Local, seismic improvement and adaptation interventions: differences and objectives


Emma Potter

Chapter 8 of the Technical Standards for Construction (NTC2018) addresses the delicate issue of interventions on existing buildingsclassifying the works in three categories:

  • «repair or local interventions: interventions that affect individual structural elements and which, in any case, do not reduce pre-existing safety conditions;
  • improvement interventions: interventions aimed at increasing pre-existing structural safety, without necessarily reaching the safety levels set out in § 8.4.3;
  • adaptation interventions: interventions aimed at increasing the pre-existing structural safety, achieving the safety levels set out in § 8.4.3.»

Punctual or local interventions

Certainly the most common are the so-called “point-in-time or local interventions”described in par. 8.4.1 «interventions of this type will concern individual parts and/or elements of the structureThey must not significantly change the overall behaviour of the construction and are intended to achieve one or more of the following purposes:

  • restore the initial characteristics of damaged elements or parts, compared to the configuration prior to the damage;
  • improve the resistance and/or ductility characteristics of elements or parts, even if not damaged;
  • prevent local collapse mechanisms;
  • modify an element or a limited portion of the structure.»

To give some examples, replace a floor or roof, repair a cracked picture, consolidate a beam or pillar, insert a tie rodthey are interventions called local because they concern individual resistant elements and not the entire structural skeleton. Although these are mainly static consolidation works, their design must also take into account the contribution offered in reducing seismic risk. A tie rod, for example, as well as a top curb can offer an effective contrast to the overturning out of plane of the walls during an earthquake.

Even if a comprehensive modeling and analysis of the building is not required, «in the case of local strengthening interventions, aimed at improving the mechanical characteristics of structural elements or limiting the possibility of local collapse mechanisms, it is necessary to evaluate the increase in the local safety level.» Remembering, in fact, that most of the serious seismic damage it happens for out-of-plane kinematicswhich can be easily countered with local interventions.

Improvement interventions

If the goal is instead a Increased seismic resistance of the entire structure, the intervention falls within the improvement and must concern the reinforcement of the mechanical performance of the entire resistant skeleton. injections or the application of reinforced plasters on the walls, or the gluing of carbon fabrics on the reinforced concrete elements are some examples of processes that, involving all the static elements, contribute to increasing the seismic safety of the entire building. Pay attention to do not limit the improvement intervention on only one portion of the structure, because it would result in a excessively rigid part compared to the unconsolidated one, creating imbalances in the dynamic response.

In the field of seismic improvement, as well as for adaptation, the essential premise is that the structure has the necessary box-like behaviorwhich can be acquired by carrying out local interventions, such as to allow the activation of the floor mechanisms only. The designer is required to build the calculation model of the entire building.

The par. 8.4.2 iindicates the minimum seismic safety objectives to be pursued with seismic improvement interventions, assessable in z ratioAND between the maximum seismic action that the structure can withstand and the maximum seismic action that would be used in the design of a new building. «For class III buildings for school use and class IV buildings the value of zANDfollowing the improvement interventions, must still be not less than 0.6, while for the remaining class III buildings and for those of class II (civilian dwelling, for example) the value of zANDalways following the improvement interventions, must be increased by a value in any case not less than 0.1».

Local interventions, improvement and seismic adaptation: differences and objectives Photo 3

The adaptation interventions

The par. 8.4.3finally, introduces the interventions of adjustment as the last stage of the seismic risk reduction process to bring the existing building to safety standards equal to or close to those of new construction, required in the following cases:

  • superelevation (zAND ≥ 1);
  • volumetric expansion without seismic joint (zAND ≥ 1);
  • changes in intended use with increases in global vertical loads on the foundation greater than 10%(zAND ≥ 0.8);
  • structural interventions aimed at transforming the building through a systematic set of works that lead to a structural system different from the previous one (zAND ≥ 1);
  • make changes to the use class that lead to class III buildings for school use or class IV buildings (zAND ≥ 0.8).

The designer will therefore have to justify the choice of one of the three typologies of intervention in relation to the aims of the project, remembering that it remains his responsibility to identify the right degree of seismic safety which the building must guarantee in relation to the intended use and to thecrowding.