2. INTRODUCTION
• Chimneys are slender structures
• Vulnerable to seismic excitation
• Calculate the structural response
• Corresponding shear forces and bending
moments
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3. FACTOR AFFECTING MAGNITUDE AND
INTENSITY OF EARTHQUAKE
• Depth of focus
• Distance from epicentre
• Characteristics of the path of travel of seismic
waves
• Soil strata on which the structure stands
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4. FACTOR AFFECTING STRUCTURAL
RESPONSE TO EARTHQUAKE
• Foundation
• Soil
• Material
• Form
• Size
• Mode of construction
• Duration and characteristics of ground motion
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5. IS 1893 (PART 4) : 2005
(Industrial Structures Including Stack-like Structures)
Earthquake resistant design of industrial structures
including stack-like structures
Stack-like structures
Cooling towers and drilling towers
Transmission and communication towers
Chimneys and stack-like structures
Silos (including parabolic silos used for urea
storage)
Support structures for refinery columns, boilers,
crushers etc.
Pressure vessels and chemical reactor columns
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6. ASSUMPTIONS
No resonance (No time to build up resonant
response since the period and amplitude are
constantly changing)
Earthquake does not occur simultaneously with
maximum wind, flood or sea wave
Elastic modulus same as that for static analysis
unless more definite value is available
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7. IS 1893 (PART 4) : 2005
Section I – Industrial structures (Clause 7 to 12)
Section II – Stack-like structures (Clause 12 to 18)
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12. NUMERICAL EXAMPLE
Consider 130 m tall RC cylindrical chimney like structure
of uniform cross section (A = 8.16m2, I = 87.9 m4) with
weight of 25 kN/m3 and E = 3.4X104 n/mm2. Evaluate the
base moment and shear under earthquake conditions. The
structure is located in seismic zone III and is supported on
raft foundation. Take damping factor as 5% and
importance factor as 2. Use both response-spectrum
method and modal analysis technique.
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13. RESPONSE-SPECTRUM METHOD
H = 130 m
A = 8.16 m2
ρ = 25 kN/m3
W = 25 X 8.16 X 130 = 26.52 X 103 kN
I = 87.9 m4
Ec = 3.4 X 104 N/mm2
Zone III
Raft foundation
Damping factor (β) = 5 %
Importance factor (I) = 2
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23. TIME HISTORY RESPONSE ANALYSIS
Response of the structure evaluated by subjecting
the mathematical model to design earthquake for
each incremental time interval
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24. IS 1893 (PART 4) : 2005
Special design considerations for RC stacks (Clause 18)
Total vertical reinforcement not less than 25% of concrete
area
2 layers of vertical reinforcement – outer layer not less than
50% of the reinforcement
Total circumferential reinforcement not less than 0.2% of
concrete area
2 layer of circumferential reinforcement – reinforcement on
each face not less than 0.1% concrete area at that section
Circumferential reinforcement at 0.2 dia from top -twice
the normal reinforcement
Extra reinforcement around openings
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