EARTHQUAKE DESIGN CONSIDERATIONS OF BUILDINGS By Ir. Heng Tang Hai
SYPNOSIS 1. Earthquake-Induced Motions 2. Building Configurations 3. Effectiveness Of Shear Walls 4. Enhancement Of Ductility In Buildings 5. Mitigation Of Earthquake-Induced Vibrations 6. Cracking In Buildings 7. Tremor Design Forces For Buildings 8. Structural Adequacy Of Existing Buildings 9. Conclusions
Earthquake-Induced Motions In Multistory Buildings a) Possible ground movement-normally accelerations In the horizontal plane are the largest and most significant. b) Typical vibration modes for a tall building subjected to varying horizontal ground accelerations.
Fundamental Period Of Buildings 40 story Citicorp Equipment Buildings 10-20 story 4 story 1 story Seconds 0.05 0.1 0.5 1.0 2.0 7.0
Symmetrical Buildings Centroid of resisting forces No torsional effects develop Centroid of applied forces a) Symmetrical buildings do not experience exceptionally high torsional forces and are hence preferred to nonsymmetrical buildings.
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 b) Buildings that are nonsymmetrical because of either their basic configuration or the nonsymmetrical placement of lateral-load-resisting elements typically experience high torsional forces which are very destructive. Nonsymmetrically placed masses can also lead to similar torsional effects.
Nonsymmetrical Buildings With Reentrant Corners 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 In Vertical Direction 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.
Damage Due To Soft Story Chi-chi Earthquake, Taiwan Sept 21, 1999 Izmit Earthquake, Turkey Aug 17, 1999
Torsional Failure Gualan Earthquake, Guatemala 4 February, 1976
Elongated Buildings Seismic joint (actually quite narrow) High-damaged zone a) Not desirable b) Preferred. 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.
Slender Buildings Possible overturning High forces Lower forces a) Not desirable. b) Preferred. Relatively slender buildings are less able to resist efficiently the overturning movements cause by earthquakes than are shorter and more compact configurations.
Small Separation Between Buildings Clearance Pounding a) Small separation-not desirable. b) Large separation-preferred. Adjacent buildings should be adequately separated so that buildings do not pound against each other during seismic events.
Pounding Damage Prince William Sound Earthquake, Alaska 24 March, 1964 Izmit Earthquake, Turkey 17 August, 1999
Rigid Frame Buildings Frame Plastic hinges a) Frame b) Post-and-beam assembly Rigid frame buildings are generally preferable to pin-connected ones because the plastic hinges that necessarily form in rigid frame buildings before they collapse absorb large amounts of energy.
Collapse of Columns Taiwan
Shear Failure & Short Columns Failure Shear Failure, Northridge Earthquake Short Column Failure Santa Monica, Northridge Earthquake 17 January 1994 (Magnitude 6.8)
Rigid Floor Diaphragm Fig.7 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.
Failure Of Soffit Façade and soffit damage, Northridge Earthquake, 14 January, 1994 (magnitude 6.8) Fallen Soffit at Entrance, Northridge Earthquake
Strong-Column-Weak-Beam Strategy 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).
Collapse of Upper Floor Kobe, Japan 1995
Regular Building Configurations Shear Walls/Moment-Resistant Frames/Braced Frames Low Height to Base Ratios Equal Floor Heights Symmetrical Plans Uniform Sections and Elevations Maximum Torsional Resistance Short Spans and Redundancy Direct Load Paths
Regular Building Configurations Shear Wall Braced Frames Moment Resistant Frames
Irregular Building Configurations Soft First Story : Discontinuity of Strength & Stiffness for lateral load. Discontinuous Shear Walls. Variation in Perimeter Strength & Stiffness. Problematic Stress Concentrations & Torsion
Irregular Building Configurations Building with Irregular Configuration L-Shaped Plan Cruciform Plan U-Shaped Plan T-Shaped Plan Other Complex Shape Unusual Low Story Unusual High Story Multiple Tower Split Levels Setbacks Outwardly Uniform Appearance but Non-uniform Mass Distribution or converse
Irregular Building Configurations Building with Abrupt Changes in Lateral Resistance Interruption of Beams Soft Lower Levels Openings in Diaphragm Large Openings in Shear Walls Interruption of Columns
Irregular Building Configurations Building with Abrupt Changes in Lateral Stiffness Shear walls in some stories, Moment Resisting frames in others Interruption of Vertical Resisting Elements Abrupt Changes in Size of Member Drastic Changes in Mass/Stiffness Ratio
EFFCTIVENESS OF SHEAR WALLS
Structural Framings 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.
Fundamental Period Shift & Damping
DUCTILITY OF SHEARWALLS AND BEAM & COLUMN CONNECTIONS
Arrangement of Reinforcement In Shear Wall Opening Shallow lintel Anchorage length Additional diagonal bars in deep lintels Additional closely Spaced link Additional reinforcement for high base shear Reinforcement concentrated at extremities of wall Shear reinforcement Anchorage length Foundation
Detailing Requirements For Potential Yield Zones <200 >200 >200 Close tie Compression yield strain may be exceeded within these limits
Shear In Joint Col. Steel C 2 = T 2 V col T 1 = aa st f y A s t V 1 beam SHEAR CRACK V beam A sb T 2 = aa sb f y C 1 =T 1 V col. a) Forces in members at joint b) Shear stress in joint
Reinforcement Details At Joint Splice not permitted in joint, splices must be made outside joints. Const. Joint Additional closely spaced link Provide ties to carry 1.5 times horizontal component of thrust in offset bars. If offset bend occurs below beam longitudinal bars.
Transverse Reinforcement Details 6 d b ( 75mm) 6 d b extension X X X X X Example for transverse reinforcement in columns; consecutive crossties engaging the same longitudinal bars must have 90 hooks on opposite si des of columns.
MITIGATION OF EARTHQUAKE-INDUCED VIBRATION
Lateral Ground Movement Isolation Building Loads Lateral ground movement is quieted within building by the isolation bearing. La Installation of New isolation Bearing Deformation of isolation bearing during lateral ground movement. Footing
Lead Rubber Bearings, Bhuj District Hospital, India 2002
Damper (Energy Absorber/Dissipator)
STRUCTURAL CRACKING IN CONCRETE
Diagonal Tension Cracking In R.C. Beams Web-shear crack Flexural crack (a) Web-shear cracking Flexure shear crack Flexural crack (b) Flexure-shear cracking
Splitting Of Concrete Along Reinforcement Splitting Splitting (a) (b)
Torsional Cracks In R.C. Beam θ T (b)
Failure Of A Tied Column
Flexural Cracking In Slabs Nonparallel supports (a) (b)
Flexural Cracking In Slabs Simple supports all sides Simple supports all sides (c) (d)
Flexural Cracking In Slabs Axes of rotation Four columns (e) Fixed supports two sides Free edge Column Free edge (g) (f) Fixed supports two sides
NON-STRUCTURAL CRACKING IN CONCRETE
Plastic Shrinkage Cracking In Slabs
Crack Formed Due To Obstructed Settlement
Typical Crack Patterns At Reentrant Corners
Severe Cracking in Unreinforced Masonry Wall
Reinforcing In-Filled Brickwalls And Opening Typical lintol details Typical details of r.c. stiffener and horizontal beam.
TREMOR DESIGN FORCES FOR BUILDINGS
Kulim 700km Tremor Acceleration at Malaysia Seismic Stations Station Kulim Ipoh 26 Dec 2004 N-S : 0.01332g E-W : 0.01067g V : 0.01957g N-S : 0.01317g E-W : 0.01231g V : 0.0129g 28 Mar 2005 N-S : 0.00915g E-W : 0.01284g V : 0.02153g N-S : 0.013g E-W : 0.00905g V : 0.02628g Ring of Fire
COMPARISON BETWEEN AMERICAN 1994 UBC SEISMIC LOADS AND BRITISH BS8110 NOTIONAL 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 Height Of Building (Apartment, Hotel, Office) Total Service Horizontal Loads (kn( kn) American 1994 UBC Seismic Loads British BS8110 Notional Loads ( Multiply By 1.5/1.4) 10-storey 1.225 % DL 1.071 % DL 20-storey 1.013 % DL 1.071 % DL 30-storey 0.897% DL 1.071 % DL
COMPARISON OF TOTAL BASE MOMENTS IN CORE WALLS Height Of Building (Apartment, Hotel, Office) American 1994 UBC Seismic Loads Total Service Base Moment (kn( kn-m) British BS8110 Notional Loads Difference 10-storey 10071 7110-42 % 20-storey 18936 15152-25 % 30-storey 22950 27714 +17 %
CONCLUSIONS No Earthquake In Peninsular Malaysia. Only Tremor Is Felt. BS8110 Horizontal Notional Loads > Max. Tremor Force Of 0.01332g. Existing Buildings Have Adequate Lateral Resistance At This Moment. Regular Building Configurations Have Better Tremor Resistance. Ductile Structural Design And Detailing Will Help In Resisting The Tremor.
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