SEI07

1
ASSESSMENT OF RESPONSE REDUCTION FACTOR FOR RC
BUILDINGS IN PERFORMANCE BASED SEISMIC DESIGN
FOR INDIA
Sachin M. Pore, Vipul Prakash and
G.I. Prajapati struct_engg_at_yahoo.co.in,
vipulfeq_at_iitr.ernet.in, giprafeq_at_iitr.ernet.in Dep
artment of Earthquake Engineering, Indian
Institute of Technology, Roorkee 247 667, India

This picture in background is of famous Roorkee
Lion that is along the Ganga Canal

• Problem Definition and Solution
• The information on building stock of the country
  reveals that most buildings are four storied,
  buildings with five to ten floors can be grouped
  as few, while those having more than ten stories
  are rare. Among these multistoried buildings, use
  of steel frames and/or special moment resisting
  frames (SMRF) in concrete is still rare. This
  means that majority of our RCC framed buildings
  can be classified as medium rise buildings and
  requires more attention. In this context, a
  module of 4 m side in plan and 3 m in height (4 X
  4 X 3) was integrated in space to obtain 30
  geometries under three categories as shown below.

Three types of designs (i) Gravity load case
involving only dead and imposed loading (ii)
Design for seismic loading as OMRF type and (iii)
design for seismic loading as a SMRF type were
carried out for each of the frame and the cost
comparison was done between the later two types.
The first case was included to arrive at
preliminary sizing of members. The comparison of
cost for the structural members is shown in
graph below.
? With increase in size of the structure there is
decrease in slope of trend line, indicating that
in general the SMRF designs lead to reduction in
cost by about 20 due to higher response
reduction, though the design provisions are
stringent. ?To verify whether the assumed
response reduction is available, NSP is used in
accordance with ATC 40. ? Table below show the
results for the 3 storied frames, which were
close to the average values for the entire set
IV. Design Practice in India

• ?Emergence of IS 13920-1993, the code of practice
  exclusively dealing with design and detailing of
  RC members for ductility attracted the attention
  of practicing engineers towards the concept of
  ductile designs In India, though IS 4326-1976
  contained similar provisions.
• ?The criteria for earthquake resistant design of
  structures is specified in IS 1893 (Part 1)- 2002
  (hereafter referred to as IS 1893), published by
  Bureau of Indian Standards (BIS). The latest
  version of the code, explicitly prescribed the
  response reduction factors through reliance on
  ductile deformation or frictional energy
  dissipation in the cracks.
• ?In previous versions of IS 1893 the response
  reduction factor was incorporated in the values
  for average acceleration coefficient based on the
  natural period and damping of the structure.
  Being implicit, the response reduction factor did
  not require justification. Explicit provision of
  the response reduction factor in IS 1893 (Part
  1)2002 created the need for its justification.
  The codes (1893 and 13920) gave birth to two
  main issues
• how to ensure the performance of structure in
  post elastic range i.e. is it
• possible to ensure the assumed level of
  reduction of design forces?
• (2) What makes a better choice among the OMRF and
  SMRF?
• ? Absence of formal guidance addressing above
  issues led to continuation of use of ordinary
  moment resisting frames for medium rise
  buildings.

Comparison of Pushover Analysis For Bare and
Infilled Frames
If the infills are considered to offer a role of
secondary lateral load resisting system forming a
dual system with RC frame, its contribution can
be accounted towards redundancy factor. In this
case the Redundancy Factor Rr is considered as
the ratio of base shear of infilled frame to the
ratio of base shear of bare frame corresponding
to DBE and MCE levels separately.

• Conclusions

• It is economical to design the MRF systems as
  SMRF.
• The drift levels attained in SMRF cases are
  higher than the corresponding OMRF cases, meaning
  that SMRF option is offering economical solutions
  at the cost of reduced lateral stiffness. The
  regular configuration show similar distribution
  of material consumption.
• Performance in DBE remains near end of linear
  range of capacity curve in all the cases
  considered. However, the reduction factors
  evaluated in accordance with the available
  literature, are much lower than the factors
  specified by the IS 1893 in case of bare frame
  analysis even for simple, regular and symmetric
  geometric configurations
• If the contribution of infills is accounted then
  the situation improves but yet is not satisfactory

Acknowledgements The authors gratefully
acknowledge the AICTE and MHRD of Government of
India for funding of this research.
Presented at SEI
2007 Structure Congress Long Beach, California.
May 16-19, 2007