Simplified Base Isolation Design Procedure. Gordon Wray, P.E.

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1 Simplified Base Isolation Design Procedure Gordon Wray, P.E.

2 SEAONC Protective Systems Subcommittee Objectives > Current Unique Code Requirements More sophisticated engineering analysis Geotechnical need site specific study Peer review is required by the code and needs to be done concurrently with the design. > Why Simplify the Process? Base isolated structure is a structural system that is closest to SDOF Only structure in US codes that often requires non-linear time history analysis > SEAONC PSSC believes that the design and analysis process must be simplified for more widespread use of the technology

3 Breaking Down the Process > Input Parameters V y : Nominal (Target) system yield strength as a fraction of total building weight Force V y Displacement T 2 : Nominal (Target) second slope system period Force K 2 Displacement T2 = 2π W K g 2

4 Lead Rubber Bearing V y = 0.9*Dp 2 K 2 = G r A r /h K 2 = W/R Friction Pendulum Bearing R V y = Friction Coefficient High Damping Rubber V y based on rubber properties

5 Breaking Down the Process > Consider Property Variation Upper Bound Properties Increase Base Shear Accounts for aging, contamination, first cycle effects, specification tolerance 1.33 for LRB, FPS 1.50 for HDR Lower Bound Properties Increase Maximum Displacement Accounts for specification tolerance 0.85 for all systems > Displacement due to Accidental Torsion D TM includes 1.2 amplification factor on D M D D TM M

6 Breaking Down the Process Sample Result T2 = 3 seconds 0.3 V max Nominal Upper Bound (x1.33) Lower Bound (x0.85) Base Shear (g) Displacement (in) D M D TM

7 Design Process Determine S M1 & D TM Use Table to determine minimum V y or & V y Use Table to determine D TM Use Chart to Determine V max Determine Design Shear, V s Distribute Forces Vertically Check Isolator Tension/Uplift Design Structure Isolator System Layout

8 Design Response Spectra Spectral Acceleration (g) S M1 = 0.80g Period (sec)

9 Design Process Determine S M1 & D TM Use Table to determine minimum V y or & V y Use Table to determine D TM Use Chart to Determine V max Determine Design Shear, V s Distribute Forces Vertically Check Isolator Tension/Uplift Design Structure Isolator System Layout

10 Determine V y T2 SM1 DTM Max Vy If S m1 >= 0.7 then minimum V 0.08 y >= 0.04, otherwise minimum V y >= T2 SM1 DTM Max Vy Gray area not permitted, values included for interpolation only T2 SM1 DTM Max Vy Notes applicable on all tables

11 Design Process Determine S M1 & D TM Use Table to determine minimum V y or & V y Use Table to determine D TM Use Chart to Determine V max Determine Design Shear, V s Distribute Forces Vertically Check Isolator Tension/Uplift Design Structure Isolator System Layout

12 Determine V max Unreduced Isolation System Base Shear, Vm versus S Vm = 0.6 x Sm Vm (V/Wt) Vm = 0.45 x Sm Vm = Sm1 / T2 = 2 sec T2 = 2.5 sec T2 = 3 sec T2 = 4 sec T2 = 5 sec T2 = 6 sec Vm = Sm1 / T S1 (g)

13 Design Process Determine S M1 & D TM Use Table to determine minimum V y or & V y Use Table to determine D TM Use Chart to Determine V max Determine Design Shear, V s Distribute Forces Vertically Check Isolator Tension/Uplift Design Structure Isolator System Layout

14 Design Base Shear > Select Structural System Determine R i (typically 2) Concrete Shear Wall, R i = 2.0 Ordinary Braced Frame, R i = 1.6 > Calculate Design Base Shear V s = V max /R i Vs = 0.17g in example (0.27/1.6) Vs = 0.21g for fixed base OCBF, type B soil > Check Minimum Base Shear Requirements Vs > 1.5* Vy Vs > Wind Load Vs > Base Shear for a fixed base structure with Period T D

15 Simplified Modeling Procedure Horizontally Rigid Isolator Elements (pins)

16 Design Process Determine S M1 & D TM Use Table to determine minimum V y or & V y Use Table to determine D TM Use Chart to Determine V max Determine Design Shear, V s Distribute Forces Vertically Check Isolator Tension/Uplift Design Structure Isolator System Layout

17 Vertical Distribution Low Strength, V y < 0.04W High Displacement High Strength, V y > 0.06W Low Displacement >Current code distribution approximates dynamic response of high strength system >Low Strength, High Displacement results in better performance

18 70 Overturning Moments Comparison Moment Frame, T 2 = 3 seconds Height (ft) V y 0.03 Dynamic y = V y 0.08 y = Dynamic Dynamic V y = V y 0.08 V= y 0.03 = Code 0.03 Dynamic Code Overturning Moment (k-ft/w)

19 Design Process Determine S M1 & D TM Use Table to determine minimum V y or & V y Use Table to determine D TM Use Chart to Determine V max Determine Design Shear, V s Distribute Forces Vertically Check Isolator Tension/Uplift Design Structure Isolator System Layout

20 Check Isolator Tension/Uplift Fm3 0.8D 0.8D 0.8D 0.8D 100 psi Fm2 Fm1 Tension >Check with Manufacturer for Isolator Tension Capacity Sliding isolators cannot resist uplift

21 Check Isolator Tension/Uplift Fm3 0.8D 0.8D 0.8D 0.8D Fm2 Fm1 >Check strength/stability after progressively removing isolator elements.

22 Design Process Determine S M1 & D TM Use Table to determine minimum V y or & V y Use Table to determine D TM Use Chart to Determine V max Determine Design Shear, V s Distribute Forces Vertically Check Isolator Tension/Uplift Design Structure Isolator System Layout

23 Design Structure Fm3 1.2D+L 1.2D+L 1.2D+L 1.2D+L Fm2 Fm1 P P = D TM P /2 P /2 >Design framing above isolators for Vs

24 Design Process Determine S M1 & D TM Use Table to determine minimum V y or & V y Use Table to determine D TM Use Chart to Determine V max Determine Design Shear, V s Distribute Forces Vertically Check Isolator Tension/Uplift Design Structure Isolator System Layout

25 Isolator System Layout > Use Spreadsheet to Layout Isolators Arrange Sizes, Types, Lead Core Locations Friction Isolator > Vy: Friction Coefficient a function of bearing stress > T2: Function of Weight/Radius Lead Rubber Isolator > Vy: Function of Lead Core > T2: Function of Rubber Area and Height Confirm Properties with Manufacturer for varying axial loads Sum properties and confirm system meets V y and T 2 requirements

26 Isolator System Layout > Locate Center of Stiffness/Center of Mass Design for Least Amount of Accidental Torsion D TM /D M assumption uses 1.2 factor as limit for torsionally regular buildings Arrange isolators to align center of stiffness (K 2 ) and center of strength (V y ) to center of mass. Committee working on recommendations for maximum allowed eccentricity from center of mass.

27 Isolator System Layout Y Location (ft) X Location (ft) Isolator Type A Isolator Type B Center of Mass Center of K2 Center of Vy

28 Summary > Isolator System Properties Site dictates S M1 or C v Engineer chooses T 2, D TM Easily determine V max, V y Useful preliminary design tool > Alternative Modeling Choose structural system Build static model with horizontally rigid isolators Apply static loads, including P load case Layout isolators and confirm properties with manufacturer > Work in Progress Modification to vertical distribution Confirmation of allowed center of K 2, center of V y eccentricity

29 Questions

30 Property Modification Factor Table LRB FPS HDR K2 Qd K2 Qd K2 Qd Contamination Aging Scragging Upper Bound Factor from Nominal Properties System Property Modification Factor Maximum Upper Bound Factor from Nominal Properties Adjusted Upper Bound Factor System Upper Bound Specification Tolerance System Lower Bound Specification Tolerance Final Upper Bound Factor From Nominal Properties Final Lower Bound Factor From Nominal Properties

CHAPTER 5. T a = 0.03 (180) 0.75 = 1.47 sec 5.12 Steel moment frame. h n = = 260 ft. T a = (260) 0.80 = 2.39 sec. Question No.

CHAPTER 5. T a = 0.03 (180) 0.75 = 1.47 sec 5.12 Steel moment frame. h n = = 260 ft. T a = (260) 0.80 = 2.39 sec. Question No. CHAPTER 5 Question Brief Explanation No. 5.1 From Fig. IBC 1613.5(3) and (4) enlarged region 1 (ASCE 7 Fig. -3 and -4) S S = 1.5g, and S 1 = 0.6g. The g term is already factored in the equations, thus