StateoftheArtTechnologies Wray

8/11/2019 StateoftheArtTechnologies Wray

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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


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



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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








Design Structure


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








Design Structure


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








Design Structure


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








Design Structure


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



0

10

20

30

40

50

60

70

0 2 4 6 8 10


Hei

ght(ft)

Vy = 0.03 Dynamic

Vy = 0.03 Code



0

10

20

30

40

50

60

70

0 2 4 6 8 10


Height(ft)

Vy = 0.08 Dynamic

Vy = 0.03 Dynamic



0

10

20

30

40

50

60

70

0 2 4 6 8 10


Height(ft)

Vy = 0.08 Dynamic

Vy = 0.08 Code


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Determine SM1








Design Structure


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








Design Structure


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








Design Structure


Design Process


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> 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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> 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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-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

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