Avoiding a Flexible Storey Plastic Mechanism – Eurocode 8
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4.4.2.3 Global and local ductility conditions (4) Unless otherwise specified in Sections 5 to 8, to satisfy the requirement of (3)P, in frame buildings, including frame-equivalent ones as defined in 5.1.2(1), with two or more storeys, the following condition should be satisfied at all joints of primary or secondary seismic beams with primary seismic columns:
ΣMRc is the sum of the design values of the moments of resistance of the columns framing the joint. The minimum value of column moments of resistance within the range of column axial forces produced by the seismic design situation should be used in expression (4.29); and ΣMRb is the sum of the design values of the moments of resistance of the beams framing the joint. When partial strength connections are used, the moments of resistance of these connections are taken into account in the calculation of ΣMRb. |
Strong Column Weak Beam
To favour energy dissipation through plastic hinges in beams, EC8 imposes, for High and Medium Ductility structures, that the flexural resistance of beams, in a seismic situation, must be 1.3 times greater than the resistance of columns. As illustrated in the image below, it is clear that in a framed or frame-equivalent structure, this condition ensures greater energy dissipation until a mechanism is reached.
Implementation in SAP2000 and ETABS
This condition is checked during the structural design (Concrete Frame Design). If both the Column and Beam elements have been defined for design as "To be Designed," the program will determine the necessary reinforcement at the ends of both elements to satisfy the required condition.
Similar to what is presented in the article Seismic Capacity Design – Eurocode 8, as armaduras de flexão das vigas podem ser fixadas a um limite inferior.
No SAP2000 esta opção será útil caso se pretenda estabilizar uma solução de armadura para as vigas e assim evitar que a cada iteração de cálculo seja gerada nova quantidade de armaduras nas vigas.
O ETABS apresenta uma opção de considerar ou não esta verificação no dimensionamento das armaduras dos pilares:
Implementation in VIS
In complementarity or independently, the Software VIS Concrete Design presents itself as a powerful and reliable alternative for designing reinforced concrete structures following all the provisions of the current Eurocodes.
On this specific topic, VIS has the added value of being able to simply change the reinforcement of a given beam without compromising the already stabilized final reinforcement solution.
