Energy Dissipation System Configurations for Improved Performance

发布于:2021-10-14 11:23:52

Energy Dissipation System Configurations for Improved Performance Michael C. Constantinou and Ani Natali Sigaher Abstract Energy dissipation systems are being employed in the United States to provide enhanced protection for new and retrofit building and bridge construction. The hardware utilized includes yielding steel devices, friction devices, viscoelastic solid devices and mostly, so far, viscous fluid devices. This hardware has been used in either diagonal or chevron brace configurations. Two new developments in the field of energy dissipation systems utilize unusual configurations which substantially increase the effectiveness of the system. These configurations are presented in the paper and their utility is demonstrated. Introduction In conventional construction, earthquake-induced energy is dissipated in components of the gravity-load-resisting system. The action of dissipating energy in framing such as beams in a moment-resisting frame produces damage in those components. Repair of such damage after an earthquake is typically expensive and often requires evacuation of the building while repair work on the gravity system is undertaken. The objective of adding energy dissipation hardware to new and existing construction is to dissipate much of the earthquake-induced energy in elements not forming part of the gravity framing system. Key to this philosophy is limiting or eliminating damage to the gravity-load-resisting system (FEMA, 1997). Engineers are familiar with and have extensively used diagonal and chevron brace configurations for the delivery of forces from energy dissipation devices to the structural frame (Soong and Dargush, 1997; Constantinou et al., 1998). New configurations have been developed which offer certain advantages, either in terms of cost of the energy dissipation devices, or in terms of architectural considerations such as open space requirements. Particularly, stiff structural systems under seismic load or structural systems under wind load undergo small drift and the required damping forces are large. This typically results in increased volume of fluid viscous damping devices and accordingly cost. In other cases, energy dissipation devices cannot be used in certain areas due to open space requirements and the ineffectiveness of damping systems when installed at near-vertical configurations. Two recently developed configurations, the toggle brace and the scissor-jack energy dissipation system configurations, offer advantages that overcome these limitations. Bother utilize innovative mechanisms to amplify displacement and accordingly lower force demand in the energy dissipation devices. However, they are more complex in their application since they require more care in their analysis and detailing. This paper presents these new configurations, compares them with the familiar chevron

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brace and diagonal configurations and presents samples of experimental and analytical results on their behavior. Description of Toggle-Brace and Scissor-Jack Damper Configurations The toggle-brace and scissor-jack systems are configurations for magnifying the damper displacement so that sufficient energy is dissipated with a reduced requirement for damper force. Conversely, they may be viewed as systems for magnifying the damper force through shallow truss configurations and then delivery of the magnified force to the structural frame. Figure 1 illustrates various damper configurations in a framing system. Let the interstory drift be u, the damper relative displacement be u D , the force along the axis of the damper be FD and the damping force exerted on the frame be F. It may be shown (Constantinou et al., 1997) that

uD = f u F = f FD

(1) (2)

where f = magnification factor. Expressions for the magnification factor of various configurations are shown in Figure 1. The significance of the magnification factor may be best demonstrated in the case of linear viscous dampers, for which
 FD = C o u D

(3)

 where u D = relative velocity between the ends of the damper along the axis of the damper. The damping ratio under elastic conditions for a single-story frame (as shown in Figure 1) with weight, W, and fundamental period, T, is:

β=

C o f 2 gT 4πW

(4)

That is, the damping ratio is proportional to the square of the magnification factor. The toggle-brace and scissor-jack systems can achieve magnification factors larger than unity. The systems can be typically configured to have values f = 2 to 3 without any significant sensitivity to changes in the geometry of the system. By contrast, the familiar chevron-brace and diagonal configurations have f less than or equal to unity. For the purpose of comparison, consider the case of the use of a linear viscous damper with C o = 160 kN-s/m (= 0.9 kip-s/in) in the framing systems of Figure 1 with weight W = 1370 kN (= 308 kip) and T = 0.3 second. The resulting damping ratios are shown in Figure 1. The effectiveness of the toggle-brace and scissor jack systems is clearly demonstrated. It should be noted that the configurations for these

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two systems are identical to those tested at the University at Buffalo (Constantinou et al., 1997). It is clear in the results of Figure 1 and in equations (1), (2) and (4) that the togglebrace and scissor-jack configurations may provide substantial energy dissipation capability with the use of low output force devices. This may result in an important cost advantage in systems that undergo small drifts such as stiff structural systems under seismic load and most structural systems under wind load. Such cases of small drift lead to a requirement for increased volume of fluid viscous devices and accordingly increased cost. The use of the new configurations eliminates the necessity for large volume damping devices and may result in reduced cost. Moreover, the scissor-jack system may be configured to allow for open space, minimal obstruction of view and slender configuration, which are often desired by architects. As an example, Figure 2 illustrates the scissor-jack system tested at the University at Buffalo. The open bay configuration, the slenderness of the system and the small size of the damper are apparent.
Some Experimental Results on the Scissor-Jack System

Testing of the scissor-jack system has been recently conducted at the University at Buffalo. The study included shake table testing of a stiff structural system consisting of two identical frames with the geometry shown in Figure 2. The frames carried on their tops a concrete weight of 143 kN (32 kip), resulting in a fundamental frequency, in the absence of the damping system, of 3.2 Hz. The damping system included two linear viscous dampers with constant C o = 26 N-s/mm (150 lb-s/in). Transfer functions obtained in the shake table testing revealed the dynamic characteristics of the structural system without and with the scissor-jack system. Shown in Figure 3, these transfer functions reveal: (a) for the structure without the damping system, a fundamental frequency of 3.2 Hz and damping ratio of 0.04, and (b) for the structure with the damping system, a fundamental frequency of 4.0 Hz and damping ratio of 0.15. It is interesting to note that the increase in frequency (stiffening) is caused by the flexibility of the scissor-jack system (large forces in toggles cause deflections of the beam), so that a component of the damping force occurs in-phase with the restoring force (Constantinou et al., 1998). The model structure was tested on the shake table utilizing a length scale factor of 2 and a time scale factor of 2 . A sample of recorded results for the 1940 El Centro earthquake, component S00E with peak acceleration of 0.17g is shown in Figure 4. The figure shows the recorded histories of interstory drift and beam acceleration for the structure without and with the damping system. The effectiveness of the scissorjack system is evident.
Conclusions

Two new energy dissipation system configurations, toggle-brace and scissor-jack, were described. These systems may offer the advantages of reduced cost of fluid damping devices in applications of small structural drift (as those of stiff structural

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systems) and of open bay configuration, slender construction and minimal obstruction of view. The efficacy of these configurations was demonstrated by application to a stiff structural system in which multi-fold increases in damping ratio with respect to that provided by conventional damper configurations were shown to be possible with the toggle-brace and scissor-jack configurations. A sample of experimental results obtained in the shake table testing of a large scale steel model demonstrated the effectiveness of the scissor-jack system.
References

Constantinou, M.C., P. Tsopelas, and W. Hammel (1997), Testing and Modeling of an Improved Damper Configuration for Stiff Structural Systems, Center for Industrial Effectiveness, University at Buffalo, SUNY, Buffalo, NY. Constantinou, M.C., T.T. Soong, and G.F. Dargush (1998), Passive Energy Dissipation Systems for Structural Design and Retrofit, MCEER Monograph, Multidisciplinary Center for Earthquake Engineering Research, Buffalo, NY. FEMA (1997), NEHRP Guidelines for the Seismic Rehabilitation of Buildings, Report No. FEMA 273 (Guidelines) and FEMA 274 (Commentary), Federal Emergency Management Agency, Washington, DC. Soong, T.T. and G.F. Dargush (1997), Passive Energy Dissipation Systems in Structural Engineering, J. Wiley, England.
Authors

Michael C. Constantinou, Member ASCE is Professor and Chairman, Department of Civil, Structural and Environmental Engineering, University at Buffalo, State University of New York, Buffalo, NY 14260. Phone: (716) 645-2114 ext. 2404. Fax: (716)-645-3733. E-mail: constan1@civil.eng.buffalo.edu Ani Natali Sigaher, Student Member ASCE is Graduate Research Assistant, Department of Civil, Structural and Environmental Engineering, University at Buffalo, State University of New York, Buffalo, NY 14260. Phone: (716) 645-2114 ext. 2445. Fax: (716)-645-3733. E-mail: sigaher@acsu.buffalo.edu

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W

u F Co

θ = 37 o
f = cos θ f = 0.799

Diagonal

θ

β = 0.017

W

u F Co

Chevron

f = 1 .00

f = 1.00

β = 0.027

W

u F

Lower Toggle

θ1 = 31.9 o , θ 2 = 43.2 o
f = sin θ 2 cos(θ1 + θ 2 ) f = 2.662

θ2 θ1
90° Co

β = 0.194

W

u F

Upper Toggle

θ1 = 31.9 o , θ 2 = 43.2 o
f = sin θ 2 + sin θ1 cos(θ1 + θ 2) f = 3.191

Co 90°

θ2

β = 0.279

θ1
u F

W

Scissor-Jack

θ 3 = 9 o , ψ = 70o
f = cosψ tan θ 3 f = 2.159

θ3
Co

Ψ

β = 0.126

Figure 1. Effectiveness of Damper Configurations in Framing Systems

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c L

W8x21 BEAM

STIFFENER BOTH SIDES

2'-1 3/64"

70° TS 2x2x1/4"
1'-9"


DAMPER

5'-3 9/16''



W8x24 COL. (TYP.)

/4" 10 3




6'-3 7/8" 1 29/32"

TS 2x2x1/4" SHAKE TABLE OR BEAM

9 1'" 3/4

20°

8'-4"

Figure 2. Tested Scissor-Jack Damper Configuration

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9 9/16''

Structures 2000

20 AMPLITUDE OF TRANSFER FUNCTION
RATIO OF BEAM/COLUMN JOINT ACCELERATION TO TABLE ACCELERATION

15
W/OUT SCISSOR-JACK DAMPER SYSTEM (FOR 0.10g WHITE NOISE)

10

5

WITH SCISSOR-JACK DAMPER SYSTEM (FOR 0.30g WHITE NOISE)

0 0 5 FREQUENCY (Hz) 10

Figure 3. Amplitude of Transfer Function of Tested Structure with and w/out Scissor-Jack Damper System

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15 10 DRIFT (mm) 5 0 -5 -10 -15 0 15 10 DRIFT (mm) 5 0 -5 -10 -15 0 0.8 ACCELERATION (g) 10 20 30 40 10 20 30 40
East Frame West Frame

WITHOUT

WITH

WITHOUT

0.0

-0.8 0 0.8 ACCELERATION (g) 10 20 30 40

WITH

0.0

-0.8 0 10 20 TIME (sec) 30 40

Figure 4. Recorded Histories of Drift and Acceleration of Structure without and with Scissor-Jack System in 1940 El Centro Earthquake (PGA=0.17g)

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