Study of a large plate

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Research on the behavior of a reinforced concrete slab reinforced with EPSTAL steel with high ductility in an emergency situation caused by the removal of the edge support.

Due to the small amount of information regarding the behavior of the plate-and-column system at the time of the failure, CPJS in cooperation with the Department of Building Structures of the Silesian University of Technology re-examined the research started in 2012. Their aim was to observe the behavior of emergency work of the edge section of the plate-column system caused by the removal of the edge support.

In the course of the research, information about the mechanism of destruction after removal of the support and the influence of the amount and ductility of the reinforcing steel on the destruction of the edge part of the plate-column system were also to be determined.

Nowadays, plate-and-column systems are increasingly being built. It is known that the resistance of such structures to exceptional loads is much lower than the resistance to such loads of longwall buildings or post-and-beam frame structures. With the increase in the number of such facilities, their threat increases due to the possibility of more frequent exceptional burdens, such as impacts with means of transport, internal gas explosions or terrorist attacks. Such hazards also include such factors as the use of materials with low mechanical parameters, or, for example, too early use of the structure.

Description of the research model

Geometry

Regarding the research methods of plate-and-column structures, depending on the expected results, professional literature indicates specific rules for the selection of the dimensions of research elements. In situations where separate structural elements are considered, eg poles, beams, or their interconnections (eg plate-to-pole connections), it is recommended to perform such elements on a real scale, i.e. 1:1, possibly slightly reduced, however not less than 1:2. Conducting research on entire structural systems is largely related to research capabilities. Most often, such tasks are carried out on models made in the scale of 1:2, much less frequently in the 1:1 scale and on existing constructions. In the described research, the research model was designed in such a way as to best reflect the work of the real nine-pole plate and column structure, made in the 1:2 scale. A research model with an axial spacing of supports 3000×3000 mm was adopted, which was articulated by means of dynamometers on 16 prefabricated supports with a height of 2400 mm. The thickness of the model was assumed to be equal to 1/30 of the roof span between supports. As a result, the overall dimensions of the model were 9,300×9,300×100 mm.

Reinforcement

When designing - determining the amount of reinforcement in the model - it was assumed that all calculations will be carried out in accordance with the PN-EN standards. The following values were assumed in the load statement:

Own weight of the model: gk1 = 2.5 kN/m2
Permanent load resulting from the floor layers: gk2 = 0.5 kN/m2
Usable load: qk = 3.0 kN/m2
Total characteristic load: gk1 + gk2 + qk = 6.0 kN/m2
Proportion of alternating load to constant load: 1:1
Safety factor for permanent loads: 1.35
Safety factor for variable loads: 1.50

According to EC 2, an additional lower coronal reinforcement has been calculated, which should transfer the force resulting from the removal of the edge column. For the diagram as in the picture above, the value of this force is determined by the formula:

Fx =1,6 [(gk1+gk2+qk)lx]lx*

where:

gk1 , gk2 , qk - load values
lx - span of the plate in the axes of columns
lx * - load span (assumed lx* = 0,5 lx + 0,15 m = 1,65 m)

Based on the calculations made in Model 1, two bars with a diameter of 8 mm were used as the coronary reinforcement. In Model 2, with unchanged number and arrangement of the basic reinforcement, two bars with a diameter of 16 mm were used as the coronary reinforcement. The views of the arrangement of the lower and upper reinforcement are shown in the figures below.

Upper reinforcement mesh for Model 1 and Model 2

Upper reinforcement mesh for Model 1 and Model 2

Lower reinforcement mesh Model 2

Loading system

Due to the anticipated values of displacement of the upper surface of the model to 900 mm and a load in the range of up to 2.5T at one point of load application, it was decided to use a hydraulic system. Bidirectional action cylinders were used (Fig. 18), characterized by nominal values of pulling force 3.5T, with a possible extension value of up to 1000 mm. The load of each analyzed separate model consisted of four independent systems - the gravitational load system "A" and three hydraulic load systems "B", "C" and "D". The figure below presents a simplified view of the arrangement of individual systems together with the values of the maximum evenly distributed loads that can be obtained from individual systems.

The gravitational load system "A". The gravitational load P1 was realized in the form of concrete weights of 200 kg, which were suspended in 132 points.
External hydraulic load system "B". The hydraulic load P2 consisted of a set of 12 hydraulic cylinders.
Internal hydraulic load system "C". The hydraulic load P3 consisted of a set of 9 hydraulic cylinders.
Hydraulic support system of the "D" model. The hydraulic load P4 consisted of one actuator (long extension cylinder) with a nominal extension range of 1200 mm and a load capacity of 150T located at the point of the planned loss of support.

The test procedure

After the preparatory works (model rectification) and gravity loading were suspended, the proper part of the study was started, which for each of the examined fields consisted of two stages - figure below.

Stages of loading individual Models 1 ÷ Model 2 (view of the model in section through the removed support):

First stage A - zeroing of force meters and inductive sensors, suspension of gravitational load.
First stage B - initial hydraulic loading of the model to the level of 2kN.
The second stage - lowering the edges and increasing the hydraulic load until the destruction (the long-lift hoist was used only as a safeguard and was kept constantly several cm below the bottom edge of the plate).

In the first stage "A" each model was loaded with a preload value equal to 2 kN. Then, the support reactions and displacements of the upper surface of a given model were read. In the second stage, when the preloading value was set, the edge support was lowered, which was accompanied by the leveling of the assumed load (equalization of the hydraulic load value was related to the fact that with the lowering of the support the oil pressure in the actuator decreased, and thus the value decreased the load applied to the model). Then, as shown in the figure above, the load was gradually increased until the moment of destruction, which occurred at the strength of: 9.15 kN (16.27 kN/m2) in the case of Model 1 and 13.32 kN (23.68 kN/m2) in the case of Model 2. The given values of forces were measured on the internal hydraulic circuit, on the outer circumference the load value was twice lower.