Punching shear test - part 1

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Research on the behavior of the support zone of monolithic plate and column structures in the emergency stage.

Most of the tests carried out on the plate-column connection concerned various reinforcement methods and verification of calculation models and methods. The analysis also covered various ways to reinforce these construction zones. Little attention was paid to the preservation of the half-timbered zone in the emergency stage - after punching shear. In 2008, at the request of the Center for Quality Promotion of Steel, tests were carried out to check the work of the plate-column connection in an emergency state. The research took place in the Laboratory of the Department of Building Structures of the Silesian University of Technology.

Work in an emergency state of a slab-and-column connection at the design stage is usually not considered. It is assumed that during the connection the error will not be made and any emergency state will be signaled by large deflections and cracks. In special situations, such as:

the use of low strength concrete in relation to the designed value,
freezing concrete during hardening,
too early removal of forms
or a local explosion,

The connection between the plate and the column may be destroyed. Surviving the structure in this case can ensure the presence of a lower reinforcement crossing the column. The obvious condition is that the reinforcement would be able to take over the loads resting on the ceiling - despite the destruction of the support zone. The takeover of forces by the lower reinforcement and the development of the tensile structure is conditioned by the appropriate limit elongation of the reinforcing steel. The greater the elongation of the reinforcing steel, the greater the deformation and hence the smaller forces in tensile system are to be expected.

DESCRIPTION OF ELEMENTS SUBJECT TO TESTS

GEOMETRY

These elements were characterized by the same: geometry, concrete strength, upper reinforcement structure and load method. Two models were tested. They differed only in the structure, diameter and the steel class of the lower plate reinforcement. A single research model was a part of the ceiling in the plate-and-column structure and consisted of a reinforced concrete slab with dimensions 2,65 x 2,65 m, with a thickness of 0.2 m, with a centrally located 0,5 m long column from the bottom and with a square cross-section with dimensions 0,4 x 0,4 m. The model's plate was attached to the test stand by 65 mm diameter screws. It was assumed that the models will be loaded with concentrated force applied to the column base in accordance with the diagram shown in Fig. 2. The reinforcement bottom bars crossing the column were released beyond the outline of the model and anchored in a special holder attached to the test bench.

REINFORCEMENT

The bottom reinforcement of the models consisted of longitudinal straight reinforcement bars with a diameter 12 mm made, depending on the model from steel SI (model PI/16-1) or SII (model PII/12-1). The spacing of the bottom reinforcement in the slab was the same in both models and amounted to 95 ÷ 203 mm (on average every 173 mm). The average percentage of bottom reinforcement in both models was the same and amounted to: ρS2 = 0.47%. As reinforcement crossing over the column, both directions were used:

In the model PII/12-1, 2 bundles of 2φ12 in each, SII steel (cold-rolled steel).
In the model PI/16-1, 2 bars φ16, SI steel (hot-rolled steel EPSTAL).

This reinforcement was calculated in accordance with the recommendations of the American standard ACI and Model Code standard. The axial spacing between bars and bundles was the same and amounted to 200 mm. Parallel to the I-I axis, the lower reinforcement bars were placed closest to the bottom surface of the slab, while the bars along axis II-II were on them.

Model PI/16-1

Model PII/12-1

The top reinforcement of both models was this same and consisted of straight, perpendicular to each other bars with diameter 16 mm and made of steel SI. Bar spacing was analogous to that for bottom reinforcement, and the average reinforcement percentage (from two orthogonal directions) was ρS1 = 0.76%. The models were designed in such a way that their destruction by punching shear would first occur and the slab was not destroyed bent. The amount of bending reinforcement was calculated as for the actual slab-and-column structure with a 6 × 6 m column grid with an live load of 5 kN/m2. In the corners, similarly to the bottom, bars of 16 mm diameter made of steel SI were used. Parallel to the II-II axis, the top reinforcement bars were placed closest to the top surface of the slab, while the bars along axis II-II were on them.

Model PI/16-1 and model PII/12-1

The column reinforcement consisted of 8 straight bars, 20 mm in diameter, distributed around the perimeter. Transverse reinforcement was made as closed stirrups with bars of 10 mm diameter made of EPSTAL steel. The spacing of stirrups in the middle section of the column was constant and amounted to 150 mm, while at the connection of the column with the plate and at the base (at the point of application of the concentrated load from the hydraulic cylinder) it was compacted to 50 mm. The cover of main reinforcement bars, located closest to the surface (bottom or top) and column stirrups (diameter 10 mm) amounted cnom = 20 mm, and the cover of reinforcement bars of the column amounted cnom = 30 mm.

List of slab reinforcing bars

Plate reinforcement parameter / Model	PI/16-1	PII/12-1
Steel grade bottom reinforcement	SI (Class A steel)	SII (Class C steel - EPSTAL)
Diameter of bottom reinforcement (in both directions)	φ12	φ12
Spacing of bottom reinforcement (in both directions)	95-203 mm (average every 173 mm)	95-203 mm (average every 173 mm)
Steel class of integrating reinforcement (located in the bottom reinforcement mesh) and crossing over the column	SI (Class A steel)	SII (Class C steel - EPSTAL)
Diameter of integrating reinforcement (in both directions)	2 bars φ16	two bundles 2φ12 each
Spacing of bundles / bars (in both directions)	200 mm	200 mm
A grade of reinforcing steel top reinforcement	SII (Class C steel - EPSTAL)	SII (Class C steel - EPSTAL)
Diameter of top reinforcement (in both directions)	φ16	φ16
Top reinforcement spacing (in both directions	95-203 mm (average every 173 mm)	95-203 mm (average every 173 mm)

LOAD SYSTEM

The research of each model was carried out through 2 cycles of load and unloading. In the first cycle, the elements were loaded to a value of approximately 40 kN, controlling the indications of the measuring apparatus and adjusting the movable elements of the stand to the initial positions. In the second cycle, the load was increased in steps of 20 kN until the moment of punching shear (phase I) and further geodetic control of the displacement growth (phase II) until the maximum tension capacity of the reinforcement crossing over the column was reached - Fmax,s. In element PII/12-1 reinforced cold-rolled steel (steel class A) in phase II after reaching Fmax,s the bolts fixing the model of the stand were loosened, with further loading the model was connected to the stand via anchored bottom bars protruding beyond the model. Further load in this phase was carried out until reaching the load capacity of the tension (membrane) system Fmax. In the element PI/16-1, with reinforcement crossing over a column made of EPSTAL steel, in phase II after obtaining the load capacity of reinforcement crossing over the column Fmax, loosening of the bolts and testing in the tensile structure, due to the significant vertical deflection of the column, was impossible. During the tests, both in phase I and in the emergency state (phase II), the vertical force F was measured using a force meter with the range of 2000 kN and accuracy 0.001 kN. In Phase I - up to the moment of punching shear, at each load level, through the automatic measurement station, the registration of center slab deflections along the theoretical axes II and II-II via 13 sensors (7 on axis II-II and 6 on the II axis) was performed with a reading accuracy of 0.002 mm and an indication range of ± 50 mm. Additionally, geodetic measurement of displacements of the bottom surface of the slab and the column was carried out, using benchmarks. The position changes of the benchmarks were documented with the leveling instrument with an accuracy of ± 0.5 mm. From the moment the first visible cracks appeared to the moment of occurs punching shear, the measurement of their opening width was carried out with the help of a Brinell magnifying glass, with an accuracy of 0.05 mm.

Video from the course of the research