A metal structure can collapse at lower temperatures of the merger point: that’s why

The recent collapse of the advertising signs on the Torre Generali of Citylife in Milan designed by Zaha Hadid brought to light the possible problems of metal structures Compared to the actions produced by high temperatures. Although it is not yet confirmed (or denial) that this is the cause of the actual collapse of the reticular structure that supports the advertising sign, The news said thus aroused doubts among the non -professionalsstrange from the fact that despite having far -far temperatures far from the steel melting point, these have even been able to generate a structural crisis. Well, what has been said is probably possible and the reasons have little to do with the metal melting temperatures. We try to clarify in a simple way.

The difference between material and structure

To fully understand the conceptual difference that exists in the problem we are facing, a difference between what happens at the level of material and what happens at the structure level.

At the level of material

What happens in terms of material is what we can physically represent physically independently From how this is then employed in reality. The steel material, in this case, can be represented by the so -called constitutive bond. What does it mean? It means that when the temperature varies, the constitutive bond of the metal changes, but for having appreciable reductions in resistance or stiffness of the metal We must exceed temperatures that go beyond 200-300 ° C. In the reasoning we are carrying out, we can quietly To say that 0, 50 or 150 ° C are – at the level of material – the same temperature.

At the structure level

But what happens at the structure level? We must not forget that, any element made with any material, If subject to a variation of temperature compared to its initial stateYes deform exhibiting movements that depend on his thermal expansion coefficient. To circumscribe the examples that we will deal with in this article, we will talk about elements with prevalent linear development and, by reflected, we will refer to the so -called linear thermal dilation coefficient.

For example, a 5 -meter long steel bar, if subject to a thermal excursion of 25 ° C (call thermal distortion), undergoes a linear thermal dilation that leads it to reach a length of 5.015 meters: that is, it extends by 1.5 mm even if there are no applied external forces.

The structures then they stretch and shorten if subject to thermal excursions also relatively modest (20-25 ° C)! But what happens if these extensions are for some reason prevented? If the bar cannot freely extend, the structure reacts with one opposition force to lengtheningwhich is all the greater the more rigid the bar is. For example, let’s think of a steel bar forced between two walls of bond: as the temperature increases The bar would like to stretch but The walls prevent this extension and exert on the bar an action that “brake“The elongation. These forces that are born must obviously respect the resistance of the material and – when this does not happen – the structure, or part of it, can collapse due to lack of resistance.

When this phenomenon can happen

We must therefore understand if and when we are dealing with a structure that, for some reason, is unable to freely deform by supporting the effect of Thermal distortions. To understand it, the concept of Hyperstaticity: a structure defines itself hyperstatic If its degree of constraint exceeds that strictly necessary to guarantee its balance. For these structures, which are practically 99% of those around us, the effects of thermal distortions they always translate into a regime of internal stresses which can assume, depending on the degree of hyperstaticity, also values ​​that reach the resistance of the material in some point.

So why don’t we observe crisis of all hyperstatic structures? In reality, The structures deform And with them they also deform the constraints that support them. In fact, there is never a wall capable of totally blocking the lengthening of the bar. As the temperature increases, it will also be a bit the wall to give in and to allow, of reflection, part of the thermal expansion. This mutual interaction between constraints and elements smut The birth of reactive forces significantly. For this reason, the effect of these thermal distortions becomes in most cases not dominant Compared to the other external actions that govern the project. Nonetheless, sector regulations prescribe appropriate values ​​of temperature differences to be taken into account, in the project, to estimate the structural effects of any applied thermal distortions (25 ° C is a widely used calculation value).

The problem can become important when the lengths in the game are high: in the case of the bridges, for example, the bond systems are often designed for fully support these movements.