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[subhash_mehrotra]

Dear All,
As per IS Code we are designing for special R.C. moment-resisting frame  only 10% of maximum considered earthquake (Design Basis earthquake = ½ maximum considered earthquake and response reduction factor considered for special RC moment resisting frame is 5).  Thus we depend heavily on ductility of the structure.
Where as under dead, live & wind load, structure undergoes elastic deformation, but under earthquake it has to undergo inelastic deformations.
For modern building, avoidance of structural collapse alone is not enough.
The cost of non-structural components is much higher than the cost of the structure itself and must be protected.
Alternate Solution is establish performance based design criteria.  IS 1893 allows use of energy absorbing devices.  
In typical structure without dampers, the inherent damping is merely 2-5% of critical.  With the introduction of supplemental damping of 10-15% of critical, the forces and deformations on the structure can be significantly reduced.  Dampers generally in use are :
a)      Friction Dampers
b)      Viscous dampers
Similar to automobiles, the motion of vibrating building can be slowed down by dissipating seismic energy through friction. One of the Friction type dampers is Pall Friction damper developed by Dr. A S Pall an Indian (by origin) structural engineer & settled in Canada.  I have used these in large number of residential and office buildings in the storey ranging 10 – 20 for last 5 years.  These Friction Dampers consists of series of steel plates, which are specially treated to develop friction.  These plates are clamped together and allowed to slip at a predetermined load.  The Friction Dampers are provided in a structural steel diagonal brace or a cross brace or a chevron brace.
The friction dampers are designed not to slip during wind, so energy absorption will remain in the elastic range.  During a major earthquake, they slip prior to yielding of structural members.   
National Earthquake Hazards Reduction Program (NEHRP) guidelines require that friction dampers are designed for 130% MCE displacement.
Structures with friction dampers require nonlinear time history dynamic analysis.  Although the brace remains elastic, but the slipping of damper constitutes nonlinearity.
The structural analysis, design and testing of energy dissipating devices is explicitly covered in Regulations for New Buildings and Other Structures, Part 1: Provisions, Report No. FEMA 450, Federal Emergency Management Agency, Washington, D.C., U.S.A., 2004. (FEMA 450-2004).  
The major difference from conventional design is that it is difficult to cater for maximum Considered Earthquake (MCE) but with use of friction damper we cater 30% more of Maximum Considered Earthquake (MCE).
Regards,
S C Mehrotra

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[ishacon]

Dear All,

I agree that performance based criteria is more important for tall buildings
since the sway at top storey may trigger alarms in senior citizens living at that height.

Also for Hospital buildings, where Importance factor is 1.5, even Zone 5 will translate into
large seismic forces. This necessarily requires additional measures to ensure minimum drift
which can be achieved effectively with dampers.
Also with possible changes in IS 1893 where we may have to design for MCE, architects will
need to change their mindsets in permitting vertical braces to accommodate these dampers.

We have been lucky to have been associated with retrofit at Apollo Hospital, Sarita Vihar
where Fluid Viscous dampers were used effectively in the central block,
to ensure that ICU patients do not feel any discomfort at the upper levels.
Non linear analysis using ETABS is a necessity in such cases.

V.P. Agarwal

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