CUSTOMER REQUEST:

We are often asked why it is not sufficient to perform the rollover check alone.

OUR ANSWER:

Because there are three failure modes of a ballasted system:

• overturning, in which the action of the wind causes the panel to rotate around the centre of rotation typically corresponding with the edge of the front ballast.
• lift, in which the vertical action of the wind overcomes the gravitational force acting on the panel/ballast system.
• Scroll, in which the horizontal action of the wind overcomes the friction force acting on the panel/ballast system.

Le variables which intervene in the quantification of the entity of the three phenomena are:

• wind load
• panel dimensions
• Panel Orientation
• Panel tilt
• ballast position
• ballast weight
• coefficient of friction

Without a timely assessment, it is risky to assume which is the most critical failure mode, which depends on specific system data.

Our approach is to verify always all ballasted systems according to all failure modesOur long experience in the sector leads us to say that often the most demanding mode is lifting.

The images show the before and after of a photovoltaic system where the lifting failure occurred.

Wind tunnel testing they confirm that the first visible effect of the wind action on the panel is a loss of ballast adhesion (reduction of the friction coefficient) due to the wind lifting force which then causes a uncontrolled horizontal translation of ballast.

PRACTICAL EXAMPLE:
Row of 12 vertical panels, inclined at 6° in Rivergaro (PC).
The minimum ballast to satisfy the static verification at lift results 1001kg.
The minimum ballast to satisfy the static verification alone overturning results 806kg.

By sizing for tipping only, I reduce the amount of ballast but expose the system to the risk of failure due to lifting.