Research and Development of Seismic Base Isolation Technique

Research and Development of Seismic Base Isolation Technique
for Civil Engineering Structures
SUN Hongling1, DOU Weiling2
1. School of Civil Engineering and Architecture, Southwest Petroleum University, Chengdu, Sichuan,
610500
2. Construction Superintend Station of Shunqing District Nanchong City. Nanchong, Sichuan, 637000
[email protected]
Abstract: Base isolation is one of the most promising alternatives among the structure control methods.
In recent decades, base isolation has been seriously considered for civil structures, such as buildings and
bridges, subjected to ground motion. Seismic isolation technique had been applied successfully abroad,
especially in Japan, and buildings with seismic isolation design had good performances in the
earthquakes before. Seismic isolation technique has developed well in recent years in our country. After
the Wenchuan earthquake, some departments revised the Code for Seismic Design of Buildings and
relaxed some restraint of seismic isolation design. Using the isolation design correctly, the buildings can
get a good capability of seismic resistance. It is a good choice for the reconstruction of the important
buildings. Focus on description of the concept of seismic design and the design method of base-isolat
edstructure of Zundao Central School and hospital.
Keywords: seismic base isolation, civil structures, development, applicati on seismic isolation design
1 Introduction
In the past two decades, increasing attention has been paid by scientists and engineers to the mitigation
of damage caused by earthquakes. Earthquake is one of the most serious calamities that threaten life and
property of the people. To eliminate the damage, the traditional method is to undertake seismic resistant
design for buildings. But the performances of the internal ductile structures during major earthquakes
have proved to be unsatisfactory and indeed far below expectation. To enhance structural safety against
severe earthquakes, more effective and reliable techniques for aseismic design of structures based on
structural concepts are desired. Among the structural control schemes developed, base isolation has been
well known as one of the most promising alternatives.
A significant portion of this effort is devoted to the study and application of base isolated systems.
Among various base isolation systems, the lead-rubber bearing (LRB ) has been extensively used.
Lead-rubber bearings are relatively easy to manufacture, have no moving parts, are unaffected by time,
and are very resistant to environmental degradation. It can be adopted for new structures as well as the
retrofit of existing buildings and bridges. The base isolation technology developed rapidly from 1970s in
many countries, such as in America, Japan, and New Zealand. The theory and technology of modern
base isolation have been studied in China since 1980s. So far, isolation technology has been applied in
26 provinces and cities in China. There were more than 700 buildings using rubber isolation bearings in
some way in the isolation system.
Base isolation achieves reducing earthquake force by shifting the fundamental frequency of the structure
far from the dominant frequencies of the ground motions and increasing energy dissipation by
introducing relatively flexible isolation devices between the super structure and its foundation. In this
approach, the structure is decoupled from the horizontal components of the earthquake ground motion
by isolator with low horizontal stiffness between the structure and the foundation. So the fundamental
frequency of structure is much lower than its fixed base frequency. The first dynamic mode of the
isolated structure involves deformation only in the isolation system. The higher modes do not participate
in the motion. So if there is high energy in the ground moti on at these higher frequencies, this energy
cannot be trans mitted into the structure.
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If a building structure is torsionally unbalanced due to asymmetry in geometry, stiffness or mass
distributions, a coupled lateral-torsion motion will occur when subjected to a horizontal ground motion.
Tests and theoretical analyses[ 1 ] on coupled lateral-torsion motion indicated that this effect is sometimes
one of the main factors to cause structural damages. In recent years, asymmetric base-isolated structures
have been increasingly studied and some important conclusions have been obtained.
2 Analysis on the Damages of the RC Frame Structure
、
The cost of the building with RC frame structure is the highest in brick-masonry structure brick-timber
structure and RC frame structure, so it is commonly used in the school buildings in the urban districts
and seldom used at countryside. Associating with local economic conditions, all of the buildings mostly
were built after 2000 concentrative. Unlike the buildings at countryside, the school buildings in urban
districts usually are designed by professional and technical personnel and the construction and
organization management is more regularly and strictly, so the damages of the buildings with this
structure are the slightest. In the total 112576m2 buildings mentioned above, 7300m2 were destroyed
severely, as a percentage of 6.5, 99355m2 were damaged moderately, as a percentage of 88.5, and the
rest of were damaged slightly. There wasn’t any building with RC frame structure collapsed completely.
The damages of these structure buildings concentrate on: the fill walls cracked or collapsed in large
quantity (especially the arc and the other irregular fill walls.), the joints between the columns or beams
and the fill walls cracked in vertical or horizontal, the concrete of the columns and beams nodes
damaged and fell down with the reinforcements inside yielding. Lots of the“beam hinges” appeared in
the damaged buildings, instead of the “column hinges” expected in design. The concept of “strong
columns and weak beams” hadn’t been embodied at all [5] (Fig.1). The important reason of fill walls
damaged severely is lacking reliable joints—there isn’t any wall binding rebar or wall binding rebar is
inadequate according to the code. As the first defensive line for reducing earthquake, the damages and
the collapse of them are in accordance with expectations. But we must point out that the damaged or
collapsed wall caused numerous injuries and deaths also for happening at the escaping time. Problems of
stability and seismic resistance capability of RC frame structure need to be discussed and studied
further.
Fig.1 Destructional forms of reinforced concrete frame
3 Recommendations of Seismic Isolation Design in Reconstruction
3.1 Introduction of seismic Isolation Design
Unlike the traditional seismic design methods, seismic isolation design sets a seismic isolation layer
between the foundation and the superstructure to isolate them to reduce the response of the earthquake,
which makes the buildings move slightly in the earthquake and keep safe[6]. Laminated rubber bearings
developed best and are used widely most inall seismic isolation methods [7]. There are more than 5000
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buildings with seismic isolation design all over the world so far. Seismic isolation buildings have bore
the earthquakes before successfully with good performances. Seismic isolation design has been the
dominant seismic design method and has been used in buildings higher than 50 floors in Japan and
USA.
3.2 Seismic Isolation Design has Developed Fully in Recent Years in China
Researches on seismic isolation technique are full-fledged in China, so far. Especially, laminated rubber
bearings have gone into the application step. We have obtained the industrial standards and regulations
progressively for ourselves under the hard work continued from 1990s of insiders. Rubber isolation
bearings for buildings (JG118-2000), effected on Dec. 1st, 2000, and Technical specification for
seismic-isolation with laminated rubber bearing isolators CECS 126:2001 [10], effected on Nov. 1st,2001.
Code for seismic design of buildings (GB50011-2001) added a chapter of seismic isolation design
absorbing the full-fledged parts of researches home and abroad. Furthermore, the code revised in 2008
relaxed some restrictions after Wenchuan earthquake [11]. There are more than 1200 buildings with
seismic design in China now, mostly covering all the high-intensity areas in mainland China. Seismic
isolation design will certainly get in the spring of its as the development of the economy and technology
in China for its an unparalleled advantages [12].
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3.3 Seismic Isolation Design Recommendation for the Important Buildings in the Reconstruction
1 Seeing from the earthquakes
Reconstruction areas must be the hard hit areas, while most of the hard hit areas locate the
Longmenshan fault or its influence areas exactly. That is to say, reconstruction areas mostly are
high-intensity areas. Longmenshan fault is very active in 21st century. We have recorded more than 4.7
earthquake 27 times, 5-5.9 earthquake 5 times and 6-6.5 earthquake 5 times before Wenchuan
earthquake [13]. Earthquake prediction is still a worldwide problem as we know. That is to say,
earthquakes might take place in the hard hit areas, while won’t be able to predict those exactly in a long
time. So it is a great challenge for us to reconstruct our homes. Besides the construction problems, the
intensity of the areas epicenter of or near the earthquake is much higher than that of design is also a
primary cause of the damages of the buildings [14]. Code for seismic design of buildings revised the
intensities of some hard hit areas in 2008 and raised the safety requirements of buildings, but it didn’t
solute the problem of the actual intensity is higher than that of design.
2 Compared with traditional design on obtaining seismic resistance capability
Seismic isolation is applicable to the low-rise or tier buildings and it can get optimum efficiency when
used in a building (designed by traditional method) with a fundamental period of 4s. The experimental
results show that the horizontal earthquake response of buildings with seismic isolation design reduces
to 1/3~1/7 of that designed in tradition and the efficiency in strong earthquakes is better than in weak [15].
The first building with laminated rubber bearings was built in 1993 in Shantou city in China. In order to
be compared with the traditional buildings on seismic resistance capability, constructed a building with
traditional seismic design at the same site (16m apart), at the same time, and with the same
structure—RC frame, 8 floors(Fig.2 &3). A M=7.3 earthquake took place on Sep. 16th, 1994 in Taiwan
Strait. Shantou is 140km away from the epicenter of the quake and the intensity was mostly 6. Buildings
designed in tradition shook heavily and some of them cracked, inclined or damaged in levels. People
fled in panic and 100 more injured or died.
Shantou seismic office made an inspection after the earthquake. They found that people in traditional
buildings shook so severely that couldn’t stand and most of them felt nervous and scared. But people in
the building with seismic isolation design seldom felt the shake. They knew the earthquake form others
downstairs. People highly praised the building with seismic isolation design on reducing the earthquake
response.
The traditional seismic design resists the earthquake by magnifying the dimensions and ratios of
reinforcements of the members. It causes the rigidities of the members larger and larger as the magnified
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dimensions. With the higher and higher earthquake response, they become a vicious circle. It decreases
the buildings’ safety with the high “seismic resistance” cost. Although the seismic isolation design needs
to add isolators, the response effecting on major structure decreases a lot. Therefore, the dimensions and
ratios of reinforcements can be reduced to increase building areas. Furthermore, the foundation
quantities will be lower for the lighter superstructures. As with all things above, the cost of the building
with seismic isolation design will be lower on the contrary. Compared with the traditional design
buildings on cost in Shantou, Guangzhou and Xichang, etc, the seismic isolation can save by 3-6% in 7
intensity areas, 8-14% in 8 intensity areas and 15-20% in 9 intensity areas with better seismic
performance.
3 Compared with traditional design on the economy.
Give a cost analysis on a sample building with seismic isolation design followed. Project profile: a
commercial domestic house, 45.2m long, 12.9m wide, 5+1 floors, the first floor 3.6m high, 2-5 floors 3m
high, the top floor 2.5m high, height in total 18.1m, RC frame structure, thickness of 300mm fill walls
(concrete hollow blocks and clay bricks), thickness of 200mm interior walls, intensity 8, C-class
building, -class site. There are two proposals to choose, traditional design in intensity 8 and seismic
isolation design in intensity 7with 39 isolators.
Proposal 1 Traditional design, according with Code for seismic design of buildings (GB50011-2001) &
Code for design of concrete structures(GB50010-2002), in intensity 8.
Proposal 2 Seismic isolation design, superstructure is designed in intensity 7, C-class frames, the
rigidities and bearing capacities of the beams and slabs at the top of the seismic isolation layer should be
greater than those of the ordinary. Meet the requirements of Code for seismic design of buildings
(GB50011-2001) & Code for design of concrete structures (GB50010-2002).
Compared on economy: proposal 1: Direct construction cost of the superstructure 528 yuan/m2. Proposal
2: The construction cost of the superstructure decreases by 95.7 yuan/m2 for decreasing the column
dimensions and reinforcements ratio, while increases 71.9 yuan/m2 for increasing cost of seismic
isolation bearing isolators and isolator installations. The cost is 504.3 yuan/m2.in total, reducing 23.8
yuan/m2, saving by 4.5% approximately.
Taking most of the reconstruction areas are in high intensity and the big financing gap for reconstruction
into consideration, seismic isolation design is a good choice for important buildings reconstruction.
Seismic isolation design is a good choice for the reconstruction of important building. In addition,
damages and collapse of lifeline systems had given us so much trouble in rescue and relief work. It is
significant to build the lifeline systems with seismic isolation design for the disaster reduction work in a
long time. For the other important buildings, seismic isolation design is also a good and economical
choice.
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：
：
Fig.2 The layout plan of the two
compared buildings
Fig.3 The elevation and section plan of a seismic isolation
building built earliest in China with laminated rubber bearings
3.4 Design and application of base isolation in the post-disaster reconstruction projects in Zundao
The old center school in Zundao is frame structure and brick masonry wall. The concrete frame
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appeared damaged and the wall of the bottom appeared severe crack. It's typical seismic damage of the
frame structure. So the old school must be demolition and reconstruction. The old hospital in Zundao is
frame structure and mix-structure. It appeared damaged, though not collapse, but cannot use.
The project sponsor establish the design of the safety, security and the refuge after earthquake since the
plan began to do. They proposed the seismic fortification intensity is not less than 9 degree, seismic
requirements(the value of the basic design earthquake acceleration is 0.40 g, the second earthquake
grouped, characteristic period of ground motion is 0.40 seconds), so the seismic fortification intensity is
higher than the national criterion(the local seismic fortification intensity is 7 degree, the value of the
basic design earthquake acceleration is 0.15 g ), the building structure complied the seismic fortification
criterion of key structure to design.
The newly built teaching building is a three layer structure construction. The upper structure applies
reinforced concrete frame structure, cast reinforced concrete floor and rubber bearing set between the
upper structure and base, the seismic energy is limited to flow into the upper structure. Prolong the
natural period of vibration of the structure by the isolation layer and increase damping, weaken or
change the intensity and ways of ground motion to the structure. All the ways are to reduce structure
vibration.
According to the seismic design code, we must select appropriate bearings and some members of initial
stiffness to resist foundation micro-vibration and wind loads, and the two composition consist the
isolation layers. At the same time, isolation bearings must be checking calculation of the vertical bearing
capacity and the horizontal displacement of the rare earthquake. In order to reduce the seismic shear on
the upper structure, the design of isolated structures as bellows: The maximum layer shear ratio
between isolation and non-isolated structure is 0.35; The horizontal damping coefficient is 0.5;
The conversion intensity of upper construction after seismic isolation is 8 degrees (reduce 1 degree).
We use Etabs software in actual calculation. The space model is that the structure is seen to be rigid in
all level floor plates, and it consider two horizontal movements component, a plane of rotation and a
vertical component.
We calculate Respectively the story shear force of the isolated structure model and the non-isolated
structure model, and the results must meet the requirements that the story shear force ratio not more than
0.35 between the isolated and the non-isolated structure. The horizontal displacement of isolation
bearings, which caused by the horizontal shear of the rare earthquake, should not more than the
minimum value both 0.55 times of effective diameter of the bearing and 3.0 times of all the rubber
underlayer thickness of the bearing. By the analysis of isolation design, we select 32 lead rubber
bearings(LRB700) and 12 natural rubber bearings(RB700). Isolation bearings arrange as the figure2.
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Fig.4 Isolation bearing
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Fig.5 The calculation diagram of isolation bearing structure
As considering the horizontal damping coefficient of the upper structure, the calculation method of
upper structure is same to the general structure. The calculation model of the upper structure adopts the
structure above the isolation layer and the bearings see as hinged to use PKPM software to analyse and
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calculate. The horizontal seismic action of the upper structure accorde to reduce 1 degree of seismic
fortification intensity(as 8 degree); the vertical seismic action of the upper structure accorde to 9 degree.
According to the seismic design code, it must consider the standard value of the vertical seismic action
is less than 40% of the gravity loading representative value of the structure above the isolation layer.
The checking calculation of column head and bean-column of the upper isolation bearings, must
consider the effect of the isolation bearings shear and additional bending moment in 9 degree rare
earthquake.
The foundation of the teaching building is strip foundation. The upper of foundation and the bottom of
isolation layer stick out column head, and embed steel sheet to connect isolation cushion. The checking
calculation of the structure under the isolation layer should consider the shear of the isolation bearings in
the 9degree rare earthquake. The checking calculation of the column head of the bottom of the isolation
bearings should consider the shear and additional bending moment of the isolation bearings in the
9degree rare earthquake. The intensity checking calculation of the foundation must accord to seismic
action of fortification intensity.
The connection of the up-down plate of the isolation bearings and up-down construction use bolted
connection, which use the interchangable external-insert bolts. The connection of isolation cushion and
beam column use reliable compressed stirrup and configure net-sharp steel.
All of connection bolts must meet the action of the 9 degree rare earthquake, which generate horizontal
shear moment and possible strain force.
Isolation cushion appear greater flexible deformation in the time of massive earthquake. It must set
definite horizontal isolation joint in the upper structure above the isolation layer and nearby any fixing
element. The joint width should not less that 1.2 times of most horizontal displacement of all isolation
cushion in massive earthquake
As the structure use the newly technique of base isolation, at the same time, the enclosure wall and
partition wall use the lighten steel wallboard, the seismic performance of the construction improve
greatly. Even if the rare earthquake happened, the isolation structure does not occur damaged of
life-threatening safety and loss of using function.
、
、
Fig.6 The arrangement of isolation bearings
4 Conclusion
（1）Taking De-yang city for example, the damages of the school buildings are different as the different
construction times and structures. Seeing from the structures, buildings with brick-timber structure
damaged most severely for poor seismic capability. Buildings with brick masonry structure are used
commonly in school buildings and damage severely too, especially the buildings dissatisfying the
requirements of the codes. Buildings with RC frame structure are usually in urban district for its high
cost. They gave the best performances in the earthquake, but we must point out that the damages and
collapse of the fill walls caused numerous injuries and deaths still. The buildings were built more early
damaged more severely in the earthquake seeing from the construction time, which associate with the
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local economic development close.
2 Compared with the traditional design, seismic isolation design has more advantages on the lower
cost and better seismic resistance capability. Seismic isolation design has been applied successfully at
many places with a blank in hard hit areas in Wenchuan earthquake.
3 Most of the reconstruction areas are in high intensity. Taking the uncertainty of the earthquakes and
the particular properties of the important buildings(school buildings and lifeline systems etc.)in
consideration, seismic isolation design is a good choice for the reconstruction of the important buildings
.
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