Analysis and modelling of the seismic behaviour of high ... - Ingegneria
Analysis and modelling of the seismic behaviour of high ... - Ingegneria
Analysis and modelling of the seismic behaviour of high ... - Ingegneria
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5. SEISMIC BEHAVIOUR OF RC COLUMNS EMBEDDING STEEL PROFILES<br />
design rules able to improve <strong>the</strong> global safety <strong>of</strong> reinforced concrete structure,<br />
without great changes <strong>of</strong> <strong>the</strong> traditional constructional practice. The introduction <strong>of</strong><br />
this new concept can obviate <strong>the</strong> most frequent failure mode <strong>of</strong> RC buildings, <strong>the</strong><br />
so-called s<strong>of</strong>t storey mechanism, which consists in a localisation <strong>of</strong> buildings<br />
<strong>seismic</strong> deformations <strong>and</strong> rupture in <strong>the</strong> one or two lower storeys. The cause <strong>of</strong> <strong>the</strong><br />
manifestation <strong>of</strong> this mechanism is <strong>the</strong> surplus effort <strong>the</strong> columns at ground level<br />
have to st<strong>and</strong>, basically due to a big difference in stiffness between <strong>the</strong> first <strong>and</strong> <strong>the</strong><br />
upper storeys. After some <strong>seismic</strong> events, experience on <strong>the</strong> spot provided<br />
evidence <strong>of</strong> a collapse <strong>behaviour</strong> not predicted at <strong>the</strong> design stage for many RC<br />
buildings. More precisely, some local brittle mechanisms occurred in <strong>the</strong> columns,<br />
generating <strong>the</strong> complete collapse <strong>of</strong> most buildings: 90% <strong>of</strong> failures in Kocaeli 1999<br />
Turkey earthquake were <strong>of</strong> this nature, as illustrated in Figure 5.2.<br />
Figure 5.2. Collapse <strong>of</strong> building in Kocaeli Turkey earthquake (1999) due to s<strong>of</strong>t-<br />
storey mechanism<br />
The “dirty job” is completed by <strong>the</strong> poor capacity <strong>of</strong> <strong>the</strong> reinforced concrete RC<br />
columns to develop adequate ductile resources as plastic elongation <strong>and</strong> by <strong>the</strong><br />
difficulty in realising RC structures that behave in accordance with <strong>the</strong> Strong<br />
column–Weak beam Principle. The first aspect is well known by <strong>the</strong> in field<br />
engineers since <strong>the</strong> only ductile resource for concrete members, when subjected to<br />
tensile <strong>and</strong> compressive stresses due to cycling loading, arises from <strong>the</strong> plastic<br />
yielding <strong>of</strong> <strong>the</strong> steel re-bars; <strong>the</strong> concrete material alone showing typical brittle<br />
failure. The second aspect may be fulfilled following <strong>the</strong> prescriptions stated by <strong>the</strong><br />
Capacity Design Principle built-in most Codes, which recommend <strong>the</strong> prevention <strong>of</strong><br />
any kind <strong>of</strong> global collapse by concentrating <strong>the</strong> hinges development in some<br />
strategic points <strong>of</strong> <strong>the</strong> structure. In few words, <strong>the</strong> dissipating mechanisms are to<br />
form in <strong>the</strong> beams <strong>and</strong> not in <strong>the</strong> columns or joints, providing ductility for beams<br />
<strong>and</strong> a sort <strong>of</strong> over-strength for columns <strong>and</strong> joints. An explanation <strong>of</strong> this S<strong>of</strong>t<br />
Storey Mechanism was pursued <strong>and</strong> <strong>the</strong> answer obtained, taking into account <strong>the</strong><br />
role played in practice by <strong>the</strong> in-fill walls combined with <strong>the</strong> usual weakening <strong>of</strong> <strong>the</strong><br />
lower storeys due to <strong>the</strong> presence <strong>of</strong> wide openings as generally requested by<br />
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