SEISMIC APPROACH DESIGN COMPARISON BETWEEN

Transcription

1 IABSE ANNUAL MEETING, LONDON, 19 TH SEPTEMBER 2011 SEISMIC APPROACH DESIGN COMPARISON BETWEEN IBC AND ITALIAN DM2008 Ing. Luca Zanaica Senior Structural Engineer Ing. Francesco Caobianco Senior Structural Engineer 1

2 OUTLINES The two codes frameworks Study case project Seismic input parameters definition Elastic spectra & Design spectra Results for the case project Cantilever walls investigation Final results Force-based Vs. Displacement-base 2

3 THE TWO CODES FRAMEWORKS DM 2008 IBC 2009 Decreto Ministeriale International Building 14/01/2008 Code 2009 EN : :2005 Eurocode 8 - Design of structures for earthquake resistance Part 1: General rules, seismic actions and rules for buildings ASCE 7 Minimum Design Loads for Buildings and Other Structures ACI 318 Building Code Requirements for Structural Concrete 3

4 CASE PROJECT Military facility Shear wall seismic resistant t structure Asymmetric plan shape Short walls 4

5 SEISMIC INPUT PARAMETERS DM 2008 IBC 2009 Use Class II: structure with regular crowd C U =1 Occupancy Category II: 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 acceleration amplification factor F O ; spectrum constant-velocity period start T C * Mapped spectral response accelerations: S S &S 1 Site 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: Seismic-force-resisting system: shear walls 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 5

6 ELASTIC & DESIGN SPECTRA 0, g 0,7 IBC R=1 0,8 DM 14/01/2008 q=1 DM 14/01/2008 q= g 07 0,7 IBC R=6 0,6 0,6 0,5 0,5 0,4 0,4 0,3 0, g 0,2 0, g 0,1 0,

7 RESULTS FOR THE CASE PROJECT DM 2008 IBC 2009 Design Base shear: 4150 kn REASONS: Design Base shear: 4400 kn Structure high stiffness: very low period moves the study onto the PGA zone Facility study case is not well representative for this CODES comparison Further study is required 7

8 CANTILEVER WALLS INVESTIGATION m = 60 tons P = 200 kn h storey = 3m T A =0.3s T A =0.7s T A =1.6s T A =2.6s T A =3.3s T A =4.0s [Priestly, Calvi, Kowalsky Displacement-Based Seismic Design of Structures ] 8

9 CANTILEVER WALLS FINAL RESULTS MV base base IBC DM Base Base Moment Shear [kn] [knm] Μ V base base DM2008 IBC Storey Nu Nu umber Base Moment [knm] Base Shear [kn] 9

10 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 Allocating seismic force between elements 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 type and material is at least disputable. Displacement check is performed at last [Priestly, Calvi, Kowalsky Displacement-Based Seismic Design of Structures ] ] 10

11 FORCE-BASED VS. DISPLACEMENT-BASED SEISMIC CODES PROVIDE A VARIETY OF DESIGN DISPLACEMENT [Priestly, Calvi, Kowalsky Displacement-Based Seismic Design of Structures ] ] why not starting straight from a design displacement? 11

12 FORCE-BASED VS. DISPLACEMENT-BASED Estimate Structural Dimensions: Calculate yield displacement y Select the ductility level l µ and the max permitted drift Θ Calculate the design displacement d =min{θh;µ y } Calculate the design ductility µ d = d / y Calculate the effective stiffness K e (m e,t e ) Calculate the design forces and moments: e.g. K e d & K e d H Capacity design with particular attention to material properties, over strength factors and P- Calculate the updated plastic displacement for the obtained section: d,ls DISPLACEMENT-BASED REMARKS Constant yield curvature behaviour for a given geometrical section Empirical calculation (through calibrated laws) of ξ hyst Use of elastic displacement spectra with adequate damping: NO R or q force reduction factor The design is made onto the secant stiffness K e Calculate the secant-stiffness equivalent damping ξ eq = ξ el + ξ hyst d,ls = d N Y END Calculate the effective response period T e ( d,ξ e ) Calculate the updated design displacement d NEW =min{θh; d,ls } 12

13 FORCE-BASED VS. DISPLACEMENT-BASED DO YOUTHINKA DIRECT DISPLACEMENT-BASED METHOD IS GOING TO BE THE FUTURE FOR SEISMIC DESIGN OF NEW STRUCTURES? IN ANY CASE IT APPEARS MORE RATIONAL 13

14 IABSE ANNUAL MEETING, LONDON, 19 TH SEPTEMBER 2011 THANK YOU! 14