IABSE ANNUAL MEETING, LONDON, 19 TH SEPTEMBER 2011
SEISMIC APPROACH DESIGN COMPARISON BETWEEN IBC AND ITALIAN DM2008 IBC AND ITALIAN DM2008
Ing. Luca Zanaica g Ing. Francesco Caobianco
Senior Structural Engineer
g
Senior Structural Engineer
OUTLINES
• The two codes’ frameworks
• Study case project
• Seismic input parameters’ definition
El ti t & D i t
• Elastic spectra & Design spectra
• Results for the case project
• Results for the case project
• Cantilever walls investigation g
• Final results
• Force-based Vs. Displacement-base
THE TWO CODES’ FRAMEWORKS
DM 2008
Decreto Ministeriale 14/01/2008
IBC 2009
International Building Code 2009
14/01/2008 Code 2009
EN 1998-1-1:2005
ASCE 7
Minimum Design Loads EN 1998 1 1:2005
Eurocode 8 - Design of structures for earthquake
i t
Minimum Design Loads for Buildings and
Other Structures resistance
Part 1: General rules, seismic actions and
ACI 318 Building Code seismic actions and
rules for buildings
Building Code
Requirements for
Structural Concrete
CASE PROJECT
Milit f ilit
• Military facility
• Shear wall
• Shear wall seismic
i t t resistant structure
• Asymmetric
l h
plan shape
• Short walls
• Short walls
SEISMIC INPUT PARAMETERS
DM 2008 IBC 2009
Use Class II: Occupancy Category II:
structure with regular crowd Æ C
U=1
p y g y
buildings not designated as essential nor
representing a substantial hazard to human life in the event of failure
Nominal Service Life V
N=50years Seismic Importance Factor I=1 Mapped parameters: PGA; horizontal spectral
l ti lifi ti f t F t
Mapped spectral response accelerations: S
S& S acceleration amplification factor F
O; spectrum
constant-velocity period start T
C*
& S
1Site Class C: Site Class D:
coarse-grained thickener soil or fine-grained stiff soil (180≤v
s≤360 m/s)
stiff soil (180≤v
s≤360 m/s)
Seismic-force-resisting system: g y Seismic-force-resisting system:
shear walls
g y
special reinforced concrete shear walls
Structural Factor q=3 Response Modification Factor R=6 Over strength factor Ω
0=MIN{q; 1,2} for squat
walls
Over strength factor Ω
0=2.5
ELASTIC & DESIGN SPECTRA
0,8
DM 14/01/2008 q=1 IBC R 1
0.797g
0 777g
0 70,8
DM 14/01/2008 q=3 IBC R=6
0,6
0,7
0.777g
IBC R=10,6
0,7 IBC R=6
0,5 0,5
0,4 0,4
0,2 0,3
0,2
0,3
0.259g
0,1 0,1
0.133g
0
0 1 2 3 4
0
0 1 2 3 4
RESULTS FOR THE CASE PROJECT
DM 2008 IBC 2009
Design Base shear: Design Base shear:
Design Base shear:
4150 kN
Design Base shear:
4400 kN
R EASONS :
• Structure high stiffness: very low period moves the study onto the PGA zone
moves the study onto the PGA zone
• Facility study case is not well representative for this CODES’ comparison
Further study is required
• Further study is required…
CANTILEVER WALLS INVESTIGATION
• m = 60 tons
• P = 200 kN
• ∆h storey = 3m
TA=0.3s TA=0.7s TA=1.6s TA=2.6s TA=3.3s TA=4.0s
[Priestly, Calvi, Kowalsky “Displacement-Based Seismic Design of Structures”]
CANTILEVER WALLS’ FINAL RESULTS
DM2008
Vbase
DM2008
Mbase
IBC
20 16 12 8 4 2
IBC
20 16 12 8 4 2
∆V
baseDM2008‐IBC
0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Base Shear [kN]
∆Μ
baseDM2008‐IBC
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 Base Moment [kNm]
mber 20
umber
20 16
Storey Nu 16
12 8 4
Storey Nu 16
12 8 44
22
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300
Base Shear [kN]
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000
Base Moment [kNm]
FORCE-BASED VS. DISPLACEMENT-BASED
FORCE-BASED METHOD CRITICISMS Stiffness is estimated to determine the
period T.
Stiffness is dependent on strength which cannot be know until the end of the design process be know until the end of the design process Allocating seismic force between elements based on initial stiffness is illogical because based on initial stiffness is illogical because
different elements might not yield simultaneously
The assumption that unique force-reduction factors are appropriate for a given structural
t d t i l i t l t di t bl type and material is at least disputable.
Displacement check is performed at last
FORCE-BASED VS. DISPLACEMENT-BASED
S EISMIC CODES PROVIDE A V ARIETY OF D ESIGN D ISPLACEMENT
[Priestly, Calvi, Kowalsky “Displacement-Based Seismic Design of Structures”]
[ y y p g ]
… why not starting straight from a design displacement?
FORCE-BASED VS. DISPLACEMENT-BASED
Estimate Structural Dimensions:
Calculate yield
Calculate the effective stiffness Ke(me,Te)
DISPLACEMENT-BASED REMARKS Constant yield curvature behaviour for
a given geometrical section
Calculate yield displacement ∆y
S l t th d tilit l l
Calculate the design forces and moments: e g K ∆d& K ∆dH
Empirical calculation (through calibrated laws) of ξhyst
U f l ti di l t t ith
a given geometrical section
Select the ductility level µ and the max permitted drift Θ
moments: e.g. Ke∆d& Ke∆dH
Capacity design with particular attention tomaterial properties
Use of elastic displacement spectra with adequate damping:
NO R or q force reduction factor
Th d i i d t th t
Calculate the design displacement
∆d=min{ΘH;µ∆y}
attention to material properties, over strength factors and P-∆
Calculate the updated plastic
The design is made onto the secant stiffness Ke
Calculate the design ductility µd=∆d/∆y
p p
displacement for the obtained section: ∆d,ls
Y Calculate the secant-stiffness
equivalent damping ξeq=ξel+ξhyst
∆d,ls=∆d
N END
Y
Calculate the effective response (∆ ,ξ
Calculate the updated design displacement ∆ NEW=min{ΘH;∆ }
