case study of a 40 story brbf building located in los angeles
TRANSCRIPT
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CASE STUDY OF A 40 STORY BRBF BUILDING LOCATED IN LOS ANEGELES
Anindya Dutta, Ph.D., S.E. Ronald O. Hamburger, S.E., SECB
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Background
• Study performed on behalf of PEER under its Tall Building Initiative
• SGH designed two prototypes of this tall BRB for a generic site in downtown Los Angeles
• Two other firms performed similar designs of reinforced concrete buildings located at the same site
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Background – Contd.
• Two design prototypes • Standard code based design • Performance based alternative
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• Design parameters (SDs, SD1 etc.) and approximate floor plans were provided by PEER
• Designers were instructed to create a system for the code-based design that would have a fundamental period of about 5 seconds
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Code Based Design
• Building located in downtown Los Angeles with SDS = 1.145 and SD1 = 0.52
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• Building design to follow the provisions of all applicable building codes and standards
except
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Code Based Design – Contd.
• Ignore the height limitation
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Information from PEER
• Approximate building floor plan – Tower to have plan dimensions of 170 ft X 107 ft – Podium with four levels of subterranean basement to have plan
dimension of 227 ft X 220 ft
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Information from PEER - Contd.
• Gravity loading data
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Description/Location Superimposed Dead
Live Load Reducability
Roof 28 psf 25 psf Yes
Mechanical, Electrical at Roof Total of 100 kips - -
Residential including Balconies
28 psf 40 psf Yes
Corridors, Lobbies and Stairs 28 psf 100 psf No
Retail 110 psf 100 psf No
Parking Garage, Ramp 3 psf 40 psf1 Yes
Construction Loading 3 psf 30 psf No
Cladding 15 psf - -
PEER document showed 50 psf. SGH considered 40 psf in keeping with ASCE 7-05
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Information from PEER - Contd.
• Response spectrum for code design
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• Information for wind design (Basic Wind Speed, Occupancy etc.)
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Detailed Code Design
• Preliminary sizing of the gravity framing performed in RAM Structural System
• Lateral Analysis and Design performed in ETABS using 3D response spectrum analysis
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Detailed Code Design – Wind Design
• Wind forces computed using Method 2 of ASCE 7-05
• Requires application of horizontal X and Y direction pressures in combination with torsion
• Gust factor (Gf) computed using 6.5.8.2 for dynamically sensitive structures with 1% damping
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Detailed Code Design – Wind Design (Contd.)
• Wind loads were statically applied in ETABS and brace forces computed
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Parameter Value Basic Wind Speed, 3 sec. gust (V) 85 mph
Basic Wind Speed, 3 sec gust (V), for serviceability wind demands based on a 10 year mean recurrence interval 67 mph
Exposure B
Occupancy Category II
Importance Factor (Iw) 1.0
Topographic Factor (Kzt) 1.0
Exposure Classification Enclosed
Internal Pressure Coefficient (GCpi) ± 0.18
Mean Roof Height (h) 544’-6”
Wind Base Shear along Two Orthogonal Directions 1436 kips and 2629 kips
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Detailed Code Design – Seismic Design
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• Seismic analysis performed using the response spectrum provided by PEER
• Base Shear scaled to 85% of the static lateral base shear obtained from equivalent static lateral force analysis
• Base Shear is the story shear immediately above podium
Basement walls and floor masses modeled
Restraint provided only at wall base
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Detailed Code Design – Seismic Design
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Parameter Value Building Latitude/Longitude Undefined Occupancy Category II Importance Factor (Ie) 1.0 Spectral Response Coefficients SDS = 1.145; SD1 = 0.52 Seismic Design Category D
Lateral System Buckling restrained braced frames, non moment resisting beam column connections
Response Modification Factor (R) 7 Deflection Amplification Factor (Cd) 5.5 System Overstrength Factor (Ω0) 2.0 Building Period (T) using Cl. 12.8.2 3.16 sec1
Seismic Response Coefficient Cs (Eq. 12.8-1) 0.051 W (Governed by Cs-min from Eq. 12.8-5)
Scaled Spectral Base Shear 3504 kips (85% of Static Base Shear)
Analysis Procedure Modal Response Spectral Analysis 1. Actual period from dynamic model: TY = 5.05 sec; TX = 3.62 sec
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Detailed Code Design – Member Design • Member design performed using ANSI/AISC 341-05
• Beams designed for unbalanced force corresponding to adjusted brace strength
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βωRyPysc ωRyPysc
Assumed ω = 1.25, β = 1.1 Ry =1.1 and Fy = 38 ksi
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Detailed Code Design – Member Design
• Columns designed for accumulated force (sum of vertical components)corresponding to adjusted brace strengths
• Led to large compression and tension design forces for columns and foundations (Note: Attachment of columns to foundations needs to be designed for same forces used for column design)
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Detailed Code Design – Member Design
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• Accommodation of the large forces required use of steel box sections filled with concrete
• Upside: Using Chapter I of AISC 13th Ed. a composite EIeff can be used. This contributed significantly to the
lateral stiffness.
• Braced frame beams were sized for horizontal adjusted brace forces and unbalanced loading.
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Detailed Code Design – Typical Member Sizes
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Transverse Frame (Below 10th Floor) Longitudinal Frame (Below 10th Floor)
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Performance Based Design
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• PBD based on LA Tall Building Guidelines
• Seismic analysis to disregard all code requirements. Design to be verified using non linear analysis.
• Wind and Gravity Design to follow code.
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Performance Based Design – Contd.
• Generally two levels of hazard considered in the design
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Level of Earthquake Earthquake Performance Objectives
Frequent/Service : 25 year return period, 2.5% damping
Serviceability: Essentially elastic performance with minor yielding of brbf-s. Drift limited to 0.5%
Maximum Considered Earthquake (MCE): As defined by ASCE 7-05, Section 21.2, 2.5% damping.
Collapse Prevention: Extensive structural damage, repairs are required and may not be economically feasible. Drift limited to 3%
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Performance Based Design – Service Level
• Either linear response spectrum or non linear time history is allowed per LATBSDC document
• Structure to remain essentially elastic • Used linear response spectrum analysis in ETABS.
Max drift was 0.34%(<0.5%)
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• Minimum brace sizes governed by wind design
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Performance Based Design – General
• Member sizes were more economical. Additional bays required in the transverse direction below 10th
floor were eliminated.
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Performance Based Design – MCE Analysis
• A non linear response history analysis performed using CSI Perform (Tx = 6.5s, Ty = 4.5s)
• Used 7 ground motion pairs provided by PEER
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Performance Based Design – General Modeling
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• Non linear model created in CSI Perform
• Modeled the non linear backbone curve of the BRBs
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Performance Based Design – General Modeling
• Columns modeled as linear elastic elements. PMM interaction was monitored for elastic performance
• Diaphragms modeled as rigid in upper floors. Ground floor and below modeled with shell elements with 30% EIeff
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Performance Based Design – Acceptability Criterion
• Drift criterion based on LATBSDC document
3% for Maximum Considered Earthquake
• Acceptability criterion for BRB based on limiting average strain to 10 times yield (~0.013). This is based on observance of data from a large number of tests.
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Performance Based Design – Selected Results
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Performance Based Design – Selected Results
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BRB Strain Demand Capacity Ratio
Maximum DCR ~ 0.85
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Performance Based Design – Selected Results
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Performance Based Design – Typical Member Sizes
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Transverse Frame (Below 10th Floor) Longitudinal Frame (Below 10th Floor)
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Summary & Conclusions
• Two prototype designs developed – 2007 CBC design (without height limit) – Performance-based per LATBSDC document
• Performance-based Design resulted in more economical member sizes and more practical column base connection
• Building code for BRBs seems to be overly conservative for high rise structures – Assumption that all braces yield simultaneously incorrect
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