CASE STUDY OF A 40 STORY BRBF BUILDING LOCATED IN LOS ANEGELES

Transcription

1 DESIGN INVESTIGATE REHABILITATE CASE STUDY OF A 40 STORY BRBF BUILDING LOCATED IN LOS ANEGELES Anindya Dutta, Ph.D., S.E. Ronald O. Hamburger, S.E., SECB

2 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 2

3 Background Contd. Two design prototypes Standard code based design Performance based alternative 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 3

4 Code Based Design Building located in downtown Los Angeles with SDS = and SD1 = 0.52 Building design to follow the provisions of all applicable building codes and standards except 4

5 Code Based Design Contd. Ignore the height limitation 5

6 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 6

7 Information from PEER - Contd. Gravity loading data Description/Location Superimposed Dead Live Load 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 psf 1 Yes Construction Loading 3 psf 30 psf No Cladding 15 psf - - Reducability PEER document showed 50 psf. SGH considered 40 psf in keeping with ASCE

8 Information from PEER - Contd. Response spectrum for code design Information for wind design (Basic Wind Speed, Occupancy etc.) 8

9 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 9

10 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 for dynamically sensitive structures with 1% damping 10

11 Detailed Code Design Wind Design (Contd.) Parameter Basic Wind Speed, 3 sec. gust (V) Basic Wind Speed, 3 sec gust (V), for serviceability wind demands based on a 10 year mean recurrence interval Value 85 mph 67 mph Exposure Occupancy Category Importance Factor (I w ) 1.0 Topographic Factor (K zt ) 1.0 Exposure Classification Wind loads were statically applied in ETABS and brace forces computed B II Enclosed Internal Pressure Coefficient (GC pi ) ± 0.18 Mean Roof Height (h) Wind Base Shear along Two Orthogonal Directions 1436 kips and 2629 kips 11

12 Detailed Code Design Seismic Design 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 12

13 Detailed Code Design Seismic Design Parameter Value Building Latitude/Longitude Undefined Occupancy Category II Importance Factor (I e ) 1.0 Spectral Response Coefficients S DS = 1.145; S D1 = 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 (C d ) 5.5 System Overstrength Factor (Ω 0 ) 2.0 Building Period (T) using Cl sec 1 Seismic Response Coefficient C s (Eq ) W (Governed by C s-min from Eq ) Scaled Spectral Base Shear 3504 kips (85% of Static Base Shear) Analysis Procedure 1. Actual period from dynamic model: T Y = 5.05 sec; T X = 3.62 sec Modal Response Spectral Analysis 13

14 Detailed Code Design Member Design Member design performed using ANSI/AISC Beams designed for unbalanced force corresponding to adjusted brace strength βωrypysc ωrypysc Assumed ω = 1.25, β = 1.1 Ry =1.1 and Fy = 38 ksi 14

15 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) 15

16 Detailed Code Design Member Design Accommodation of the large forces required use of steel box sections filled with concrete Upside: Using Chapter I of AISC 13 th 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. 16

17 Detailed Code Design Typical Member Sizes Transverse Frame (Below 10 th Floor) Longitudinal Frame (Below 10 th Floor) 17

18 Performance Based Design 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. 18

19 Performance Based Design Contd. Generally two levels of hazard considered in the design Level of Earthquake Frequent/Service : 25 year return period, 2.5% damping Maximum Considered Earthquake (MCE): As defined by ASCE 7-05, Section 21.2, 2.5% damping. Earthquake Performance Objectives Serviceability: Essentially elastic performance with minor yielding of brbf-s. Drift limited to 0.5% Collapse Prevention: Extensive structural damage, repairs are required and may not be economically feasible. Drift limited to 3% 19

20 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%) Minimum brace sizes governed by wind design 20

21 Performance Based Design General Member sizes were more economical. Additional bays required in the transverse direction below 10 th floor were eliminated. 21

22 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 22

23 Performance Based Design General Modeling Non linear model created in CSI Perform Modeled the non linear backbone curve of the BRBs 23

24 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 24

25 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. 25

26 Performance Based Design Selected Results 26

27 Performance Based Design Selected Results BRB Strain Demand Capacity Ratio Maximum DCR ~

28 Performance Based Design Selected Results 28

29 Performance Based Design Typical Member Sizes Transverse Frame (Below 10 th Floor) Longitudinal Frame (Below 10 th Floor) 29

30 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 30