International Concrete Abstracts Portal

In 1988, the collapse of a supermarket roof in Burnaby, Canada, highlighted the need to distinguish between the specified strength and the actual strength of a material when evaluating existing structures. It is unsafe to calculate resistances using the actual strength instead of the specified strength in conven-tional resistance equations with customary resistance factors. This is the rationale behind current criteria for the evaluation of existing bridges in Canada, that require in-place strength data to be converted to equivalent specified strengths. In this paper, the statistical relationship between the specified strength and the actual in-place strength of concrete is examined. A method is summarized which enables concrete core compression strength data to be converted to an equivalent specified strength for structural assessment purposes. The method is illustrated by a case study involving 1200 40-year-old precast stringer bridges in Alberta, Canada.

The CSA Standard A23.3-94 permits a moment magnifier approach for design of slender composite steel-concrete columns. This approach is strongly influenced by the effective flexural stiffness (EI) of the columns. However, the EI expressions given in CSA A23.3-94 are quite approximate when compared with the values of EI derived from axial load-moment-curvature relationships. This study was undertaken to determine the influence of different variables on EI of slender tied rectangular composite columns in which steel sections are encased in concrete. The columns studied were subjected to uniaxial bending in symmetrical single curvature under short-term loads. Approximately, 8,000 simulated columns, half of which subjected to bending moment about the major axis and the remaining half subjected to bending moment about the minor axis of the steel section, were used to generate the stiffness data. The EI expressions were then developed for use in slender composite column designs. A design equation is proposed in this paper.

Leadership, combined with innovation and integrity, are qualities inherent in James G. MacGregor’s personal and professional activities. These attributes led a group known as the “Friends of Jim” to prepare and hold a national symposium of the American Concrete Institute in Seattle to honor Jim MacGregor. The symposium offers papers on diagonal tension, reliability, columns, and engineering procedures. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP198

The evolution of strut and tie models is reviewed and their characteristics are discussed. From their evolution it is seen that more design effort should be expended on developing a reasonable truss than on refined calculations of nodal stresses and permissible concrete stresses. Strut and tie models are design tools. They allow engineers to put an appropriate amount of material in an appropriate place, and expeditiously demonstrate that there is at least one way for the structure to safely carry the design load. They are not very useful for checking designs. When one is planning the structural analysis, one needs to know if the design will be done on the basis of strut and tie models. Different models may be required for different load cases. Rather than modeling the whole structure, models may be applied to specific regions of a structure. Designs based on strut and tie models may require slightly more reinforcement than designs based on other methods since, in general, the contribution of concrete tensile strength is ignored. Finely tuned empirical equations will use less reinforcement but are restricted by the limits of the test data. Strut and tie models are general rational design tools that do not have such limitations.

Professor Jim MacGregor began his doctoral studies at the University of Illinois just as his research advisor had become concerned with the safety provisions of the ACI building code. As MacGregor began his teaching and research career at the University of Alberta, developments in the reliability area became suitable for structural applications, especially in providing a basis for load and resistance factors. MacGregor was able to combine his exceptional talents in reinforced concrete behavior and design with the reliability-based code developments. This combination of expertise in reinforced concrete with an interest in safety requirements of codes has resulted in significant advances in the safe and economical design of concrete structures.