Earthquake Engineering

Essentials of Earthquake Engineering for Architects and Engineers Architect's and Engineer's Meet at Patiala 27th Aug. 2007 Dr. Hardeep Singh Rai Professor and Head, Civil Engg. Dept. Guru Nanak Dev Engg. College Ludhiana

Agenda: Vulnerable environment ● Govt's role ● Professional's role ● Planning ● Design ● Construction ●

India One of the most disaster prone countries ● 85% area is vulnerable to one or multiple ● disasters 57% area is in high seismic zone ●

Govt's contribution Prepration of standards ● IS 1893: Criteria for Earthquake Resistant ● Design of Structures 1962: `Recommendations for Earthquake – Resistant Design of Structures' Revised in 1966, 1970, 1975, 1984 – 2002: (Part 1) General provisions and ● Buildings; 2005 (4) Industrial Structures Including Stack Like Structures

Govt's contribution (contd.) Part 2: Liquid Retaining Tanks – Elevated ● and Ground Supported Part 3: Bridges and Retaining Walls ● Part 5: Dams and Embankments ● IS 4326:1993 Earthquake Resistant Design ● and Construction of Buildings Code of Practice

Govt's contribution (contd.) IS 13828:1993 Improving Earthquake ● Resistance of Low Strength Masonry Buildings – Guidelines IS 13920:1993 Ductile Detailing of ● Reinforced Concrete Structures Subjected to Seismic Forces – Code of Practice IS 13935:1993 Repair and Seismic ● Strengthening of Buildings – Guidelines

IS: 4326 Eq. Res. D&C bldgs CP General Principles: ● Lightness – Continuity of constrction – Projecting and suspended parts – Configuration – Strength in various directions – Foundations – Ductility – Damage to nonstructural parts – Fire safety –

Affect of Architectural Features on Bld during EQ Size of the Building 

Affect of Architectural Features on Bld during EQ * If we have a poor configuration to start with, all the engineer can do is to provide a bandaid – improve a basically poor solution as best as he can. Conversely, if we startoff with a good configuration and reasonable framing system, even a poor engineer cannot harm its ultimate performance too much. Henry Degenkolb

Affect of Architectural Features on Bld during EQ Horizontal layout of the building 

Affect of Architectural Features on Bld during EQ Vertical layout of buildings 

Affect of Architectural Features on Bld during EQ Adjacency of Buildings 

What are the Seismic Effects on Structure Horizontal and Vertical Shaking 

Affect of Architectural Features on Bld during EQ Suggestions  Architectural features detrimental to EQ response of building ● should be avoided. If not they must be minimised In case irregular features included in building higher level of ● engineering efforts is required in structural design Decision made at the planning stage on building configuration are ● very important Building with simple architectural feature will always behave ● better during EQ

How building twists during Earthquake Contd………

Seismic Design Philosophy for Building Severity of ground shaking at a given location during an earthquake can be minor, moderate and strong. Relatively speaking, minor shaking occurs frequently, moderate shaking occasionally and strong shaking rarely.  For instance, on average annually about 800 earthquakes of magnitude 5.05.9 occur in the world while the number is only about 18 for magnitude range 7.07.9

Seismic Design Philosophy for Building • Don’t attempt to make EQ proof building (Bld. Will be too robust and too expensive) • Engineering intention shall be to make EQ resistant building

Seismic Design Philosophy for Building Earthquake Design Philosophy 

Seismic Design Philosophy for Building Earthquake Resistant Design 

Seismic Design Philosophy for Building Acceptable Damage:Ductility 

Behaviour of Brick Masonary Houses during EQ Behaviour of Wall  • Masonary Bld. Most vulnerable under EQ shaking(Brittle Structure) • Wall is most vulnerable component of the Bld due to horizontal force (EQ) • Wall offers greater resistance if pushed along its length (Strong Direction) • Wall topples easily if pushed in a direction perpendicular to its plan(Weak Direction)

Behaviour of Brick Masonary Houses during EQ Behaviour of Wall  • All walls if joined properly to the adjacent wall ensures good seismic performance • Walls loaded in weak direction take advantage of the good lateral resistance offered in their strong direction • Walls need to be tied to the roof and foundation to reserve their overall integrity

Simple Structural Configuration required for Masonary Building Box Action in Masonary Bld.  • Separate block can oscillate independently and even hammer each other (If too close during EQ) • Adequate gap required betn such blocks • Gap not necessary if horizontal projections in Bld are small • An integrally connected inclined stair case slab acts like a cross brace betn floors • It transfers large horizontal forces at the roof and the lower level (Area of Potential Damage)

Vertical Band necessary in Masonary Building Protection of Opening in Walls  •Most common damage observed after an EQ is diagonal Xcracking of wall pier, inclined cracks at corners of doors and window opening. • A square opening become rhombus during EQ Shaking • The corners that come closer develop cracks, Cracks are bigger when the opening sizes are large •Steel bars provided all around the opening restrict cracks (corner)

Seismic Design Philosophy for Building EQ Resistant Design of Building 

Effect of Earth Quake on RC Building Strength Hierarchy  • If this strategy adopted in design & beam detailing done properly Building as a whole can deform by large amount despite progressive damage caused due to consequent yielding of beams If columns are made weaker, it suffer local damage at the top and bottom of a particular storey • This localised damaged can lead to collapse of building

How do Beams in RC Bld resist EQ Reinforcement and Seismic Damage  • Long straight bars (longitudinal bars) placed along its length • Closed loop of small diameter steel bars (Stirrups) placed vertical at regular intervals along its length

How do Beams in RC Bld resist EQ Reinforcement and Seismic Damage  • Two basic types of failure in beams: a) Flexural (Bending) failure b) Shear failure

How do Beams in RC Bld resist EQ Reinforcement and Seismic Damage  • FLEXURAL (BENDING) FAILURE • Beam can fail in two ways a) Brittle failure (b) Ductile failure • Brittle Failure: Relatively more steel is present on the tension face, concrete crushes in compression which is undesirable

How do Beams in RC Bld resist EQ Reinforcement and Seismic Damage  • Ductile Failure: Relatively less steel is present on the tension face, steel yield first and the re distribution occurs in the beam until eventually concrete crushes in compression, is desirable • Characterised with many vertical cracks starting from the stretched beam face and going towards its mid depth

How do Beams in RC Bld resist EQ Reinforcement and Seismic Damage  • SHEAR FAILURE: A shear crack, inclined at 45 degree to the horizontal, develops at mid depth near the support and grows towards the top and bottom face Closed loop stirrups are provided to avoid such shearing action Shear damage occurs when area of shear stirrup is insufficient A Brittle failure, must be avoided

How do Beams in RC Bld resist EQ Stirrup helps beam in three ways  • It carries the vertical shear force, thereby resist diagonal shear crack • It protect the concrete from buldging outwards due to flexure • It prevents the buckling of compressed longitudinal bars due to flexure

How do Beams in RC Bld resist EQ Longitudinal bars  • Provided to resist flexural cracking on the side of the beam that stretches • Requires on both faces at the ends and on the bottom face at mid length

How do Beams in RC Bld resist EQ Longitudinal bars  • As per ductile detailing code: At least two bars shall go through the full length of the beam at the top as well as the bottom of the beam At the end of the beams, the amount of steel provided at the bottom is at least half that at the top

How do Beams in RC Bld resist EQ Requirements related to stirrups in RC Beams  • Φof Stirrups – 6 mm minimum • Φ o f Stirrups – 8 mm , if beam>5m. • Both ends of a vertical stirrups should be bent into 135 degree hook and extend sufficiently beyond this hook to ensure that stirrups does not open out in an earthquake

How do Beams in RC Bld resist EQ Requirements related to stirrups in RC Beams  • Max. spacing of stirrups is less than half the depth of beam • For a length twice the depth of beam from the face of the column, the spacing should not be more than one fourth the depth of beam

How do Beams in RC Bld resist EQ Requirements related to stirrups in RC Beams  • At the location of the lap, the bars transfer large forces from one to another • Laps of the longitudinal bars are: a) Made away from the face of col. b) Not made at locations where they are likely to stretch by large amounts and yield (eg. Bottom bars at mid length of the beam) • At the location of laps, vertical stirrups should be provided at closer spacing

How do Columns in RC Bld resist EQ Possible EQ Damage  • Column can sustain 2 type of damage: a) Axial Flexural (Combined Comp. Bending) failure b) Shear Failure (Brittle Damage) & must be avoided by providing transverse ties at closer spacing • Minimum width of the column = 300 mm, and if the unsupported length of column <4 meter and beam length< 5 m., width up to 200 mm is allowed

How do Columns in RC Bld resist EQ Possible EQ Damage  • Purpose of horizontal ties a) Carry horizontal shear force induced by EQ and thereby to resist diagonal shear crack b) Hold together the vertical bars and prevent them from buckling c) Contain the concrete in the column within the closed loops • The ends of the ties must be bent as 135 degree. The length of the ties beyond hook bend must be atleast 10d of steel bar ( close ties) but not less than 75 mm.

How do Columns in RC Bld resist EQ Possible EQ Damage  • In column where spacing between the corner bar exceeds 300 mm “Additional links with 180 hook ends for ties to be effective in holding the concrete in its place and to prevent the buckling of vertical bars”

How do Beam Column Joins in RC bld Resist EQ EQ behaviour of Joints  • Column beam joint have limited force carrying capacity when forces larger than these are applied during EQ, joints are severely damaged • Repairing damage joints is difficult, so damage must be avoided • Under EQ shaking, the beam adjoining a joint are subjected to moments in the same direction

How do Beam Column Joins in RC bld Resist EQ EQ behaviour of Joints  Under these moments, the top bar in the beamcolumn joint are pulled in one direction & the bottom one in opposite direction. The forces are balanced by bond stress developed between concrete and steel in the joint region If there is insufficient grip of concrete on steel bars in such circumstances, the bar slip inside the joint region, the beam loose their capacity to carry load

How do Beam Column Joins in RC bld Resist EQ EQ behaviour of Joints   Under this pull push forces at top and bottom ends joint undergo geometric distortion  One diagonal length of the joint elongates and the other compresses. If the column cross sectional size is insufficient, the concrete in the joint develops diagonal cracks

How do Beam Column Joins in RC bld Resist EQ EQ behaviour of Joints   Problem of diagonal cracking & crushing of concrete in the joint region can be controlled by a) Providing large column size b) Providing closely spaced closed loop steel ties around column bars in joint region Ties hold together the concrete in the joint and also resist shear force.

How do Beam Column Joins in RC bld Resist EQ

How do Beam Column Joins in RC bld Resist EQ  Anchorage of beam bars in exterior joints  Anchorage of beam bars in interior joints

Why are Soft storey building vulnerable in EQ

Why are Short Columns more Damaged During EQ Short Column Behaviour   Bld resting on sloped ground consisting of short & long column, when shakes, all column move horizontally by the same amount along with floor slab at a particular level  Short column effect also occurs in columns that support mezzanine floor or loft slabs that are added in between two regular floors.

Why are Short Columns more Damaged During EQ Short Column Behaviour   A tall column & a short column of same cross section move horizontally by same amount during EQ  Short column is stiffer than long column(Stiffness of column means resistance to the deformation)  Larger is the stiffness, larger is the force required to deform it

Why are Short Columns more Damaged During EQ Short Column Behaviour   If a short column is not adequately designed for such large force, it can suffer significant damage during EQ  Short column attracts several times larger force and suffer more damage as compare to taller ones.  This behaviour of short column is called short column effect and often the damage is in the form of X –shaped cracking (Shear Failure)

Why are Short Columns more Damaged During EQ Short Column Behaviour   Special Confining reinforcement is to be provided over the full height of column that are likely to sustain short column effect  Special confining reinforcement must extend beyond the short column into the column vertically above and below by certain distance

Why are Short Columns more Damaged During EQ The Solution   In new building, short column effect should be avoided to the extent possible during Architectural design itself For short columns in the existing building retrofit solutions can be employed to avoid damage in future Earth Quake The retrofit solution should be designed by a Qualified structural Engineer with requisite background

How to reduce EQ effects on Buildings Why EQ effects are to be reduced   Lifeline structures like hospitals etc are remain to be functional in the aftermath of EQ Special techniques are required to design such life line structures which usually cost more than normal bld do Two basic technology are a) Base isolation device b) Seismic Dampers

How to reduce EQ effects on Buildings Why EQ effects are to be reduced  a) Base isolation device Idea behind base isolation is to detach (isolate) the buildings from the ground in such a way that EQ motions are not transmitted up through the building or at least reduced b) Seismic Dampers Special devices introduced in the building to absorb the energy provided by the ground motion to the building

How to reduce EQ effects on Building Base isolation in real buildings  • Over 1000 blds across the world have been equipped with seismic base isolation • In India base isolation technique was first demonstrated after 1993 Killari EQ • Two single storey bld (one school and another shopping complex bld) were built with rubber base isolators resting on hard ground • The four storey bhuj hospital bld was built with base isolation technique after 2001 bhuj EQ

How to reduce EQ effects on Building Seismic Dampers  • Another approach for controlling seismic damage in bld is by installing seismic dampers in place of structural elements such as diagonal braces • These dampers act like hydraulic shock absorbers and absorbs part of the seismic energy transmitted through them, thus damps the motion of the building

Resource Material EERI, (1999), Lessons Learnt Over Time – Learning from Earthquakes Series: Volume II Innovative Recovery in India, Earthquake Engineering Research Institute, Oakland (CA), USA; also available at http://www.nicee.org/readings/EERI_Report.htm. Hanson,R.D., and Soong,T.T., (2001), Seismic Design with Supplemental Energy Dissipation Devices, Earthquake Engineering Research Institute, Oakland (CA), USA. Skinner,R.I., Robinson,W.H., and McVerry,G.H., (1999), An Introduction to Seismic Isolation, John Wiley & Sons, New York. IITK & BMTPC Earthquake Tips; available at http://www.nicee.org/

For a Building to Perform well in an Earthquake it should have Six main attributes Lightness ● Simple and Regular Configuration ● Adequate Lateral strength ● Stiffness ● Continuity of Construction ● Ductility ●

Contact: Email: ● hardeep . rai @ gmail . com – hsrai @ gndec . ac . in – hs @ raiandrai . com – hsrai @ yahoo . com – hsrai @ grex . org – hsrai @ gmx . net –

Thank you.

Earthquake Engineering

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