Punching shear test - part 2

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Research on the behavior of plate-column connections reinforced with EPSTAL steel with high ductility in the emergency stage caused by puncture.

The study of the supporting zone of the plate-and-column structure (described in the Technical Bulletin No. 2) carried out in earlier years concerned the columns placed inside the structure. The way of working in the emergency state of the plate-column connection located on the edge of the floor was a noteworthy and not yet investigated problem. This task was undertaken by the employees of the Department of Building Structures of the Silesian University of Technology on behalf of the Center for the Promotion of Steel Quality. The study took place in the Laboratory of the Building Department in a specially designated research stand.

The biggest threat to the slab-pillar construction is the possibility of the ceiling penetrating in the area of the column as a result of exhausting the tensile strength of the concrete. The breakdowns caused by the puncture occur almost without any initial symptoms, because the deflections are small, and scratches on the upper surface of the board are not usually visible due to the floor layers used. A local ceiling failure caused by a puncture at one of the columns causes an increase in lateral forces in neighboring supports, which may cause a failure of these areas and, as a result, collapse of the entire structure.

Description of the research model

Geometry

Laboratory tests were carried out on models designed to reproduce a fragment of the plate-column system. They consisted of a rectangular, reinforced concrete slab with a fixed height with a column with a square cross-section at the bottom. Three of the four edges of the board were fixed (with screws placed along the edge) to the station in order to reproduce the work of such a connection in the real system. The fourth non-attached edge of the model, under which the pole was located, was the outer edge of the ceiling. Due to the location of the post relative to the center of the geometric plate, two positions were distinguished:

axial (models with the P-I designation),
eccentric (models with the P-II designation).

The eccentricity in the P-II model was set in the direction parallel to the edge of the non-fixed model and was 283 mm. The elements subjected to the tests had the same geometry, strength of the concrete, the structure of the upper reinforcement and the way of loading.

Reinforcement

The reinforcement of the models was designed so that their puncture first occurred and the plate was not damaged flexibly. The amount of bending reinforcement was calculated in accordance with the provisions of Eurocode 2 and Eurocode 1 as for the actual plate-and-column structure with a column mesh 6.0 x 6.0 m with a service load of 5 kN / m2. In each of the research models, as additional protection against a progressive disaster, additional reinforcement was applied in the bottom grid of the slab reinforcement, in the form of bars passing directly above the column post. Reinforcement as well as the entire reinforcement of the board was made of high-ductility steel of the EPSTAL brand. This solution was taken from the Canadian CSA A23.3 standard and recommendations to the American ACI 318 standard. This reinforcement was referred to as integrating reinforcement. The integrating reinforcement was routed in parallel and perpendicular directions to the unscrewed edge of the model and then led out of the model and anchored in the test stand. The reinforcement carried perpendicular to the model's edge was made in the form of a loop.

Loading system

In order to map the state of displacements occurring during the puncture, the load was carried out by the concentrated force applied to the column base by means of an actuator. During the whole destruction of the model, two phases of the connection operation were identified. "Phase I" - occurring before puncture and "Phase II" - after puncture to total destruction of bottom reinforcement. Due to the distinction between the two ways of working the connection after the occurrence of the puncture, the following were distinguished:

MODEL TYPE A - after the occurrence of the puncture in load transfer, both the reinforcing reinforcement (lower reinforcement bars crossing the column) and the remaining reinforcement of the slab and concrete cooperate. This work pattern was implemented by leaving the fastening screws in the "Phase II" test.
MODEL TYPE B - after the puncture occurs, the load is transferred only by the lower reinforcement bars crossing the column. This work pattern was implemented by unscrewing the fastening screws in the "Phase II" test.

As a consequence, the destruction of 4 models being the resultant of the way the column is positioned and the way of loading. The course of their destruction is presented in the following films from the study. Depending on the research model: