International Concrete Abstracts Portal

Presents a simple design aid for predicting long-term (up to 50 years) movements in reinforced concrete columns and bridge beams made of normal and lightweight aggregate concrete. The method is based on the principle of superposition using a creep factor chart, which takes into account varying sizes of members, age at loading, exposure conditions, and the percentage of reinforcement, and it requires only a knowledge of the concrete strength and the loading history of the member. The method is developed from the study of in situ movements in two reinforced concrete structures subjected to increment loading. The shrinkage strains in columns are predicted using a shrinkage chart, which requires only a knowledge of elastic modulus of concrete at 28 days. The predicted load-induced and basic strains show excellent agreement with measured strains in the two structures, and the method shows good agreement with literature. The paper demonstrates how the simple method of predicting long-term movements in buildings and bridges can be utilized by the structural engineer as a designer's tool.

In seismic zones, severe earthquakes occur within a certain period. However, the important functions of a concrete structure must be maintained after the earthquake. Hence, structures must be designed for safety during the earthquake and serviceability after the earthquake. The acceptable level of damage can be varied in accordance with the type and importance of the structure. When a reinforced concrete structure suffers significant plastic deformation, residual deformation and large crack opening in the structure are impaired. A new and rational seismic design method was proposed. The peculiarity of the concept of the design method is as follows: Seismic design should be performed to fulfill required serviceability after the design earthquake as well as required safety during the earthquake. High magnification factor due to dynamic response was introduced according to actual observation in the earthquakes. Reduction factor referred to the acceptable level of damages in the structure after the earthquake was introduced. The importance of design details was emphasized. Furthermore, the influence of axial compressive force on the ductility was pointed out.

SP-117 If you are a structural engineer or a contractor, this important new volume will enhance your understanding of the most up-to-date data and techniques for assessing the long-term serviceability of new and existing concrete structures.

Differential elastic, creep, shrinkage, and thermal deformations of vertical concrete elements, columns, and walls in tall building structures require special attention to insure proper behavior for both strength and serviceability of the structure and the attached nonstructural elements. The long-term serviceability problems include out-of-level floors in both concrete and composite buildings, and cracking and deformations of internal partitions and external cladding elements. A procedure is developed to predict the long-term deformations of reinforced concrete columns, walls, and composite columns. The procedure incorporates the effects of concrete properties, construction sequence, and loading history. For composite columns, the effects of load transfer from the steel erection column to the reinforced concrete column are also included. Methods to minimize differential shortening of columns and walls are discussed. The methods involve corrections during both design and construction phases. Differential shortening effects for three tall buildings, in Chicago, which were designed using the procedure, are discussed. Results from six years of field measurements of column shortening are compared with predicted values.

Long-term behavior of in situ reinforced concrete columns in two structures during construction, occupancy, and service is reported. Extensive strain and moisture movements were made up to about 10 years of service, and long-term movements at 25 to 30 years were then estimated. The stress history experienced by the columns in the second structure was monitored through a stress meter embedded in one of the columns, and the influence of reinforcement and the time-dependent movements on the stress history is described. The gradual but significant stress redistribution with time and the resulting concrete-steel load transfer is also discussed. Results show that the time-dependent deformation in in situ columns occurred over a very long period of time and continue to occur at a very small rate. However, the majority of movements in the columns occurred during the first 5-year period after construction of the columns. It is shown that dead load appears to be the predominant load carried by the columns. The design steel stress has been exceeded in several columns within 10 years of service life; however, none of the steel is expected to yield in 50 years of service life. Based on the in situ performance of columns along with other available data, a design recommendation is made to incorporate the effects of load transfer from concrete to steel at the design stage.