Keywords: added viscous dampers, structural deformability, toggle brace system, amplification of damper motion, parametric study.

When a civil engineering structure is subjected to a seismic event, primary and secondary seismic waves hit the structure and a huge amount of energy is introduced into the structure. Within the seismic effects reduction campaign for buildings, the energetic approach proposed by Constantinou and Symans [5] sets as the design scope the realization of structures able to store or to dissipate an amount of energy higher than the energy value furnished by the seismic event. In this way, the structural performance is considerably improved if a part of the energy is absorbed not by the structure, throughout by the creation of plastic hinges and the consequent inevitable damage of the structural elements, but through additional devices. Such an approach is called the "dissipative approach" and is based on the use of mechanical devices incorporated in the structure that dissipate energy in different ways: by steel yielding, sliding friction, movements of a piston inside a viscous fluid, passage of a fluid through orifices and the viscoelastic behaviour of polymeric materials.

The use of damping devices in building structures for the mitigation of the seismic action has been the object of many research and applicative works, proving its efficiency [1,2,3,4]. Among all possible kinds of damping devices, viscous dampers have already proven to be beneficial in mitigating the effects of earthquakes upon structures and have been widely used, successfully, in several buildings, above all in the USA and Japan [7,8].

However, for structures characterised by a relatively small deformability (such as masonry structures or massive concrete frame buildings), the use of viscous dampers is somehow hampered by the small elongation of the dampers which either (a) reduce considerably their efficiency in the mitigation of the seismic action, or (b) require the use of specially designed dampers which can be quite expensive. An economic solution to this problem can be found in the use of the so-called "toggle-brace" mechanical system [6,9], which permits the amplification of the relative motion of the two damper ends with respect to the relative motion of the two storeys to which the damper is attached (typically, two adjacent storeys). Chevron braced systems are characterised by a damper elongation which is equal to that of the two storeys to which the damper is attached, and diagonal bracing systems are characterised by damper elongation which is inferior to that of the two storeys to which the damper is attached.

This paper presents the results of an extensive parametric study which permits the determination of the toggle brace system which, for given geometric conditions are imposed by the structure to which it is attached. Thus the maximum amplification of the motion of the damper ends with respect to the inter-story drift can be obtained.

The results indicate that it is possible to obtain amplification factors up to 3.5 and that the best results are achieved when the two inextensible arms have similar inclinations with respect to a horizontal axis. Also, these findings are in complete agreement with the results obtained from experimental shaking table tests available in the literature [6,9].

1

D.P. Taylor, "Buildings: Design for Damping", Technical Report, Taylor Devices Inc., 90 Taylor Drive, North Tonawonda, NY 14120-0748, www.taylordevices.com, www.shockandvibration.com, www.seismicdamper.com.

2

G.C. Hart, K. Wong, "Structural Dynamics for Structural Engineers", John Wiley & Sons, New York, 2000.

3

G.D. Hahn, K.R. Sathiavageeswaran, "Effects of added-damper distribution on the seismic response of buildings", Computers & Structures, Vol. 43, No. 5, pp 941-950, 1992. doi:10.1016/0045-7949(92)90308-M

4

I. Takewaki, "Optimal damper placement for critical excitation", Probabilistic Engineering Mechanics, Vol. 15, pp 317-325, 2000. doi:10.1016/S0266-8920(99)00033-8

5

M.C. Constantinou, M.D. Symans, "Seismic response of structures with supplemental damping", The Structural Design of Tall Buildings, Vol. 2, pp 77-92, 1993. doi:10.1002/tal.4320020202

6

M.C. Constantinou, P. Tsopelas, W. Hammel, A.N. Sigaher, "Toggle-Brace-Damper Seismic Energy Dissipation System", Journal of Structural Engineering, Vol 127, No. 2, 2001. doi:10.1061/(ASCE)0733-9445(2001)127:2(105)

7

P. Brown, I.D. Aiken, F.J. Jafarzadeh, "Seismic Retrofit of the W. F. Bennett Federal Building", Modern Steel Construction, URL, 2001.

8

P.W. Clark, I.D. Aiken, E. Ko, K. Kasai, I. Kimura, I., "Design Procedures for Building Incorporating Hysteretic Damping Devices", Proceedings, 68th Annual Convention, Structural Engineers Association of California, Santa Barbara, California, 1999.

9

S. Marcelin, "Parametric Study on the Effectiveness of The Toggle Brace Damper", Department of Civil Engineering, Florida Agricultural and Mechanical University/ Florida State University College of Engineering, Tallahassee, FL, 32310, 2002.

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