Frequent Occurrences of Major Earthquakes

Question: The easiest way to define the ductility is in terms of displacements, as the maximum displacement divided with the displacement during the first yield. Answer: Eurocode 8 intents to assure life safety along with damage restriction that can be caused by frequent occurrences of major earthquakes. Standards laid down allow dissipation of seismic forces either through ductile damping or elastic behaviour, with preference towards the first technique. Ductility is the ability of a structure and its parts to resist damage through extreme yet effective deformations and in seismic engineering, expression of ductility is done through ductility demand, a term used to define maximum ductility a structure can achieve and by available ductility, that is the maximum zero damage deformation a structure can sustain. Thus, creation of a stable and reliable energy absorption system that has no impact on any critical inertial load bearing component is the goal of the standard and rules have been laid down for such ductility model designs that dont inhibit loading. For reinforced concrete structures, delay circles can reduce capacity of such critical zones and ensure plastic behaviour and efficient designing rules can prevent destruction associated with brittle structural failures like concrete shearing, crushing and reinforcement bending. Three levels of energy absorption are adapted:- Low Class Ductility with no delayed ductile properties and the structures capacity resists seismic forces. Medium Class Ductility with high ductile levels that have flexible designs and design requirements. High Ductility Class defined by very high ductility levels and have strict and complex designs and design requirements. Low Ductility Class estimates seismic loading design of structural members through design seismic actions with behaviour factor of q=1.5 and reinforcement calculations for normal situations albeit with some material limitations like minimum concrete quality of c16/20. . EC8 suggestions deal with DCL design limitations; these are applicable only for low seismic activity regions with ground acceleration of 0.10g and less. Areas with high activity should not have DCL designed buildings as it would be catastrophic from a safety and financial point of view. For higher ductility classes, the standard lays down designs of a stable and secure seismic energy absorption model in enumerated critical areas of structure and these models are to have a behaviour factor of more than 1.5. There are obvious differences between the two higher classes of ductility, in case of, the steel strain and associated geometrical and material restrictions.; difference in factors of design loading effects ; and finally, different rules of design for capacity and local ductility level. The behaviour factor of the different ductility classes can be varied with respect to the variations in the horizontal directions of a particular structure, irrespective of the fact of ductility being equal and same in all directions of the structure. The higher two classes of ductility are quite similar and equivalent in terms of structural performance during any kind of seismic action and activity that affects the structures design. It is quite easy and simple to accomplish and implement a medium class ductility design at an instant and this kind of design yields better result in cases of medium level seismic activity. The higher class ductility designs are deemed to be able to provide greater and better levels of safety y and security against case of localized or total and complete collapse of a particular structure during the occurrence of major earthquakes of high Richter scale magnitude that exceeds the theoretical load bearing yield point of the structural elements. Eurocode 8 standards do not connect or relate the comparative choices between the two ductility classes with any kind of seismic activity and action in that area or region, neither is any connection or link made with respect to the structures importance and rele vance and does not set any kind of limit whatsoever regarding the usage of these two classes of ductility. The state members are the ones who have been given prioritising power regarding the definition of usage in different areas and the various kinds of structures where these two classes can and should be used. It should be ensured that if the design forces calculations are done in accordance with the ductile responsive demand, then the structure must fail in a ductile and controlled way; this idea is the primary driving force behind capacity designing. Contents of capacity designing can be laid down in a few points:- Plastic hinges should be fixed on beams not columns. Dense steel stirrups to be used for adequate hear reinforcement Steel members should fail away from any points of connections. Considerable structural irregularities should be avoided Tensile capacity must exceed shear capacity