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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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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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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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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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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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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.pdf

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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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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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Seismic Design Principle

A well designed structure can withstand ahorizontal force several times the design forcedue to:

Overstrength

Redundancy

Ductility

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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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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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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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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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)(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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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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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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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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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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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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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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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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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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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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Building Plans with Orthogonal Systems

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Building Plans with Non-Orthogonal Systems

walls

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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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Additions to Existing Structures (Cl. 6.1.7)

Read this clause carefully. It is an important clause.

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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?

is1893_lecture3

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