Luz testigo cargador de batería (D)

 The test slab was only restrained vertically. As the panel deformed the flexible supporting rods would allow the edges to move inwards. The deformation can be visualized as a square piece of paper forced to take a conical shape, which is only possible if the edges move towards the centre as the centre displaces vertically, thus no membrane forces could develop. This also meant that the slab would move horizontally underneath the LVDTs. Since LVDT 2 & 3 were screwed into Perspex plates, the movement of the slab caused the stanchions on the LVDTs to bend – Fig 6.8.

Fig 6.8 Bending of LVDT stanchions

 Due to this movement the LVDTs were placed on the concrete. Subsequently the measured deflection values underwent a jump in value and needed some adjustment. The adjustment was done by inspection and adding/subtracting a constant value to the affected measurements to render a more realistic curve. The measured and adjusted behaviours of Corner 1 and Corner 2 can be seen in Fig 6.9 and Fig 6.10 respectively.

Fig 6.9 Corner 1 – Measured & Adjusted Deflection Values

Fig 6.10 Corner 2 – Measured & Adjusted Deflection Values

 Due to the friction on the steel rods and the crushing of little concrete imperfections on the slab, the first loading sequence shows a fairly jumpy load vs. deflection curve – Fig 6.11.

Fig 6.11 Load 1 – Column Load vs. Corner Support Deflections

From Fig 6.11 it is evident that the deflection behaviours of LVDTs 2 & 3 do not follow the same trend as that of LVDTs 4 & 5. Even though the latter two are more affected by the friction between the supporting rods and the slab, they follow a more acceptable linear behaviour of load vs. deflection. Consequently all further calculations are based upon the average deflections of LVDTs 4 & 5.

From Fig 6.12 the following can be concluded:

 0mm to ±2.5mm deflection, 0kN to ±350kN column load

The stiffness of the slab is initially parabolic and then settles to a linear trend. The first visible cracking took place at approximately 1.5mm deflection and 250kN – Fig 6.13 (a) & (b)

The horizontal movements on the graph are due to observers bumping the test panel while inspecting and marking the newly formed cracks.

(a) (b)

(c)

Fig 6.13 First visible cracking on the concrete surface

From 250kN to 350kN the crack pattern grew into a radial pattern, indicative of flexural cracking – Fig 6.13 (c).

 ±2.5mm to ±8.5mm deflection, ±350kN to ±550kN column load The panel continued to behave with a linear increase in deflection for the growing load. Once again the horizontal movements on the graph are indicative of the stages where new cracks were inspected and marked on the slab.

Fig 6.14 Growing flexural cracks

The flexural cracks continued to grow in a radial pattern towards the edges, as seen in Fig 6.14. At approximately 475kN the first shear crack appeared around the column – Fig 6.15.

Fig 6.15 Appearance of first shear cracks at 475kN

Before unloading a distinct radial crack pattern is visible – as can be seen in Fig 6.16. The increased load caused the cracks formed at lower loads to open up more significantly – Fig 6.17.

Fig 6.17 Increased crack width

Due to behaviour of the LVDTs at Corner 1 & 2, the test was terminated at approximately 550kN.

 ±8.5mm to ±3mm deflection (unloading)

Removing the applied column load caused an approximately linear elastic unloading behaviour of the test specimen – Fig 6.12. The positive residual deflection of the centre of the slab indicates plastic deformation and a certain degree of damage (cracking) already inflicted on the slab.

6.3.5. Original Panel – Load Application 2

Due to the fact that initial cracking had already taken place, the response of the test panel was practically linear with the second load application – Fig

6.19.

Punching shear failure was not yet achieved at the maximum loading capacity of the original test setup. Consequently, it was decided to unload the slab and modify the test equipment to increase its loading capacity.

After unloading, it is clear that further plastic deformation took place. It should be noted that the elastic limit for the bending steel has not yet been reached. Should the slab start yielding due to flexural failure; the Load vs. Deflection curve would form a plateau with increasing deflection, i.e. ductile failure.

Fig 6.18 Load 2 – Column Load vs. Corner Deflections

With the second load application the effect of the misaligned support at Corner 3 (LVDT 4) can be seen clearly. The friction causes the support to have sudden deflections as the frictional forces are overcome at distinct instances. As seen in Fig 6.18, the load-deflection behaviour of Corner 3 differs considerably from that of the other corners. Subsequently the average value of the corner deflections was calculated using only Corners 1, 2 & 4.

The second load application caused the existing cracks to become more pronounced. As the jack approached the end of its capacity, pumping became more strenuous to the operator. The lower rate of load application seems to cause the response curve to flatten – as seen in Fig 6.19 from ±5mm deflection to unloading.

Fig 6.19 Load 2 – Column Load vs. Average Relative Middle Deflection

6.3.6. Original Panel – Load Application 3

In order to push the test panel to punching failure, the maximum loading capacity of the setup was increased by introducing a second hydraulic jack (Fig 6.20) and a bigger capacity hydraulic hand pump.

Fig 6.21 Load 3 – Column Load vs. Corner Deflections

Once again the measurements taken at LVDT 5 (Corner 3) differed

substantially from the other three corner measurements – Fig 6.21. In order to do plot the load-deflection behaviour of the slab panel, the average value of corners 1, 2 & 4 (LVDTs 2, 3 & 5) were used – Fig 6.22.

Fig 6.22 Load 3 – Column Load vs. Average Relative Middle Deflection

For the third load application the following significant stages can be highlighted:

 0mm to ±6mm deflection, 0kN to ±725kN

Reloading of the slab shows a fairly linear relation between the applied load and measured deflections.

Cracking of concrete was quite audible towards 725kN.

 ±6mm to ±8mm deflection, 725kN to ±850kN

The angle of response started to decrease in this stage of the load application. Between 750kN and 850kN two significant observations could be made. Firstly, crushing of the concrete at the interface of the column and the slab soffit started – Fig 6.23.

Fig 6.23 Concrete crushing at the column face

Secondly, another shear crack appeared on the concrete surface, further away from the column – Fig 6.24.

Fig 6.25 Highlighted possible punching shear cracks

 ±8mm to ±10.5mm deflection, ±850kN

Suddenly, the stiffness of the panel decreased, showing an increased deflection for a fairly constant load. The test panel could be seen deflecting, accompanied by audible cracking inside the concrete.

 ±10.5mm to ±13mm deflection, ±850kN to ±450kN

It was clear that the slab had reached its failure load. The deflection increased dramatically with a lower resistance to the column load.

 ±13mm to ±5.5mm deflection, unloading

Upon unloading the slab once again recovered in a linear fashion; however, the rate of recovery was much lower than for the previous two load applications. The residual deflection is also substantially more than for loads 1 & 2.

6.3.7. Original Panel – Combination of results – Loads 1, 2