability other than or in addition to loss of structure thickness caused by corrosion. It is realized that this would be a significant effort by highway departments. However, with the large sums of money required for large SPSCP structure rehabilitation, the effort would be well worth the time and cost because these records should be available as a matter of quality control and quality assurance for the installation of every structure, rather than relying on the assumption that the installation was undertaken as per specifications regarding backfill density, material characteristics, and so forth. Very often, CSPs and SPSCPs are damaged during the installation process, for example, by backfill material placed over the crown containing rockfill and lack of care for the structure by equipment during the installation process. In such cases, the culvert is often deemed acceptable from visual observation because the damage may be considered not to affect the hydraulic capacity much, and, being a new structure, the structural integrity aspect is not considered inadequate. For significant multiplate structures, instrumenting these structures to evaluate the in-service structural behavior with time would also be of valued importance and could be somewhat similar to the approach being used to evaluate existing piled foundations for reuse. This aspect would also be important to understanding which type of installation damage would warrant that an installation be rejected and the installation redone with new materials. This aspect is often a contentious issue between the client and contractor, with the contractor often arguing that the integrity of the culvert is not compromised whether constructed with multiplates or SPCSP. For SPCSP structures, other modes of distress such as bolt shearing, for example, resulting from ground settlement that results in distortion of the crown and invert can eventually lead to collapse of the structure. The need for a rational approach to assessment is, therefore, very relevant. The findings of this research program would tend to indicate that installation damage may have a penalty on the performance of such a structure with time because the distortion of the crown could cause deflection around the circumference of the culvert, as shown in Figs. 4 and 5 of the authors’ paper. With in-service deterioration of the plate thickness attributable to corrosion and abrasion, this would further decrease the integrity of the structure. Overall, the authors’ paper has provided very pertinent information in assessing the stability of culvert structures to determine when repair and/or replacement is required, but it also has merits in quality control and quality-assessment evaluations during installation of these structures. Procedures for use of the rational approach separately or together with the visual-inspection approach would be of benefit to practitioners. It also would have been of interest if the authors had presented in the figures the results of the numerical analyses for comparison with those obtained from the experimental program.

Limit states and reliability-based design for a non-codified problem of aqueduct buoyancy

Limit states design methods and engineering judgement have been used to assess buoyancy issues for remediation of the 85 year old Shoal Lake Aqueduct in Manitoba. The study demonstrates how these methods can be applied to non-codified engineering problems. Four separate buoyancy analyses were completed using (i) partial safety factors from the Ontario Highway Bridge Design Code, (ii) project-specific partial safety factors, (iii) Monte Carlo simulation techniques, and (iv) working stress design (WSD) methods. Engineering judgement was required to develop a buoyancy model, interpret data for modeling parameters, and provide meaningful values for parameters that could not be measured. Results from the analyses show that more uniform reliability is provided when measured variability of the model parameters is accounted for. The reliability is not quantifiable when working stress design methods are used. Key words: limit states, probability, non-codified problem, aqueduct, buoyancy.

Calibration of design code for buried structures

The reliability-based calibration procedures were applied to develop load and resistance factors for the Ontario Highway Bridge Design Code (1979, 1983, and 1991) and recently the Canadian Highway Bridges Design Code (2000). However, the load components for buried structures were not considered. The development of a statistical model for earth pressure requires a special approach. Therefore, this paper deals with the reliability-based calibration of the design code for buried (cut-and-cover) structures. A typical running structure consists of reinforced concrete walls forming a rectangular box section, while an underground station may have a one- to six-cell box. The major load components include earth pressure, water pressure and weight of the concrete. Other load components such as live load are relatively small. Statistical parameters are derived for representative structures and structural systems. The correlation between load components is estimated based on the available field data. Structural performance is measured in terms of the reliability index. Reliability indices are calculated for a representative spectrum of running structures and stations. In general, the reliability indices for existing buried structures are higher than those for bridges or buildings. The target reliability index has been selected on the basis of calculated reliability indices, comparison with other structures, and cost analysis (consequences of failure). The optimum load and resistance factors are calculated and recommended for the design code to achieve a uniform safety level.Key words: buried structure, code calibration, load models, reinforced concrete, reliability analysis, resistance models.

Experimental investigation of the role of reinforcement in the strength of concrete deck slabs

To study systematically the role of each layer of steel reinforcement in conventionally reinforced deck slabs of girder bridges, a full-scale model was built of a 175 mm thick concrete deck slab on two steel girders with a center-to-center spacing of 2.0 m. The 12 m long deck slab was conceptually divided into four 3 m long segments, identified as segments A, B, C, and D. Segment A contained isotropic steel reinforcement in two layers, conforming to the requirements of the Ontario Highway Bridge Design Code (OHBDC). Segment B contained only the bottom layer of steel reinforcement. Segment C contained only the bottom transverse steel bars. Segment D contained only bottom transverse glass fibre reinforced polymer (GFRP) bars having the same axial stiffness, but 8.6 times the axial tensile strength, as those of the steel bars in segment C. Each segment of the deck slab was tested to failure under a central concentrated load, simulating the dual tire footprint of 250 × 500 mm dimension of a typical commercial vehicle. All segments failed in the punching shear mode. The failure loads for the four segments were found to be 808, 792, 882, and 756 kN, respectively; these failure loads are similar in magnitude to that of a 175 mm thick steel-free deck slab with steel straps having nearly the same cross-sectional area per metre length of the slab as those of the bottom transverse steel bars in the first three segments. The tests on the four segments of the full-scale model have confirmed that (i) only the bottom transverse reinforcement influences the load carrying capacity of a reinforced concrete deck slab and (ii) the stiffness of the bottom transverse reinforcement, rather than its strength, is of paramount importance.Key words: arching, deck slab, FRP, shake down, slab-on-girder bridge.

Long-Span Deep-Corrugated Structural Plate Arches with Encased-Concrete Composite Ribs

The design and construction of two long-span reinforced deep-corrugated structural plate arch culverts are described. The arch structures have maximum spans of 23.3 and 19.0 m. They were constructed on the Alaska Highway in northern British Columbia, Canada, during the fall of 1998. The 23.3-m-span arch is the largest of its kind in the world. Both structures are intermittently reinforced using a system of encased-concrete stiffeners. This system employs concrete pumped into the cavities formed by opposing deep-corrugated steel plates fitted with shear studs. The reinforcement for the 19.0-m structure was terminated at a location above the springline of the arch. The reinforcement for the 23.3-m-span arch extended to the top of the foundation. The arches were designed in accordance with the provisions of the Ontario Highway Bridge Design Code and the new Canadian Highway Bridge Design Code for Buried Structures. They were also reviewed using a nonlinear frame analysis (Structural Optimization Design and Analysis, or SODA) and a finite-element analysis (Nonlinear Soil-Structure Interaction Program, or NLSSIP). The deflected shapes of the two arches were monitored by instruments in the field during the construction and backfilling operation. The design results and field-measured deflections are evaluated.

Polypropylene FRC Bridge Deck Slabs Transversely Prestressed with CFRP Tendons

Compressive membrane action in bridge deck slabs has been shown to considerably increase the failure loads of such slabs. Thus, the reinforcing requirement can be reduced if a system is configured to ensure development of compressive membrane action. Recent research has shown that the use of transverse prestressing in bridge deck slabs can achieve even greater savings by reducing slab thickness and reinforcing requirements while enhancing serviceability. This paper describes an investigation of a deck slab system totally devoid of steel reinforcement. A one-quarter-scale mode of a typical deck slab supported on three steel girders was constructed in the laboratory and transversely prestressed with carbon-fiber-reinforced plastic tendons. Polypropylene-fiber-reinforced concrete was used for temperature and shrinkage crack control. The deck slab panels in the model bridge were tested to failure under simulated concentrated wheel loads. The observed failure mode of all panels was that of punching shear, at loads in excess of the scaled Ontario Highway Bridge Design Code specified design service load. Load-deflection curves and failure loads are presented and compared. The test results show that transverse prestressing enhances, considerably, the punching failure loads of deck slabs. It was also observed that bridge deck slabs totally devoid of any steel reinforcement perform satisfactorily in both strength and deformation.

Equivalent area of voided slabs

In order to calculate prestress losses in the transverse prestressing of voided concrete slabs, it is sometimes convenient to estimate the thickness of an equivalent solid slab. The Ontario Highway Bridge Design Code, as well as the forthcoming Canadian Highway Bridge Design Code, specifies a simple expression for calculating this equivalent thickness. This expression is reviewed in this technical note, and a simple alternative expression, believed to be more accurate, is proposed, along with its derivation. It is shown that the equivalent solid slab thickness obtained from consideration of in-plane forces is also applicable to transverse shear deformations, provided that the usual approximations of elementary strength of materials are used in both cases.Key words: axial stiffness, equivalent area, shear deformation, transverse prestressing, voided slab, slab.

Bridge superstructure: new developments

This book is based on lecture notes for short courses offered by the authors in several Asian countries over the past two decades on various aspects of bridge engineering, in which the authors have developed special expertise. The result is an account of recent developments in selected aspects of highway bridge engineering which have not been compiled in bookform before. The book comprises eleven free-standing chapters and four appendices. Each chapter focuses on a narrow but important aspect of bridge engineering providing sufficient information to permit direct adoption of the new concepts and methods into the practice of bridge engineering. Several of these concepts and methods have been incorporated in Canadian bridge codes. Chapter 1 explains how bridge design live loads are formulated for codes of practice. The chapter provides the basics of the probabilistic methods which are used to quantify safety in modern design codes. Chapter 2 presents the details of some methods to analyse a bridge by hand calculations. The simplified methods for load distribution of North American codes are discussed, and more sophisticated new methods are proposed. In Chapter 3, a versatile method for bridge analysis by microcomputer is presented. Because of its accuracy and efficiency, the isemi-continuum methodi has been adopted by the Ontario Highway Bridge Design Code since 1992, and more recently by the AASHTO Load and Resistance Factor Design Specifications. Chapters 4 and 5 deal with deck slabs for slab-on-girder bridges. Chapter 4 explains the mechanics of internal arching in deck slabs, and presents two sets of design provisions: one for slabs with considerably reduced steel reinforcement as compared to slabs designed for pure flexure, and the other for slabs that may be entirely free of tensile reinforcement with an external tension tie below the slab. Chapter 5 then deals with the flexural analysis of cantilever slabs subjected to concentrated sheet loads making extensive use of the design charts of the authors.

Étude paramétrique du facteur d'amplification dynamique des charges pour des ponts à travée simple en béton armé

It is generally accepted that several parameters influence the dynamic amplification of a bridge response, such as the dynamic characteristics of both the vehicles and the bridge, the initial loading conditions, the surface profile of the pavement, etc. The objective of this paper is to use a tridimensional finite element model (vehicle and bridge) to investigate the effect of some significant factors such as the bridge geometry, the type of measures that form the basis for computing the dynamic amplification factor (DAF), the loading conditions, and the velocity of trucks, on the DAF applied to static loadings. We limited ourselves to the study of 20 to 32 m simple span reinforced concrete bridges. The simulations show that the DAF proposed in the standards Calcul des ponts routes (CAN/CSA-S6-88), Ontario Highway Bridge Design Code, and Canadian Highway Bridge Design Code is not always safe for calculating the stresses in the type of bridges investigated herein and that it decreases for high loads and velocities. Using two DAFs, one for bending and the other for shear and bearings, would certainly be more adequate for the design and assessment of bridges.

Girder moments in simply supported skew composite bridges

The evaluation of girder moments in composite bridges becomes more urgent with the trend to increasing truck loads. The method specified by the American Association of State Highway and Transportation Officials for such an evaluation depends only on the centre-to-centre girder spacing. This method does not account for skew and therefore is extremely conservative for skew composite bridges, since the presence of skew reduces the longitudinal moments in the girders. The method proposed by the Ontario Highway Bridge Design Code (OHBDC) depends on the longitudinal and transverse rigidities of the bridge in addition to the girder spacing. However, this method is limited to bridges with skew parameters less than a certain value specified in the code. In this paper, the influence of skew on the moment distribution factor is investigated. Furthermore, the influences of other factors such as girder spacing, bridge aspect ratio, number of lanes, number of girders, and intermediate transverse diaphragms on the moment distribution factor are examined. An experimental program was conducted on six simply supported skew composite steel–concrete bridge models. The finite element method was used for the theoretical analysis. Good agreement is shown between the experimental results and the theoretical results. In addition, the finite element method was employed to conduct an extensive parametric study on more than 300 prototype composite bridge cases. The data generated from the parametric study were used to deduce expressions for the moment distribution factor for OHBDC truck loading and for dead load. An illustrative example is presented. Key words: bridges, codes of practice, composite, distribution, moment, reinforced concrete, skew, structural engineering, tests.

Revisiting arching in deck slabs

The arching action in concrete deck slabs of girder bridges is generally recognized and is utilized by the Ontario Highway Bridge Design Code, and some other codes, to specify an empirical design method which leads to considerable savings in the amount of reinforcement. Despite this general recognition, there are some aspects of the arching action that are yet to be explored. To the knowledge of the author, all reported laboratory and field tests on deck slabs exploring its arching action under applied loads have been conducted by measuring strains in the bottom transverse reinforcement midway between the girders. Based on the results of tests on a full-scale model of a deck slab, it has been confirmed in this note that the transverse bottom reinforcement in the deck slab acts as a tie to the internal transverse arch in the slab. Because of embedment in concrete, the force in this reinforcement is the smallest midway between the girders, and not the largest as would be the case if the slab were in pure bending. Key words: arching in slabs, deck slabs, girder bridge, punching shear, steel-free deck slabs.

Shear distribution in simply supported skew composite bridges

Composite steel–concrete bridges remain one of the most common types built. Proper design of new bridges and evaluation of existing bridges requires accurate prediction of their structural response to truck loads. The American Association of State Highway and Transportation Officials has traditionally applied a load distribution factor for both moment and shear. The Ontario Highway Bridge Design Code (OHBDC) considers several parameters in establishing load distribution factors for moment. However, the method is limited to bridges with skew parameters less than a certain value specified in the code. The presence of skew reduces the longitudinal moments in the girders. However, it also causes high concentration of shear in the girder closest to the obtuse corner and reduces shear concentration in the girder closest to the acute corner as well as in the interior girders. Therefore, shear should be considered in the design of such bridges. In this paper, the influence of skew on the shear distribution factor is investigated. The influences of other factors such as girder spacing, bridge aspect ratio, number of lanes, number of girders, end diaphragms, and intermediate cross-beams are presented. An experimental program was conducted on six simply supported skew composite steel–concrete bridge models. Results from a finite element analysis showed excellent agreement with the experimental results. An extensive parametric study was conducted on prototype composite bridges subjected to OHBDC truck loading. The parametric study included more than 400 cases. The data generated were used to develop empirical formulas for shear distribution factors for OHBDC truck loading and also for dead load. An illustrative example is presented. Key words: bridges, codes of practice, composite, distribution, reaction, reinforced concrete, shear, skew, structural engineering, tests.

Design of horizontally curved composite box-girder bridges: a simplified approach

Because of their excellent torsional capacity, box girders are used extensively in modern bridge construction having curved alignments. Applications of most design codes have been limited to bridges where the radius of curvature is much greater than the span length and cross-sectional dimensions. To meet the practical requirements arising during the design process, simple design methods are needed for curved bridges. This paper presents the results of a parametric study on the relative behaviour of curved and straight box-girder bridges and on the development of a simplified design method for the combined longitudinal moment of curved bridges. The combined moment includes the effects of flexure, torsion, and distortion. Three simply supported concrete-steel composite bridge models, including single-cell, twin-cell, and three-cell box girders and subjected to loadings as specified in the Ontario Highway Bridge Design Code, were analyzed using the finite strip method. The parameters considered in the study include types of cross section; types, locations, and magnitudes of loads; span lengths; and radius of curvature. Preliminary analysis of the results suggests that the behaviour of horizontally curved box-girder bridges is dependent on a variety of parameters, but most importantly on the span-to-radius ratio. Empirical relationships for combined longitudinal moment between curved and straight box-girder bridges are also proposed. Key words: bridge, curved, composite, design, finite strip.

Calibration of the Ontario Highway Bridge Design Code 1991 edition

The paper describes the calculation of load and resistance factors for the Ontario Highway Bridge Design Code (OHBDC) 1991 edition. The work involved the development of load and resistance models, the selection of the reliability analysis method, and the calculation of the reliability indices. The statistical models for load and resistance are reviewed. The considered load components include dead load, live load, and dynamic load. Resistance models are developed for girder bridges (steel, reinforced concrete, and prestressed concrete). A reliability analysis is performed for selected representative structures. Reliability indices are calculated using an iterative procedure. The calculations are performed for bridge girders designed using OHBDC 1983 edition. The resulting reliability indices are between 3 and 4 for steel girders and reinforced concrete T-beams, and between 3.5 and 5 for prestressed concrete girders. Lower values are observed for shorter spans (up to 30–40 m). The acceptance criterion in the selection of load and resistance factors is closeness to the target reliability level. The analysis confirmed the need to increase the design live load for shorter spans. Partial resistance factors are considered for steel and concrete. The criteria for the evaluation of existing bridges are based on the reliability analysis and economic considerations. Key words: bridge code, calibration, load factor, resistance factor, reliability index.

Load model for bridge design code

The paper deals with the development of load model for the Ontario Highway Bridge Design Code. Three components of dead load are considered: weight of factory-made elements, weight of cast-in-place concrete, and bituminous surface (asphalt). The live load model is based on the truck survey data. The maximum live load moments and shears are calculated for one-lane and two-lane bridges. For spans up to about 40 m, one truck per lane governs; for longer spans, two trucks following behind the other provide the largest live load effect. For two lanes, two fully correlated trucks govern. The dynamic load is modeled on the basis of simulations. The results of calculations indicate that dynamic load depends not only on the span but also on road surface roughness and vehicle dynamics. Load combination including dead load, live load, dynamic load, wind, and earthquake is modeled using Turkstra's rule. The maximum effect is determined as a sum of the extreme value of one load component plus the average values of other simultaneous load components. The developed load models can be used in the calculation of load and resistance factors for the design and evaluation code. Key words: bridge, dead load, live load, dynamic load, load combinations.

Live load model for highway bridges

Load models are developed for highway bridges. The models are based on the available statistical data on dead load, truck loads and dynamic loads. The paper deals mostly with the static live load. The model is derived from truck surveys, weight-in-motion measurements and other observations. Simple span moments, shears and negative moments are calculated for various spans. Extreme 75 year loads are determined by extrapolation. The important parameters also include girder distribution factors and multiple presence (more than one truck on the bridge). Multiple presence is considered in lane and side-by-side with various degrees of correlation between truck weights. The maximum load is calculated by simulations. The developed live model served as a basis for the development of new design provisions in the United States (LRFD AASHTO) and Canada (Ontario Highway Bridge Design Code).

Combined wind and ice loading on antenna towers

The present study contains an analysis of the recorded and predicted values of ice accretion for the meteorological stations in Canada; also, the value of the wind pressure associated with the maximum ice accretion is studied. The values were compared with those specified in CAN/CSA S37-M86. Changes to the ice map of Canada are proposed based on the climatic data analyzed, and this map is compared with the one specified in the 1991 edition of the Ontario Highway Bridge Design Code. Also, some modifications to the method of calculating radial ice thickness are recommended to include the size of the member. Key words: antenna towers, environmental loads, guyed towers, ice loads, self-supporting towers.

Evaluation of Impact Factors for Horizontally Curved Steel Box Bridges

This paper presents the dynamic impact factors for horizontally curved steel box‐girder bridges. The dynamic impact is presented as a function of the span length and the first fundamental frequency. The results are discussed and compared with the impact provisions as applied in the previous AASHTO specifications and the Ontario Highway Bridge Design Code. It is concluded that the impact functions previously specified by AASHTO exhibit shortcomings and do not apply for a large percentage of designs. Consequently, an alternate set of impact factors for horizontally curved steel box‐girder bridges is developed.

Simplified methods of bridge analysis for the third edition of OHBDC

Research conducted since the publication of the second edition of the Ontario Highway Bridge Design Code (OHBDC) in 1983 has paved the way for further simplification of the simplified methods of bridge analysis specified by the code. The third edition of the OHBDC (1991) incorporates simplified methods, most of which have been made even more "user friendly" in the light of this research. This paper gives the details of these new methods of analysis along with their developmental background. It is shown that the low volume of traffic on bridges on lightly travelled roads can be accounted for conveniently through the simplified methods of analysis by incorporating in them the lower values of the modification factors for multipresence of vehicles specified in the OHBDC (1991). The paper also discusses those major pieces of contemporary research work in load distribution analysis, the end product of which was considered but could not be utilized in the OHBDC. Key words: analysis, bridge, load distribution, multi-beam bridges, multipresence of vehicles, slab-on-girder bridges, timber bridges.