Assessment of the seismic performance of steel frames using OpenSees

Assessment of the seismic
performance of steel frames
using OpenSees
Sara Oliveira, Filippo Gentili, Ashkan Shahbazian, Hugo Augusto, Ricardo Costa,
Carlos Rebelo, Yukihiro Harada and Luís Simões da Silva
20-06-2017
OpenSees Days Europe
June 19-20, 2017
Porto, Portugal

Institute for Sustainability and Innovation in Structural Engineering
2|Assessment of the seismic performance of steel frames using OpenSees Sara Oliveira
1. Introduction
2. Simplified numerical models
2.1 Parametric study on D-CBF
2.2 Modal analysis
2.3 Pushover analysis
2.4 Incremental dynamic analysis
3. Conclusions
Contents

1. Introduction
 Seismic behaviour of steel frames
 Deformation of the panel zone of the beam-to-column joint region
 Global structural model
 Eurocode EN 1998-1
EC8-1 allows the formation of plastic hinges in the joints in case of partial strength
and/or semi-rigid joints, provided that the following requirements are verified:
 The connections have a rotational capacity consistent with the global
deformations;
 Members framing into the connections are demonstrated to be stable at the
ultimate limit state
 The effect of connection deformation on global drift is taken into account using
non-linear static global analysis or non-linear time history analysis
 EQUALJOINTS – European pre-QUALified Steel JOINTS
 To estimate the seismic demand of the semi-rigid partial strength joints in
typical Moment Resisting Frames (MRF) and Dual Concentrically Braced
Frames (D-CBF)

1. Introduction
Fig: Joint modelling strategies considered in this study

Dual Concentrically Braced Frame (D-CBF)
 Design and configuration
 Braces located at the central bay
 Inverted “V” (Chevron)
 Braces with square hollow sections
 Behaviour factor - 1=2.5 (braces)
 Verifications
 Member strength and stability checks
(EN 1993-1-1)
 Seismic action effects (EN 1998-1)
Fig: Dual Concentrically Braced Frame (D-CBF)

 Joint typologies considered
 EH-S: Full-strength with strong panel zone
 ES-B-E: Equal-strength with balanced panel zone
 E-B-P(0.6): Partial-strength with balanced panel zone
 E-B-P(0.8): Partial-strength with weak panel zone
 Structural configuration considered
 Ten D-CBF configurations studied
 Level of seismic hazard
 Number of storey
 Number of bay
 Length of span

Scissor model
 External spring
 Column web in compression
 Column web in tension
 Remaining connection components
 Column flange in bending
 End-plate in bending
 Bolts in tension
 Internal spring
 Column web panel in shear
Fig: Scissor model

 Connection springs
 Elastic-plastic behaviour
 Post-yield hardening – 1%
 Pre-capping plastic rotation capacity – a=18 mrad
 Strength and stiffness values of the rotational spring – Krawinkler
(Charney and Downs, 2004)
Fig: Generalized force-deformation relation for steel
elements or components (ASCE 41-13, 2004)
Fig: Backbone curve

Scissor model
 Connection behaviour
 Bilin material - modified Ibarra-Medina-Krawinkler model (Ibarra et al. 2005)
 Column web panel zone spring
 Tri-linear model by Krawinkler
(Gupta and Krawinkler, 1999)
 Strength value – second yield point
(plastic hinges at column flange or
continuity plates)
 Post-yield hardening of 1.5%
Fig: Backbone curve for hysteretic models
(Ibarra et al., 2005)

2.2 Modal analysis
 Modal analysis
 Seismic masses were assigned to the joint nodes at each floor
 Calculation of modes and eigen periods
 1st, 2nd and 3rd period of the frames
 In general, frames with shorter span are stiffer comparing to ones with
larger span, while 5-bay frame shows the smallest stiffness
Table: Periods of frames (Gentili et al., 2016)
storey-bay-span

 Pushover analysis
 Pushover analysis performed according to EN 1998-1
 Uniform and modal distribution of lateral forces along the height of the
building
 Target top displacement – horizontal displacement of the last floor
 Pushover curves
 V/Vd ratio vs Top-Displacement
 After the first plastic event, a sudden reduction in the lateral resistance of
the frames occurs – buckling phenomena on the brace in compression
 Following this decrease, an increase of lateral stiffness is experienced

 Conclusions
 Sudden decrease in lateral stiffness influences more frames with larger span
(8m), while 5-bay frames are not significantly affected by this behaviour
storey-bay-span
Fig: Normalized pushover curve – Modal distribution (Gentili et al., 2016)

 Conclusions
 In general, 6-storey frames have larger V/Vd ratio than the 12-storey frame
story-bay-span
6-storey frames 12-storey frames

 Conclusions
 In general, V/Vd ratio is slightly higher for frames with shorter span (6m)
storey-bay-span
8m span 6m span

 Incremental dynamic analysis (IDA)
 Two sets of seven acceleration records (PEER NGA database)
 Type of spectra – Type 1
 Ground type – Type C
Medium seismic hazard
(MH)
High seismic hazard
(HH)
Magnitude M 5.0 to 6.5 higher than 6.5
Distance from fault 10km to 100km 20km to 100km
Shear wave velocity Vs 180 m/s to 800 m/s 180 m/s to 800 m/s
Target spectrum PGA0=0.25g PGA0=0.35g

 Verifications
 Damage Limitation (DL) – intensity 50%
 Significant Limitation (SL) – intensity 100%
 Near Collapse (NC) – intensity 175%
 Local behaviour of D-CBF
Limit values (rad) DL SL NC
Maximum connection rotation 0.01 0.009 0.010-0.018
Maximum panel rotation 0.02 0.018 0.087-0.159
Maximum beam rotation 0.035 0.023 0.106-0.194

 IDA curves
 Each record was applied in increments of 0.25 PGA (0.25 PGA to 4.0 PGA)
 Ground Motion Intensity vs Interstorey Drift Ratio
 Conclusions
 Beam rotations satisfies always the criteria
 Seismic demand for the connections is too high for many of the frames with
12 storeys
Fig: IDA curves in terms of max interstorey drift ratio for D-CBF (Gentili et al., 2016)
storey-bay-span

3. Conclusions
 Ten D-CBF with different values of selected parameters were studied
 Level of seismic hazard
 Number of storeys
 Number of bays
 Span length
 Pushover analysis
 Sudden reduction in the lateral resistance when brace in compression buckles
 This decrease is immediately followed by an increase of lateral stiffness
 Incremental dynamic analysis
 12-storey frames have higher seismic demand comparing to 6-storey frames
 Frames designed for HH show higher seismic demand comparing to those
designed for MH

Thank you for your kind attention!

Assessment of the seismic performance of steel frames using OpenSees

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