International Concrete Abstracts Portal

The time-dependent behaviour of concrete structures subjected to thermal gradients and, in addition, to mechanical loading is considered. Redistribution of stress with time results from two kinds of nonhomogeneity; firstly, there is a stress transfer from concrete to steel and secondly, the spatial variation of creep parameters (resulting from the temperature variation) causes stress redistribution. The constitutive laws for concrete creep are discussed in the light of recent experimental findings and a simple constitutive equation is found to account for the major features with sufficient accuracy in the analysis of static temperature fields. Because of the form of the equation, viscoelastic methods can be used; based on this fact, two approximate methods are proposed. Both use effective modulus calculations which must incorporate a spatially varying elastic modulus to account for the spatially varying creep properties; this is easy to achieve through the use of the finite element method. Two examples are given; the first is a reinforced cylinder subjected to internal pressure and a temperature gradient and the second is concerned with prestressed beams subjected to external loads and a temperature gradient. A comparison of the approximate methods with step-by-step calculations is given and good agreement is demonstrated.

An 859-ft building was instrument deformations of column of individual member displacements were measured (262 m) high reinforced concrete ted to m easure the time-dependents and walls. Vertical shortening S and their relative vertical sured at selected floor levels. Laboratory tests were the field. The elastic modulus, coefficients and shrinkage with age each different concrete laboratory tests and vertical shortening of values were compared performed on concrete obtained in iation 0f compressive strength, icient of thermal expansion, creep, of conc rete were determined for rete st rength. Based on the detailed construction records, the column S was calculated. These with those measured in the building.

The long-term movements of in-situ reinforced concrete columns in two structures are reported through the construction stage to occupancy and subsequently in service. The columns were extensively instrumented to measure strains, temperature and moisture changes; in addition, a stress meter was incorporated in the second structure at the base of an internal column to relate the field movements and their effects to the load actually carried by the column. The results show that the inelastic movements in concrete cause a gradual load transfer to the steel reinforcement. In one structure steel stresses in excess of the permissible design values were noted. The assumed design dead load components of the working loads were realized in the lower columns of the structure but not in the upper storeys. After nearly five years in service, the concrete in the columns of the second structure were found to carry loads varying from 50% to 80% of the measured load carried by the columns. It is shown that a major proportion of the loads carried by the columns result from dead loads and that only about 10%-20% of the total design loads constitute imposed loads.

The principles are presented for the main types of the dif- . ferentiated, structural fire engineering design systems, in practice at present or anticipated to be applied in the future. Such design systems are generally based on real fire exposure characteristics, given by the gastemperature-time curves of the complete fire process and specified in detail with respect to the influence of fire load and the geometri-cal, ventilation and thermal properties of the fire compartment. The design procedure can be in its entirety analytical or combined analyti-cal and experimental. In the latter case, real fire exposure conditions can be transferred to the heating conditions according to the standard fire resistance test via the concept equivalent time of fire duration. Starting from the present state of knowledge, the possibilities are discussed for a practical application of a complete analytical, diffe-rentiated design in regard to fire exposed, reinforced and prestressed concrete structures. Finally, the various sources and kinds of uncer-tainty in the differentiated design procedure are briefly dealt with within the framework of the structural fire safety problem.

Analytical predictions of thermal and mechanical behaviour of reinforced concrete structures exposed to differentiated complete fire processes including the cooling phase are presented and verified by tests. The modelling of the fire response comprises a heat flow ana-lysis in the first step and a structural analysis in the second step, based on two separate computer programs. The evaluated structural fire response is compared with the measured behaviour in a great number of experimental tests in which, the fire process and the external load level are widely varied. The experimental investigation refers to a well-defined hyperstatic structure, viz. a reinforced concrete plate strip fire-exposed on one side and completely fixed against rotation at both ends while axial movement is free to develop. The outline of the project is built on the philosophy of a functionally based, dif-ferentiated design procedure for fire exposed, load-carrying and separating structures. Such a design procedure refers to performance criteria and postulates that the real physical processes with res-pect to fire exposure, heat transfer and structural behaviour are predicted as far as possible.