Seismology Civil Engineering
SEISMIC RESISTANT REINFORCED CONCRETE STRUCTURES-DESIGN PRINCIPLES
SUMMARY:Earthquakes cause considerable economic losses.It is possible to minimize the economic loses by proper seismic design.In this paper basic principles for seismic design are summarized.There are three basic requirements to be satisfied;(a)strength,(b)ductility and(c)stiffness.In the paper these are briefly discussed.
In the second part of the paper the author summarizes his views on the damages observed in the past earthquakes.He concludes that most of the damages have been due to,(a)bad configuration,(b)inadequate detailing and(c)inadequate supervision.In the paper these are discussed,pointing out the common mistakes made and damages observed as a result of these mistakes.In the last part of the paper some simple recommendations are made for producing seismic resistant reinforced concrete structures,emphasizing on detailing and proportioning.
Key Words:Seismic resistance,reinforced concrete.
1.INTRODUCTION
Every year more than 300 000 earthquakes occur on the earth.Many of these are of small intensity and do not cause any damage to our structures.However,earthquakes of larger intensity in the vicinity of populated areas cause considerable damage and loss of life.It is estimated that on the average 15000 people have been killed each year throughout the world because of earthquakes.
Since ancient times mankind has sought ways and means of minimizing the damage caused by earthquakes.The great masters of the art of building have been able to build structures which have withstood many severe earthquakes for centuries.Magnificent mosques and bridges in the Middle East built by our ancestors are still in service,These masters did not know seismic analysis,but were able to evaluate past experience with their excellent engineering intuition and judgement.Mosques,bridges and schools(Medrese)built by Sinan in Istanbul and Edirne are not only beautiful,but are also engineering masterpieces.
Today we have great advantages as compared to our ancestors.We have more experience,we have highly developed analytical tools and considerable experimental data.It should also be noted that computers enable us to consider more variables and several alternatives in the analysis.
The main objective of this paper is to lay down some basic principles for producing earthquake resistant reinforced concrete structures.These are simple principles and easy to apply.They have been developed in the light of analytical and experimental research done and on observations made from past earthquakes.
2.BASIC PHILOSOPHY AND REQUIREMENTS
Design principles cannot be laid down unless there is a well defined design philosophy.The design philosophy generally accepted is summarized below:
-Buildings should suffer no structural damage in minor, frequent earthquakes. Normally there should be no nonstructural damage either.
- Buildings should suffer none of minor structural damage (repairable) in occasional moderate earthquakes.
- Buildings should not collapse in rarely occurring major earthquakes. During such earthquakes structures are not expected to remain in the elastic range. Yielding of reinforcing stell weill lead to plastic hinges at critical sections.
The general design philosophy will not have much practical use unless design requirements are developed in parallel with this philosophy.The author believes that the design requirements can be summarized in three groups.
a.Strength requirements
b.Ductility requirements
c.Stiffness requirements(or drift control).
These three requirements will be briefly discussed in the following paragraphs.
2.1.Strength Requirements
Members in the structure should have adequate strength to carry the design loads safely.Since the designers are well acquainted with this requirement,it will not be discussed in detail.However,it should be pointed out that the designer should avoid brittle type of failure,by making a capacity design(1).The basic principles in capacity design are illustrated for a beam in Figure 1.If the design shear is computed by placing the ultimate moment capacities at each end of the beam,the designer can make sure that ductile flexural failure will take place prior to shear failure.
2.2.Ductility Requirements
In general it is not economical to design R/C structures to remain elastic during a major earthquake.It has been demonstrated that structures designed for horizontal loads recommended in the codes can only survive strong earthquakes if they can have the ability to dissipate considerable amount of energy.The energy dissipation is provided mainly by large rotations at plastic hinges.The energy dissipation by inelastic deformations requires the members of the structure and their connections to possess adequate"ductility”.Ductility is the ability to dissipate a significant amount of energy through inelastic action under large amplitude deformations,without substantial reduction of strength.
Adequate ductility can be accomplished by specifying minimum requirements and by proper detailing(2).
2.3.Stiffness Requirements
In designing a building for gravity loads,the designer should consider serviceability in addition to ultimate strength.In seismic design,drift limitations imposed might be considered to be some kind of a serviceability requirement.However,the drift limitation in seismic design is more important than the serviceability requirement.
The limiting drift is usually expressed as the ratio of the relative storey displacement to the storey height(interstorey drift).Excessive i
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