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Section 13. Technical sciences
Comparison of vertical stress values in fig. 5a and fig. 6a shows that the installation of the frame leads to a more uniform distribution of the normal vertical stresses in the plane of the wall even if the wall openings. The maximum compressive stresses in the lower part of the building without a skeleton near the doorway, reaching values of 0,06 MPa (fig. 5a). In the case of a frame, receiving the load from the ceiling, the compressive stresses induced by the action of its own weight only, two orders of magnitude less. Approximately the same quantitative ratio observed in buildings without a frame and with frame and shear stresses in the planes of the longitudinal and transverse walls. At the same time the largest in magnitude shear stresses arise in the front wall in the areas immediately adjacent to the corners of openings (fig. 5b). The presence of the frame not only reduces shear stresses, but also leads to a more even distribution of them on the walls
of the plane (fig. 6b). Concrete frame reduces the stress produced even doubled comparing with the structure having a wooden frame. In general, the vertical static load of its own weight and the weight of the slab does not cause a high tension of the walls of the building, not even a reinforced frame. Of course, this applies to the particular case, when the overlap weight, as stated above, is equal to 52 kN. [1, 158-169].
In general, based on the analysis of the stress-strain state of a single-storey building with an internal partition, you can draw the following conclusion:
Availability carcass unites longitudinal and transverse walls and floor in a single spatial system that has increased resistance applied static load, resulting in movement and level of stresses arising in the walls are greatly reduced in comparison with the same characteristics in the walls are not supported by the frame.
References:
1. Razzaqov S. J. The earthquake-resistance and stability of buildings and structures built from clay. Moderner Lehmbau - 2003. Nachhaltiger Wonungsbau-Zukunft Ökologisches Bauen. Fraunhofer IRB Verlag. Auferstehungs-kirche-Berlin, Germany. - S. 158-169.
Sagatov Bahodir Uktamovich, Tashkent Architecture and Construction Institute, Senior Fellow Researcher, E-mail: sagatov_b88@mail.ru
About transfer of effort through cracks in ferro-concrete elements
Abstract: The paper discusses new mechanisms of nonlinear behaviour of RC with regard tostress transfer across the cracks. It also gives the results of testing and realization of contact interaction model in cracks. Keywords: ferroconcrete, nonlinear behavior, cracks.
Qualitative change is intense-deformed state ferroconcrete elements after formation of cracks is connected with considerable anisotropy of properties of a material, display of nonlinear deformations, and also variety influence insufficiently known features of teamwork of concrete and armature. Uncertainty these factors bring the greatest at calculations of the ferroconcrete designs having the difficult physical mechanism of destruction as, for example, it takes place at shift or a cross-section bend. For the account of nonlinear properties of ferroconcrete, besides more exact estimation of its fundamental properties, it is necessary to pay attention to creation of models and methods of calculation of the ferroconcrete, reflecting the valid character of their behaviour under loading and a physical essence of problems arising thus.
At calculation of ferroconcrete designs with cracks numerical methods of final differences, the variation — differential and final elements are usually used. As a rule, convergence of iterative process is defined by accuracy of calculations on efforts of values hardly which essentially differ for stages before formation of cracks. In existing programs the account cracks formations is made by various models of the discrete crack which development on border of final elements is represented rupture of communications in knots. Common faults of this approach are restriction of a direction of development of a crack of orientations of knots of a final element and discount contact interaction of coast of a crack. Partially these restrictions are eliminated by «spreading» cracks on element volume in the assumption that directions of the main pressure or are parallel or perpendicular orientations of the cracks which surface is not capable to transfer stretching or shifting efforts. It automatically excludes what or redistribution of efforts after cracks formations, and the module of shift rigidity G thus is accepted equal to zero.
Other extreme measure, i. e. maximum resistance to a cut after cracks formations, is offered in norms CEB — FIP [3]. Probably, the decision at which decrease in rigidity of an element to certain size depending on width of disclosing of the cracks formed in it would be considered is compromise.
The made observations specify in extreme importance of researches of the mechanism of transfer of pressure through cracks in ferroconcrete elements. Such researches demand studying various mechanical and geometrical parameters in this connection working out of corresponding mathematical models should lean against adequate experimental data. First of all it concerns researches of the mechanism and features of transfer of shift pressure through a crack in the course of contact interaction of its coast. An important step forward in this direction was the deformation theory ferroconcrete with the cracks. In it ferroconcrete considers as physically nonlinear anisotropic material, and receiving on its basis of dependence and the calculation program on the computer are confirmed experimentally and spreading in designing practice. If at compression and a stretching mechanism transfers of pressure through cracks has found a sufficient experimentally-theoretical substantiation at a cut it is investigated obviously insufficiently. Here it is a question of the new factors shown in cracks at mutual shift of their coast: tangents of forces hitchs and treenail actions armatures cores. Cracks in concrete, developing, pass through a cement stone, grains of a filler and a contact zone, forming two cooperating rough surfaces of difficult geometry. They also provide transfer of shifting pressure through cracks by mechanical gearing and a friction. Researches have shown [2] that the assumption of full restraint of tangents of displacement in cracks at such gearing does not represent the facts. More over, displacement tangents can serve more exact exponents
About transfer of effort through cracks in ferro-concrete elements
presence of shift pressure in cracks, than width of disclosing of a crack. Treenail action of armature is shown in a local bend, a cut and an excess of the cores crossing a crack.
Some underestimation of a role of tangents of forces of gearing in cracks at designing of ferroconcrete designs with short of disperse reinforcing, characteristic for covers, box-shaped beams and plates, retaining walls, beams-walls, pressure vessels etc., is based on a popular belief that the friction in cracks are sizes of a variable and he can be neglected in stock durabilities. However last researches [1; 2] have revealed an inaccuracy of such argument. The matter is that at mutual tangential displacement 5crc crack coast happen its normal disclosing acrc (dilatancy) owing to mutual gearing of roughnesses on crack surfaces (fig. 1a). Therefore the width of its disclosing in a stage exploitation can appear considerably bigger, than it is supposed calculation on effective standards. Typical results of tests of the reinforced samples-disks on shift (fig. 1b) have shown that in armatures the cores crossing such crack, there can be considerable additional pressure.
Revealing of models of display of forces of gearing in a crack at shift for forecasting of rigidity and limit resistance of the mechanism of contact interaction in cracks demands special researches. Such as models should reflect influence structural peculiarity concrete and consider the mechanism axial and tangential rigidity of the armature crossing a crack. Widely used and become classical the concept about width of disclosing of cracks in ferroconcrete is defined as
mutual equal to displacement of its coast in a normal direction. For the general case when crack coast along with the normal test also tangential mutual mixtures, this concept should include dilatancy, defining essential distinction in width cracks on various sites on its length. At practical using models of the mechanism of gearing the knowledge of dependences t = f (5 , a ) and a = f (5 , a )
o r crc v crc crc' crc v crC crc'
for four variables (fig. 1bB) is necessary: Tangents and normal pressure (t , a ) and displacement corresponding to them (5 , a ).
^ crc crc' L I. o \ crc crc'
Such dependence will reflect one of fundamental physic-mechanical properties of ferroconcrete as cracking a material, defining its behaviour under loading. It does by its most convenient tool in realisation of the concept of the "smeared" cracks at calculations of ferroconcrete designs by numerical methods.
In work [1, 2] the detailed analysis of researches is carried out according to forces of gearing in cracks which can be divided conditionally on groups with following characteristic test specifications: at external relations (draughts) of constant rigidity; at internal "reinforcing" of variable rigidity; at constant controllable disclosing of a crack (acrc); at constant controllable normal pressing acrc; at the fixed constant width of disclosing of a crack with the controllable relation t /a = const. The similar
crc crc
analysis of researches treenail armature actions has allowed to reveal following groups: direct tests for a cut of samples-disks; tests of fragments of beams; tests of full-scale beams with treenail loose leaves; tests of samples-blocks.
Fig. 1. Contact interaction in cracks at shift (a), typical test pieces on shift (b) and schedules of dependence of displacement in a crack from pressure (c)
The analysis of results of researches has shown that normal disclosing of a crack is the key factor in the mechanism of transfer of tangents of forces of gearing through cracks. Shift rigidity grows in a crack with increase percent of reinforcing and that more than above durability of concrete and is better its coupling with armature. It is thus noticed that behaviour samples at powerful "reinforcing" of a crack or high significance acrc practically did not differ from behaviour of samples without cracks.
In the spent researches of the mechanism of transfer of pressure through cracks [2] skilled special disks with the initiated crack were made of easy, heavy and high-strength concrete and were tested on shift under the scheme on fig. 1b. Samples concerned the first series without cross-section reinforcing with free normal displacement of coast of a crack. The second series of samples tested at the fixed values of initial width of a crack which was regulated by screws on steel draughts with controllable normal stretching pressure. Thus, besides pressure of shift the normal pressure arising from dilatancy of disclosing of a crack were supervised. Samples of this series have
been intended not only for definition of limiting durability of gearing in the cracks testing action normal closing-up but also also for revealing of character of dependence "t — 5 ". The third series of
crc crc
samples armatured cores of class A-I, A-III and A-IV. On each series of samples the family of skilled curves t = f (5 , a ) and a = f
L ' crc v crc crc crc
(5 , a ) taking into account influence of a kind and durability of
crc crc
concrete, width of disclosing of a crack, size acrc and percent of cross-section reinforcing (fig. 1b) is received.
Results of tests have shown that the concrete kind influences both limiting resistance to shift, and on deformation behaviour samples. Despite various behaviour under loading, for each kind of concrete the limit of shift durability which at ceramic-concrete has appeared much more low, than at heavy, even at considerable to smaller width of disclosing of a crack is characteristic. At the big disclosing of cracks in samples with a considerable quantity armature cores in these sections shift rigidity was observed less. The average width of disclosing of a crack in ceramic-concrete samples of the second series has appeared almost identical, but in spite of
