Seismic response controlling of structures with a new semi ...waliaj.com/wp-content/2015/Special Issue 1, 2015/06 2015-31-S1-pp.30-38.pdf · Seismic response controlling of structures

WALIA journal 31(S1): 30-38, 2015

Available online at www.Waliaj.com ISSN 1026-3861

© 2015 WALIA

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Seismic response controlling of structures with a new semi base isolation devices

Mehdi Ezati Kooshki 1,*, Abbas Abbaszadeh Shahri 2

1Department of Civil Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran

2Faculty of Civil Engineering, Rudehen Branch, Islamic Azad University, Tehran, Iran

Abstract: The numbers of devices which are used to control of seismic in structures have been developed during

past decades and base isolation design is a new technology on seismic control of structures, the important reasons for the delay of use new technology of base isolation system almost referred to the mentality of the builder

(especially in underdeveloped country) and high cost of the new technology, This study is an attempt to introduce

an effective way to protecting of structures against grand motions by new method (semi base isolation system). In the new way structures isn’t completely decouple of bases and it changed natural frequency of structures due

earthquake by changing horizontal stiffness. The proposed semi base isolation (SBI) system were applied to a one story frame and compared with end fixed frame and the time history analysis was conducted on record of Kobe

earthquake (1995), San Fernando (1971) and Santa Barbara (1978), by used finite element software (ABAQUS 6-10-

1). The analysis results can shows that the efficiency reduced the floor acceleration and displacement and velocity. This study shows that (SBI) system has great potential in future application of seismic isolation technology.

Key words: Semi base isolation system (SBI); Seismic; Natural frequency; Horizontal stiffness; Finite element software

(ABAQUS 6-10-1)

1. Introduction

*Controlling structures against earthquake

techniques can be classified too several types: use passive control, semi active control, active control

and base isolation method. In active and semi active and also passive control part of energy which income

to structures dissipate by added some devices to structures and in base isolation method by decouple

structures from bases avoided the energy income to structures. In the new method semi base isolation just limited from high seismic hazard that income to

structures and the structures aren’t completely separated from the bases with the low horizontal

and high vertical stiffness.

2. Literature and background of the study

The term of base isolation uses the word isolation in its meaning of the state of being separated and

base as a part that supports beneath or serves as a foundation for an object or structure. The base isolation systems have been traditionally applied to various structures under earthquake loads. Research has also been extended to investigate the applicability of these systems in designing low-rise structures (Singh et al. 1996). Base isolation is a

passive structural control technique where a collection of structural elements is used to

substantially decouple a building from its foundation resting on shaking ground, thus protecting the

* Corresponding Au thor.

buildings structural integrity, New Zealand is a leader in base isolation techniques, following pioneering work (Skinner et al., 2000).

In the recent years, numerous catastrophic structural failures due to severe and impulsive seismic events have been occured and reported (Abbaszadeh Shahri, 2010). As an example, Fig. 1 shows the damage of the Imperial County Services Building subjected to 1979 Imperial Valley earthquake. The support pillar failed at the east end of the imperial country services building in El Centro for 6-story reinforced concrete frame and shear wall structure completed in 1971 at a construction cost of $ 1.87 million, was designed to be earthquake resistant, the concrete at the base of columns was shattered and the vertical reinforced bars were severely bend, allowing the building to sag about 30 cm (the building was later destroyed and rebuilt).

2.1. Base isolation devices

Base isolating a building enhances the

performance of the structure by reducing its response to ground accelerations. Base isolation

devices helps in reducing the design parameters for example base shear and bending moment in the structural members above the isolation interface by around 4-5 times. The displacements increases but relative displacements are reduced thus reducing the damage to the structures when the subjected to an earthquake. The shear and bending moments are reduced due to the higher time period of the base isolated structure which results in lower

Mehdi Ezati Kooshki, Abbas Abbaszadeh Shahri / WALIA, 31(S1) 2015, Pages: 30-38

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acceleration acting on the structure and also, due to the increased damping in the structure due to the

base isolation devices (Thakare et al. 2011).

Fig. 1: Imperial County Services building 1979

Two type of base isolation system are well known seismic protection in the world, the elastomeric rubber-steel plate bearing system and roller bearing

or friction pendulums also figure 2 shows a pre-compressed spring box usually used as base isolate

for structures in this device several helical springs compressed in steel box and Pre-Compressed Elastomer Box, these devices consisted two boxes

and elastomers pad between them always useful for natural frequency of structures.

Fig. 2: Typical lead-rubber bearing and sliding bearing and helical spring (Blandford et al. 2009)

As illustrated in Fig. 2 several common base isolation devises (lead-rubber bearing and sliding bearing and helical spring) shows, lead-rubber bearing can be divided into three sub-categories lead

rubber(LR), high-damping rubber (HDR) and natural rubber (NR), any elastomeric isolation bearing is

constructed of alternating layer and rubber pad and steel shims. The rubber part provided horizontal

flexibility and also its able to return to first position after each cycle of loading and in some situation the rubber come back to right position slowly and the main rod made from lead because of its plastic property and it can deformed during earthquake and steel layer between rubbers also allowed to base isolation devices to move in horizontal direction and

stiff in vertical direction, another devices which control of motion, of sliding bearing is achieved by

designing the shape of the contact surface, by designing the friction coefficients between the

contact surfaces, the friction Pendulum family of bearings that dominates that isolates structures

against horizontal motions during earthquake shows in figure.3 the concave surface of the bearing forces it to also upward, against gravity (Blandford et al.

2009). The single-concave friction pendulum bearing is

the original friction pendulum system described by (Zayas et al.1987) and represented the first

manufactured sliding bearing to make use of the pendulum concept. This bearing consists of an

articulated slider resting on a concave spherical surface. The slider is coated with a woven PTFE (Polytetrafluoroethylene) composite liner, and the

spherical surface is coated with a polished stainless steel overlay.

Fig. 3: Friction pendulum TM bearing (Blandford et al.

2009)

3. Research models

3.1. Semi base isolation (SBI) system

As a mentioned several practical base isolation systems have been developed and implement in recent years. Many of base isolation systems use materials which are not traditionally used in structural engineering, such as natural or synthetic rubber or polytetrafluoroethylene (PTFE). An often expressed may not have a design life as long as other structural components, usually considered to 50 years or more (Trevor et al. 2001).

In semi base isolation (SBI) method structures separated from the harmful horizontal motion of the


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ground by providing flexibility in horizontal direction so by this method horizontal excitation which induced by earthquake significant redaction before income to structures. In figure 4 illustrated two layouts of semi base isolation system, left figure

shows square section and right one circulars model, both of them useful but in this paper investigated on square section layouts because of simplify in manufactures.

Fig. 4: two concept of semi base isolation (SBI) with supposes steel frame

As illustrated in Fig. 4 the semi base isolation (SBI) structures little allowable for horizontal excitation by added specific plate to the base of structures. The bearings are very stiff in the vertical and flexible in the horizontal direction, in Fig. 5-a shows package of (SBI) and in Fig. 5-b shows assembly figure of this package, as appear this device made from two steel boxes those sliding on two sliding palates which shows in Fig. 5-c and attach together by four steel spring plates (brown plates) the elliptical hole shapes in spring plates provided vertical movement, and otherwise steel spring resisted from moment of inertia (around center of column) and tensile forces.

Flat plate sliding bearings have been used in many applications to support structures. This kind of bearing has no inherent centering action, so the isolated structure can end up with considerable residual displacement after the earthquake is over. The structure may have to be forced back to its original position with hydraulic jacks, but in new

devise presser load on two plate which shows in figure 5-c the plates are cone shaped for adjustment

after displacement, bottom plate has circular graves for lubrication between two plats (with silicon

greases) and declined friction. As mentioned when an earthquake occurs, base

isolated structures freely move in horizontal directions. Therefore, the structures need to be

returned to their original neutral positions after the earthquake stops. For that purpose, the new method equipped with this type of recovery system. The main advantages of the sliding plate base isolation system are its low cost and it’s excellent in low rise structures. The isolated structure can pull away from the bearing and then settle back down with no other devices. It is possible to add more spring plates or compression fluid viscous dampers in the vertical support system for the bearing pads to insure against impact damage when the bearing comes back together.

Differences between traditional methods and new method (SBI system) divided to three categories: 1. In new method attempted to use traditional

material in industry and inexpensive, simplicity the structural flexure, reachable in all worth wide, safety and reliability but other base isolation like rubber bearing more expensive.

2. The semi base isolation system has recentering

behavior of spring sheet with corporation cone shape sliding plates.

3. The base isolation system have problem associated with adverse torsional motion in asymmetric

structures (Elgamal et al. 2004) but in the semi base isolation method is not.


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(a) (b) (c)

Fig. 5: Semi base isolation (SBI)

3.2. Steel spring and sliding plats materials

Conventional base isolation system is so expensive to implement and therefore only important structures have been so protected with this devices.

The steel spring plate which mentioned before use in auto mobile industries and especially in heavy

track, in fact this isolator plats in semi base isolation work in a similar way to track suspension, which allow to them move on bumpy road with comfort for drivers and passengers. Conventional steel leaf springs are manufactured by 55SiCr7 or 50CrMoCV4 or EN45 standard. The composition of material is given below (Dubey et al. 2013).

Table 1: The composition of sheet spring (Dubey et al., 2013)

Grade C% Mn% Si% Cr% Ni% Mo% S.P%(max)

En45 0.45-0.55 0.50-0.80 Max 0.50 0.80-1.20 - - 0.050

Table 2 Specification of steel spring (Dubey et al. 2013).

parameter Specification

Material Steel (55Si2Mn90) Tensile Strength 1962 N/mm^2

Yield strength 1470 N/mm^2

Young’s Modulus (E) 2.1e5 N/mm^2 Density 0.0000785 Kg/mm^3

In automobile industries and in order of conserve

weight reduction and natural resources recently focused on new materials like composite spring plate, Aluminium Foam, Fiberglass also used for suspension system in wheels of heavy tracks.

For sliding plates regarded to lubrication grove

and cycles of loading of structures stainless steel material or hadfield steel (13% manganese steel)

suggested for this purposes, these steels resistance to abrasion (high stress abrasion) and grinding, as

when two surfaces rub together in the presence of 13%Mn steel has excellent in metal-to-metal wear. It is used in several applications such as railway crossing, and crawler treads for tractors and impact hammers, because of its excellent work-hardening rate, high toughness and high wear resistance (Qian et al., 2011). The material number is DIN 1.3401(X 120Mn12).

Table 3 The composition of hadfiel slide plate.

C Si Mn P S

1.0-1.35 1.0max 11.00min 0.050max 0.050max

3.3. Fixed base structure modeling

Fig. 6: Lumped mass model of fixed-base for three story

structure (Halit et al. 2006)

Dynamic equation of the system model can be written as follows (Kaplan et al. 2000):

�� + C�(x�� -� �(t)) + C(x�� -x� ) + K�(x�-x) = 0 (1) � �� + C(x� -x�� ) + C�(x� -x�� ) + K(x-x�) + K�(x-x�) = 0 (2) �� + C�(x�� -x� ) + K �(x�-x) = 0 (3)


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Where � is the ground displacement, ��, � , and

�� are mass, C�, C, and C�are damping, K�, K, and K� are stiffness, and x�, x , and x�displacement

values for each floor, respectively. Modifying system by use semi base isolation:

� � � � K��x� � x�� + C�(x�� -x�� ) + μ� �� + K�(x�-x�) = 0 (4)

Where is � is the mass of the base, ��is the

mass of the structure, and g is the gravitational acceleration. Note that Equation 4 is valid when the spring is being compressed. The vertical acceleration at the base, � � , and the total horizontal force acting

on the base (Halit et al., 2006).

Fig. 7: Lumped mass model of (SBI) system for three story structure

4. Semi base isolation (SBI) configuration and analyzing

The (SBI) system able proven to be effective against a wide range of ground excitations. By new method can potentially reduce the associated acceleration at base and slightly increase the displacement. The physical model of (SBI) system

shows in figure 8 it was designed to produce a maximum damping in all vertical direction.

Figure 8 is a pictorial representation of the action of semi base isolation with shows several practically pictures during one famous earthquake (Kobe earthquake), as a appear two steel boxes slide together on two slide plate (cone shape plates), and springs plates allowed to the both boxes move in vertical direction by deformation (elastic deformation).

Fig. 8: motion configuration of semi base isolation system (SBI) in 5 steps

The analyses of this device involve a 3-D

analytical model. Comparison of one story frame with corporation (SBI) end fixed frame for several earthquakes (Kobe, San Fernando and Santa Barbara) shows in graphs below. data of the time history graphs for 3 ground motion earthquake are available on http://peer.berkeley.edu/. (Peer the Pacific Engineering Research Center of the University of California at Berkely).

As the analyzing with software (Abaqus) led to the below result:

The time history response of structure for semi base isolation (SBI) compare to fixed base steel frame shown in Fig. 9 to Fig. 17, as mentioned the single story steel frame analyzed under Kobe, Sanfernado, Santa Barbara ground motions and

shows significant decline in all graphs in roof level

acceleration and displacement and also in velocity, Table 4 illustrated percentage effect of new devise.

5. Conclusion

Semi base isolation (SBI) system incorporates spring plates and sliding plates was suggested for earthquake protection of structures. The result analyzing demonstrated that the measured maximum acceleration, velocity and displacement significant declined with application new method.

Conclusion from this study is summarized as follows:


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1. The semi base isolation package reduced maximum acceleration, displacement and velocity decreased significantly.

2. The cost of semi base isolation system is less than conventional base isolation system and also reachable.

3. Material behavior familiar to practicing engineers and safety.

4. Reentering and no residual strain remain (long

life) and Energy dissipates in any direction.

5. No activation mechanism is required and don’t need to maintenance and Insensitivity to ambient temperature.

6. Capability to changes their stiffness by changes thicknesses of spring plate or added more spring plate to the packages or change the diameter of elliptical shaped in spring plats.

Fig. 9: Comparison of Acceleration Time History (for Kobe earthquake) between semi base isolation (blue line) and end fixed

frame (red line)

Fig. 10: Comparison of Displacement Time History (for Kobe) between semi base isolation (blue line) and end fixed frame

(red line)

Fig. 11: Comparison of Velocity Time History (for Kobe) between semi base isolation (blue line) and end fixed frame (red

line)


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Fig. 12: Comparison of Acceleration Time History (for San Fernando earthquake) between semi base isolation (blue line) and

end fixed frame (red line)

Fig. 13: Comparison of Displacement Time History (for San Fernando earthquake) between semi base isolation (blue line)

and end fixed frame (red line)

Fig. 14: Comparison of Velocity Time History (for San Fernando earthquake) between semi base isolation (blue line) and end

fixed frame (red line)

Table 4: Effect of semi base isolation method on (pick time history graphs) reducing response of one-story structure

Earthquake

Factor Kobe Sanfernado Santa Barbara

Acceleration 72% 48% 40%

Displacement 43% 42% 40% Velocity 33% 42% 42%


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Fig. 15: Comparison of Acceleration Time History (for Santa Barbara earthquake) between semi base isolation (blue line) and

end fixed frame (red line)

Fig. 16: Comparison of Displacement Time History (for Santa Barbara earthquake) between semi base isolation (blue line)

and end fixed frame (red line)

Fig. 17: Comparison of Velocity Time History (for Santa Barbara earthquake) between semi base isolation (blue line) and end

fixed frame (red line)

In general Semi base isolation system

considerably reduce earthquake induced load on structures, and it’s shown that semi base isolation (SBI) is effective in reducing the response as compared to fixed base system.

Acknowledgment

The authors would like to acknowledge the invaluable assistance, feedback and contribution of

Dr. Hasan Pourmohamad Assistant Professor Department of Azad University of Karaj.

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