How to stiffen existing floors

How to stiffen

The DPCM 09/02/2011technical directive on the evaluation of the seismic safety of historic buildings under protection constraint, recalls that «(…) The role of the floors in the seismic behavior of masonry constructions is to transfer the horizontal actions of their competence to the walls arranged in the parallel direction to the earthquake (…) The floors must be effectively connected to the wall walls, through support (…) they can constitute a further constraint for the walls solicited by orthogonal actions on their plan (…) ” .

Before being stiffened, the attic needs to be improved connections of his beams on the support wallsthrough constraints that eliminate the danger of fray during seismic action. In the case of a wooden floor, this can take place, for example by inserting punctual metal garrisons (Figure 1, as an example of the possibility of intervention).

How much stiffen? And how? It depends on the type of existing floor. Pre-modern floors such as those in putrelle and tavelloni (or voltons) can be stiffened with theInserting a concrete slab (better lightened) Army with metal or fiberglass network. The intervention acquires a sense only if the slab becomes Collaborating with the putrelle by applying metal pegs on them, and if it comes connected to all perimeter walls through punctual connections. It is always necessary evaluate the increase in seismic masses resulting from this solution within the overall dynamic behavior of the structure (Figure 2).

This technique of EstraDossal Reinforcement It can also be applied to the latest lateral-cement floors, often present in historical construction, for which there is also the variant today through chemical connections with the application on the extradossal surface of details Anchoring resins the co -faulty concrete jet instead of applying mechanical anchors.

Diversified instead the Solutions for the ancient wooden floorsif they are still in static conditions to be reused. In addition to the same intervention concerning the collaborating concrete slab («(…) The effects of this intervention must be evaluated in relation to the specific conservation needs»), There are more traditional solutions applicable to dry such as the laying of a double or triple crossed tablepossibly also made collaborating with the beam by inserting connectors (Figure 3). The latter solutions are certainly more compatible with the original construction techniques and bring less seismic masses to the structure.

How much stiffen?

The old NTC08 required par. 7.2.2 which, for the purposes of planimetric symmetry, “(…) The horizons can be considered infinitely rigid on their plane with respect to vertical and sufficiently resistant elements. (…) ” A rather rigid out out, in which An adaptation of the old floors to the same stiffness of a modern deck in lamentocement was strictly required.

“(…) The horizons can be considered infinitely rigid in their plane, provided that they are made of reinforced concrete, or in lateral-cement with at least 40 mm thickness, or in a mixed structure with reinforced concrete slab of at least 50 mm of thickness connected by cutting connectors appropriately sized to the structural elements in steel or wood and as long as the openings present do not significantly reduce the stiffness significantly (…) “. Although the latter statement is still present and valid in par. 7.2.6 of the current NTC18, however it is fortunately changed the final goal limiting himself to a Improvement of the stiffness of the decksince in many cases this “therapeutic fury” could lead to invasive interventions and increased seismic masses.

As in any project of consolidation and seismic improvement on the existing building heritage, Each solution must be evaluated and compared in compliance with the building type. “(…) For the aforementioned reasons, a limited stiffening of the floors is useful, whose effects must be evaluated, which is inevitably associated with an increase in the resistance of the elements. Only in particular cases, on the other hand, a significant stiffening of the floors on their plan is necessary, with the aim of starting the seismic action between the different walls; In most cases this distribution leads to concentrating forces on the strictest elements, anticipating its breakage, and on the perimeter elements, in the case of planimetric irregularities with accentuation of the torsional effects (…) ” (DPCM 09/02/2011).