Living Box – Prefabricated modular wood-house system - Part VII - KEP energy Magazine - KEP energy

Living Box – Prefabricated modular wood-house system

English translation from a paper published in 'Bollettino degli Ingegneri n. 6/2016 pp. 3-18'

Structural hints

Not knowing the Living Box geographic location, the project is intrinsically decontextualized from the structural point of view. As a consequence, it has been necessary check the possibility of install the building in various geographic locations, taking in account at the same time various living units combinations. To this aim, an upper limit of the structure bearing capacity has been established, referred to the steady structural stresses. The upper limit has been calculated in reference to a multi-floors configuration, composed from the vertical overlap of four living units.

The structure bearing capacity, referred to the dynamic structural stresses, also has been calculated, in order to verify the building ability of hold out to the earthquakes stresses. To this aim it has been hypothesized to build the Living Box in the city of Reggio Calabria, that is in a city characterized from a high earthquakes dangerousness. The Italian Institute of Geophysics and Volcanology ranks this city in the “1st seismic zone”, having Peak Ground Acceleration (PGA) beyond 0,25 g (g is the gravity acceleration, having conventional value equal to 9,8 m/s2).

The building is composed from the combination of two different structure typologies:

1 – “A-type” cell: discrete structure, formed by a frame of lamellar timber pillars and beams;

2 – “B-type” cell: continuous structure, formed by a box of lamellar timber crossed plates.

The “A-type” cell frame is formed by GL24H lamellar timber, assembled on-site, whilst the “B-type” cell box is formed by C24 lamellar timber crossed plates, known as “X-lam” or “Cross Laminated Timber”, assembled in factory. The box is split in two different parts, which must be connected on-site, due to the size limit of the standard containers.

The building ability of hold out to the earthquakes stresses has been verified through a finite elements method analysis. The analysis has been carried out compiling some software libraries, specifically developed for the lamellar timber structure components.

The structural stresses, both in the Stati Limite Ultimi (SLU) condition and in the Stati Limite di Esercizio (SLE) condition, have been calculated respectively through two different building models, diversified between them on the basis of the structure stiffness properties. In the first one case (SLU calculation) the structure is modelled as non-deformable, that is without take in account the real operational deformation of the metallic joints. The stiffness of each floor is due both to the lamellar timber plates stiffness and to the junctions one. In the second one case (Stato Limite di Danno - SLD calculation of the shifts) the metallic joints stiffness Kser is calculated in the modelling first stage, and subsequently a virtual material is created, through which are modelled the lamellar timber plates. The mechanical properties of this virtual material are representative both of the timber deformability and of the metal one, in order to define an equivalent cut resistance Geq.

After the lamellar timber frames and plates calculation, have been sized the metallic joints.

The joints among pillars and beams, and these among beams and plates have been calculated on the basis of assembling simplicity and quickness. Respectively, in the first case they are formed by embedded aluminium brackets and junction plugs, whilst in the second case from 45° sloped self-tapping screws.

The joints among the lamellar timber plates which compose the box structure of the “B-type” cells are formed by conventional metallic junctions, as hold-downs, brackets and screws, in order to equilibrate the sliding and lifting pushes due to the horizontal stresses. During the sizing stage has been particularly verified the joints resistance to the building uplift. The finite elements method analysis showed that the first three building floors are strongly stressed, from a structural point of view. Consequently, it is not possible use standard systems for connect the lamellar timber plates among them, as perforated stripes or commercially available hold-downs.

However, the ad-hoc design of brackets able to resist to the building floors stresses has been considered not suitable. The brackets sizes would have been disproportionate in respect to the plates sizes. Moreover, these components would have been expensive in the manufacturing phase and non-manageable in the installation phase.

In order to solve the problem, a continuous floors connection has been calculated, formed by high-resistance threaded bars, which starts from the building foundation. The lamellar timber plates are connected at each floor to the threaded bars through commercially available hold-downs, whilst the vertical junction among the threaded bars is formed by threaded sleeves.

In this way the lifting force pushing on the hold-downs is only that of the specific floor, and the amount of the traction stresses is balanced from the threaded bars. In the on-site installation phase, the component assembly is easy, because the “B-type” cells come to the yard already equipped with hold-downs. So, the component assembly is only the cells overlap and the connection among the specific floor hold-downs and the threated bars.

Finally, the interaction between the bearing structure and the foundation ground has been analysed, considering two different foundation structure configurations: the first one is formed by a reinforced concrete slab, whilst the second one is formed by a reinforced concrete square mesh of T-inverted section beams. The reason of these two configurations is the intrinsic building decontextualization, or rather the lack of specific information about the Living Box geographic location.