Andrzej S. Nowak

Currently in the United States, there are more than 623,000 bridges of which 6.8% are in poor condition. Over 100 million trips are taken across these structurally deficient bridges every day and the bridge-related system rehabilitation need is estimated at $191 billion. In 1990’s, calibration of AASHTO LRFD Bridge Design Code involved extrapolation of distributions to evaluate the mean maximum 75-year live loads. However, this derivation was based on a survey of Ontario trucks with small sample size. In the meantime, the Weigh-in-Motion technology improved, and millions of vehicles are measured in various locations on a continuous basis. The objective of this study is to examine site-specific live load spectra based on WIM database in Alabama. Massive volume of data collected by the Department of Transportation is used to compute moments and shears for spans ranging from 30 ft (9 m) to 200 ft (60 m). Cumulative distribution functions of bias ratios (WIM Truck/HL-93 Loading) were plotted on normal probability paper and extrapolated. The findings show that AASHTO provisions for Strength I Limit State are not representative of vehicles at WIM sites located on interstate highways. The findings confirm the importance of continuous traffic data collection and use of reliability analysis procedures.

W artykule przedstawiono wyniki badań dotyczących właściwości kompozytów geopolimerowych zawierających kruszywo pochodzące z recyklingu betonu cementowego. Analizowano wpływ zawartości kruszywa wtórnego na skurcz, wytrzymałość na ściskanie oraz nasiąkliwość materiału. Wyniki wykazały, że wzrost udziału kruszywa z recyklingu prowadzi do zwiększenia skurczu i nasiąkliwości oraz obniżenia wytrzymałości mechanicznej kompozytu. Mimo to zastosowanie kruszyw wtórnych wpisuje się w założenia gospodarki o obiegu zamkniętym. Wskazano również na potrzebę modyfikacji składu mieszanki geopolimerowej w celu poprawy parametrów fizyczno-mechanicznych kompozytów.

Excessive weight and a growing number of permit vehicles can cause an accelerated bridge damage. Therefore, there is a need to better control operation of heavy permit vehicles. This paper presents an approach to assess bridge life consumption by permit loads. An economic lifetime of a bridge is considered in terms of the number of load cycles to failure so that the structure has to be replaced. Each passage of a truck causes one or more load cycles and thus reduces or consumes a part of the total remaining fatigue life. An incremental consumption equation is derived, and it allows to convert fatigue damage to dollar damage by considering the bridge construction cost, traffic volume, and bridge parameters. The incremental consumption covers only weight exceeding the legal limits. This consumption approach is presented for representative average values and the individual consumption approach with the GIS system using Florida as an example. A developed approach can help state agencies to establish a rational and fair permit fee structure. The methodology is transparent and can be used by any state utilizing the available databases.

This study aims to develop Load and Resistance Factor Design (LRFD) provisions for movable bridges’ mechanical components. It requires the formulation of the limit state functions, determination of the statistical parameters of loads and resistances, development of the reliability analysis method, selection of target reliability indices, and calibration of load and resistance factors. The service and overload limit states for electric and hydraulic motors were considered. Movable bridge owners and operators were surveyed to gather information about the bridge’s condition. The survey results helped to define the limit state functions and the acceptability limits. The target reliability index was established based on movable bridge performance determined by the bridge owner survey. It was concluded that the target reliability index is the same as is used in current practice. Finally, load and resistance factors were calculated and verified so that designed movable bridges’ mechanical components meet minimum safety requirements. The project results will be submitted for consideration as updates to the AASHTO LRFD Movable Highway Bridge Design Specification.

To ensure bridge safety under vehicular traffic, the owners must systematically verify the adequacy of bridge live load-carrying capacity. Bridges are evaluated for design, legal, and permit loading. The legal load rating uses AASHTO legal trucks, which meet the Federal Bridge Formula requirements. However, each state has a federal law exemption for truck weight and size. The state-specific grandfather provisions allow the operation of legally overloaded vehicles. Thus, the objective of this study is to determine whether the current live load factors provide adequate safety for exempted vehicles because there are no provisions for overloaded vehicles under grandfather provisions. This study presents a reliability-based analysis to determine state-specific live load factor for bridge rating on an example of Montana’s 20% overload allowance for agricultural vehicles. This approach does not require a reanalysis of the state bridge inventory. Linear scaling of existing rating results from bridge records can be used to determine ratings for agricultural vehicles. The study provides a calibration method to maintain the required minimum safety level with the minimum cost and effort.

Roads and bridges are designed to meet the transportation demands for traffic volume and loading. Knowledge of the actual traffic is needed for a rational management of highway infrastructure. There are various procedures and equipment for measuring truck weight, including static and in weigh-in-motion techniques. This paper aims to compare four systems: portable scale, stationary truck weigh station, pavement weigh-in-motion system (WIM), and bridge weigh-in-motion system (B-WIM). The first two are reliable, but they have limitations as they can measure only a small fraction of the highway traffic. Weigh-in-motion (WIM) measurements allow for a continuous recording of vehicles. The presented study database was obtained at a location that allowed for recording the same traffic using all four measurement systems. For individual vehicles captured on a portable scale, the results were directly compared with the three other systems’ measurements. The conclusion is that all four systems produce the results that are within the required and expected accuracy. The recommendation for an application depends on other constraints such as continuous measurement, installation and operation costs, and traffic obstruction.

Twenty-five years ago, a bridge live-load model was developed along with the calibration of the AASHTO LRFD Specifications. The statistical parameters of truck loads were based on the Ontario, Canada, truck survey performed in the 1970s. The objective of this paper is to review the available weigh-in-motion (WIM) data and update the live-load model accordingly. The considered WIM database includes over 200 million records, from 44 WIM locations around the USA, with about 1–2 million vehicles per year for each location. The actual number of records depends on traffic volume expressed in terms of an average annual daily truck traffic (AADTT) and local conditions, such as type of road, local facilities, seasonal factors. The cumulative distribution functions of load effect are extrapolated to predict the maximum expected live load for extended time periods. The statistical parameters of live load are determined for the static component of the live-load effect for one lane in simple-span bridges based on the actual WIM data. The analysis showed that there is a 15%–20% increase in live load in comparison to Ontario data. An updated design tandem is proposed to adequately represent the traffic load for short simply supported spans.

There has been a considerable progress in the reliability-based code development procedures. The load and resistance factors in the AASHTO bridge design code were determined using the statistical parameters from the 1970's and early 1980’s. Load and resistance factors were determined by first fixing the load factors and then calculating resistance factors. Load factors were selected so that the factored load corresponds to two standard deviations from the mean value and the resistance factors were calculated so that the reliability index is close to the target value. However, from the theoretical point of view, the load and resistance factors are to be determined as coordinates of the so-called “design point” that corresponds to less than two standard deviations from the mean. Therefore, the optimum load and resistance factors are about 10% lower than what is in the AASHTO LRFD Code. The objective of this paper is to revisit the original calibration and recalculate the load and resistance factors as coordinates of the “design point” for prestressed concrete girder bridges. The recommended new load and resistance factors provide a consistent reliability and a rational safety margin. Odnotowano znaczący postęp w procedurach tworzenia norm opartych na niezawodności. Częściowe współczynniki bezpieczeństwa od obciążenia i nośności do projektowania mostów według AASHTO zostały określone przy użyciu parametrów statystycznych z lat 70-tych i wczesnych 80-tych. Początkowo ustalono współczynniki obciążenia, a następnie obliczono współczynniki nośności. Współczynniki obciążenia dobrano tak, aby obciążenie odpowiadało dwóm odchyleniom standardowym od wartości średniej, a współczynniki nośności obliczono tak, aby wskaźnik niezawodności był zbliżony do wartości docelowej (optymalnej). Jednakże, z teoretycznego punktu widzenia współczynniki obciążenia i nośności należy określić jako współrzędne tzw. „Punktu projektowego”, które mogą odpowiadać mniej niż dwóm odchyleniom standardowym od wartości średniej. W związku z tym, optymalne współczynniki obciążenia i nośności są o około 10% niższe niż te podane w normie AASHTO LRFD. Celem tego artykułu jest ponowna analiza kalibracji i obliczenie współczynników obciążenia i nośności jako współrzędnych „Punktu projektowego”, dla sprężonych mostów belkowych. Zalecane nowe współczynniki obciążenia i nośności zapewniają stałą niezawodność i racjonalny margines bezpieczeństwa.

Stability is one of the most important problems of soil mechanics related to the condition of excessive settlements. A reliability-based approach can be applied for designing shallow foundations for which their design needs knowledge about the distribution of settlement under a given footing is considered. The performance of geotechnical structures, including shallow foundations depends mostly on loads and soil properties that are the random variables described by statistical parameters. The objective of this study is to develop the methodology for risk analysis procedures of shallow foundations. This required the development of the reliability models that can be applied to shallow foundations and involved the establishment of serviceability limit state functions. Reliability is expressed in terms of reliability indices, which are calculated for total settlement of shallow bridge foundations. The most important part of this study is the development of statistical models for total settlements of shallow foundations on cohesionless soils by Monte Carlo simulations technique. The study verifies the most accurate settlement prediction methods for calculating immediate settlement of spread footings for each site by using compiled field data from research projects based on vertical settlements measured on 23 sites of footings for ten bridges in Northeast U.S. and two bridges in Ohio. Reliability indices are determined for settlement of the spread footings assigned in this study. CE Database subject headings: Reliability analysis; Shallow foundations; Immediate settlement; Cohesionless soils; Settlement prediction methods; Statistical parameters; Reliability index.