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Seismic Design of RC Structures
Fundamental Concepts
Md. Abdul Malek Sikder
Superintending Engineer PWD Design Circle-I
Md. Rafiqul Islam
Executive Engineer PWD Design Division-III Most Earthquake Codes require that structures be able to resist:
Minor earthquakes without any damage
Moderate earthquakes with negligible
structural damage and some nonstructural damage.
Major earthquakes with some structural and
non structural damage but without collapse. Building behavior during Earthquakes Schematic representation of seismic forces Strong Column-Weak Beam Concept
For a frame undergoing lateral displacement, the flexural
capacity of the members at a joint should be such that the columns are stronger than the beams.
Hinges will form in the beams rather than the columns,
minimizing the portion of the structure affected by non linear behavior.
Maintains the overall vertical load capacity, preventing collapse.
Seismic Design Methods:
1. Equivalent static lateral loads method
2. Response spectrum analysis
3. Time history analysis of the
dynamic response of the structure Equivalent static lateral loads method Using Newtons second law of motion, the total lateral seismic force, also called, base shear is determined by the relation : V = Ma
= a
=W Cs = Seismic coefficient = WCs
a = the maximum acceleration of the
building g = acceleration due to gravity W = Weight of building The seismic coefficient is modified by factors which take into account:
1. Dynamic properties of the structure i.e. natural period of
vibration, modal shapes and damping. 2. Type of construction, with particular reference to ductility. 3. Importance of the structure. 4. Seismicity of the region. 5. Subsoil conditions. 6. Allowable stresses & load factors. The design Base Shear, V is calculated for a given direction according to the equation : Z = Seismic Zone factor I = Importance factor R = Response modification coefficient C = Coefficient based on site V= W coefficient, S and period of structure, T W = Total seismic dead load C= T = Fundamental period of structure hn= Height of structure in ft. T = Ct (hn)3/4 Ct = 0.03 for RC moment resisting frame, 0.02 for all other RC buildings