International Concrete Abstracts Portal

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.

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.

A difference in temperatures exist among the various components of a flanged concrete structure which are exposed to different ambient temperatures. An experimental study indicates that a substantial temperature differential between the top flange and the bottom portion of a prestressed concrete box girder bridge superstructure can exist due to the daily and seasonal variations of the surrounding atmospheric temperatures. This paper presents methods which may be used for the computation of deformations and stresses in concrete structures due to temperature differentials. Analyses of a T-beam and of a box girder bridge superstructure are also presented as illustrative examples. These examples indicate a need of revising the design codes to emphasize the effects of the temperature differential in flanged concrete structures.

Economical design of large span slabs which are required to support partitions can be controlled by deflections and other serviceability considerations rather than strength. This design is further complicated by considerations of variations in strength of concrete, placement of steel, etc. even for constant dead and live loads (short term, long term). Even in the deterministic case when the-parameters of long term live load, short term live load, fc, w,fy, maximum deflection limitations (due to loading, creep and shrinkage) are known, the solution for finding effective thickness and depth is found by trial and error amenable for iterative computerized approaches. This problem is especially severe when the partitions are installed while the slab is still shored. A probabilistic approach with variation in concrete strengths for given coefficient of variation to deflection controlled one way R. C. slab is developed with numerical examples and degenerate case of the-deterministic specified strength f'c. Comparison is made between the probabilistic design for long term deflection control and the professional practice oriented usual design procedure for numerical cases. This could be applied to both structural lightweight and normal weight concrete. Extension to the more generalized case of variability of loads, variability of methods of assessing creep and shrinkage deflections, etc. are also explored.

The new draft Australian Concrete code contains sub-stantial revisions to the requirements for deflection and cracking of beams and slabs. The author was closely involved in the prepar-ation of this section of the draft code. The principle changes are explained in this paper and include: Load factors are specified for service loads. The adoption of the ACI (Branson) effective moment of inertia for deflection calculations for both reinforced and partially prestressed beams. The replacement of the allowable span-to-depth tables with revised formula consistent with the ACI formula. The method for slab deflection is related to equivalent beams. More stringent requirements are given for cracking in the secondary direction in slabs. Flexural cracking requirements are specified by bar spacing rules or limits on tensile stresses or bar stress increments.