Probabilistic Load Model Research Articles

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.

Combining Snow and Earthquake Loads for Limit States Design

ASCE Standard 7, “Minimum Design Loads for Buildings and Other Structures,” and the “NEHRP/BSSC Recommended Provisions for Seismic Regulations for Buildings” both contain load requirements for earthquake-resistant structural design. Many of the NEHRP provisions are being implemented in building codes and are the basis for the seismic load provisions in Chapter 9 of ASCE 7. However, the treatment of combinations of earthquake and snow loads differs in the ASCE 7 and NEHRP documents. Concerns regarding these differences were raised during the balloting of both the ASCE 7-93 and ASCE 7-95 provisions. To address these concerns, the load combination requirements involving dead, snow and earthquake load in these two resource documents were examined using modern probabilistic load models and load combination techniques. It is found that approximately 20% of the nominal snow load should be considered in combining snow and earthquake loads.

Simplified load combination factor for snow load

The load combination factor for snow load is formulated by a simplified development based on Turkstra's rule for load combination and the Gumbel distribution for the snow load model. The probability of exceedance expression is compared with an existing probabilistic load model. The load combination factor is expressed in terms of the period of snow duration and the ratio between the standard deviation of the principal load effect and the accompanying snow load effect. Numerical examples are presented and discussed with the load combination factor used in Japanese current practice.

Transmission loss evaluation based on probabilistic power flow

A simulation method for composite power systems is proposed for the purpose of evaluating the probability distribution function of transmission losses. The method accounts for the uncertainty of the electric load, availability of generating units, nonlinearities in the power flow equations, and major operating practices. The method is based on the following procedure. First, given the probabilistic electric load model, the probability distribution function of the power injection at generation buses is computed by taking into consideration the availability of generating units and economic dispatch practices. Next, transmission losses are expressed as a piecewise linear function of power injections at generation buses. Subsequently, the probability distribution function of transmission losses is computed. Validation of the method was performed via a Monte Carlo simulation. The method was applied to the 24-bus IEEE reliability test system, and the results are validated by comparing it to Monte Carlo simulation results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Rational methods for fatigue design and inspection planning of offshore structures

This paper reviews the current usage of probabilistic fracture mechanics in fatigue assessment of offshore structures and summarizes practical usage of probabilistic load and response analysis models used in conjunction with fracture mechanics models. These models are then used in conjunction with probabilistic models for inspection methods used in the offshore and marine industries to assess the probability of failure, derive optimum inspection plans and derive optimum design considering cost of design, cost of inspection, cost of repair and consequential costs of failure. Comments are made on the status of such reliability based optimization techniques in design and on the need for further work. Some examples of use are demonstrated.

Safety assessment of urban expressway bridges based on probabilistic modeling of multiple load environments

The first part of this paper describes the results of a field measurement and detailed investigations on actual load environments of urban expressway bridges. Based on these results their probabilistic load models are developed. The second part presents the results of critical appraisal of the safety of structures designed under the current Japanese code. The difference in safety margins under various actual loads among typical structural components are studied in detail, and on this basis load factors that distribute the safety uniformly to respective structural members of a bridge subjected to combined loads are quantitatively evaluated.

A new probabilistic power flow analysis method

A method for the simulation of the composite power system is proposed for the purpose of evaluating the probability distribution function of circuit flows and bus voltage magnitudes. The method consists of two steps. First, given the probabilistic electric load model, the probability distribution function of the total generation of generation buses is computed. Second, circuit flows and bus voltage magnitudes are expressed as linear combinations of power injections at generation buses. This relationship allows the computation of the distribution functions of circuit flows and bus voltage magnitudes. The method incorporates major operating practices such as economic dispatch and nonlinearities resulting from the power flow equations. Validation of the method is performed via Monte Carlo simulation. Typical results are presented, showing that the proposed method matches the results obtained with the Monte Carlo simulations very well. Potential applications of the proposed method are: composite power system reliability analysis and transmission loss evaluation. >

Effect of a Photovoltaic Power Generation System on Power Plant Mix

In this paper, by using real data of electricity load and output of Photovoltaic Power Generation system(PV system), the effect of PV system on power plant mix is evaluated mainly from the viewpoints of power reduction and capacity replacement in the conventional power generation system. For the purpose of this evaluation first, we examine statistically periodic variation and random variation in both the output of PV system and the load, including relationship between them. Second, referring to the result of this statistical analysis, a probabilistic load model is built in which the output of PV system and the electricity load are treated as mutually correlated random variables. Due to this probabilistic model, a load-duration curve(LDC) is derived for the power system equipped with PV system Finally, the result obtained from an optimal power plant mix with this LDC is compared with that obtained from the LDC of power system without PV system. The study is earned out by using the real data of electricity load, solar radiation and daily average temperature in the Kansai region, Japan.

Operational Cost Savings of Direct Load Control

Direct load control -- load which is under the control of the system dispatcher, such as could be the case under say demand subscription service -- can be used to reduce system spinning reserve. Such a reduction may constitute a significant additional saving in system variable cost. The objective of this paper is to evaluate the viability of this philosophy. A quasi-static system simulation algorithm is employed to compute the potential variable cost saving for a sample system consisting of thermal generation and direct load control. This algorithm considers probabilistic load models, outage probabilities and reliability economics.

Operational Cost Savings of Direct Load Control

Direct load control -- load which is under the control of the system dispatcher, such as could be the case under say demand subscription service -- can be used to reduce system spinning reserve. Such a reduction may constitute a significant additional saving in system variable cost. The objective of this paper is to evaluate the viability of this philosophy. A quasi-static system simulation algorithm is employed to compute the potential variable cost saving for a sample system consisting of thermal generation and direct load control. This algorithm considers probabilistic load models, outage probabilities and reliability economics.

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.

On Combinations of Random Loads

Structures are subject to changing loads from various sources. In many instances these loads fluctuate in time in an apparently random fashion. Certain loads vary rather slowly (called constant loads); other loads occur more nearly as impulses (shock loads). Suppose that the stress put on the structure by various loads acting simultaneously can be expressed as a linear combination of the load magnitudes. In this paper certain simple but somewhat realistic probabilistic load models are given and the resulting probabilistic model of the total stress on the structure caused by the loads is considered. The distribution of the first time until the stress on the structure exceeds a given level x, and the distribution of the maximum stress put on the structure during the time interval $( 0,t]$ are studied. Asymptotic properties are also given. It is shown that the asymptotic properties of the maximum stress are related to those of the maxima of a sequence of dependent random variables. Classical extreme value ty...

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.