EKU curtain wall seismic test – Interview with Engineer Antonio Bonati, Researcher for ITC-CNR

Engineer Antonio Bonati, Head of the SG1 Research Unit “Materials, Components and Technologies of New Conception for Safe Construction with Elevated Performance”, explains in detail the nature of the new equipment for seismic testing and the test carried out on the EKU 50 Glass curtain wall.

How long has this new seismic testing equipment been in operation? What renders it innovative in respect to the tests of other international testing laboratories?

The façade seismic testing equipment was evaluated in February 2017. The factors that render it unique are linked to the dimensions, the displacement capacity of the seismic beams and the possibility of assessing the performance against external atmospheric agents, both before and after seismic simulation. The equipment allows you to stress both portions of the curtain wall across two storeys in both a static and dynamic manner.

The two “seismic” beams, which simulate the first and second level slabs above the starting slab, are equipped with hydraulic jacks that allow them to be moved in and out of the plane, reaching frequencies of 20 Hz. The control system also makes it possible to reproduce displacements deriving from the accelerograms of real seismic events.

What kind of test was carried out on 24th May with the curtain wall of the Profilati Group?

A test was carried out on the curtain wall of the Profilati Group in compliance with the American standard AAMA 501.6-01 – “Recommended dynamic test method for determining the seismic drift causing glass fallout from a wall system”. This test is able to determine the storey drift (as the relative displacement between the two levels of the building to which the curtain wall is bound) due to which fragments of the façade fall. To obtain this critical displacement, a temporal history of displacements (a “crescendo test”) is applied to the portion of the curtain wall, through the seismic beams.

The crescendo test is characterised by a concatenated series of sinusoidal cycles with a gradually increasing amplitude up to 150 mm, having a variable frequency from 0.8 Hz (for displacements up to 75 mm) to 0.4 Hz (for displacements greater than 75 mm). It must be conducted without interruption until one of the critical conditions indicated in the AAMA standard occurs, with the most important of which being glass fragments falling.

What degree of resistance did the curtain wall provide? Is it possible to say that this curtain wall resists a seismic event up to x degrees on the Richter scale?

During the “increasing test”, the first glass to fall was upon a displacement equal to 120 mm. It is not possible to generically refer to a value of the magnitude of a seismic event expressed through the Richter scale. To reproduce the behaviour of the curtain wall on the test equipment and render it similar to what would be obtained on a real building, it is actually necessary to analyse the conduct of the structure of the building itself when subjected to seismic actions, given that the storey displacements are relative to such. It must be said that displacements equal to those obtained in the test are so high that most of the buildings would already be severely damaged structurally, rendering meaningless the achievement of a better curtain wall performance.

How should a curtain wall behave during an earthquake? What is the greatest danger?

The approach to the assessment of non-structural elements that are potentially dangerous in terms of safety (think of glass or a portion of the curtain wall falling on people fleeing the building) can be carried out by setting – in analogy with the analysis of structural elements – service limits and ultimate limits. If, upon small events (actions), the maintenance of all or the most important functions of a curtain wall (such as water tightness and air permeability) are required, for events of greater intensity, greater damage will be considered as acceptable (the loss of local stability that shall require more or less significant maintenance operations).

What does Italian legislation establish in this regard? And what about international legislation?

The Technical Standards for Constructions (Ministerial Decree 14.01.2008), which is currently in force, require the verification of construction elements without any structural function, the damage of which can cause harm to things or people (such as the façades, for example), due to seismic actions relating to each limit state taken into consideration (being operational or ultimate). Such elements must be able to maintain their self-supporting capacity against vertical loads, even in conditions of high deformity.

At an international level, there are different approaches for the design and verification of non-structural components, also generally based on the definition of limit state criteria and on the requirements that such components must satisfy for the seismic actions specified, imposing precise limits on inter-storey displacements and particular tolerances in the design of construction details.

Are any other tests foreseen? What research objectives have you set yourselves?

Research in the field of seismic behaviour of curtain walls and more generally of non-structural elements that can impact on safety, is of great interest today, as is the assessment of the resilience of building structures and, on a larger scale, of portions of buildings. The experiments thus far carried out on the seismic behaviour of these systems are, in fact, very limited and in most cases, limited only to certain components (such as the glass-frame interaction).

However, in view of the increased frequency of earthquakes occurring and the extent of the damage that such events cause on people and buildings, there is an exponentially greater interest of both the regulatory bodies and manufacturers in the construction sector regarding the study and analyses of the consequences that these catastrophic events can generate on the constructed environment. The innovative equipment that has just been created in the ITC laboratories will certainly render it possible to make a strong contribution to research in this field.