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Simplified BaseIsolation DesignProcedureGordon Wray, P.E.
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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 needsto be done concurrently with the design.
> Why Simplify the Process?
Base isolated structure is a structural systemthat is closest to SDOF
Only structure in US codes that often requiresnon-linear time history analysis
> SEAONC PSSC believes that the design andanalysis process must be simplified for morewidespread use of the technology
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Breaking Down the Process
> Input Parameters Vy: Nominal (Target) system yield
strength as a fraction of total buildingweight
T2: Nominal (Target) second slopesystem period
Displacement
Force
Vy
Displacement
Force
gK
WT
2
2 2=K2
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Vy= 0.9*Dp2
K2= GrAr/h
Vy= Friction
Coefficient
R
K2
= W/R
Lead Rubber Bearing
Friction Pendulum Bearing
High Damping Rubber Vy based on rubber properties
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> Consider Property Variation Upper Bound Properties Increase Base Shear
Accounts for aging, contamination, firstcycle 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 DM
Breaking Down the Process
DMDTM
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Sample ResultT2 = 3 seconds
0
0.05
0.1
0.15
0.2
0.25
0.3
0 5 10 15 20 25
Displacement (in)
BaseShear(g)
Uppe
rBou
nd
(x1.33)
Lowe
rBou
nd
(x0
.85)
Nomina
l
Vmax
DM
DTM
Breaking Down the Process
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Determine SM1
Choose T2 & DTM Choose T2 & Vy
orUse Table to determineminimum Vy
Use Table todetermine DTM
Use Chart to Determine Vmax
Determine Design Shear, Vs
Distribute Forces Vertically
Check Isolator Tension/Uplift
Design Structure
Isolator System Layout
Design Process
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Design Response Spectra
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
0.0 1.0 2.0 3.0 4.0
Period (sec)
SpectralAcceleration(g
)
SM1= 0.80g
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Determine SM1
Choose T2 & DTM Choose T2 & Vy
orUse Table to determineminimum Vy
Use Table todetermine DTM
Use Chart to Determine Vmax
Determine Design Shear, Vs
Distribute Forces Vertically
Check Isolator Tension/Uplift
Design Structure
Isolator System Layout
Design Process
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Determine VyT2 SM1 Max Vy
12 18 24 30 36 42
0.4 0.027 0.065
0.5 0.048 0.023 0.08
3.0 0.6 0.078 0.037 0.021 0.08
0.7 0.117 0.056 0.031 0.080.8 0.166 0.079 0.045 0.028 0.08
0.9 0.108 0.061 0.038 0.08
DTM
T2 SM1 Max Vy12 18 24 30 36 42
0.4 0.030 0.035
0.5 0.054 0.029 0.06
4.0 0.6 0.087 0.046 0.028 0.075
0.7 0.130 0.069 0.041 0.027 0.08
0.8 0.098 0.059 0.038 0.08
0.9 0.134 0.080 0.052 0.036 0.08
DTM
T2 SM1 Max Vy12 18 24 30 36 42
0.4 -
0.5 0.062 0.034 0.021 0.035
5.0 0.6 0.098 0.054 0.033 0.022 0.06
0.7 0.079 0.049 0.032 0.07
0.8 0.110 0.068 0.045 0.032 0.080.9 0.091 0.061 0.043 0.031 0.08
DTM
If Sm1>= 0.7 then minimum
Vy>= 0.04, otherwise
minimum Vy>= 0.03
Gray area not permitted, valuesincluded for interpolation only
Notes applicable on all tables
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Determine SM1
Choose T2 & DTM Choose T2 & Vy
orUse Table to determineminimum Vy
Use Table todetermine DTM
Use Chart to Determine Vmax
Determine Design Shear, Vs
Distribute Forces Vertically
Check Isolator Tension/Uplift
Design Structure
Isolator System Layout
Design Process
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Determine Vmax
Unreduced Isolation System Base Shear, Vm versus S1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1
S1 (g)
Vm(V
/Wt)
T2 = 2 sec
T2 = 2.5 sec
T2 = 3 sec
T2 = 4 sec
T2 = 5 sec
T2 = 6 sec
Vm = 0.6 x Sm1 - 0.035
Vm = 0.45 x Sm1 - 0.020
Vm = Sm1 / 3 + 0.002
Vm = Sm1 / T2
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Determine SM1
Choose T2 & DTM Choose T2 & Vy
orUse Table to determineminimum Vy
Use Table todetermine DTM
Use Chart to Determine Vmax
Determine Design Shear, Vs
Distribute Forces Vertically
Check Isolator Tension/Uplift
Design Structure
Isolator System Layout
Design Process
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> Select Structural System Determine Ri (typically 2)
Concrete Shear Wall, Ri = 2.0
Ordinary Braced Frame, Ri = 1.6
> Calculate Design Base Shear Vs = Vmax/Ri
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 TD
Design Base Shear
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Simplified Modeling
Procedure
Horizontally Rigid Isolator Elements (pins)
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Determine SM1
Choose T2 & DTM Choose T2 & Vy
orUse Table to determineminimum Vy
Use Table todetermine DTM
Use Chart to Determine Vmax
Determine Design Shear, Vs
Distribute Forces Vertically
Check Isolator Tension/Uplift
Design Structure
Isolator System Layout
Design Process
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Vertical Distribution
Low Strength, Vy < 0.04W
High Displacement
High Strength, Vy > 0.06W
Low Displacement
>Current code distribution approximates
dynamic response of high strength system
>Low Strength, High Displacement results inbetter performance
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Overturning Moments Comparison
Moment Frame, T2= 3 seconds
0
10
20
30
40
50
60
70
0 2 4 6 8 10
Overturning Moment (k-ft/W)
Height(ft)
Vy = 0.03 Dynamic
Overturning Moments Comparison
Moment Frame, T2= 3 seconds
0
10
20
30
40
50
60
70
0 2 4 6 8 10
Overturning Moment (k-ft/W)
Hei
ght(ft)
Vy = 0.03 Dynamic
Vy = 0.03 Code
Overturning Moments Comparison
Moment Frame, T2= 3 seconds
0
10
20
30
40
50
60
70
0 2 4 6 8 10
Overturning Moment (k-ft/W)
Height(ft)
Vy = 0.08 Dynamic
Vy = 0.03 Dynamic
Overturning Moments Comparison
Moment Frame, T2= 3 seconds
0
10
20
30
40
50
60
70
0 2 4 6 8 10
Overturning Moment (k-ft/W)
Height(ft)
Vy = 0.08 Dynamic
Vy = 0.08 Code
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Determine SM1
Choose T2 & DTM Choose T2 & Vy
orUse Table to determineminimum Vy
Use Table todetermine DTM
Use Chart to Determine Vmax
Determine Design Shear, Vs
Distribute Forces Vertically
Check Isolator Tension/Uplift
Design Structure
Isolator System Layout
Design Process
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Check Isolator Tension/Uplift0.8D 0.8D 0.8D 0.8D
Fm3
Fm2
Fm1
Tension
>Check with Manufacturer for Isolator Tension
Capacity Sliding isolators cannot resist uplift
100 psi
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Check Isolator Tension/Uplift0.8D 0.8D 0.8D
Fm3
Fm2
Fm1
>Check strength/stability after progressively
removing isolator elements.
0.8D
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Determine SM1
Choose T2 & DTM Choose T2 & Vy
orUse Table to determineminimum Vy
Use Table todetermine DTM
Use Chart to Determine Vmax
Determine Design Shear, Vs
Distribute Forces Vertically
Check Isolator Tension/Uplift
Design Structure
Isolator System Layout
Design Process
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Design Structure1.2D+L 1.2D+L 1.2D+L
Fm3
Fm2
Fm1
1.2D+L
P
P
= DTM
P/2
P/2
>Design framing above isolators for Vs
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Determine SM1
Choose T2 & DTM Choose T2 & Vy
orUse Table to determineminimum Vy
Use Table todetermine DTM
Use Chart to Determine Vmax
Determine Design Shear, Vs
Distribute Forces Vertically
Check Isolator Tension/Uplift
Design Structure
Isolator System Layout
Design Process
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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 forvarying axial loads
Sum properties and confirm system meets Vyand T2 requirements
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Isolator System Layout
> Locate Center of Stiffness/Center of Mass Design for Least Amount of Accidental Torsion
DTM/DM assumption uses 1.2 factor as limitfor torsionally regular buildings
Arrange isolators to align center of stiffness(K2) and center of strength (Vy) to center ofmass.
Committee working on recommendations formaximum allowed eccentricity from center ofmass.
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Isolator System Layout
-20
0
20
40
60
80
100
120
140
160
180
-20 0 20 40 60 80 100 120 140
X Location (ft)
YLocation(ft)
Isolator Type A Isolator Type B Center of Mass
Center of K2 Center of Vy
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Summary
> Isolator System Properties Site dictates SM1 or Cv
Engineer chooses T2, DTM Easily determine Vmax, Vy Useful preliminary design tool
> Alternative Modeling Choose structural system
Build static model with horizontally rigidisolators
Apply static loads, including P load case
Layout isolators and confirm propertieswith manufacturer
> Work in Progress Modification to vertical distribution
Confirmation of allowed center of K2,center of Vy eccentricity
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Questions
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K2 Qd K2 Qd K2 Qd
Contamination - - 1.0 1.1 - -
Aging 1.1 1 1 1.1 1.2 1.2
Scragging 1 1.2 1 1.1 1.2 1.2
Upper Bound Factor from
Nominal Properties1.10 1.20 1.00 1.33 1.44 1.44
System Property Modification
Factor
Maximum Upper Bound
Factor from NominalProperties
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
1.10 1.10
0.85 0.85 0.85
0.850.850.85
1.25 1.34 1.42
1.10
1.44
0.66 0.66 0.66
Property Modification Factor Table
1.13 1.22 1.29
LRB FPS HDR
1.20 1.33