What to learn from structural sagging: the general sign and the complexity of the design

The General Torre (or Torre Hadid o the crooked)

It is worth remembering that the structure collapsed on itself belongs to a very complex and recently implemented work, Started in 2014 and inaugurated in 2019. The skyscraper was designed by Zaha Hadid For the headquarters of the Offices of the Generali Insurance Company in the well -known CityLife district, with a height of 177 meters, corresponding to 44 floorsmore The big teaches that brings it to 192 metersas a second highest skyscraper in the Milan skyline.

The skyscraper was immediately nicknamed the crooked for the characteristic torsional form generated by the variation of form on each floor. The structure is built in reinforced concretewith a project that in 2016 won the second prize to Emporis Skyscraper Awardin addition to the international prize Excellence in Concrete Construction Award 2019 dedicated by the American concrete institute to the category of high reinforced concrete buildings for «la la design and engineering wisdomrecognizing on the one hand the audacity and characteristics of the project, on the other the Innovative design methodologies who made it possible “.

The general sign and the solicitations of the wind

The sign of the Skyscraper Generali rests on a central steel drum, bound to the central core in reinforced concrete. From the drum is bound to custody the reticular space structure formed by tubular elementsto which I am anchored, by secondary structure, the large advertising panels characterizing the summit perimeter of the skyscraper. From the photographic report available so far on the press, it is detectable how the failure has mainly affected the reticular structure of only one part which supported two of the four large panels.

The design of high altitude signs requires A lot of attention towards the solicitations of the windof the consequent vortices, aeroelastic phenomena (interaction between surfaces and air flows) and oscillations that can be generated for the shape and height of the summit structure. The calculation of the wind load for a skyscraper is much more complex operation than the provisions for traditional civil structures. The highest structures are subject to greater wind forces due to their exposure at higher speeds. However The intensity of the wind is not only the parameter characterizing the sizing.

The Aerodynamic study It is a fundamental part of the design of high structures. The shape of a building can significantly influence the flow of the wind around it: rounded or conical shapes can, for example, help to minimize wind resistance, while acute corners can create turbulence that increase wind pressure on the structure. Architects and engineers who design skyscrapers collaborate with Aerodynamics experts To create aesthetically interesting forms of buildings, and which at the same time also react well to the complex windy stresses. Advertising signsespecially if characterizing the top of a skyscraper, are subjected to the same aerodynamic studies.

The wind represents the main solicitation for this structure, however its failure has not occurred during strong wind storms, as we are now accustomed to receiving during thunderstorms with increasingly tropical intensity. Quite the opposite, It happened at the dawn of a warm summerafter one Red sticker day for high temperatures in an atmosphere of “flat calm” about windy actions. Many comments refer the causes to high temperatures and even climate change.

Thermal variations and hyperstatic structures

The wind is certainly not the only action, in addition to its own weight, to solicit this structure at the top of the skyscraper. Also the thermal variations can exercise tension states Following the physiological dilations and thermal contractions To which the metal structures are subject during their service, due to daily and seasonal variations, also with modest excursions of the order of 20-25 ° C. These deformations must be free to practiceotherwise if the constraints are very rigid they turn into internal voltages which, depending on the degree of hyperstaticity, can reach high values.

In fact, this has implications especially in hyperstatic structureswhich have degrees of bond higher than those necessary to guarantee its balance, where the effects of the thermal distortions always translate into a regime of internal stresses. The design must take into account the effects of these thermal distortions, and in particular the constraint system must be designed to satisfy these movements.

What happenedHowever, cannot be attributed to climate change and the heat wave that is affecting Italy in this periodwith temperatures higher than the seasonal average: otherwise, if it were a general problem, we would not have witnessed the sole failure of the structure of the General Segna. There are many metal structures positioned outdoors around the world subject to more torrid climates than ours who have never expressed critical issues regarding thermal variations or high values ​​of external temperature. It is not the absolute value of the temperature, but the degree of hyperstaticity of the structure that can determine the value of the states of thermal coaction.

Furthermore, both the wind and the thermal distortions, even if within the resistance limits, can over time trigger phenomena of cyclical effort within the metal structuressuch as to lead to a collapse (partial or total) even with loads far from the limits of resistance. The structure subject to the failure is recently built, in operation for a few years, so fatigue phenomena could be induced not by the absence of maintenance but possibly from building or design defects.

Design complexity and risk margin

Useless at the moment to support theses without technical reliefs and evidencewhich will probably require a lot of time of investigation. The design of the support structure of the large general signs, also considered the position at the top of a skyscraper, takes place in a context dominated by a computational complexity. This complexity must not represent a justification for what happened, because surely the technical responsibilities will be ascertained, however it must represent a brake on immediate conclusions in front of a single photographic report reported by the press. Pending the results of the technical appraisals, it should be remembered how the complexity of the structure may have possibly induced more causes between them related.

Premises underlining how modern skyscrapers are real engineering challengeswhich as of course never lose sight of the safety priority required by the technical standards, without however forgetting that Safety is a semi -profound concept which accepts a very limited but existing margin of error. The Zero risk does not exist in any scientific and technical fields.

As repeated several times in these days by several technicians, “A tree that falls compared to a growing forest makes more noise.” In the face of the limited cases of collapses, there are millions of similar structures that perform their exercise very well, guaranteeing people with safety to people. A concept similar to aerial tragedies: despite every now and then, a plane crash occur (with almost fatal consequences), the statistics show that air transport is among the safest. Engineering, like medicine, is a reliable science but still affected by a minimum margin of human error. As there may be risks, albeit minimal, deriving from an operation in the operating room, similar risks, always limited, can also exist, also in the design of very complex structures.

The steps forward in terms of safety and reliability have matured over time also thanks to the understanding of errors. From the collapses you learn to avoid others in the future. So engineering has also progressed over time. Unfortunately in recent times we have witnessed more frequently in collapses due to the absence of maintenance: even if This is not the case of the news of the general tear, that of maintenance is on the contrary a lesson that is hard to learn.