Practice Periodical on Structural Design and Construction

Construction learning curves frequently appear in contractor claims and sometimes are the basis for calculating some portion of monetary damages. Therefore, it is incumbent on all parties to understand the proper use of learning curves and the implication of their use. Learning curves originated in the aircraft manufacturing industry and have been applied in the construction industry without much thought as to the factors that lead to true learning and whether any learning actually took place. Based on the writer’s experience, it is suggested that three things need to be present for learning to take place: (1) there should be something sufficiently complex about the operation to facilitate learning; (2) there should be repetition in the units being constructed; and (3) management must create a stable work environment. It is suggested that in most instances, the underlying factors are not present and that true learning does not occur. Most of the time, the appearance of learning is caused by disruptions that occur early in the construction timeframe. Most purported learning curves are plotted using cumulative data rather than unit productivity data. It is shown using an actual project, how cumulative data gives a distorted view of performance. It is suggested that in some instances, a learning curve is a sign of a poorly managed project. As many as 14 learning curves from actual projects, most where there were no contractor claims, are included to illustrate the points made in the paper.

The growth experienced by the city of Pamplona in the last $20\text{years}$ has resulted in the Arga River, which has historically bordered the city, being transformed into a riverbed that is fully embedded into the urban weave. However, successive urban development plans have preserved important areas of a rural nature surrounding the river and kept them free of construction. The connection of a purely urban realm with natural environments where landscape and ecosystems are to be preserved and strengthened requires the design of specific structures following mainly aesthetic and even historic and social criteria. The project of a pedestrian footbridge described in this paper has sought a design that minimizes its effects on the riverbed and the immediate environment and that also constitutes a reference point in a newly developed area that lacks landmarks to characterize it.

Contractors must often work in unfavorable weather. Two important questions arise. How much labor inefficiency will be incurred (which leads to the question of how much should the estimate be increased) and how can the negative impacts be mitigated? This paper addresses these two questions. The literature is reviewed and data from multiple projects are examined to estimate the average loss of efficiency resulting from very hot and very cold temperatures and from rain and snow. The average loss of efficiency estimates are used in a simple example to illustrate how to decide how much to increase the workhour estimate to account for adverse weather. Fundamental principles are stated so contractors can mitigate the effects of unfavorable weather conditions. It is postulated that if the principles are followed, labor inefficiencies resulting from weather-related disruptions are less likely to occur.

This paper deals with the effects of the National Electrical Safety Code (NESC)-2007 extreme ice with concurrent wind loading conditions, a new loading condition for the design of electric utility structures, hardware, and conductors. This paper provides answers to some of the questions that may be in the minds of utility engineers involved in the design of transmission line structures by comparing the effects of the new extreme ice loading condition with those of the existing NESC weather-related loading conditions. This study evaluates whether structures designed on the basis of the NESC district loading conditions or extreme wind loading conditions could sustain loads resulting from the new extreme ice loading condition and identifies the regions of the United States where the extreme ice loading conditions would not likely control the design of transmission line structures. The new NESC extreme ice loading condition is also compared with the ASCE Std7-05 ice and wind loadings for other structures.

A new strength theory for the design of concrete members subjected to triaxial compression stress has been developed which is an extension of the traditional Mohr’s strength theory. This new theory explicitly relates the maximum principal stress, ${\sigma }_1$ , to the confining pressure, ${\sigma }_3$ . It is shown that this new theory requires information about the uniaxial compressive strength, $f_c^{\prime }$ , the uniaxial tensile strength, $f_t^{\prime }$ , and a quantity $n$ relating to the shape of Mohr’s envelope of ultimate strength states which depends on the degree of confinement. Because few test results that include the values of $f_t^{\prime }$ have been reported in the engineering literature, a preliminary and simplified version of this new theory has been proposed and verified using results from 236 tests of normal strength concrete $(f_c⩽\mathrm{8,000}\mathrm{psi})$ specimens and 50 tests of high strength concrete $(f_c>\mathrm{8,000}\mathrm{psi})$ specimens previously reported in the engineering literature. Agreement between values predicted by means of the proposed theory and test results has been found to be satisfactory.