Live Load Model Research Articles

성능향상된 교량 바닥판의 확률론적 해석 및 수명연장 분석

Although the strengthening effect of deteriorated concrete bridge decks has been studied by various authors, most researches are focused on the experimental works on the pulsating loading in laboratory in spite of deterioration of deck caused by moving vehicle loads. In this research, a theoretical live load model that was proposed to reflect an effect of moving vehicle loads is formulated from a statistical approach on the measurement of real traffic loads for various time periodsin Korea. Fatigue life and strengthening effect of strengthened bridge decks strengthened with either Carbon Fiber Sheet or Grid typed Carbon Fiber Polymer Plastic by the probabilistic and the reliability analyses are assessed. As a results, secondary bridge deck (DB18) strengthened with FRP ensures a sufficient fatigue resistance against the increased traffic loads as well as load carrying capacity in life cycle.

Model Validation for Bridge-Road-Vehicle Dynamic Interaction System

The dynamic response of highway bridges subjected to moving truckloads has been observed to be dependent on (1) dynamic characteristics of the bridge; (2) truck configuration, speed, and lane position on the bridge; and (3) road surface roughness profile of the bridge and its approach. Historically, truckloads were measured to determine the load spectra for girder bridges. However, truckload measurements are either made for a short period of time [for example, weigh-in-motion (WIM) data] or are statistically biased (for example, weigh stations) and cost prohibitive. The objective of this paper is to present results of a three-dimensional computer-based model for the simulation of multiple trucks on girder bridges. The model is based on the grillage approach and is applied to four steel girder bridges tested under normal truck traffic. Actual truckload data collected using a discrete bridge WIM system are used in the model. The data include axle loads, truck gross weight, axle configuration, and statistical data on multiple presence (side by side or following). The results are presented as a function of the static and dynamic stresses in each girder and compared with code provisions for dynamic load factor. The study provides an alternate method for the development of live-load models for bridge design and evaluation.

Bridge live load models from WIM data

This paper describes the development of a new methodology for deriving highway bridge live load models for short span bridges. The development is based on a ‘repeatable’ methodology to obtain extreme daily moments and shears using 10 years Hong Kong weigh-in-motion (WIM) data as compared to the traditional normal probability paper approach. The methodology can also be applied to the development of bridge live loading models in other parts of the world. Two types of loading are proposed. A methodology based on the equivalent base length concept is used to derive the lane loading model and the standard truck loading model is developed based on a statistical approach. The developed lane and truck loadings are compared with other loading models adopted locally and overseas. It is the first time in Hong Kong the bridge design loading is developed based on actual acquired load data using statistical and probabilistic approach.

Live loads in office buildings: lifetime maximum load

This is the second of two papers that describe a study on probabilistic live loads in office buildings surveyed in Kanpur, India. The paper presents a probabilistic live load model and its application to sustained load and extraordinary load. Parameters for sustained load are established from actual survey data. Probability distribution of the sustained load is represented by log-normal and gamma distributions. Predicted extraordinary loads based on the present survey are calculated with parameters selected from the work done in Australia (Choi ECC. Struct. Engr. 67 (1989) 421–437; Proc. Instn. Civil Engrs Structs. Bldgs. 94 (1992) 307–314). A lifetime maximum load, comprising a sustained load process and an extraordinary load event, has been evaluated and compared with previous predicted values and those given in codes of practice.

Vehicular Overloads: Load Model, Bridge Safety, and Permit Checking

All states in the United States issue special permits for nondivisible and/or divisible truck overloads exceeding the weight limit of the highway jurisdiction. This causes stress levels higher than those induced by normal truck traffic. The rationality of such overstress levels has not been documented. This paper addresses several aspects of this issue. It presents (1) a method to develop live load models including overload trucks; (2) associated reliability models for assessing structural safety of highway bridges; and (3) proposed permit-load factors for overload checking in the load and resistance factor format. It shows that the proposed overload checking procedure leads to relatively uniform reliability of bridge structures. A sensitivity analysis is also presented here to assure that possible variations of the input data used to prescribe the proposed load factors will not adversely affect bridge safety. The proposed procedure is intended to be used by engineers responsible for checking overload permits. It may be included in evaluation specifications for highway bridges.

Fatigue reliability of steel bridges

A fatigue reliability analysis is performed for steel girder bridges. The data base for the live load model is obtained by weigh-in-motion (WIM) measurements. Bridge structures were selected to determine the site-specific truck parameters and component-specific stress spectra. Stress cycles are presented as cumulative distribution functions. The WIM measurements confirm a significant variation in stress spectra between girders. Fatigue limit state function is developed using the number of cycles to failure as the structural resistance, and the number of applied cycles as the load. The statistical parameters of steel are based on the available test results. Reliability indices are calculated for several equivalent stress ranges.

Repair Optimization of Highway Bridges Using System Reliability Approach

As reliability based methods gain increased acceptance, there is greater opportunity to use scarce resources more efficiently while maintaining a prescribed level of reliability of a structure throughout its service life. The goal is to provide management decisions that will balance lifetime system reliability and expected life-cycle cost in an optimal manner. This study proposes a system reliability approach for optimizing the lifetime repair strategy for highway bridges. The approach is demonstrated using an existing Colorado State highway bridge. The bridge is modeled as a series-parallel combination of failure modes, and the reliability of the overall bridge system is computed using time-dependent deterioration models and live load models. Based on an established repair criterion, available repair options, repair costs, and updating, the optimum lifetime repair strategy is developed. The sensitivity of the optimum strategy to changes in various problem parameters including the prescribed service life,...

Calibration of LRFD Bridge Code

This paper reviews the code development procedures used for the new load and resistance factor design (LRFD) bridge code. The new code is based on a probability-based approach. Structural performance is measured in terms of the reliability (or probability of failure). Load and resistance factors are derived so that the reliability of bridges designed using the proposed provisions will be at the predefined target level. The paper describes the calibration procedure (calculation of load and resistance factors). A new live load model is proposed, which provides a consistent safety margin for a wide spectrum of spans. The dynamic load model takes into account the effect of road roughness, bridge dynamics, and vehicle dynamics. Statistical models of resistance (load-carrying capacity) are summarized for noncomposite steel, composite steel, reinforced concrete, and prestressed concrete. The reliability indices for bridges designed using the proposed code are compared with the reliability indices corresponding to the current specification. The proposed code provisions allow for a consistent design with a uniform level of reliability.

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.

柱軸力計算用積載荷重の確率モデルと層数低減係数

The variation of axial compression of a column caused by live load gets smaller than the variation of a column which supports only one floor as a number of floors supported increases. Therefore, the design live load for multiple story column can be reduced in proportion to the number of floor supported by the column. The purpose of this paper is to present a live load reduction factor for multiple story column. A stochastic live load model for multiple story column is presented using the live load survey results for office buildings. The design live load for multiple story column is discussed based on the stochastic live load model.

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).

Development of loading-truck model and live-load factor for the Canadian Standards Association CSA-S6 code

Studies have shown that the MS-200 loading model in the Canadian Standards Association standard CAN3-S6-M78 for design of highway bridges no longer represents modern-day heavy trucks in Canada. For the new edition of the CSA-S6 code, based on the limit states philosophy, a new loading-truck model was developed based on the Council of Ministers' loading, which is the legal load limit for interprovincial transportation in Canada. The loading model, designated as the "CS-W loading truck," provides the flexibility to adopt a multiple-level loading system appropriate to various jurisdictions.The live-load factor was determined from a statistical approach using data from a truck survey conducted across Canada in seven provinces. Responses in simple-span bridges were determined by running one or more trucks from the survey across the bridge. Based on this study, a live-load factor of 1.60 was determined and CS-600, with a gross weight of 600 kN, was selected as the standard load level. As well, the validity of the truck model and the live-load factors were checked for continuous-span bridges. Key words: highway bridges, design loads, codes and standards, live-load models, load factors, load surveys, vehicle weight regulations.

Simulation of load-duration effects in wood

The load-duration behavior of wood floor joists was examined using the Monte Carlo method. The simulations were performed using a cumulative damage model and a stochastic floor live load model. Floor joists designed to the existing American wood design code exhibited a decreasing failure rate. Although weak members tended to be the ones to fail, a majority of the weak members in a population were still surviving after 50 years. The total number of joists to fail in 50 years is very dependent on the distributional assumption made for short term strength.

Reliability-based design criteria for timber bridges in Ontario

The new Ontario Highway Bridge Design Code (OHBDC) is based on limit states theory and therefore uses a load and resistance factor format. This paper deals with the development of the basis for the timber bridge design provisions (OHBDC). Three structural systems are considered: sawn timber stringers, laminated nailed decks, and prestressed laminated decks. The latter system has been successfully used in Ontario for the last 7 years.The acceptance criterion in calculation of load and resistance factors is structural reliability. It is required that bridges designed using the new code must have a reliability equal to or greater than a preselected target value. Reliability is measured in terms of the reliability index. The safety analysis is performed for a structural system rather than for individual members. The live load model was developed on the basis of available truck survey data. Material properties are based on extensive in-grade test results. Numerical examples are included to demonstrate the presented approach. Key words: bridge deck, design code, prestressed timber, reliability, reliability index, stringers, structural safety, timber bridges.

Safety indices for wind loading in Australia

Two probabilistic wind load models for cyclonic and non-cyclonic regions of Australia are described. These models have been used in conjunction with dead and live load models to assess the safety indices (β) of structural members designed according to the present Australian standards. New load combination formulae have then been proposed for ultimate limit-states design. Compared with the rules in existing codes, these formulae lead to a more consistent level of safety in the practical range of load combinations. Finally, a change in the return period for nominal wind gust speed from 50 years to 1000 years for the ultimate limit state has been proposed and the reasons for the change are given.

A comparison of design loads for highway bridges

Load effects for four bridge design live load models are compared over a broad span range for three simple influence line shapes. The models are American Association of State Highway and Transportation Officials (AASHTO) HS20, Ontario Highway Bridge Design Code (OHBDC), Canadian National Standard CAN3-S6-M78 MS250, and one from the American Society of Civil Engineers (ASCE). Comparisons are made on a number of grounds including maximum effect, simplicity, and accuracy. The span range 30–300 m is considered.

Analysis of Live Loads in Office Buildings

An analysis of live loads in offices is presented which applies probabilistic live load models to the results of a recent survey of office buildings in the United States conducted by the National Bureau of Standards. The results are compared to a study of live loads in offices in the United Kingdom and the design loads currently specified by ANSI A58.1-1972. On the basis of this study, a new expression for computing allowable reductions in live loads is proposed.

Live Load Effect on Dynamic Response of Structures

For the aseismic design of structures, the dynamic forces are generally replaced by equivalent static forces which are usually given as a function of the effective weight of the structure. The effective weight of a structure consists of full dead weight and a fraction of live loads as these are invariably spring and dashpot mounted. This paper presents the effect of such a mounting on the response of structures subjected to horizontal ground motion caused by an earthquake. The primary system as well as live load system have been idealized as single degree of freedom systems. This study indicates that only a part of the live load is effective under horizontal vibrations and this depends on the ratio of live to dead load and the amount of damping present in the live load model. The effect of various other parameters, namely, the spring mounting, undamped natural period of vibration of the primary system and the ground motion has also been investigated.