No matter how well designed a structure is, one or possibly more structural members will exceed their design strength, either because seismic forces might exceed the design assumptions or because of local conditions during the construction.
1st Defense mechanism: In case of an earthquake greater than the design earthquake we don’t want failure (fracture) of any member even if it remains permanently deformed, this means that we need ductile structural members.
2nd Defense mechanism: in the case of an extremely intense earthquake, where failure of some members is unavoidable, the elements that must not fail are the columns; this means that the columns must have sufficient capacity-overstrength.
In the second defense mechanism all failures must be flexural because of their ductile nature as opposed to shear failures that have a brittle behavior (i.e. sudden fracture