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EXPERIENCES OF SLIDING ISOLATION DISK BEARING IN KOREA

Bang In Seok

Biography: Bang In Seok is a director for the Research and Development Center of

ESCORTS Co., Ltd that is a leading company of structural bearing and expansion joint

business in South Korea. He has 20 year experiences in this field and is working as a R&D

coordinator between R.J. Watson, Inc. and ESCORTS Co., Ltd at present. He received his

master degree of civil engineering from Yonsei University in Korea. His research interests

include seismic isolation, seismic fragility analysis and development of new concept isolation

bearings with special materials.

ABSTRACT

Due to recent evidences of increasing activities in Korea and lessons learned from the several

big earthquakes in the neighboring countries, research and application of seismic protection

systems have become more active in Korea. Among such systems, seismic isolation bearings

and passive energy dissipation devices are generally accepted as most effective systems

considering the low to moderate seismicity of Korea. For the most of long span bridges, the

concepts of base isolation and passive energy dissipation have been strongly introduced to

Korean engineers for seismic protection and economic bridge design. Currently, seismic

isolators such as lead rubber bearings, elasto-plastic steel dampers and sliding isolators have

been or are to be installed many bridges in Korea. Among them, sliding isolation disk bearing

(SIDB) has been emerged and in a good reputation as a seismic protection system to Korean

engineers. This paper will review the experiences of the SIDB that have been evaluated by

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the dynamic test according to AASHTO Guide Specifications for Seismic Isolation Design

and installed in the bridges in Korea.

Key words: sliding, bearing, isolation, friction, damping, polyurethane, seismic, PTFE.

INTRODUCTION

There is a long-standing belief that Korea is free from earthquakes. But in recent times,

powerful earthquakes have been occurred more frequently throughout the world and there are

many earthquake occurrences in neighboring countries like Japan, China and Taiwan.

According to the report from Korea Meteorological Administration, there were 90

earthquakes beyond 3.0 on the Richter scale from 2000~2010 in Korean territory. Among

them, Pyeongchang earthquake in 2007 and Shiheung earthquake in 2010 are prominently

registered 4.8 and 3.0 on the Richter scale. Considering these situations Korea is not free

from the risks of high earthquakes but there is very poor preparation to protect from

earthquakes and one research says only 10% of whole structure in Seoul is built with

earthquake resistant design.

In Korea spherical bearing, POT bearing and Disk bearing are normally used as the

earthquake-resistant bridge bearings for anti-earthquake reinforcing structures with short span

bridges. And LRB, mild steel damping bearing (MSDB) and sliding isolation disk bearing

(SIDB) are mainly applied as the isolation bearings which have energy dissipation capability

for seismic isolation structures with medium to long span bridges.

Pin and roller supports were usually used when earthquake-resistant and isolation concepts

were not universally applicable in Korea. Since the late 1980s, the bearing has been used in

earnest. The spherical bearing shown in figure 1(a) has hemispheric shape bronze plate,

which accommodates 0.03 Rad. rotational capacities and is dominantly used for short span

railway bridges at present. The application of Pot bearing shown in figure 1(b) has been

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started for normal highway bridges in the early 1990s. A rubber disk confined in the steel pot

case accommodates rotational movement. Afterwards, the disk bearing shown in figure 1(c)

using a polyurethane disk to allow rotational movement has been appeared in the early 2000s.

(a) Spherical bearing (b) POT bearing (c) Disk bearing

Figure 1 – Anti earthquake bearings

Since the late 1990s, Korea has started applying the isolation bearings with an energy

dissipation capacity for medium to long span bridges to protect against big seismic force.

Unlike the prior conservative bearings which utilize coupling between super and sub

structures needed to increase sections of the structure, the isolation bearings which induce to

decouple structures from the earthquake ground motion can make the structure slender and

reduce the cost of construction.

LRB shown in figure 2(a) is the first isolation bearing applied in Korea that has the damping

and restoring properties by combination of lead's plasticity and rubber's elasticity behavior. In

the late 1990s, MSDB shown in figure 2(b) has been used at the long span bridges. This

isolation bearing with elasto-plastic behavior of the mild steel element doesn’t have the

restoring function. Once the steel element deforms up to plastic area by dynamic action it has

to be replaced with new one for next dynamic actions. In the early 2000s, SIDB shown in

figure 2(c) was introduced in Korea. The basic components of SIDB consist of a sliding

multirotational disc bearing with damping and polyurethane springs with recentering

capability. A PTFE (PolyTetraFlouroEthylene) / stainless sliding surface allows for high

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levels of energy dissipation to occur during seismic event, while spring units provide a

restoring force.

(a) LRB (b) MSDB (c) SIDB

Figure 2 – Isolation bearings

Since 2002, when SIDB installed first in Korea, the use of SIDB has been increased

prominently. As shown in figure 3 with the annual records, 2587 SIDBs were installed at 49

projects by the year 2010 and this figure was more than half of isolation bearings installed in

Korea.

(a) Projects with SIDB (b) Quantity of SIDB installation

Figure 3 – Annual record of SIDB in Korea

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SLIDING ISOLATION DISK BEARING

SIDB consists of top plate, bottom plate, polyurethane disk pad, PTFE, and side polyurethane

springs as shown in figure 4. The top and bottom plates are connected to superstructure and

substructure, respectively. The polyurethane disk pad, high strength hard rubber disc (Shore

62D) used to allow for bearing rotation. Average allowable compressive stress is 5.0Ksi. Test

specimens have survived pressures of more than 100 ksi. Each disc is custom detailed for

each sized bearing, including inner and outer grooves and recess to allow for proper

deflection geometries and properties and rotational clearances. Standard discs incorporate a

0.02 radian design rotation, although more can be designed for if necessary. Material

specifications can be found in the AASHTO Standard Specifications for Highway Bridges.

The PTFE performs friction damping function. The side polyurethane spring that supplies a

restoring force and determines a secondary stiffness consists of a shaft, and load plate with

recessed PTFE. In this application, these springs must withstand several hundred thousand

high speed impacts, making them a highly reliable component in the less demanding bridge

bearing environment. Spring material properties are a proprietary polyurethane blend (Shore

95A) similar to those covered in the AASHTO Standard Specifications for Highway Bridges.

Figure 4 – A constitution of SIDB

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The adequacy of each SIDB is evaluated by the dynamic test according to AASHTO Guide

Specifications for Seismic Isolation Design and then they are supplied to the sites. The test

machine shown in figure 5 has vertical capacity 18000kN, horizontal capacity of actuator

1500kN with maximum testing velocity 120mm/sec and displacement +/-200mm.

Figure 5 – Dynamic test machine

Additionally to evaluate the dynamic behavior of two SIDBs to be shipped to Chumdan

Bridge and Gwangro#7 line projects, the dynamic tests shown in figure 6 were performed at

the University of California San Diego Charles Lee Powell Structural Research Laboratories,

Seismic Response Modification Device (SRMD) test facility. SRMD test machine has

vertical capacity 53400kN, horizontal capacity of actuator 8900kN with maximum testing

velocity +/-1778mm/sec and displacement +/-1220mm. SIDB for Chumdan Bridge was the

maximum vertical capacity 45000kN, dead load 28500kN and the maximum seismic

displacement ±40mm. The horizontal testing velocity of actuator was 100mm/sec with the

test frequency 0.4Hz.

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Figure 6 – The Dynamic test view

The tested SIDB met the requirements of the project specification and had sufficient energy

dissipation capacity as shown in figure 7. The EDC (Energy Dissipated per Cycle) and

equivalent viscous damping were 350,000~300,000kNmm and 52% respectively.

Figure 7 – Hysteresis curve of dynamic behavior test

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PROJECT EXPERIENCES OF THE SIDB

Ganeung Overpass Bridge

The first Korean project that the SIDB has been installed in 2002 is Ganeung Overpass

Bridge shown in figure 8. The bridge is steel box girder type and its total length is 578m

(2@34, 2@52+73+2@52, 52+73+2@52). This bridges have 52 SIDBs with the capacities

300~1500ton and several SIDBs equipped with STU(Shock Transmission Unit) to allow big

thermal movements were installed on the expansion joint piers.

(a) EQS bearing (b) EQS+STU bearing

Figure 8 – Ganeung Overpass Bridge

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Seoul-Incheon Ara Canal

The Seoul-Incheon Ara Canal which boats are running through with the length 18km, the

width 80m is a project linking Seoul to Incheon port terminals. There are 3 bridges crossing

over this canal and the SIDBs were installed in the Sicheon Bridge and Hwangyeong Bridge

among them. The dynamic behavior test of the bearing installed in Seoul-Incheon Ara Canal

was conducted for quality assurance as shown in figure 9.

Figure 9 – Dynamic behavior test

The SIDB with the vertical load capacity 8500kN was tested on the displacement control

method with +/- 70mm seismic displacement. The maximum horizontal actuator velocity was

100mm/sec and the test frequency was 0.227Hz. Figure 10 is the hysteresis curve of the

dynamic behavior test. The equivalent viscous damping calculated from the test result was

about 40%.

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Figure 10 – Hysteresis curve

A SIDB used at Hwangyeong Bridge which has 85000kN capacity is shown in figure 11 and

a view of installation is shown in figure 12.

Figure 11 – The SIDB at Hwangyeong Bridge

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Figure 12 – Installation of the SIDB

It is a PC box girder bridge with 535m length. Maximum vertical and horizontal loads of the

bearing installed at the bridge are 85000kN and 8700kN respectively. SIDB's maximum

horizontal allowable displacement is ±100mm in all directions. This is the biggest SIDB with

the dimensions of 3m x 3m in plan view, 1m height among sliding isolation bearings installed

in Korea so far.

Incheon Grand Bridge

The Incheon Grand Bridge is the world fifth longest cable-stayed bridge on the sea that

connects Incheon International Airport to Incheon Songdo city. The total length of this bridge

is 21.38km and the main span length is 800m. The SIDBs were installed at JCT1, JCT2 in the

main line, Haean IC1 and Haean IC2 .

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Figure 13 – JCT bridges in Incheon Grand Bridge

Figure 14 – The SIDBs installed on the column

The bridge with PC box girder in figure 13 is JCT bridge constructed in 2008 and which

length is 9.04km. The SIDBs installed on column are shown in figure 14 and the maximum

vertical load capacity is 20000kN.

Singyeongju station

Since the Korea High-speed rail service started in 2004, routes of the service have been

gradually increased. The second phase construction of Seoul-Busan high-speed rail service

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begun in 2004 and the service opened in November 2010. The Singyeongju station is located

on that way and there was application of the SIDB. The station appearance and the bearings

installed in station are shown in figure 15 and figure 16.

Figure 15 – Singyeongju high-speed railway station

Figure 16 – The SIDB in Singyeongju Station

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The construction period of Singyeongju station with two floors is 4 years. The 300 SIDBs

were installed on the 5.5 km section whose bridge girders are the concrete box and T-beam

type and the maximum vertical load capacity of SIDB is 6000kN. SIDB designed to resist

high horizontal braking force 200kN from the 300km/hr speed rail train within 1mm and to

achieve this strict requirement the polyurethane springs of SIDB were pre-compressed up to

260kN as figure 16.

Figure 16 – The Sliding Isolation

The SIDBs installed in Singyeongju station designed to perform not only isolation against

earthquake action but also noise and vibration reduction function. The SIDBs with 6000kN

and 2800kN were installed under the track bed with the vertical stiffness of the polyurethane

disk (Shore 62D) 1350kN/mm and 895kn/mm respectively. Other SIDBs with 1750kN and

1000kN were located under the slab of the station waiting room with the vertical stiffness of

the polyurethane disk (Shore 95A) 56kN/mm and 35kN/mm each. A noise and vibration

measurements were conducted during the transportation of high speed rail trains as shown in

figure 17.

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Figure 17 – Noise and vibration measurement in the station

A measured vibration acceleration level was 0.0026~0.008g. It is far below the vibration

standards of Korea and the vibration acceleration level measured on the place at which the

SIDBs installed was only one-fourth of the one measured on the other place where the slab is

connected to column with monolithic structure. As shown in figure 18, noise level in station

is 63~64db and it is also below the noise standards of Korea.

Figure 18 – Result of noise test

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CONCLUSION

Based on the results of these experiences, the following conclusions are drawn

1. Since the late 1980s, the bearing has been used in earnest and the isolation system started

to apply in the late 1990s in Korea. In recent times, the SIDB is most widely used as an

isolation bearing in Korean territory.

2. The SIDB has high damping effect as shown in dynamic behavior test results and has been

applied in various construction fields.

3. The SIDB performs well not only isolation against earthquake action but also noise and

vibration reduction function.

REFERENCE

1. Ronald, J. Watson., “The Design of Steel Plate Girder Bridges using Sliding Isolation

Bearings”, 13th World Conference on Earthquake Engineering, Canada, 2004, pp. 490.

2. G. Benzoni. and D. Innamorato., K., “RJ Watson Eradiquake System EQS4500 Bearing

Test Gwangju Chumdan Project”, Report No. SRMD-2007/11-2, 2007

3. H. G. Park., Y. I. Lee., D. Y. Jung., “Experimental and Analytical Study on Dynamic

Behavior of Polyurethane Spring Restoring Disk Bearing”. Journal of Earthquake

Engineering Society of Korea, Korea,

4. Farzad Naeim and James M. Kelly., “Design of Seismic Isolated Structures”, John Wiley

& Sons Publisher, New York, 1999.