9TH EUROPEAN WORKSHOP ON THE SEISMIC ......9TH EUROPEAN WORKSHOP ON THE SEISMIC BEHAVIOUR OF...

9TH EUROPEAN WORKSHOP ON THE SEISMIC BEHAVIOUR OF IRREGULAR AND COMPLEX STRUCTURES

15-16 Dec 2020

Seismic behaviour of torsionally-weak buildings with and without base isolators

Juan C. Reyes, Ph.D.Associate Professor and Laboratory Director

Universidad de los Andes, ColombiaColombia

9TH EUROPEAN WORKSHOP ON THE SEISMIC BEHAVIOUR OF IRREGULAR AND COMPLEX STRUCTURES 15-16 Dec 2020, LISBON

Structural systems (UBS‐85)

A03 B03 C03

T1=1.34 sec T2=1.29 sec T3=0.80 sec

A03 UBC85

Mode 1 Mode 2 Mode 3

C.M.

T1=1.10 sec T2=0.96 sec T3=0.89 sec

B03 UBC85

T1=1.32 sec T2=0.93 sec T3=0.85 sec

C03 UBC85

Beams: trilinear model with strength loss. Columns: PMM interaction using plasticity

theory. Panel zone: trilinear model with strength loss. Isolators: trilinear model with high‐strain

stiffening.


Ground motions

ID Earthquake Name Station Name 30m/s

Site km

1 1994 Northridge 6.7 Beverly Hills ‐ 14145 Mulhol 356 D 9.40

2 1999 Duzce, Turkey 7.1 Bolu 326 D 12.02

3 1979 Imperial Valley 6.5 Delta 275 D 22.03

4 1995 Kobe, Japan 6.9 Shin‐Osaka 256 D 19.14

5 1999 Kocaeli, Turkey 7.5 Duzce 276 D 13.60

6 1989 Loma Prieta 6.9 Oakland ‐ Outer Harbor Wharf 249 D 74.16

7 1999 Chi‐Chi, Taiwan 7.6 TCU095 447 C 45.20


Design of the isolation system

Criteria: The bearings are Bridgestone high‐damping rubber bearings. The average pressure on the bearings is 6.89 MPa (1.00 ksi). The maximum seismic shear strain is not to exceed 150%. The design response spectrum is the specified for the site. This spectrum has constant velocity

of 0.95 m/s (37.4 in/sec). The maximum axial load is 1944 kN (437 kips), the minimum is 645 kN (145 kips), and the

average is 1272 kN (286 kips). These axial loads are computed on one isolator, and correspond to dead load plus 0.50 times live load.

Final devices: Diameter = 0.51 m (20 in). Thickness of rubber (12 layers of 5/8”) = 0.19 m (7.50 in). Period of the isolated building = 2.6 sec. Bearing displacement = 0.29 m (11.25 in). Base shear (fixed‐base ζ=2%) / Based shear (Isolated ζ=14%)

between 3 and 4.


Parametric linear analysis

0 00 00 0

0

00

Equations of motion for undamped base isolation system considering rigid super‐structure:

If the structure is regular and all isolators are equal, and are equal to zero and the system would be torsional stiff with negligible rotational mass participation in the fundamental mode.

However, for highly irregular structures, the isolation system may have some eccentricity to compensate the super‐structure eccentricity.


Optimum eccentricity

ex1

s1,1

CMm1

ex2

s1,2

CMm2

exb

s1,b

CMmb

ex3

s1,3

CMm3

ex1

s1,1

CMm1

ex1

s1,1

CMm1

ex2

s1,2

CMm2

ex2

s1,2

CMm2

exb

s1,b

CMmb

exb

s1,b

CMmb

ex3

s1,3

CMm3ex3

s1,3

CMm3

#

0

0.0%

0.6%

1.2%

1.8%

2.4%

-3.5% -2.5% -1.5% -0.5%

Base‐isolation system eccentricity

Rotatio

nal m

ass p

artic

ipation mod

e 1

B03 building


Effects of structural nonlinearities


Conclusions

Using similar isolators for all columns of a unsymmetric‐plan building reduces, but not eliminates torsional behaviour of the structure leaving a residual torsion that gets amplify by higher mode effects and nonlinearities of the super‐structure.

The flexural stiffness of the first‐story beams has an important effect on the magnitude of story drifts of isolated buildings.

The first‐story beams should be as rigid as possible to neglect rotation of the columns at the base.

Colombia


Than you for your attention

Colombia

9TH EUROPEAN WORKSHOP ON THE SEISMIC ......9TH EUROPEAN WORKSHOP ON THE SEISMIC BEHAVIOUR OF...

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