David V. Rosowsky

Time-dependent reliability of deteriorating reinforced concrete bridge decks

A structural deterioration reliability model is developed to calculate probabilities of structural failure (flexure) for a typical reinforced concrete continuous slab bridge. Corrosion may be initiated from the application of de-icing salts or atmospheric exposure in a marine environment. It is assumed that corrosion will lead to a reduction in the cross-sectional area of the reinforcing steel. Monte Carlo simulation is used to calculate probabilities of failure for annual increments over the lifetime of the structure (75 years). The application of de-icing salts or atmospheric marine exposure is shown to cause significant long-term deterioration and reduction in structural safety. Reducing the cover from that currently specified for design results in a significant increase in failure probabilities.

Long-term hurricane risk assessment and expected damage to residential structures

Abstract This paper presents results from a study to evaluate long-term hurricane risks in the Southeastern United States using event-based simulation procedures. These risks are defined by (1) the statistical extreme wind climate, and (2) the expected insured losses from damage to residential structures. A probabilistic hurricane event model developed by the authors is used to evaluate long-term risks. The event model parameters were derived from a statistical analysis of storms affecting the Southeastern United States and include radius of maximum winds, central pressure difference, landfall location, storm track, and decay rate. The 50-year mean recurrence interval (MRI) gradient-level and surface gust wind speeds are evaluated for the region investigated using results from the simulation analysis. When coupled with a damage model, also developed by the authors, the results from the event-based simulation analysis are used to provide estimates of the expected losses. The states of North Carolina, South Carolina, and Florida are used to demonstrate the applicability of this procedure for evaluating expected losses. Implications for setting design wind speeds as well as risk-consistent insurance rates are discussed.

Hurricane simulation techniques for the evaluation of wind-speeds and expected insurance losses

Abstract This paper describes the development of event-based hurricane simulation techniques for the evaluation of long-term risks in the Southeastern United States. Numerical simulation techniques were used to evaluate the 50-yr mean recurrence interval gradient wind-speeds for hurricane-prone regions in the study area. Using a damage model derived from actual insurance loss data and developed by the authors, the expected annual insurance losses were evaluated. The state of South Carolina was used as a case study to demonstrate the applicability of this system.

Reliability-based bridge assessment using risk-ranking decision analysis

Information about present and anticipated bridge reliabilities can be used in conjunction with decision models to provide a rational decision-making tool for the assessment of bridges and other structural systems. The present paper presents a broad overview of reliability-based assessment methods and will then focus on decision-making applications using updated time-dependent estimates of bridge reliabilities considering a risk-ranking decision analysis. A practical application of reliability-based safety assessment is illustrated herein which relates the effects of bridge age, current and future (increasing) traffic volume and loads, and deterioration on the reliability and safety of ageing RC bridges.

Load duration effects in wood members and connections: order statistics and critical loads

Abstract Load duration behavior, arising from creep-rupture, is one of the most significant effects distinguishing wood materials from other structural materials. The phenomenon of creep-rupture has been widely studied over the past two decades. Recent experimental programs have focused on duration-of-load (DOL) effects in full-size lumber and a number of different cumulative damage models have been proposed. These models have been used in reliability analyses that take into account the stochastic nature of the loading process to evaluate appropriate load duration adjustment factors for use in design. More recently, the stochastic damage accumulation process itself has been investigated. This has resulted in proposals for simplified cumulative damage analyses and the re-emergence of the ‘killer pulse’ concept for load duration effects in wood. Other recent studies have focused on evaluating analogous load–time effects in mechanical connections in wood. While the mechanisms in connections are recognized as different from those in wood members, the treatment of time effects in design are (at least at the present time) similar. With the tendency toward engineered design of wood structures subject to natural hazards loadings, such as wind and seismic loads, load duration effects in both the primary framing members and the structural connections may be of particular importance. The evolution of the new LRFD standard for wood provides a good opportunity to re-visit this important issue. This paper will briefly review DOL research, with particular emphasis on the work used as the basis for the time effects factors in the LRFD standard, and describe some recent work in (a) a simplified approach to cumulative damage analysis using order statistics, and (b) load–time effects in simple wood connections. It is shown that reliability analyses including DOL behavior can be performed more simply (efficiently) using an order statistics approach or even a simple FORM analysis. Thus, it may be possible to evaluate load duration factors for design without having to perform complex stochastic cumulative damage analyses.

Probabilistic analysis of time-dependent deflections of RC flexural members

Abstract This paper presents a procedure for the probabilistic analysis of time-dependent deflection of a reinforced concrete flexural member subject to a time-varying load process. Particular consideration is given to simply supported, singly reinforced rectangular beams. The algorithm takes into account concrete maturation, nonlinear creep and shrinkage behavior, and stochastic load process models for both construction and service. While this paper describes some practical implications of early-age effects (i.e., high early-age construction loads on immature concrete) for serviceability design, the emphasis of this paper is on the probabilistic time-dependent deflection analysis technique.

Estimation of design loads for reduced reference periods

Abstract Traditionally, loads specified for structural design correspond to maximum lifetime loads. These values are appropriate for lifetime (or “ultimate”) design in which the relative safe performance of the structural element or system is of concern. However, the use of these values for design for reduced reference periods (i.e., “shorter exposure periods”) may be excessively conservative. A number of design cases can be identified for which a 50-year reference period may not be appropriate, such as (1) buildings during construction, (2) temporary structures, and (3) serviceability. From a design standpoint, it may be most appropriate to provide modification factors to be used with existing (i.e., 50-year) design values. In this paper, one approach to determining appropriate design load values for design situations other than the full design-life is presented. This may be especially useful for the design of structures exposed to environmental loadings such as wind, snow and earthquake for relatively short exposure periods.

Time-dependent service-load behavior of wood floors: analytical model

Abstract A preceding companion paper [1] examined the experimental time-dependent service-load response of wood joist floor systems, including an assessment of time-dependent deflections and load distribution. The results of the experimental program are used herein to develop an analytical model for the time-dependent response of wood joist floors. Time-dependent component (i.e. joist, sheathing and connection) models are developed from component tests. These component models are then integrated into a system model. The focus of this paper is to present the theoretical basis for the time-dependent system model, and illustrate its predictive capabilities. The system model approximates the time-dependent system creep of parallel-member wood systems to an acceptable degree of accuracy. Moreover, the model requires relatively little computer time, thus making it suitable for use in a Monte Carlo simulation

EARLY-AGE LOADING AND LONG-TERM DEFLECTIONS OF REINFORCED CONCRETE BEAMS

This paper examines the effects of early-age (construction) loads on the long-term deflections of reinforced concrete (RC) flexural members. Particular consideration is given to simply supported, singly reinforced rectangular beams. The probabilistic analysis of time-dependent deflections takes into account concrete maturation, nonlinear creep and shrinkage behavior, and load process models for both service and construction. While this paper summarizes the time-dependent analysis technique (reported in detail elsewhere), the focus of this paper is on evaluating the effects of early-age loading on the long-term deflection and on practical implications for deflection serviceability design of RC members.

Load monitoring and hazard warning systems for buildings under construction

The construction phase of a building is one in which the structure is rather vulnerable to damage and/or collapse. The potential for human and economic loss, as well as the potential for reduced construction costs, presents an opportunity for developing construction structural load and response monitoring systems that can possibly be effective in reducing the frequency of collapses and/or damage. This paper presents the results of a study in which such systems were developed and applied to slab-style concrete building construction. The shoring systems that are used to support fresh concrete floor slabs are instrumented with strain gage based load cells to measure the loads during construction operations. The loads on the shores have been measured during the construction of three different buildings. This information is being used for construction procedure code development. The next phase of this research is to apply the sensing system so that it can provide an early warning for potential collapses of the partially- built structure. Mechanical models of the structural system indicate that load monitoring of individual shoring members is probably insufficient because the major structural collapses are due to a global instability. It appears that a combination of strategically-placed load, temperature and deflection monitoring instrumentation combined with a real-time analysis of the data may be necessary. The design and use of such systems are discussed.