Resistance hierarchy: how to design seismic safety in the disproceive field

The importance of the ductility of the structures

Let’s start with the assumption that In the case of high magnitude earthquakes, new buildings are alsodesigned with modern anti -seismic criteria, However, they will suffer damagein accordance with the semi -probabilistic design for which it is Impossible to reduce damage to zero In case of a strong earthquake, if not with economic costs (and sizing) incompatible with the risk.

BetterIndeed, allow the structure a “controlled” dissipation of energy in the form of deformation and therefore of damagein ways and limits such as to always preserve at least the safeguarding people’s lives And, in the possible, reduce the extent of the damage to easily repairable costs and times. This is mainly valid for seismic events of power equal to or higher than that expected in the specific geographical area in which the structure is located, that is, for very important earthquakes, whose event takes place statistically within a return period that can also be superior to the useful life of the building.

In the seismic field there is a need for greater ductility of the structures own To avoid fragile breakswhich do not give notice of notice before occurring, but such as to irreparably compromise the overall safety of the building towards the risk of total collapse. By flexibility, however, we mean the ability to deform in the plastic fieldbeyond the elastic limit, dissipating a large amount of energy through a progressive damage that offers warning signals And it grants people to save themselves, without drastic resistance falls.

There design in capacity interests i new buildings both in reinforced concrete (ca) and in steel or woodeven if the defects in the absence of it, characterizing the existing building heritage, mainly concern the buildings in CA

Fragile collapse e soft plan

One of the recurring examples of fragile collapse after a strong earthquake is the so -called soft planwhen The building “sued” by actually crushing a plannormally the ground floor (pilotis floor) or the immediate first floor. The structure of the other floors does not seem to suffer irreparable damage, except unfortunately of the fatal collapse of the soft plan with very serious consequences for the safety of people who live in that plan, in addition to the remaining people who together with the structures undergo the lowering of a share (fig. 1 and 2). This dangerous collapse mechanism is due to the construction of the past, without anti -seismic details, to the fact that The fragile damage (often by cutting) of the pillars takes place before the flexible one of the beamswhich in these cases almost always remain almost intact (fig. 2).

Design in capacity with the hierarchy of resistance

On the contrary, the design in capacitythrough the principle of resistance hierarchy, guarantees that the resistance of fragile elements is greater than the ductile onesfor the purpose of exploit all ductility and possible dissipation of the structure avoiding the trigger of fragile mechanisms.

There resistance hierarchy allows you to Designing the armor of the frame in CA so that the fragile breakdown mechanismslike those from Taglieti actions, can only occur after having exhausted all the dissipative capacity of the more ductile breaks in flexion and die -castle.

The regions affected by the formation of plastic hinges must be equipped with adequate ductility and dissipative capacity, guaranteed through constructive details in particular in Pilastro -pillar connection nodes. The nodal areas must in fact have to be able to transfer cyclical toy actionsinduced by the plasticization of the beams, without undergoing excessive deformations or bars (fig. 3) through adequate confinement with specific transversal armor.

More precisely, within the same ductil mechanisms, one is established hierarchy for which the plastic hinges are activated in the dynamic response of the structural frame (a sort of fuses) At the ends of all beamsand only After their complete plasticization, the plastic hinges at the base of the pillars can be formed.

This design hierarchy identifies the ductual local mechanisms to be favored and the structural elements in which they must take place within the structuretherefore preventing that during the seismic action the pillars plasticino before the beams giving triggering the dangerous effects of the soft plan.