Static and seismic functions of the floor
THE attics in fact they don’t have Alone the function of supporting the vertical loads weighing on them, they also play a crucial role in correct distribution of horizontal actionssuch as seismic ones, to be directed effectively towards the resistant vertical elements on which the floors are supported.
This diaphragm task is more crucial within masonry buildings, whose box-like behaviour, i.e. the mutual binding of all the resistant elements (horizontal and vertical) can have a significant impact on reducing seismic damage (fig. 1a-b).
The Prime Ministerial Decree 02/09/2011 “Guidelines for the assessment and reduction of seismic risk of cultural heritage aligned with the new technical standards for construction” remember that «(…) the role of floors in the seismic behavior of masonry buildings is that of transfer horizontal actions of their competence to the walls arranged in the direction parallel to the earthquake (…) The floors must be effectively connected to the wallsthrough a support (…) furthermore they can constitute a further constraint for the walls stressed by actions orthogonal to their plane (…)”.
When designing the static reinforcement of the floor it is therefore important to evaluate whether it is possible also improve the connections of the deck to the perimbehind the wallusing constraints that prevent the walls from tipping out of plane (Fig. 1a) and the sliding of the beams during seismic action (Fig. 1c).
Reinforcements for wooden floors
In the case of a wooden atticthis can happen for example by inserting punctual metal devices installed at the head of the beams (Fig. 2) or binding the added planking at 45° to the wall perimeter (with a stiffening function) using simple L-shaped metal profiles anchored to the wall and nailed to the boarding (Fig. 3).
The main problem of wooden structures lies in the rot and material degradationdue to humidity, infiltration and attack by plant organisms and in the excessive deformations that can accumulate over the centuries. However, wooden structures do not always need to be replaced: if the deflection and degradation are not particularly pronounced and do not compromise a significant residual resistance of the wooden element, it can be statically supported by additional structural elements collaborating with it, such as for example metal profiles supporting the main beams and/or the installation of a second collaborating plank (Fig. 3).
It is also possible to intervene with the casting of a collaborating slab 5 cm thick in lightweight concrete (or NHL structural mortars), reinforced with galvanized metal or fiberglass mesh, made integral with the beams using specific connectors (Fig. 4) grouted with resin on the extrados of the wooden element. In this case the resistant section is modified, which changes from a single one to being a mixed wood-concrete one, with benefits in terms of reduction of deformability and better exploitation of the mechanical resistance of the two materials since it will make the concrete work in compression and the wooden beam predominantly at traction.
Connection between floor and walls
The connection with the perimeter walls it will take place through bars embedded in the walls with a constant pitch and superimposed on the slab reinforcement for an appropriate length, or with specific “dovetail” recesses in the masonry in which the connection reinforcement with the slab is inserted. It is very important to make the connection between the load-bearing sections, otherwise the slab above will only constitute an additional weight weighing on the original wooden structure.
The solution adopted therefore presupposes a static involvement of the floor beams, the effectiveness of which must be previously assessed by the designer based on the level of degradation of the wood.
Application to brick-cement floors
This technique of extradosal reinforcement it can also be applied to the most recent brick-cement floors, often present in historic buildings, for which today there is also the variant through chemical connections with the application on the extrados surface of particular resins anchoring the collaborating concrete cast instead of applying anchors mechanics.
The proposed interventions also aim to increase stiffness of the deck as required by the current NTC2018, with the warning not to seek infinite stiffness at all costs, as required by the previous 2008 technical standards, but rather intervene to improve it especially on very deformable floors. «(…) For the aforementioned reasons, a limited stiffening of the floors is useful, the effects of which must be evaluated, which is inevitably associated with an increase in the resistance of the elements. Only in particular cases is it necessary to significantly stiffen the floors in their own plane, with the aim of distributing the seismic action between the different walls; in most cases this distribution leads to concentrating the forces on the more rigid elements, anticipating their failure, and on the perimeter elements, in the case of planimetric irregularities with accentuation of the torsional effects (…)” (DPCM 02/09/2011).
As in every structural recovery project on existing building stock, every solution must be evaluated and related to static critical issues respecting the original construction typology, avoiding interventions that are too invasive.
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