Canadian Highway Bridge Design Code Research Articles

Live load distribution factors for simply-supported composite steel I-girder bridges

The Canadian Highway Bridge Design Code (CHBDC) provides formulas for determining the live load distribution factors for composite slab-on-girder bridges. These formulas were developed by neglecting the contribution of secondary elements such as lateral bracing in bridges, leading to conservative design in some cases and unsafe design in others. This paper presents a practical-design-oriented parametric study, using three-dimensional (3D) finite-element modeling, to investigate the live load distribution factors of composite braces steel I-girder bridges under CHBDC truck loading conditions at ultimate, serviceability and fatigue limit states. The key parameters considered in this study included girder stiffness and spacing, span length, number of girders, and number of design lanes. The comparison of results suggested that the CHBDC equations underestimate the girder moments for the 15 m spans and overestimates the response for greater spans. On the other hand, CHBDC generally overestimates the shear distribution factors, with significant difference observed at fatigue limit state. Based on the database generated from this study, a set of empirical expressions were developed for the moment and shear distribution factors for rational prediction of the girder design moment and shear force. A design example is presented to illustrate the use of the developed equations.

Modified spectral format based on probability level using site-specific uniform hazard spectrum

Deficiencies of the four spectral formats i.e., 2%/50-yr, 5%/50-yr, 10%/50-yr and AASHTO (American Association of State Highway Officials) 2009 demand modification of the spectral formats for bridge design application in Canada. Among them the 10%/50-yr spectrum is dropped from current investigation as its difference with Canadian Highway Bridge Design Code (CHBDC) 2006 is too large to modify. This study introduces an approach calibrating on the values of elastic seismic response coefficient to provide a new shape of the Canadian bridge design spectrum based on modified 2%/50-yr, modified 5%/50-yr and modified AASHTO spectral formats. A Digital Visual Fortran program was prepared to determine the optimum values of the modification factors incorporated into the three spectral formats calibrated on the data of 389 cities of Canada. Thus, here it is developed the strategies of modifying the three spectral formats based on the best probability level for CHBDC using site-specific Uniform Hazard Spectrum (UHS). Finally, select the most suitable spectral format to apply for the design base shear calculations for the bridges of Canada.

Seismic fragility and damage assessment of reinforced concrete bridge pier under long-duration, near-fault, and far-field ground motions

Observations from recent earthquakes around the world have revealed the destructive nature of near-fault ground motions that cause significant damage to structures. On the other hand, the effect of long-duration motions on seismic demands has attracted a lot of interest over the last decade. This study aims to numerically explore how long-duration (LD) motions and near-fault (NF) ground motions affect the seismic performance of a seismically designed bridge pier and compare the response with short duration far-field (FF) motions. A fiber based nonlinear finite element model of a bridge pier is used for evaluating the damage potential of different types of ground motions. The bridge is assumed to be a major route bridge located in Western Canada and is designed following the performance-based seismic design requirements of the Canadian Highway Bridge Design Code. A set of 20 long-duration motions, 20 spectrally equivalent near-fault and far-field motions are used to study the effect of ground motions on drift and damage measures of the bridge pier. The results indicate that since the current design guidelines consider neither the ground motion duration effect nor the pulse effect of near-fault motions, damage measures for performance-based design should be reviewed to take into account the duration effect as well as the velocity pulse associated with NF motions.

Field Monitoring and Numerical Analysis of Large-Span Three-Sided Reinforced Concrete Culvert

This paper investigates the structural behavior and soil–structure interaction mechanism of a three-sided culvert (TSC) at service and ultimate load levels. Field measurements were used to verify a two-dimensional (2D) finite-element model (FEM) and evaluate the backfill height at which the TSC would fail. In addition, experimental data available in the literature of an arched TSC segment loaded to failure was adopted to verify a three-dimensional (3D) FEM for the analysis at the ultimate limit state (ULS). The results demonstrated that the Canadian Highway Bridge Design Code overestimates the earth pressures on the top slab. It was also found that the connection at the sidewall base does not behave as a perfect hinge. Finally, the nonlinearity of the reinforced concrete (RC) was evaluated throughout the loading spectrum and it was concluded that simplifying the behavior of RC using a linear-elastic model is only adequate at the service load levels.

Live load factors for military traffic in bridge evaluation

Military vehicles are sometimes required to transit bridges owned and operated by civilian bridge authorities. Using available data regarding the gross vehicle weight and associated axle loads of military traffic, live load factors, calibrated to the Canadian Highway Bridge Design Code, are proposed for bridge design and evaluation. This paper recommends live load factors for three categories of military vehicles: (i) wheeled-transport vehicles; (ii) wheeled-fighting vehicles; and (iii) tracked-fighting vehicles. The values are derived for interior girders of simply supported slab-on-girder bridges subjected to a single lane of traffic loading and are believed to be generally applicable for other structural elements and bridge types. Inherent differences between fighting vehicles, which are heavily armoured, and transport vehicles, which although armoured have high payloads, suggest that highway bridges should be evaluated separately for military fighting vehicles and military transport vehicles using distinct live load factors.

A case for adding an inspection level related to structural health monitoring (SHM) for bridge evaluation

The Canadian Highway Bridge Design Code (CHBDC) uses the concept of a target reliability index for evaluating the load carrying capacity of existing bridges. This index, which is based on risk to human life, relates to three aspects of uncertainties inherent in a bridge: (a) element behaviour, (b) system behavior, and (c) inspection level, where the inspection level currently refers to only manual inspections. Citing examples of tests on many instrumented bridges, the paper proposes an additional inspection level for inspections done with the help of electronic instruments and tests under controlled vehicle loads. The paper proposes simple additions to clauses of the CHBDC, for use when determining the optimum load carrying capacities of existing bridges where structural health monitoring information is available.

Seismic vulnerability and loss assessment of an isolated simply-supported highway bridge retrofitted with optimized superelastic shape memory alloy cable restrainers

Restrainers, being of relatively low cost and easy to install, are often used to prevent unseating of bridge spans. The potential of using superelastic shape memory alloy (SMA) restrainers in preventing such failure has been discussed in the literature; however, the impact of such smart restrainers with optimized configurations in reducing the failure probability of bridge components and system as well as the long-term economic losses given different earthquake scenarios has not been investigated yet. This study presents a probabilistic seismic fragility and long-term performance assessment on isolated multi-span simply-supported bridges retrofitted with optimized SMA restrainers. First, SMA restrainers are designed following the displacement-based approach and their configuration is optimized. Then, seismic fragility assessment is conducted for the bridge retrofitted with optimized SMA restrainers and compared with those of the original bridge and the bridges with elastic restrainers (steel and CFRP). Finally, long-term seismic loss (both direct and indirect) are evaluated to assess the performance of the retrofitted bridges in a life-cycle context. Results showed that among three considered restrainers, SMA restrainers make the bridge less fragile and help the system lower long-term seismic loss. The design event (DE, 2475-year return period) specified in Canadian Highway Bridge Design Code (CHBDC, CSA S6-14 2014) may underestimate the long-term seismic losses of the highway bridges. Under DE, the damage probability of the bridge retrofitted with optimized SMA restrainers experiencing collapse damage is only 0.7%. Under the same situation, its expected long-term loss is approximate 17.6% of that with respect to the unretrofitted bridge.

Validation of fatigue design wind loads for natural wind gusts and for truck-induced wind gusts using full-scale measurements

Full-scale instrumentation is carried out for a typical variable message sign overhead truss support structure and a typical simply supported tri-chord sign overhead truss support structure commonly found in Ontario, Canada. The measurements are used to develop wind loads and responses induced by natural wind gusts and by truck induced gusts. The results are used to investigate the modelling error, and to validate fatigue design wind loads in the context of the Canadian Highway Bridge Design Code. Comparison of the measured responses to the predicted responses obtained by using finite element (FE) models indicates that the actual responses of the instrumented structures are, on average, about 70% of that calculated by using the FE models, if the effects of aerodynamic admittance and spatially incoherent winds are not considered. The bias is practically eliminated if these effects are included. The modelling error is uncertain with a coefficient of variation of about 0.2. The statistics of the truck-induced wind pressure coefficient based on the full-scale measurements are consistent with those reported in the literature. They are used to support the calibrated fatigue design loads for natural wind gusts and for truck-induced wind gusts for Canadian practice.

Comparison of Canadian Highway Bridge Design Code and AASHTO LRFD Bridge Design Specifications regarding pile design subject to negative skin friction

Negative skin friction acting on piles has long been included in the design of bridge foundations subject to ground settlement. However, currently there are inconsistencies in how negative skin friction and drag force are incorporated into the calculation of the geotechnical ultimate limit state (ULS), partly due to differences in the design codes. The latest editions of the Canadian Highway Bridge Design Code and AASHTO LRFD Bridge Design Specifications are compared with the analysis of a hypothetical steel H-pile, driven through a settling clay layer into a dense, nonsettling layer. The results show that foundation designs can be significantly more conservative and costly when adhering to the AASHTO code because this code includes the drag force in the geotechnical ULS. It is concluded that adhering to the CHBDC can result in a reduced foundation system by considering the actual force distribution in the pile.

The first rolling load simulator (ROLLS) for testing bridges in Canada and its application on a full-scale precast box girder

This paper describes the development of a unique rolling load simulator (ROLLS) for testing bridge superstructure with a footprint up to 4 m ×17 m, and its first application to test a full-scale 1220 mm ×900 mm ×16000 mm B900 prestressed concrete box girder. This facility at Queen’s University in Kingston, Ontario, is the first of its kind in Canada. ROLLS can apply cyclic loading in a controlled laboratory environment, under realistic highway scale ‘rolling wheel loads’, in lieu of the conventional ‘pulsating stationary loads’. It has two half-axles of a large tandem, each comprising a dual 1140 mm diameter air-inflated tires spaced at either 1.2 or 2.4 m. Each half-axle can apply up to 125 kN, representing the heaviest half-axle load of the CL-625 design truck of the Canadian Highway Bridge Design Code (CHBDC). The maximum travel range and speed are 14.9 m and 6 m/s, respectively. A case study involving analysis of a bridge with eight adjacent B900 box girders of 27.6 m span was carried out prior to experimentally testing one of the girders using ROLLS. Load distribution analyses were conducted using both (i) a finite element model of the full bridge under various CL-625 truck loading configurations and (ii) the CHBDC load distribution method, and both agreed well. Scaling analysis of the girder load share was then conducted to account for shortening it to 16 m to fit in the laboratory, resulting in two-115 kN ROLLS design loads, 1.2 m apart. Multiple passes were conducted at various loads of 40%–100% of the design load, at speeds of 1–5 m/s to examine the machine and girder behaviours. It was found that the applied load fluctuates by less than 10% of full capacity and a 0.13 s/cycle time lag occurs. The measured girder deflection and elastic strains were 11%–20% lower than predicted theoretically. With the two half-axles assembly spaced at 1.2 m, the apparatus has the ability to complete three million cycles in approximately 4.5 months if ran continuously at 5 m/s.

GFRP Bars Anchorage Resistance in a GFRP-Reinforced Concrete Bridge Barrier.

In the present paper, experimental and numerical investigations were conducted on concrete bridge barriers utilizing glass fiber reinforced polymer (GFRP) bars with a hook at their ends. Implementation of these hooked bars instead of the bent bars or headed-end bars in the bridge barriers presented in the Canadian Highway Bridge Design Code (CHBDC) was investigated on American Association for State Highway and Transportation Officials (AASHTO) test level 5 (TL-5) concrete bridge barriers. This research aimed to reach a cost effective and safe anchorage method for GFRP bars at the barrier–deck junction, compared to the conventional bend bars or headed-end bars. Therefore, an experimental program was developed and performed to qualify the use of the recently-developed, small radius hooked bars at the barrier–deck junction. The experimental findings were compared with the design factored applied transverse load specified in CHBDC for the design of the barrier–deck junction as well as factored applied bending moment obtained at the barrier–deck junction using a recently-conducted finite-element modeling. Satisfactory behavior for the developed hooked GFRP bars as well as their anchorage resistance was established and a reasonable factor of safety in design of barrier–deck joint was achieved.

Calibration of resistance factors for geotechnical seismic design

The next edition of the Canadian Highway Bridge Design Code will contain a table of geotechnical resistance factors to be used for seismic design. This paper will estimate the geotechnical resistance factors for shallow foundations required to achieve various target maximum acceptable failure probabilities, which in turn may depend on the assumed design earthquake return period. The investigation will include consideration of design lifetime, uncertainty in the magnitude of the maximum lifetime earthquake event, and the uncertainty in ground properties. The results suggest resistance factors that are lower than commonly used at the moment in Canada and that the failure probability is not greatly dependent on the return period of the design earthquake. The paper will present recommendations on geotechnical resistance factors for seismic design that can be used to guide and calibrate future editions of civil design codes in Canada.

Some observations on BWIM data collected in Manitoba

Three highway bridges in the Canadian province of Manitoba are being monitored continuously not only for their long-term performance but also for bridge weighing-in-motion (BWIM). Data collected for the BWIM study has led to some observations that have far-reaching consequences about the design and evaluation loads for highway bridges. This paper presents the well-known concept of equivalent base length, Bm, as a useful tool for comparing trucks with different axle weight and spacing configurations as they influence load effects in all bridges. It is discussed that the statistics of gross vehicle weights (GVWs), W, collected over a one-month period is not significantly different from that for the GVW data collected over a longer period. A rational method concludes that the value of W for the CL-W Truck, the design live load specified by the Canadian Highway Bridge Design Code, is 555 kN for Manitoba. The observed truck data in Manitoba presented on the W–Bm space is found to be similar to that collected in the Canadian province of Ontario more than four decades ago. It was also found that the multi-presence factors, accounting for the presence of side-by-side trucks in two-lane bridges, specified in North American bridge design and evaluation codes are somewhat conservative.

Comparison of different approaches for determining the residual post-cracking strength index of fiber reinforced concrete for bridges

In Canada, there are different approaches to evaluate the performance of fiber reinforced concrete (FRC) for bridges based on the residual post-cracking strength index (Ri). They involve different combinations of test methods ASTM C78, ASTM C1399, and ASTM C1609. The aims of this study were to assess all possible (existing and new) methods to determine the Ri values, and capture the major differences between the current Canadian Highway Bridge Design Code (CHBDC S6-14) and the City of Winnipeg specification’s approaches, as a case study. Flexural tests (ASTM C78, ASTM C1399, and ASTM C1609) were performed on 60 FRC beams (100 mm × 100 mm × 350 mm) prepared from concrete provided by four ready-mix concrete suppliers according to City of Winnipeg’s bridge deck specifications for a project built in Winnipeg. The results showed that all the methods implemented herein for calculating the Ri of FRC gave comparable results. However, by using Method V, all required parameters (first peak load and residual loads at specified deflections) could be directly extracted from one load–deflection curve obtained from ASTM C1609. In addition, when using this method, the Ri can be calculated for each specimen, which enables quantifying the magnitude of variation from average values. Since this approach also requires fewer number of specimens, reducing time and cost of testing, it has been adopted by the City of Winnipeg in bridge specifications for FRC.

Reliability-Based Design (RBD) For Everyone: (No Monte Carlo Simulation Required!)

In North America, the design of earth structures for transportation applications is most often carried out using load and resistance factor design (LRFD). In the U.S., the AASHTO LRFD Bridge Design Specifications are followed. In Canada, the primary code is the Canadian Highway Bridge Design Code. The premise behind LRFD is that when a limit state design equation is used with prescribed load and resistance factors, a minimum margin of safety, expressed as a reliability index b, or probability of failure, is assumed to be assured. Unless the design is at the point where the limit-state equation is satisfied, the true margin of safety in probabilistic terms is unknown. Furthermore, designers may have a choice of load and resistance models to use in a limit-state design equation. Because these models will have different accuracies, using different design models with the same load and resistance factors will result in different true margins of safety in probabilistic terms.

Shear strengthening of concrete members with unbonded transverse reinforcement

This paper examines the behaviour of thick concrete members strengthened in shear with unbonded transverse reinforcement. The retrofitting technique consists of placing unbonded vertical bars with steel end plates or torque controlled expansion end anchorages in pre-drilled holes of existing thick members. To study the behaviour of these members, loading tests as well as numerical analyses were carried out. Shear capacities were compared to the predictions using the shear design approach in the Canadian Highway Bridge Design Code. The design equations which are intended for traditional stirrups reinforcement overestimates the shear capacities of the members strengthened with unbonded transverse reinforcement. However, numerical analyses provided very accurate predictions of the shear capacities. A finite element parametric study examines the effects of the shear span-to-depth ratio, vertical prestressing, shear reinforcement ratio and the stiffness of the vertical reinforcement. The stiffness of the shear strengthening system and the effects of prestressing significantly affect the shear capacity. The shear capacities were predicted well when a minimum amount of vertical prestressing was provided.

Structural Qualification of a Developed GFRP-Reinforced Concrete Bridge Barrier using Ultimate Load Testing

Corrosion of reinforcing steel bars is the main factor affecting durability and service life of steel-reinforced bridge barriers in North America. The use of glass fiber reinforcing polymer (GFRP) bars as non-corrosive material has emerged as an innovative solution to corrosion related problems. A recent cost-effective design of PL-3 bridge barrier was developed at Ryerson University incorporating high-modulus GFRP bars with headed ends. An experimental program was conducted to investigate the load carrying capacity of the developed barrier wall. A 40-m long barrier was constructed and tested at four different locations to investigate its structural behavior, crack pattern and ultimate strength when subjected to the equivalent static loading simulating vehicle impact. Experimental results were compared with the design values specified in the Canadian Highway Bridge Design Code. Experimental findings showed a large margin of safety for the proposed GFRP-reinforced barriers. The failure pattern due to transversal loading the longitudinal barrier over 2400 mm length was initiated by a trapezoidal flexural crack pattern at front face of the barrier, followed by punching shear failure at the transverse load location. Comparison between the available punching shear equations in the literature and the punching shear failure developed in the barrier wall was conducted.

Ultimate capacity of barrier–deck anchorage in MTQ TL-5 barrier reinforced with headed-end, high-modulus, sand-coated GFRP bars

This paper presents an experimental program to justify the barrier design at the barrier–deck junction when compared to the factored applied transverse vehicular loading specified in the Canadian Highway Bridge Design Code (CHBDC). Compared to the dimensioning and the glass fibre reinforced polymer (GFRP) bar detailing of a recently crash-tested GFRP-reinforced barrier, the adopted barrier configurations in this paper were similar to those specified by Ministry of Transportation of Québec (MTQ) for TL-5 barrier except that the base of the barrier was 40 mm narrower and the deck slab is of 200 mm thickness, leading to reduction in the GFRP embedment depth into the deck slab. Four full-scale TL-5 barrier specimens were tested to collapse. Correlation between the experimental findings and the factored applied moments from CHBDC equivalent vehicle impact forces resulting from the finite-element modelling of the barrier–deck system was conducted followed by recommendations for use of the proposed design in highway bridges in Québec.

Method for developing and updating deterioration models for concrete bridge decks using GPR data

Abstract This study presents a workable procedure for developing and updating deterioration prediction models based on the results of a Non-Destructive Evaluation (NDE) technique, Ground Penetrating Radar (GPR). To this end, a system reliability based deterioration model was developed for a simply-supported reinforced concrete bridge deck, designed according to the Canadian Highway Bridge Design Code (CHBDC-S6). The superstructure element-level deteriorated conditions for different time intervals were applied in the non-linear finite element model developed for the entire bridge deck, and the system level reliability indices were estimated accordingly. The salt contamination was assumed to affect the top surface, soffits of the deck, and beams due to infusion of de-icing salt material. The resulting primary deterioration model was updated based on a GPR test localized defect map that was obtained for an existing bridge with similar plan dimensions. The results indicate that the assumed conservative uniformly defected superstructure is reliable in this case, that is, the estimated reliability indices for the uniform and localized defect plans do not differ considerably.

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

<i>Corrigendum:</i> Reliability-based geotechnical design in 2014 Canadian Highway Bridge Design Code