Appearance of exceptional loads in the construction system, such as vehicle impacts, internal gas outbursts, human errors, as well as terrorist attacks, may trigger the phenomenon of a progressive disaster. According to the standard criteria and design recommendations, it is initiated by local destruction of one structural bearing element (a column, wall fragment), which spreads to other supporting elements connected to the initially damaged element. As a result, either a complete collapse or a disproportionately large part of the structure collapses, an example of which is the building disasters made both in plate-and-column and plate-wall technology.
The research model was designed in such a way as to best reflect the work of the corner fragment of the actual, nine-pole plate and column structure made in the 1/2 scale during the research. A research model with an axial spacing of supports 3000×3000 mm was adopted. However, to make it possible to simulate the continuous system, it was necessary to make a larger model - a flat reinforced concrete slab measuring 4000×4000×100 mm was made for the tests. Taking into account the influence of the rest of the system was achieved by installing five steel rods with a diameter of 28 mm. As a result of the calculation of the bearing capacity of support zones due to the puncture, the model was supported by four prefabricated elements of two-phase operation. In the first phase - to be puncture - the calculated control circuit and the load-bearing capacity calculated on its basis were determined for the designed dimensions of the 200×200 mm support (steel element dimension). In the second phase - after puncture - as a result of a small vertical displacement (~ 5 mm), the falling part of the model was meant to stop on a steel element, enlarging the control circuit to 500×500 mm. The reactions generated by the model through the force meters located under the intermediate supports were transferred to fixed column supports with a height of 2,400 mm.
Model reinforcement was determined according to EC2 recommendations taking into account the encumbrances contained in EC1. The following values were assumed for the combination of loads: own weight 2.5 kN / m, constant load gk = 200 kg per one tie rod, variable load qk = 400 kg per one tie rod and load factors γgmax = 1.35, γgmin = 1.0, γqmax = 1.5.
The obtained values of bending moments obtained in the way of static calculations made it possible to determine the number and diameter of reinforcement, which were then made in the form of two parallel grids. The figure below presents a plan of the bottom reinforcement, which were made of bars with a diameter of 8 mm in two spacing. According to the standard recommendations, additional lower reinforcement was used in the axes of the columns, the aim of which was to prevent the damage typical of a progressive disaster - rods No. 2 and 3.
The upper reinforcement was made of bars with a diameter of 6 mm in spacing 100 mm above the support and 125 mm in the span, which, according to the results of typical calculations of the support zone, was reinforced with additional rods with a diameter of 12 mm. The view of individual grids and their assembly is shown in the figure below. The length of the upper reinforcement bars was selected based on the indications contained in the American ACI standard. As recommended by the standard in the support band, the reinforcement was extended beyond the outline of the column to a distance equal to 1/3 of the span's span, while the upper reinforcement in the span band was extended beyond the support's outline by a distance of 0.22 ln. Around the circumference of the plate on the free edges U-shaped stirrups were used, the length of arms was greater than 2d + 30ø. The spacing of these stirrups was assumed to be equal to the distribution of the bottom reinforcement bars.
Due to the predicted large deformations of the model - mainly vertical displacements of the anticipated range up to 1000 mm, it was necessary to perform a loading system that would allow for smooth adjustment of the load position relative to the model and transfer of weight of up to 1500 kg at each point of application. Based on these assumptions, the load of the model was realized by means of a system suspended on steel concrete ropes and steel weights of various unit weights: 25 kg, 100 kg, 200 kg.
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