International Concrete Abstracts Portal

This report summarizes available methods for calcu-lating deflections of reinforced concrete beams subjected to temperature change. Selection of design temperatures and temper-ature gradients is discussed as well as the effects of cracking on response in the service load range.

This report is in two distinct parts. Part I is a summary of published studies on slab deflections (3 from Australia, 1 from Scotland, 1 from Sweden, 2 from U.S.). The summary focuses on construction practices and materials quality. Comparison of deflections calculated by various methods with actual long-term deflections is made in some cases. Part II summarizes several construction problems and mate-rial deficiencies which can contribute to large long-term deflec-tions. Focusing on large construction loads, the authors show

The bending moments for any slab subjected to given loads are calculated by means of linear elastic methods. The required area of steel reinforcement can be calculated by using a method of ultimate strength design. Computations of slab deflections are carried out by modeling the moment-deflection relationship into a bilinear curve. This simplified approach considers the influence of reinforcement as well as the material properties of both concrete and steel. Deflections corresponding to the cracking and service loads are easily computed. Comparison with the ACI approach is also made. A computer program is written by the author and coded by FORTRAN 77 to carry out the numerical calculations. It is concluded that this method reflects the actual behavior of reinforced concrete slabs with respect to the estimated cracking and service load deflections.

The serviceability of structures such as floors and tall buidlings to dynamic loading is assessed in terms of absorbed power which is the rate of energy dissipation through a standard biomechanical model simulating the human user of the structure. Design formulae and curves were developed to assist the engineer in assessing structural serviceability of existing structures and that of structures at the design stageforperiodic and transient vibration. Abstract: Some structures vibrate perceptibly when subjected to service dynamic loads. The serviceability of such structures which include floors and tall buildings is dependent upon the imposed excitations and the characteristics of the structure such as frequency, stiffness and damping. However, structural design practice has been dominated by deflection requirements limiting the live load deflection or the span to depth ratio of the main girders. These restrictions represent essentially static criteria and thus are not adequate to provide for proper serviceability under dynamic loading. It is to be noted that the loads producing disagreable vibrations are usually different in type and intensity from the design live loads and are only a small proportion of such loads. The objective of this paper is to present 'objective' criteria based on the response of the human user, for evaluating the structural serviceability of floors and tall buildings relative to vibrations in the vertical and fore-and-aft modes. These criteria, developed to deal with periodic and transient vibrations, are expressed in terms of the human response and the major characteristics of the structure. Based on the above criteria, design curves in terms of stiffness, frequency, mass and damping were produced to assist the engineer in arriving at a design satisfying both the strength and serviceability requirements. Furthermore, the serviceability of forty floors and tall buildings was assessed using these criteria. The loading on the floors included impact loads, excitations due to human walking and high-heel impact and the forces resulting from other human activity such as dancing which may produce resonance. The results indicate excellent agreement with the reported sub-jective ratings.

A 120m tall building incorporating both in-situ and precast post-tensioned concrete components was instrumented to observe some aspects of its structural behaviour. The instrumentation was commenced more than ten years ago, it includes embedded and surface strain and temperature transducers, survey reference points, wind pressure tappings, and anemometers above the roof of the building to measure wind characteristics. The paper describes the strain and deformation instrumentation and method of data acquisition. The measured long-term vertical strains and deformations in one of the columns are presented and are compared with values predicted by two methods which are suitable for use by designers. Reference is made to the results of an extensive materials testing programme the results of which enabled the assessment of methods of prediction for the long-term creep and shrinkage properties of the concrete.