EARTHQUAKE DESIGN CONSIDERATIONS OF BUILDINGS

By Ir. Heng Tang Hai

SYPNOSIS

1.1. EarthquakeEarthquake--Induced Motions Induced Motions

2.2. Building Configurations Building Configurations

3.3. Effectiveness Of Shear WallsEffectiveness Of Shear Walls

4.4. Enhancement Of Ductility In Buildings Enhancement Of Ductility In Buildings

5.5. Mitigation Of EarthquakeMitigation Of Earthquake--Induced Vibrations Induced Vibrations

6.6. Cracking In Buildings Cracking In Buildings

7.7. Tremor Design Forces For BuildingsTremor Design Forces For Buildings

8.8. Structural Adequacy Of Existing Buildings Structural Adequacy Of Existing Buildings

9.9. ConclusionsConclusions

a) Possible ground movement-normally accelerations In the horizontal plane are the largest and most significant.

b) Typical vibration modes for a tall buildingsubjected to varying horizontal ground accelerations.

Earthquake-Induced Motions In Multistory Buildings

Fundamental Period Of Buildings

Equipment Buildings

40 storyCiticorp

10-20 story

4 story

1 story

Seconds 0.05 0.1 0.5 1.0 – 2.0 7.0

No torsional effects develop

Centroid of resisting forces

Centroid of applied forces

Symmetrical Buildings

a) Symmetrical buildings do not experience exceptionally high torsional forces and are hence preferred to nonsymmetrical buildings.

Torsion develops

Nonalignment of applied and resisting forces

Off-center stiffening elements (e.g. elevator cores)

Open-ended bearing wall building

Off-center loading

Nonsymmetrical Buildings

b) Buildings that are nonsymmetrical because of either their basic configuration or the nonsymmetrical placement of lateral-load-resisting elements typically experience high torsionalforces which are very destructive. Nonsymmetrically placed masses can also lead to similar torsional effects.

Seismic joint

c) Nonsymmetrical configuration with reentrant corners (e.g., L-or H-shaped buildings) are particularly susceptable to destructive torsional effetcs. Primary damage often occurs at the reentrant corners. Allowing separate building masses to vibrate independently by using seismic seperator joints that allow free movement to occur generally improves structural performance.

Nonsymmetrical Buildings With Reentrant Corners

Little torsion develops Excessive torsion

develops

d) Buildings that are nonsymmetrical in the vertical direction also experience destructive torsional effects. Discontinuous shear walls are particularly problematical.

Nonsymmetrical Buildings In Vertical Direction

Damage Due To Soft Story

Chi-chi Earthquake, Taiwan Sept 21, 1999

Izmit Earthquake, Turkey Aug 17, 1999

Torsional Failure

Gualan Earthquake, Guatemala4 February, 1976

High-damaged zone

a) Not desirable

b) Preferred.

Seismic joint (actually quite narrow)

Elongated buildings are more susceptible to destructive forces associated with differences in ground movements along the length of the building than are more compact shapes. Long buildings can be subdivided by using seismic joints.

Elongated Buildings

Possible overturning High forces Lower forces

Relatively slender buildings are less able to resist efficiently the overturning movements cause by earthquakes than are shorter and more compact configurations.

a) Not desirable. b) Preferred.

Slender Buildings

Adjacent buildings should be adequately separated so that buildings do not pound against each other during seismic events.

Pounding

Clearance

a) Small separation-not desirable. b) Large separation-preferred.

Small Separation Between Buildings

Pounding Damage

Prince William Sound Earthquake, Alaska24 March, 1964

Izmit Earthquake, Turkey17 August, 1999

FramePlastic hinges

b) Post-and-beam assemblya) Frame

Rigid frame buildings are generally preferable to pi n-connected ones because the plastic hinges that necessarily form in rigid frame buildings before they collapse absorb large amounts of energy.

Rigid Frame Buildings

Collapse of Columns

Taiwan

Shear Failure & Short Columns Failure

Short Column FailureSanta Monica, Northridge Earthquake

17 January 1994 (Magnitude 6.8)

Shear Failure, Northridge Earthquake

a) Typical diaphragm action : the horizontal plane acts like beam in carrying earthquake-induced forces to shear walls or other lateral-load-carrying mechanism.

b) If diaphragms are improperly designed, failure can

result in floor or roof plans.

Important of rigid floor and roof elements : for earthquake-induced inertial forces to be transferred to lateral-load-carrying elements, floor and roof elements must be capable of acting like rigid diaphragms.

Fig.7Fig.7

Rigid Floor Diaphragm

Failure Of Soffit

Façade and soffit damage, Northridge Earthquake,14 January, 1994 (magnitude 6.8)

Fallen Soffit at Entrance, Northridge Earthquake

a) Beam failure occurs first b) Column failure occurs first (very un-desirable).

Members should be designed such that failure occurs first in horizontal members rather than in vertical members (a “strong-column-weak-beam” strategy).

Strong-Column-Weak-Beam Strategy

Collapse of Upper Floor

Kobe, Japan 1995

Regular Building Configurations

�� Shear Walls/MomentShear Walls/Moment--Resistant Frames/Braced FramesResistant Frames/Braced Frames

�� Low Height to Base RatiosLow Height to Base Ratios

�� Equal Floor HeightsEqual Floor Heights

�� Symmetrical Plans Symmetrical Plans

�� Uniform Sections and Elevations Uniform Sections and Elevations

�� Maximum Maximum TorsionalTorsional Resistance Resistance

�� Short Spans and Redundancy Short Spans and Redundancy

�� Direct Load Paths Direct Load Paths

Regular Building Configurations

Shear Wall Braced Frames Moment Resistant Frames

Irregular Building Configurations

�� Soft First StorySoft First Story : Discontinuity of Strength & Stiffness for : Discontinuity of Strength & Stiffness for

lateral load.lateral load.

�� Discontinuous Shear Walls.Discontinuous Shear Walls.

�� Variation in Variation in Perimeter StrengthPerimeter Strength & & StiffnessStiffness ..

Problematic Stress Concentrations & TorsionProblematic Stress Concentrations & Torsion

IrregularIrregular Building ConfigurationsBuilding Configurations

�� Building with Building with Irregular ConfigurationIrregular Configuration

L-Shaped Plan Cruciform Plan U-Shaped Plan

Unusual Low Story

Unusual High Story

MultipleTower Setbacks

Outwardly Uniform Appearance but

Non-uniform Mass Distribution or

converse

T-Shaped Plan Other Complex Shape

Split Levels

Irregular Building ConfigurationsIrregular Building Configurations

�� Building with Building with Abrupt Changes in Lateral ResistanceAbrupt Changes in Lateral Resistance

Interruption of Columns

Large Openings inShear Walls

Openings inDiaphragm

Soft Lower Levels

Interruption of Beams

Irregular Building ConfigurationsIrregular Building Configurations

�� Building with Building with Abrupt Changes in Lateral StiffnessAbrupt Changes in Lateral Stiffness

Drastic Changesin Mass/Stiffness

Ratio

Abrupt Changesin

Size of Member

Interruption ofVertical Resisting

Elements

Shear walls in some stories, Moment Resisting frames in

others

EFFCTIVENESS OF SHEAR WALLSEFFCTIVENESS OF SHEAR WALLS

a) Shear wall : a stiff structure with a short natural period of vibration.

b) Shear wall with small openings : still a relatively stiff structure with a short natural period.

c) Frame : a flexible structure with a long natural period of vibration.

d) Combination shear wall/ frame.

Different structural responses have widely varying natural periods of vibration, an important consideration in seismic design.

Structural Framings

Fundamental Period Shift & Damping

DUCTILITY OF SHEARWALLSDUCTILITY OF SHEARWALLS

AND BEAM & COLUMN AND BEAM & COLUMN

CONNECTIONSCONNECTIONS

Arrangement of Reinforcement In Shear Wall

Opening

Add

ition

al c

lose

lyS

pace

d lin

k

Shallow lintel

Additional reinforcement for high base shear

Anchorage

length

Additional diagonal bars in deep lintels

Foundation

Shear reinforcement

Reinforcement concentrated at extremities of wall

Anc

hora

ge

leng

th

Detailing Requirements For Potential Yield Zones

Close tie

Compression yield strain may be exceeded within these limits

>200

<200

>200

a) Forces in members at joint b) Shear stress in joint

Shear In Joint

V col.

C1=T1

SHEAR CRACKV1 beam

T2 = aAsbfy

Asb

C2 = T2V col

Col. Steel

Ast

T1 = aAstfy

V beam

Splice not permitted in joint, splices must be made outside joints.

Provide ties to carry 1.5 times horizontal component of thrust in offset bars. If offset bend occurs below beam longitudinal bars.

Reinforcement Details At Joint

Additional closely spaced link

Const. Joint

Example for transverse reinforcement in columns; consecutive crossties engaging the same longitudinal bars must have 90°hooks on opposite si des of columns.

6 db (≥≥≥≥75mm)

6 db extension

X

X

XX X

Transverse Reinforcement Details

MITIGATION OF EARTHQUAKEMITIGATION OF EARTHQUAKE--INDUCEDINDUCED

VIBRATIONVIBRATION

Building Loads LaLateral ground movement is quieted within

building by the isolation bearing.

Deformation of isolation bearing during lateral ground movement.

Footing

Installation of New isolation Bearing

Lateral Ground Movement Isolation

Lead Rubber Bearings,Bhuj District Hospital, India 2002

Damper (Energy Absorber/Dissipator)

STRUCTURAL CRACKING IN CONCRETESTRUCTURAL CRACKING IN CONCRETE

Web-shear crack

(a) Web-shear cracking

Diagonal Tension Cracking In R.C. Beams

Flexural crack

Flexural crack

Flexure shear crack

(b) Flexure-shear cracking

Splitting Of Concrete Along Reinforcement

Splitting

Splitting

(a)(b)

Torsional Cracks In R.C. Beam

T

(b)

θ

Failure Of A Tied Column

Flexural Cracking In Slabs

Nonparallel supports

(a)

(b)

Simple supports all sides

(c) (d)

Simple supports all sides

Flexural Cracking In Slabs

Axes of rotation

(f)

(g)Column

(e)

Four columns

Free edgeFixed supports two sides

Fixed supports two sides

Free edge

Flexural Cracking In Slabs

NONNON--STRUCTURAL CRACKING INSTRUCTURAL CRACKING IN

CONCRETECONCRETE

Plastic Shrinkage Cracking In Slabs

Crack Formed Due To Obstructed Settlement

Typical Crack Patterns At Reentrant Corners

Severe Cracking in UnreinforcedMasonry Wall

Reinforcing In-Filled Brickwalls And Opening

Typical details of r.c. stiffener and horizontal beam.

Typical lintol details

TREMOR DESIGN FORCES FOR BUILDINGS TREMOR DESIGN FORCES FOR BUILDINGS

KulimKulim

Ring of Fire

N-S : 0.013gE-W : 0.00905gV : 0.02628g

N-S : 0.01317gE-W : 0.01231gV : 0.0129g

Ipoh

N-S : 0.00915gE-W : 0.01284gV : 0.02153g

N-S : 0.01332gE-W : 0.01067gV : 0.01957g

Kulim

28 Mar 200526 Dec 2004Station

Tremor Acceleration at Malaysia Seismic Stations

700km700km

COMPARISON BETWEEN AMERICAN 1994 COMPARISON BETWEEN AMERICAN 1994

UBC SEISMIC LOADS AND BRITISH BS8110UBC SEISMIC LOADS AND BRITISH BS8110

NOTIONAL LOADSNOTIONAL LOADS

10-Storey Apartment/Hotel/Office Building

20 to 30-Storey Apartment/Hotel/Office Building

ASSUMPTIONS MADE IN SEISMIC ANALYSIS

1. Earthquake Loads

- Seismic Loads Derived From American 1994 UBC Static Method

2. Soil Profile Type S3

- Soil Profile With 21.3m Or More In Depth Containing More Than 6.1m Of Soft To Medium-Stiff Clay But Not More Than 12.2m Of Soft Clay.

- Assume As Average Soil Condition In Klang Valley Areas.

ASSUMPTIONS MADE IN SEISMIC ANALYSIS (COTD’)

3. Seismic Zone

- ZONE 0 Peak Acceleration = 0.00g To 0.02g (Non-Seismic Areas & Design To ACI Code)

- ZONE 1 Peak Acceleration < 0.05g

- Max. Tremor Acceleration In Peninsular Malaysia = 0.01332g

- Adopt Zone 1 For Comparison

COMPARISON OF TOTAL HORIZONTAL SEISMIC & NOTIONAL LOADS AT THE BASE OF BUILDING

Total Service Horizontal Loads (Total Service Horizontal Loads ( kNkN))

1.071 % DL1.071 % DL0.897% DL0.897% DL3030--storeystorey

1.071 % DL1.071 % DL1.013 % DL1.013 % DL2020--storeystorey

1.071 % DL1.071 % DL1.225 % DL1.225 % DL1010--storeystorey

British BS8110 Notional British BS8110 Notional Loads ( Multiply By Loads ( Multiply By

1.5/1.4)1.5/1.4)

American 1994 UBC American 1994 UBC Seismic LoadsSeismic Loads

Height Of Height Of Building Building

(Apartment, Hotel, (Apartment, Hotel, Office)Office)

COMPARISON OF TOTAL BASE MOMENTS IN CORE WALLS

Total Service Base Moment (Total Service Base Moment ( kNkN--mm))

+17 %+17 %277142771422950229503030--storeystorey

-- 25 %25 %151521515218936189362020--storeystorey

-- 42 %42 %7110711010071100711010--storeystorey

DifferenceDifferenceBritish BS8110 British BS8110 Notional LoadsNotional Loads

American American 1994 UBC 1994 UBC Seismic Seismic LoadsLoads

Height Of Height Of Building Building

(Apartment, Hotel, (Apartment, Hotel, Office)Office)

CONCLUSIONSCONCLUSIONS

�� No Earthquake In Peninsular Malaysia. No Earthquake In Peninsular Malaysia. Only Tremor Is Felt.Only Tremor Is Felt.

�� BS8110 Horizontal Notional Loads > BS8110 Horizontal Notional Loads > Max. Tremor Force Of 0.01332g. Max. Tremor Force Of 0.01332g. Existing Buildings Have Adequate Existing Buildings Have Adequate Lateral Resistance At This Moment.Lateral Resistance At This Moment.

�� Regular Building Configurations Have Regular Building Configurations Have Better Tremor Resistance. Better Tremor Resistance.

�� Ductile Structural Design And Detailing Ductile Structural Design And Detailing Will Help In Resisting The TremorWill Help In Resisting The Tremor. .

THANK YOUTHANK YOU