Why do buildings fall? Seismic criticalities of reinforced concrete structures


Emma Potter

How are buildings built after the war made?

It is in fact about frame structures with beams often just present in one directionon which they are supported brick and concrete floors with joists in the transverse direction. The connection between longitudinal frames is guaranteed only by floor and edge beams.

They are structures designed for withstand only gravitational loads and horizontal wind actions. Buildings more than three stories tall had bracing partitions (stairwell-lift).

The sizing occurred by separately evaluating the individual structural elements with the related areas of influence of the loads.

There section of the pillars it was calculated taking into account only the axial force deriving from the vertical loads and overloads acting within the area of ​​influence of the floor, neglecting bending and shear stresses deriving from the overall behavior of the frame.

Likewise the beams hey attics they were sized for vertical loads, and the bracing partitions were like brackets fitted to the base, neglecting the effects due to the interaction between the other structural elements.

The infills they were made with a double lining of perforated bricks and an internal empty box, without effective connections with the structural frame. The ground floor of tall buildings, the so-called pilot planwas often free of rear-end collisions.

Buildings with a more vulnerable pilotage plan

What is described represents the most frequent construction typesized in absence of anti-seismic criteriatherefore without bidirectional frames and adequate confinement brackets, resulting very vulnerable in the presence of dynamic actions as emerged from the damage of the latest earthquakes.

In particular, the correct sizing of the brackets guarantees beams and pillars not only the required shear resistance, but also the indispensable confinement of the beam-pillar nodes for the correct dissipation of seismic energy. This way it also comes instability avoided of the compressed longitudinal reinforcement bars of the pillars under bending stress during the earthquake.

There fragility and the lack of effective connections of the external infills has often favored their expulsion in conjunction with the earthquake, with serious problems of public safety and subsequent usability of the building (Fig. 1).

THE damage assessments in seismic areas they have highlighted how solid and effectively connected brick infills to the load-bearing structure they offer a positive contribution in resistance and rigidity, especially where they are present on the ground floor. On the contrary, the buildings with pilot plan have demonstrated significant vulnerabilities in correspondence with the weak surface also due to the irregular distribution in elevation of the infills themselves.

The importance of the hierarchy of resistances

Recent technical standards have introduced the concept of hierarchy of resistance to avoid tragic “soft” collapses, i.e. due to the lowering of a floor of the building (Fig. 2).

Through the sizing and arrangement of the reinforcements, a hierarchy of the crisis of the elementswhich must always prove flexible, favoring the formation of the plastic hinges first at the ends of the beams and not in the pillars, which must have a greater moment of resistance compared to that of the beams to which they converge at the node.

Why do buildings fall?  Seismic criticalities of reinforced concrete structures Figure 2

The seismic criticalities of reinforced concrete buildings

THE greater damagetherefore, can be found in the early shear failure for small diameters and wide pitch of the brackets (Fig. 3)or to flexion of the pillars due to the inadequacy of the longitudinal reinforcements to also withstand bending stresses. In the most serious cases, with loss of balance due to rotation and consequently collapse of the structure as the confinement effect of the node is eliminated (Fig. 4).

In addition to this, the irregularities in plan and height they contribute to accentuating the damage to the structures, also due to torsional actions. The staggering of the floor levels causes a strong shear stress on squat pillars. In other situations, some buildings with very slender columns have been seriously damaged due to buckling buckling due to vertical earthquake overload.

Finally, the lower resistance of older reinforced concrete elements e without adequate maintenance over timewas also favored by serious phenomena of material degradationwith expulsion of the concrete cover due to poor design thickness and corrosion of the reinforcements (Fig. 5)nullifying any residual resistance in the event of dynamic action.

Why do buildings fall?  Seismic criticalities of reinforced concrete structures Figure 3
Why do buildings fall?  Seismic criticalities of reinforced concrete structures Figure 4
Why do buildings fall?  Seismic criticalities of reinforced concrete structures Figure 5