Span-to-height ratio limits for prestressed concrete members
P.G. Debernardi, Politecnico di Torino, Italy M. Taliano, Politecnico di Torino, Italy
This paper reports the main results of a parametric analysis which was performed to define appropriate limits for the span-to-height ratio of prestressed concrete beams or slabs that are in compliance with the deflection control required by international standards (Eurocode 2, Model Code 1990) for concrete buildings structures. Two deflections were considered: the total deflection and the active deflection that occurs after construction, in both cases under the quasi-permanent load combination. The active deflection has to be considered when fragile elements such as partitions or glazing are present on the structure. Four structural systems were considered: simply supported fully or partially prestressed beams with an I-section and simply supported or fixed-end slabs with a rectangular cross-section. The load history first included the application of the prestress and the dead load and then the application of the remaining quasi-permanent load. Taking into account the effects of imposed deformations the structure was designed to obtain the geometrical properties strictly necessary for the ultimate limit state (ULS) and for stress limitation in service under quasi-permanent and characteristic load combinations. The results allow the influence of some of the variables that most affect the deflection to be evaluated, such as the span and the height of the beams, the concrete strength, the permanent load and the variable-to-permanent load ratio.
Belgrade's Delta City shopping mall - design and construction
S.B. Marinkovic, University of Belgrade, Serbia V.M. Kokovic, University of Belgrade, Serbia I.S. Ignjatovic, University of Belgrade, Serbia V.H. Alendar, DN Engineering Consultants, Belgrade Serbia
The structure of Delta City shopping mall in Belgrade consists of two separate structures: the structure of the mall and the structure of the multi-storey open garage. The overall dimensions of the irregular layout of the structure are 210 m x 120 m, with four main levels in the mall and five parking levels in the garage. Because of the different exposure conditions, the structure of the garage is separated from the mall's structure with an expansion joint at all of the levels, except the level of the foundation slab. The mall's framed structure is designed without any expansion joints, except for temporary joints during the construction stage. It consists mainly of reinforced concrete columns cast in place, at typical spans of 8.2 m x 8.2 m, precast reinforced concrete simple beams and precast prestressed hollow core slabs. The moment resistant reinforced concrete frames cast in place are designed mostly at the facade, to provide additional seismic resistance and to control lateral deflections. The main structure of the open multi-storey garage consists of reinforced concrete frames cast in place and precast hollow core slabs, and it is designed for exposure class XD3, with special attention paid to durability requirements. Owing to the mainly precast structure, especially the hollow core slabs as a floor solution, the complete concrete structure of approximately 80 000 m2 was constructed in less then 12 months.
Bond to concrete action of reinforcing bars with different deformation patterns
A. Semchenkov, Research and Development Institute for Concrete (NIIZhB), Moscow, Russia V. Meshkov, Research and Development Institute for Concrete (NIIZhB), Moscow, Russia A. Kvasnikov, Research and Development Institute for Concrete (NIIZhB), Moscow, Russia
At present in Russia hot-rolled reinforcing bars are mainly manufactured using two deformation (rib) patterns, either with ring-like ribs of constant height or with two-row crescent-shaped ribs of 'European' style. Steel of both grades A400C and A500C can be produced with one or other of these rib patterns. Often it is difficult to identify on site an actual grade of rebars. To improve the situation a new specific deformation pattern for A500C grade steel was developed. The rebar-to-concrete interaction for the three patterns mentioned above is evaluated by means of the analysis of failure of concrete keys formed between transverse ribs of a steel bar. For this purpose the authors employed some principles of analysis that previously had been used for key joints of concrete floor panels. The bond behaviour is subsequently classified as 'rigid' or 'soft', the latter being considered as more favourable. The structural advantages of commercial production in Russia of A500CP grade reinforcing steel using the newly developed deformation pattern are described.
Determination of concrete compressive strength with pullout tests
H. Thun, Luleå University of Technology, Luleå, Sweden U. Ohlsson, Luleå University of Technology, Luleå, Sweden L. Elfgren, Luleå University of Technology, Luleå, Sweden
A pullout test using post-installed inserts has been examined as an alternative to drilled cores to determine the in-place concrete compressive strength. Tests have been carried out on eight railway bridges from 1965 to 1980 and on a one year old concrete slab. An empirical strength relationship is proposed between the compressive strength of a drilled core with the diameter and height of 100 mm and the pullout force from the pullout test. It is a power function and the relationship is valid for concrete compressive strengths up to 105 MPa; it gives higher concrete strengths than earlier proposed functions.
Behaviour of confined over-reinforced self-compacting concrete beams
R. Kumar, Indian Institute of Technology Roorkee, India B. Singh, Indian Institute of Technology Roorkee, India P. Bhargava, Indian Institute of Technology Roorkee, India
In many structural applications it is desirable to use a high percentage of steel reinforcement in beams to minimise structural depth while still providing adequate stiffness. This investigation demonstrates that by confining the compression zone of an over-reinforced beam with a steel helix, considerable strength, stiffness and ductility can be achieved, even with a tension steel content as high as 4⋅79%. This is possible because the helically confined compression concrete becomes triaxially stressed during loading, leading to a high uniaxial strain capacity at an enhanced stress level. Self-compacting concrete has been used to ensure effective concreting of the over-reinforced beams. The proposed analytical approach based on the moment-curvature characteristics of the over-reinforced section gives a reasonably accurate prediction of the load-displacement behaviour of the over-reinforced beams. The serviceability limit states of cracking and deflection have been examined and the quality of the hardened self-compacting concrete in the beams has been found to be satisfactory from ultrasonic pulse velocity measurements.
Modelling of temperature profiles in a concrete slab under climatic exposure
O. Larsson, Lund University, Sweden
Effects of surrounding climate impose temperature variations in both time and space for concrete structures. The associated thermal strains may give rise to stresses and cracking due to external or internal restraint. This will affect the performance of the structure. It is therefore important to analyse the temperature variations and the extreme situations that can occur. To predict the dynamic thermal conditions in concrete structures a finite-element model has been developed. The ability of the model to describe correctly the various boundary conditions such as solar radiation, outgoing long-wave radiation and convection is investigated in this paper. Field temperature measurements in a concrete slab placed outdoors are used for the validation. The necessary climatic input data for the model were obtained from measurements in the vicinity of the slab. The results show that the model can describe with good accuracy the effects of different climatic factors in extreme situations and is well suited to use in further studies.
