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8/10/2019 IS1893_Lecture3
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1
Lecture 3
This lecture covers
Section 6.1: General PrinciplesIS:1893-2002(Part I)
January 16, 2003
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 2
General Principles and Design Criteria (Section 6)
Four main sub-sections Cl. 6.1: General Principles
Cl. 6.2: Assumptions
Cl. 6.3: Load Combination and Increase in
Permissible Stresses
Cl. 6.4: Design Spectrum
This lecture covers the first sub-section: Cl. 6.1
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 3
Ground Motion (cl. 6.1.1)
Usually, the vertical motion is weaker than thehorizontal motion
On average, peak vertical acceleration is one-half to two-thirds of the peak horizontalacceleration.
Cl. 3.4.5 of 1984 code specified verticalcoefficient as one-half of horizontal
Cl. 6.4.5 of 2002 code specifies it as two-thirds
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 4
Ground Motion Contd
All structures experience a constant verticalacceleration (downward) equal to gravity (g) atall times.
Hence, the vertical acceleration during groundshaking can be just added or subtracted to thegravity (depending on the direction at thatinstant).
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 5
Ground Motion Contd
Example: A roof accelerating up and down by0.20g.
Implies that it is experiencing acceleration in therange 1.20g to 0.80g (in place of 1.0g that it
would experience without earthquake.) Factor of safety for gravity loads (e.g., dead and
live loads) is usually sufficient to cover theearthquake induced vertical acceleration
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 6
Ground Motion Contd
Main concern is safety for horizontalacceleration.
Para 2 in cl. 6.1.1 (p. 12) lists certain caseswhere vertical motion can be important, e.g.,
Large span structures
Cantilever members
Prestressed horizontal members
Structures where stability is an issue
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7/31Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 7
Effects other than shaking
Ground shaking can affect the safety ofstructure in a number of ways:
Shaking induces inertia force
Soil may liquefy
Sliding failure of founding strata may take place
Fire or flood may be caused as secondary effectof the earthquake.
Cl. 6.1.2 cautions against situations wherefounding soil may liquefy or settle: such casesare not covered by the code and engineer hasto deal with these separately.
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8/31Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 8
Earthquake Design Principle
Large earthquakes are infrequent as comparedto smaller earthquakes
A structure may have a design life of 50-100years. Should it be designed to remain
undamaged for a large earthquake that takesplace say once in 500 years?
Read Earthquake Tip 8
(http://www.nicee.org/EQTips/EQTip08.pdf)
http://www.nicee.org/EQTips/EQTip08.pdfhttp://www.nicee.org/EQTips/EQTip08.pdf8/10/2019 IS1893_Lecture3
9/31Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 9
Earthquake Design Principle (contd)
The criteria is: Minor (and frequent) earthquakes should not
cause damage
Moderate earthquakes should not cause
significant structural damage (but could havesome non-structural damage)
Major (and infrequent) earthquakes should notcause collapse
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10/31Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 10
Clause 6.1.3
Para 1 of this clause implies that Design BasisEarthquake (DBE) relates to the moderateshaking and Maximum Considered Earthquake(MCE) relates to the strong shaking.
Indian code is quite empirical on the issue ofDBE and MCE levels (as discussed in Lecture 2).
Hence, this clause is to be taken only as an
indicator of the concept.
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11/31Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 11
Seismic Design Principle
A well designed structure can withstand ahorizontal force several times the design forcedue to:
Overstrength
Redundancy
Ductility
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12/31Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 12
Overstrength
The structure yields at load higher than thedesign load due to: Partial Safety Factors
Partial safety factor on seismic loads
Partial safety factor on gravity loads
Partial safety factor on materials
Material Properties Member size or reinforcement larger than required
Strain hardening in materials
Confinement of concrete improves its strength Higher material strength under cyclic loads
Strength contribution of non-structural elements
Special ductile detailing adds to strength also
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 13
Redundancy
Yielding at one location in the structure does notimply yielding of the structure as a whole.
Load distribution in redundant structuresprovides additional safety margin.
Sometimes, the additional margin due toredundancy is considered within the
overstrength term.
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 14
Ductility
As the structure yields, two things happen: There is more energy dissipation in the structure
due to hysteresis
The structure becomes softer and its natural
period increases: implies lower seismic force tobe resisted by the structure
Higher ductility implies that the structure canwithstand stronger shaking without collapse
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 15
Response Reduction Factor
Overstrength, redundancy, and ductilitytogether lead to the fact that an earthquakeresistant structure can be designed for muchlower force than is implied by a strong shaking.
The combined effect of overstrength,redundancy and ductility is expressed in termsof Response Reduction Factor (R)
See Fig. on next slide.
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 16
)(FForceDesign
)(FForceElasticMaximumFactorReductionResponse
des
el
Design force
MaximumLoad Capacity
TotalHorizontalLoad
Roof Displacement ()
Non linearResponse
First
Significant
Yield
Linear ElasticResponse
max
Fy
Fs
Fdes
yw
Fel
Load atFirst Yield
Due to
Overstrength
Due to
Redundancy
Due to
Ductility
Maximum forceif structure remains elastic
0
TotalHorizontal
Load
Figure: CourtesyDr. C V R Murty
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 17
Para 2 and 3 of Cl. 6.1.3.
Imply that the earthquake resistant structuresshould generally be ductile.
IS:13920-1993 gives ductile detailingrequirements for RC structures.
Ductile detailing provisions for steel structuresare not yet available in Indian codes. Hence, reference is made to SP6 (Part 6): this
really relates to plastic design.
It is advisable to refer to internationalcodes/literature for ductile detailing of steelstructures till similar Indian codal provisions aredeveloped.
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 18
Para 2 and 3 of Cl. 6.1.3 Contd
As of now, ductile detailing provisions forprecaststructures and for prestressedconcretestructures are not available in Indian codes.
In the past earthquakes, precaststructures have
shown very poor performanceduringearthquakes.
The connections between different parts havebeen problem areas.
Connections in precast structures in high seismicregions require special attention.
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 19
Past Performance
While talking of past performance of structures,I may mention that the performance of flat platestructuresalso has been very poor in the pastearthquakes.
For example, in the Northridge (California)earthquake of 1994.
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 20
Para 4 of Cl. 6.1.3
This is an important clause for moderate seismicregions.
The design seismic force provided in the code isa reduced force considering the overstrength,
redundancy, and ductility. Hence, even when design wind force exceeds
design seismic force, one needs to comply withthe seismic requirements on design, detailing
and construction.
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 21
Soil Structure Interaction (Cl. 6.1.4)
If there is no structure, motion of the groundsurface is termed as Free Field Ground Motion
Normal practice is to apply the free field motionto the structure base assuming that the base is
fixed. This is valid for structures located on rock sites.
For soft soil sites, this may not always be a goodassumption.
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 22
Soil Structure Interaction (Cl. 6.1.4) Contd
Presence of structure modifies the free fieldmotion since the soil and the structure interact. Hence, foundation of the structure experiences
a motion different from the free field groundmotion.
The difference between the two motions isaccounted for by Soil Structure Interaction (SSI)
SSI is not the same as Site Effects Site Effectrefers to the fact that free field motion
at a site due to a given earthquake depends onthe properties and geological features of thesubsurface soils also.
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 23
SSI Contd
Consideration of SSI generally Decreases lateral seismic forces on the structure
Increases lateral displacements
Increases secondary forces associated with P-
delta effect.
For ordinary buildings, one usually ignores SSI.
NEHRP Provisions provide a simple procedure to
account for soil-structure interaction in buildings See Lecture 1 for availability of NEHRP document
at internet
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 24
Direction of Ground Motion (Cl. 6.1.5)
During earthquake shaking, ground shakes in allpossible directions.
Direction of resultant shaking changes frominstant to instant.
Basic requirement is that the structure shouldbe able to withstand maximum ground motionoccurring in any direction.
We already discussed that for most structures,main concern is for horizontal vibrations ratherthan vertical vibrations.
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 25
Direction of Ground Motion (Cl. 6.1.5) (contd)
One does not expect the peak groundacceleration to occur at the same instantin twoperpendicular horizontal directions.
Hence for design, maximum seismic force is not
applied in the two horizontal directionssimultaneously.
If the walls or frames are oriented in twoorthogonal (perpendicular) directions:
It is sufficient to consider ground motion in thetwo directions one at a time.
Else, Cl. 6.3.2: will come back to this later.
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 26
Building Plans with Orthogonal Systems
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 27
Building Plans with Non-Orthogonal Systems
walls
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 28
Floor Response Spectrum (Cl. 6.1.6)
In previous lecture, we discussed responsespectrum for ground acceleration.
Equipment located on a floor needs to bedesigned for the motion experienced by the
floor. Hence, the procedure for equipment will be:
Analyze the building for the ground motion.
Obtain response of the floor.
Express the floor response in terms of spectrum(termed as Floor Response Spectrum)
Design the equipment and its connections withthe floor as per Floor Response Spectrum.
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 29
Additions to Existing Structures (Cl. 6.1.7)
Read this clause carefully. It is an important clause.
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Sudhir K. Jain, IIT Kanpur E-Course on IS:1893 / January 2003 Lecture 3 / Slide 30
Change in Occupancy (Cl. 6.1.8)
The is an important clause. Read it carefully.
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S dhi K J i IIT K E C IS 1893 / J 2003 L t 3 / Slid 31
At the end of Lecture 3
Please let me know if the lectures so far havebeen too light or too heavy.
Any constructive suggestions on improving thelectures at this stage?