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BUILDING TORSION DUE TO EARTHQUAKES:RECENT DEVELOPMENTS
Keynote Lecture
S. A. Anagnostopoulos1 and M.T. Kyrkos2
The 5th National Conference on Earthquake Engineering and the 1st
National Conference on Earthquake Engineering and Seismology
BUILDING TORSION DUE TO EARTHQUAKES:RECENT DEVELOPMENTS
Keynote Lecture
S. A. Anagnostopoulos1 and M.T. Kyrkos2
1Professor, Dept. of Civil Engineering University of Patras,2Structural Engr. Ph.D., Attica Region, Athens,
CONTENTS-OBJECTIVES• Make a general introduction to the problem of earthquake
induced torsion in buildings, including its causes andrelated code provisions
• Review the pertinent literature and point out shortcomingsin many of the past studies as well as some contradictingresults and conclusions debated for many years
• Briefly discuss our research and results pertaining:(a) To improved design of asymmetric buildings for
uniform ductility demand distribution(b) To the code specified accidental design eccentricity
• Make a general introduction to the problem of earthquakeinduced torsion in buildings, including its causes andrelated code provisions
• Review the pertinent literature and point out shortcomingsin many of the past studies as well as some contradictingresults and conclusions debated for many years
• Briefly discuss our research and results pertaining:(a) To improved design of asymmetric buildings for
uniform ductility demand distribution(b) To the code specified accidental design eccentricity
PAST RESEARCH ACTIVITY
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2 423
51 52
79
145
97 102111
69
0
50
100
150
1951
-196
019
61-1
970
1971
-197
5
1976
-198
019
81-1
985
1986
-199
019
91-1
995
1996
-200
0
2001
-200
520
05-2
010
2011
-201
2
Year
Nu
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of
Pu
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s
PAST RESEARCH ACTIVITY
• The majority (~ 65%) of the pertinent published work isbased on simplified, one-story, shear-beam models
• About ~ 30% of the published work is based onsimplified, elastic multi-story models
• Very rare (< 5%) is the use of inelastic dynamicanalyses of realistic buildings modeled with detailedplastic hinge idealizations. (Yet the building responseunder design level earthquakes is always inelastic)
• The majority (~ 65%) of the pertinent published work isbased on simplified, one-story, shear-beam models
• About ~ 30% of the published work is based onsimplified, elastic multi-story models
• Very rare (< 5%) is the use of inelastic dynamicanalyses of realistic buildings modeled with detailedplastic hinge idealizations. (Yet the building responseunder design level earthquakes is always inelastic)
Causes of earthquake induced torsion in buildings
1. Non-symmetric arrangement of the load resistingelements (stiffness eccentricity) or non-symmetricdistribution of masses
2. Torsional motion in the ground caused by seismicwave passage and by ground motion incoherency
3. Other reasons that are not explicitly accounted for instructural design (stiffness of non-structural elementssuch as brick infill walls, non-symmetric yielding of theload resisting elements, etc.).
1. Non-symmetric arrangement of the load resistingelements (stiffness eccentricity) or non-symmetricdistribution of masses
2. Torsional motion in the ground caused by seismicwave passage and by ground motion incoherency
3. Other reasons that are not explicitly accounted for instructural design (stiffness of non-structural elementssuch as brick infill walls, non-symmetric yielding of theload resisting elements, etc.).
TORSION RELATED CLAUSES IN CODES
Regularity Criteria: Structural and geometrical
Torsional Sensitivity: Usually limits on dmax/davg
Accidental Eccentricity (eacc) Usually 0.05L ÷ 0.10L
Torsional effects Usually by moving masses by eaccor by applying a static torque
Amplification of static eccentricity Not any more
TORSION RELATED CLAUSES IN CODES
Regularity Criteria: Structural and geometrical
Torsional Sensitivity: Usually limits on dmax/davg
Accidental Eccentricity (eacc) Usually 0.05L ÷ 0.10L
Torsional effects Usually by moving masses by eaccor by applying a static torque
Amplification of static eccentricity Not any more
SIMPLIFIED, ONE-STORY SHEAR BEAM MODEL(The most general case used)
Ly
emy
esy
ey
CM
GC
x
yEL4
EL5
EL2
EL3EL1
MassEccentricity
εmx = emx / Lxεmy = emy / Ly
Lx
emxesx
ex
CR
EL6
StiffnessEccentricity
εsx = esx / Lxεsy = esy / Ly
Physicaleccentricities:
ex=esx+ emxey=esy+ emy
TYPES OF SIMPLIFIED, ONE-STORY MODELS USED
(a) (b) (c)(a) (b) (c)
(d) (e) (f)BidirectionalEccentricity
“STIFF” AND “FLEXIBLE” EDGES OF A NON SYMMETRICBUILDING (Meaningful only for static loadings)
Stiff edge: Displacement from translation and rotation aresubtracted
Flexible edge: Displacement from translation and rotationare added
Shear or Stiffnesscenter (CS or CR):Point through whicha lateral force willcause no rotation
Motion in Y direction
Shear or Stiffnesscenter (CS or CR):Point through whicha lateral force willcause no rotation
PROBLEMS OF THE SIMPLIFIED SHEAR-BEAMMODELS USED IN PAST STUDIES
1. They cannot match ALL the important properties forthe inelastic dynamic response of real, non-symmetric multistory buildings
2. Stiffness and strength of the resisting elements of theshear-beam model are typically specified andcalculated independently and only for seismic loads.In real buildings, stiffness and strength coming fromgravity loads and other design requirements lead tosignificant differences in relative strength andstiffness distributions.
1. They cannot match ALL the important properties forthe inelastic dynamic response of real, non-symmetric multistory buildings
2. Stiffness and strength of the resisting elements of theshear-beam model are typically specified andcalculated independently and only for seismic loads.In real buildings, stiffness and strength coming fromgravity loads and other design requirements lead tosignificant differences in relative strength andstiffness distributions.
3. Yielding of one element in the simplified model isequivalent to the formation of a mechanism in a framestory in real buildings, prevented in modern Codesthrough capacity design provisions.
4. Higher mode effects cannot be accounted for in thesimplified models
5. Incorrect extrapolation of conclusions to real buildings
3. Yielding of one element in the simplified model isequivalent to the formation of a mechanism in a framestory in real buildings, prevented in modern Codesthrough capacity design provisions.
4. Higher mode effects cannot be accounted for in thesimplified models
5. Incorrect extrapolation of conclusions to real buildings
CONSEQUENCES OF SUCH SHORTCOMINGS
• Contradictory results by different researchers• Persisting controversies – questionable results• Conclusions applicable ONLY to the very specific
models used and hence extrapolation to realbuildings is at best questionable• Little benefit from a very large volume of research
• Contradictory results by different researchers• Persisting controversies – questionable results• Conclusions applicable ONLY to the very specific
models used and hence extrapolation to realbuildings is at best questionable• Little benefit from a very large volume of research
CONTROVERCIES AND CONTRADICTING RESULTS
• Rutenberg, A. (1992) (review paper)“The picture emerging from the foregoing review is somewhatconfusing”…..“Several discrepancies and inconsistencies amonginvestigators have been reported in the preceding sections”.
• Chandler et al (1996) (review paper) … listed “ten areas of concernwhere the use of differing definitions or the making of divergingassumptions has resulted in a basic lack of agreement between theresults and conclusions of the research”.
• De Stefano, M. and Pintucchi, B. (2008) (review paper)“Many studies adopting more realistic multi-story models haveevidenced the shortcomings of simplified one-story models,especially in predicting qualitative features of inelastic response.”
• Rutenberg, A. (1992) (review paper)“The picture emerging from the foregoing review is somewhatconfusing”…..“Several discrepancies and inconsistencies amonginvestigators have been reported in the preceding sections”.
• Chandler et al (1996) (review paper) … listed “ten areas of concernwhere the use of differing definitions or the making of divergingassumptions has resulted in a basic lack of agreement between theresults and conclusions of the research”.
• De Stefano, M. and Pintucchi, B. (2008) (review paper)“Many studies adopting more realistic multi-story models haveevidenced the shortcomings of simplified one-story models,especially in predicting qualitative features of inelastic response.”
Classic example:
For code designed buildings, which of the two edges, thestiff or the flexible is the critical one?? (where criticalmeans “ penalized most due to torsion”, in terms ofductility demands)
Conclusions divided between the two optionswhile in some papers the answer is parameter
dependent !!!
Classic example:
For code designed buildings, which of the two edges, thestiff or the flexible is the critical one?? (where criticalmeans “ penalized most due to torsion”, in terms ofductility demands)
Conclusions divided between the two optionswhile in some papers the answer is parameter
dependent !!!
OUR VIEW1. Problem complexity - many parameters2. Failure or unwillingness of most authors to
recognize the problems and shortcomings oftheir oversimplified 1-story shear beammodel (1ST-INSB)
3. Failure of most authors to clearly state thattheir results were applicable ONLY to thespecific models used and subject to theunderlying assumptions. Instead, unjustifiedgeneralizations were made.
1. Problem complexity - many parameters2. Failure or unwillingness of most authors to
recognize the problems and shortcomings oftheir oversimplified 1-story shear beammodel (1ST-INSB)
3. Failure of most authors to clearly state thattheir results were applicable ONLY to thespecific models used and subject to theunderlying assumptions. Instead, unjustifiedgeneralizations were made.
GC
CR
x
y
CM
Ly=12m
Lx=18m
ey
ex
1 2 3 4
6 7 8
9 10 11 12
5
FR1 FR2 FR3
FR4
FR5
FR6
ANSWER TO THE CRITICAL ELEMENT CONTROVERSY
3-STORY, PH MODEL 1-STORY, SHEAR BEAM MODEL3-STORY, PH MODEL 1-STORY, SHEAR BEAM MODEL
ELEMENT PROPERTIES DETERMINATION FOR THE 1ST-INSB MODEL
BUILDINGS WITH PLASTIC HINGE IDEALIZATION
Type: one, three and five-story reinforced concretebuildings (fundamental period: 0.30s < Τ < 1.32s)
Design: according to EC2 and EC8 and also UBC97(equiv. static and dynamic response spectrumanalysis)
Idealization: each member is idealized with theplastic hinge model
Analysis: inelastic, time-history for groups of earthquakemotions matching the design spectrum
Basic Parameter: Biaxial eccentricities 0.0 , 0.10 , 0.20 , 0.30
Type: one, three and five-story reinforced concretebuildings (fundamental period: 0.30s < Τ < 1.32s)
Design: according to EC2 and EC8 and also UBC97(equiv. static and dynamic response spectrumanalysis)
Idealization: each member is idealized with theplastic hinge model
Analysis: inelastic, time-history for groups of earthquakemotions matching the design spectrum
Basic Parameter: Biaxial eccentricities 0.0 , 0.10 , 0.20 , 0.30
3-St. Bldg/ Beams / Dir. y : -------- Frame 1 (“stiff” side), -------- Frame 3 (“flexible” side)
ROTATIONAL DUCTILITY FACTORS BEAMS,3 – STORY, PH MODEL : (Results for x and y directions)
3-St. Bldg / Beams / Dir x : -------- Frame 6 (“stiff” side), -------- Frame 4 (“flexible” side)
ROTATIONAL DUCTILITY FACTORS BEAMS,5 – STORY, PH MODEL : (Results for x and y directions)
5-St. Bldg/ Beams / Dir. y : -------- Frame 1 (“stiff” side), -------- Frame 3 (“flexible” side)
5-St. Bldg / Beams / Dir x : -------- Frame 6 (“stiff” side), -------- Frame 4 (“flexible” side)
DERIVATION OF PROPERTIES OF THE SIMPLIFIED MODELS
1. MODEL SIMP1 : MATCHES PROPERTIES OF REAL BUILDING(From the properties of the PH models)
3 Lowest periods, element stiffnesses AND element strengthsStrengths from ALL loading conditions
2. MODEL SIMP3 ( Design only for earthquake, as typically
done in the past)
Identical to SIMP1 except that element strengths determinedas done in the past from earthquake loading only
1. MODEL SIMP1 : MATCHES PROPERTIES OF REAL BUILDING(From the properties of the PH models)
3 Lowest periods, element stiffnesses AND element strengthsStrengths from ALL loading conditions
2. MODEL SIMP3 ( Design only for earthquake, as typically
done in the past)
Identical to SIMP1 except that element strengths determinedas done in the past from earthquake loading only
DUCTILITY FACTOR COMPARISONS: PH, SIMP1, SIMP3 MODELS
3 story _ P-H model_Displ. Ductilities
1,000
1,300
1,600
1,900
0,00 0,10 0,20 0,30ε=e/L
Duc
tiliti
es
Fr1 (Stif f side) Fr3 (Flex side)3 story _ P-H model_Displ. Ductilities
0,500
0,800
1,100
1,400
1,700
2,000
0,00 0,10 0,20 0,30ε=e/L
Duc
tiliti
es
Fr6 (Stif f side) Fr4 (Flex side)
3 story _ SIMP1 model
0,00
0,50
1,001,50
2,00
2,50
3,00
0,00 0,10 0,20 0,30ε=e/L
Duc
tility
Fr1 (Stif f side) Fr3 (Flex side)3 story _ SIMP1 model
0,00
0,50
1,001,50
2,00
2,50
3,00
0,00 0,10 0,20 0,30ε=e/L
Duc
tility
Fr6 (Stif f side) Fr4 (Flex side)
Y-DIRECTION X-DIRECTION
3 story _ SIMP1 model
0,00
0,50
1,001,50
2,00
2,50
3,00
0,00 0,10 0,20 0,30ε=e/L
Duc
tility
Fr1 (Stif f side) Fr3 (Flex side)3 story _ SIMP1 model
0,00
0,50
1,001,50
2,00
2,50
3,00
0,00 0,10 0,20 0,30ε=e/L
Duc
tility
Fr6 (Stif f side) Fr4 (Flex side)
3 story _ SIMP3 model_eacc=0.05
1,00
1,50
2,00
2,50
3,00
3,50
0,00 0,10 0,20 0,30ε=e/L
Duc
tility
Fr1 (Stif f side) Fr3 (Flex side)3 story _ SIMP3 model_eacc=0.05
1,00
1,50
2,00
2,50
3,00
3,50
0,00 0,10 0,20 0,30ε=e/L
Duc
tility
Fr6 (Stif f side) Fr4 (Flex side)
ANSWER TO THE CONTROVERSY
• For code designed eccentric structures the“flexible” edge of is penalized more by strongearthquakes due to torsion• The oversimplified one-story model can only
predict the correct trends ONLY if its 3periods, element stiffness AND strengthsmatch those of the real buildings
• For code designed eccentric structures the“flexible” edge of is penalized more by strongearthquakes due to torsion• The oversimplified one-story model can only
predict the correct trends ONLY if its 3periods, element stiffness AND strengthsmatch those of the real buildings
LAYOUT OF 3-STORY IRREGULAR STEEL BUILDINGS
TORSIONALLY STIFF
TORSIONALLY FLEXIBLE(Uniformly distributed masses)TORSIONALLY FLEXIBLE(Uniformly distributed masses)
DETAILED , REALISTIC PLASTIC HINGE MODEL OF ABUILDING AND NON LINEAR MEMBER BEHAVIOR
y
M1
p M
y
ΔΜμ=1+p.Μ
M-θ Relationship and interaction diagramof Beam-Column members
F-δ relationship for bracing members
plu
y
u1
u
DESIGN AND MEAN SPECTRUMOF 10 SEMI-ARTIFICIAL MOTIONS
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.00 1.00 2.00 3.00 4.00 5.00
Period (sec)
Spec
tral A
ccel
erat
ion
(g) MEAN
EC8, Ag,max=0.24g
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.00 1.00 2.00 3.00 4.00 5.00
Period (sec)
Spec
tral A
ccel
erat
ion
(g) MEAN
EC8, Ag,max=0.24g
Modification procedure for torsionally stiff& flexible buildings
uy,stiff = top story displacement of the “stiff” edgeuy,flex = top story displacement of the “flexible” edgeuyO = mean story displacement
(computed by equivalent static method)
yo
stiffystiffy u
uf ,
, yo
flexyflexy u
uf ,
,
EFFECTS OF DESIGN MODIFICATIONS ON ECCENTRICITIES(Buildings with initial mass eccentricity εm=0.20)
TorsionallySTIFF
TorsionallyFLEXIBLE
MEAN NATURAL ECCENTRICITYINITIAL DESIGN MODIFIED DESIGN
εx εy εx εyTorsionallySTIFF
0.115 0.135 0.04 0.05
TorsionallyFLEXIBLE
0.165 0.145 0.10 0.10
TorsionallyFLEXIBLE
MODIFIED TORSIONALLY STIFF BUILDINGInitial mass eccentricity εm=0.20
DIRECTION X DIRECTION YAXIAL STRAIN DUCTILITY FACTOR IN BRACES
1
2
3
1 2 3 4Axial strain ductility factor
Sto
ryFlex-mod Stiff-modFlex-init Stiff-init
1
2
3
1 2 3 4Axial strain ductility factor
Sto
ry
Flex-mod Stiff-modFlex-init Stiff-init
ROTATIONAL DUCTILITY FACTORS IN BEAMS
1
2
3
1 2 3 4Axial strain ductility factor
Sto
ryFlex-mod Stiff-modFlex-init Stiff-init
1
2
3
1 2 3 4Axial strain ductility factor
Sto
ry
Flex-mod Stiff-modFlex-init Stiff-init
1
2
3
1 2Rotational ductility factor
Sto
ry
Flex-mod Stiff-modFlex-init Stiff-init
1
2
3
1 2 3Rotational ductility factor
Sto
ry
Flex-mod Stiff-modFlex-init Stiff-init
MODIFIED TORSIONALLY FLEXIBLE BUILDINGInitial mass eccentricity εm=0.20
DIRECTION X DIRECTION Y
AXIAL STRAIN DUCTILITY FACTOR IN BRACES
1
2
3
1 2 3 4Axial strain ductility factor
Sto
ryFlex-mod Stiff-modFlex-init Stiff-init
1
2
3
1 2 3 4Axial strain ductility factor
Sto
ry
Flex-mod Stiff-modFlex-init Stiff-init
ROTATIONAL DUCTILITY FACTORS IN BEAMS
1
2
3
1 2 3 4Axial strain ductility factor
Sto
ryFlex-mod Stiff-modFlex-init Stiff-init
1
2
3
1 2 3 4Axial strain ductility factor
Sto
ry
Flex-mod Stiff-modFlex-init Stiff-init
1
2
3
1 2 3Rotational ductility factor
Sto
ry
Flex-mod Stiff-modFlex-init Stiff-init
1
2
3
1 2 3Rotational ductility factor
Sto
ry
Flex-mod Stiff-modFlex-init Stiff-init
THREE AND FIVE-STORYFRAME TYPE BUILDINGS
GC
CR
x
y
CM
Ly=12m
Lx=18m
ey
ex
1 2 3 4
6 7 8
9 10 11 12
5
FR1 FR2 FR3
FR4
FR5
FR6
• frame type buildings with plastic hinge idealization• simultaneous biaxial mass and stiffness eccentricity
ey
ex
CM
GCCR
x
y
Ly=15m
Lx=21m
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
FR1 FR2 FR3
FR6
FR5
FR4
GC
CR
x
y
CM
Ly=12m
Lx=18m
ey
ex
1 2 3 4
6 7 8
9 10 11 12
5
FR1 FR2 FR3
FR4
FR5
FR6
ey
ex
CM
GCCR
x
y
Ly=15m
Lx=21m
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
FR1 FR2 FR3
FR6
FR5
FR4
3-STORY 5-STORY
PARAMETRIC ANALYSES
Variants of the 3 and 5-story buildings which are analysedwith accidental eccentricity: ± 5%
Original-physicalmass eccentricity
Inelastic analysismass eccentricity
Original-physicalmass eccentricity
Inelastic analysismass eccentricity
εm εm,1 εm,0 εm,2
0.00 -0.05 0.00 0.05
0.10 0.05 0.10 0.15
0.20 0.15 0.20 0.25
'St if f 'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00
'St if f 'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00
'St if f 'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00
Design: e = 0.00L+/-eaccAnalyzed variants:
ε=0.00 : __________
ε=0.05 : ___ __O__ ___
eacc=0.00
Design: e = 0.20L+/-eaccAnalyzed variants:ε=0.15 : _ _ __ _ _
ε=0.20 : __________
ε=0.25 : ___ __O__ ___
Design: e = 0.10L+/-eaccAnalyzed variants:ε=0.05 : _ _ __ _ _
ε=0.10 : __________
ε=0.15 : ___ __O__ ___
DUCTILITY FACTORS FOR THE BEAMS OF THETHE 5-STORY BUILDINGS, STIFF SIDE (Fr1, Dir-y)
'St if f 'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00
'St if f 'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00
'St if f 'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00
'St if f 'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00
'St if f 'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00
'St if f 'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00
eacc=0.05L
'Flex'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00
'Flex'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00 6.00
'Flex'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00 6.00 7.00
Design: e = 0.00L+/-eaccAnalyzed variants:
ε=0.00 : __________
ε=0.05 : ___ __O__ ___
eacc=0.00
Design: e = 0.20L+/-eaccAnalyzed variants:ε=0.15 : _ _ __ _ _
ε=0.20 : __________
ε=0.25 : ___ __O__ ___
Design: e = 0.10L+/-eaccAnalyzed variants:ε=0.05 : _ _ __ _ _
ε=0.10 : __________
ε=0.15 : ___ __O__ ___
DUCTILITY FACTORS FOR THE BEAMS OF THETHE 5-STORY BUILDINGS, FLEXIBLE SIDE (Fr3, Dir-y)
'Flex'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00
'Flex'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00 6.00
'Flex'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00 6.00 7.00
'Flex'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00
'Flex'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00 6.00
'Flex'side
1
2
3
4
5
1.00 2.00 3.00 4.00 5.00 6.00 7.00
eacc=0.05L
CONCLUSIONS• The many conflicting conclusions and controversies in
past publications are due to the failure of many authorsto recognize the shortcomings of their over-simplifiedmodels and from unjustified generalizations orextensions of their conclusions to real buildings
• Simplified one-story models can provide usefulqualitative results ONLY when their properties arerationally selected to match most of the basicproperties of actual buildings
• The many conflicting conclusions and controversies inpast publications are due to the failure of many authorsto recognize the shortcomings of their over-simplifiedmodels and from unjustified generalizations orextensions of their conclusions to real buildings
• Simplified one-story models can provide usefulqualitative results ONLY when their properties arerationally selected to match most of the basicproperties of actual buildings
CONCLUSIONS (Cont.)
• When asymmetric buildings, designed according toEC8, are subjected to strong earthquake motions,the ductility demands at the “flexible” edges aresubstantially and consistently greater than those atthe “stiff” edges
• A simple, one step design modification has beenproposed, improving the building’s seismicperformance significantly. This suggests that a codeimprovement is possible.
• When asymmetric buildings, designed according toEC8, are subjected to strong earthquake motions,the ductility demands at the “flexible” edges aresubstantially and consistently greater than those atthe “stiff” edges
• A simple, one step design modification has beenproposed, improving the building’s seismicperformance significantly. This suggests that a codeimprovement is possible.
CONCLUSIONS (Cont.)
• The accidental design eccentricity is not veryeffective in reducing ductility demands inasymmetric frame type buildings. In fact, in somebuilding locations the designs with zero accidentaleccentricity exhibited ductility demands, less thanthose with accidental eccentricity.
• These findings suggest that accidental eccentricityprovisions in codes, should be re-examined, in viewof the great additional computational requirementsthey impose on designers
• The accidental design eccentricity is not veryeffective in reducing ductility demands inasymmetric frame type buildings. In fact, in somebuilding locations the designs with zero accidentaleccentricity exhibited ductility demands, less thanthose with accidental eccentricity.
• These findings suggest that accidental eccentricityprovisions in codes, should be re-examined, in viewof the great additional computational requirementsthey impose on designers