The beam-column node represents a critical point of the reinforced concrete (RC) frame when subjected to seismic actions of such high intensity as to compromise structural safety.
In fact, most of the existing reinforced concrete buildings were designed in past decades when the sizing (and the related reinforcements) was intended to support mainly gravitational loads, without particular anti-seismic measures, often with poor concrete properties and the use of smooth bars, without transverse reinforcements at the nodes (the stirrups of the lower column continued in the node above with the same pitch).
These deficiencies lead, in the event of a strong earthquake, to the premature damage of the nodes and a consequent limitation on seismic capacity to dissipate energy by the structural elements of the frame.
Only the NTC 2008 introduced for the first time the design and verification of the node in application of the Hierarchy of Resistance Theoryfor which in a structural system the ductile failure mechanisms must manifest themselves before the brittle ones. Specifically for RC frames, a correct hierarchy is established whereby the plastic hinges must form first at the ends of all the beams and only afterwards the plastic hinges at the base of the pillars. In this way, soft plane collapses (strong beam / weak pillar – Fig. 1a) will be avoided. The node will be the most resistant element of the RC frame and the last to fail in the event of an earthquake (Fig. 1b).
Carbon Fiber Wraps (FRP)
THE composite materials have the advantage of not increasing mass and not modifying the rigidity of the structure. Unidirectional carbon fabrics can be used by extending the reinforcement that starts from the beam and the pillar and confining the node itself as much as possible, together with quadriaxial fabrics to be applied on the node panel (according to the methods indicated in Fig. 2) to absorb oblique stresses and tensions.
Section bulge
The section thickening It is a traditional technique that involves the use of a high-performance concrete to increase the compressive strength while the contribution of tensile strength is entrusted to the possible additional reinforcement that will be inserted inside the bulge (Fig. 3). It is a type of intervention that involves an increase in resistance by stiffening the structural frame. There are also very high-performance and very high-ductility concretes that, being fibre-reinforced, do not require reinforcement (FRC composite systems).
Steel plates or strips
There are several on the market Technical steel solutions for node confinement. For example, using a custom-made plate, anchored through a series of connectors fixed with epoxy resin (Fig. 4).
Or by laying prestressed stainless steel strips, creating an additional bracket and placing it in coaction (Fig. 5).
As always applies to existing buildings, the choice and variants of the technique will depend on the individual case and the construction typology in order to design the intervention to measure.
