Leslie G. Jaeger

Lower fractiles of material strength from limited test data

Abstract The determination of lower fractiles of material strength from a limited number of test results is usually based upon the incorrect assumption that the strengths are distributed normally in which case even the negative values of the strength are taken into consideration. A numerical procedure is presented to overcome this shortcoming of the current practice and also to give fairly stable estimates of the lower fractiles of the strength. These estimates depend upon the sample size and the smallest, middle and greatest values of the test data.

Utilization of Service Loads in Bridge Evaluation

A design or evaluation procedure has to make conservative allowances for the possibility, that a structural component may have strength that is well below the nominal. A proof test on a component, by reducing the uncertainty regarding the existence of very low strengths, can be helpful in eliminating some of these conservative allowances and thus making available some extra nominal strength for applied loads. A technique is presented by which proof loads can be used with advantage to upgrade the load carrying capacities of existing bridges. The in-service performance of a bridge is itself a kind of proof testing. A probabilistic approach is presented using which the maximum in-service proof loads can be estimated. Once the in-service proof loads are established, they can be incorporated into the analytical design procedure in the same way as are the proof loads of formal tests. The techniques presented are general in nature. However, their application requires some charts and numbers which depend not only upon local code provisions, but also upon the variability of the strengths of the components and of their load effects.

A least squares method for determination of lower fractiles from material strength data

Abstract This paper is a sequel to our article ‘Lower fractiles of material strength from limited test data’, Structural Safety, 7 (1) (1990) 67–75. This article provided an estimate of the lower fractile of material strength from limited data based upon the sample size and the smallest, middle and largest values of the test data. Unlike its predecessor, the method given in this paper makes use of the values of all the test data, and gives very stable results which are not affected significantly by even fairly large changes in individual test values.

Assessment of bridges: use of dynamic testing: Discussion

then the change in dynamic response may give valuable information about the change in load distribution effectiveness, as distinct from overall load-carrying capacity. The author has presented useful data regarding the dynamic behaviour of a bridge before and after repairs. We are in some doubt as to what is being asserted in the paper. It does not seem to suggest explicitly that dynamic testing can be used to evaluate directly the load-carrying capacity of a bridge; however, in order for the intent of the paper to be stated more precisely we believe that two familiar terms used in the title, abstract, and

The Earthquake-Resistant Design of Critically-Important Concrete Structures

In the design of concrete bridges and buildings it is becoming increasingly common in countries around the world to use limit states design methods. These methods are based upon probability and statistics, and a very important part of such design methods is the concept of the safety index. In most design codes and specifications the intensity of the earthquake that must be included as one of the design loads is specified to be that intensity which has a stated probability of exceedance at the site concerned during the design life of the structure.

EFFECT OF COMPUTERS ON ECONOMY OF BRIDGE DESIGN

The paper examines highway bridge costs in relation to the costs of an overall highway system, divides the bridge costs themselves into various categories, and examines the feasibility of reducing the costs in these various categories by adopting computer-based methods of design. The conclusion is drawn that appreciable savings are available in the cost of materials in construction, and these savings greatly outweigh any possible increase in design costs needed to bring them about. The paper also considers the effect of computer-based design on safety margins in the resulting bridges, and examines briefly the roles of the engineering profession and the drafters of code specifications in influencing designers towards adopting computer-based methods. /Author/