International Concrete Abstracts Portal

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.

The random behavior of reinforced concrete elements at de-formational limit states and at the ultimate limit state is analyzed by a second moment approximation. The deterministic and stochastic parameters involved, their functional and stochastic dependences, and their experimentally based statistics are discussed. The resulting variances of ultimate carrying capacity and ductility and of crack development are shown. It should be noted that ultimate ductility has an especially strong statistical variation independent of the amount of compressive reinforcement. A two-span beam is analytically modeled to elaborate first order approximations of the means and variances of simultaneously acting live loads and temperature effects which cause different limit states. Imposed deformations do not greatly influence ultimate load-carrying capacity provided that there is sufficient ultimate ductility. However, load-carrying capacity with respect to a limit state of allowable crack width is substan-tially reduced by simultaneously acting imposed deformations.

Field measurements on two existing reinforced concrete slabs had to show that the chosen strengthening methods were successful. This was done by determining the bending stiffnesses of the two slabs before and after strengthening. The strengthening methods and the measuring equipment are described. The results showed that the sub-sequent strengthening provided an increase of the bending stiffnesses. It was also found that to achieve good quantitatif results a load test is required, whereas the actual floor loading produces only qualitatif results.

Lapped splices play an important role in the con-struction of reinforced concrete structures. The tensile force at a reinforced bar end is transferred over the concrete to the beginning of the next bar by means of tbe bond action alone. Investigations on the capacity of lapped splices were carried out at the Institute for Structural Engineering, University of Technology, Munich. This research was initiated by Professor Kupfer. The purpose of these tests was to gain more knowledge regarding the stress in the surrounding concrete and to ascertain the force distribution along the spliced bars. The first part of the research program comprised of slab tests with full splicing of reinforced bars with large diame- ters. For the judgement of the capacity of a reinforcing bar the most important criterion is the bond behaviour of the concrete and steel along the lapping length. By means of a new method specially developed for this research it was possible to measure the slip of the spliced bars in comparison to the concrete within short distances along the bar with scarcely any bond disturbance. In connection with the steel strain, measured by bonded wire strain gauges, it was possible to ascertain the bond strain-slip relations (C-A) for different sections of the lapping length. Parallel to these tests, investigations on photoelastic models were carried out. These tests in conjunction with a special technique allowed the spacial course of stress in the vicinity of the reinforced bar to be studied. With the aid of the bond laws derived from reinforced concrete tests and the knowledge gained from the photoelastic tests the splices were calculated on the basis of the finite element method and compared with the results obtained from tests.