It is well known that the different strategies for the design of seismic-resistant structures can be framed in view of energy balance.

According to the traditional strategy for the seismic design of building structures [11-12, 18, 22], in case of frequent and occasional seismic events whose return period is comparable with the life cycle of structures, the earthquake input energy has to be completely dissipated by means of viscous damping. Therefore, the hysteretic energy Eh is equal to zero because, for such seismic events, the structure has to be designed to remain in elastic range. Conversely, in case of rare and very rare seismic events whose return period is about 500 years and even more, most of the earthquake input energy is dissipated by hysteresis, but leading to severe plastic excursions and related structural damage. Such structural damage has to be compatible with the ductility and the energy dissipation capacity of structures, because, even though structural damage is accepted, collapse prevention has to be assured and the safeguard of human lives has to be guaranteed.

Within the above framework, with reference to steel Moment Resisting Frames (MRFs), there is the need to provide the structure with sufficient lateral strength and stiffness in order to remain in elastic range under frequent and occasional seismic events. In particular, adequate lateral stiffness is needed to reduce the damage to non-structural components which is a fundamental requirement for the check against serviceability limit states. Conversely, in case of destructive earthquakes, MRFs have to be designed in order to dissipate the earthquake input energy at the beam ends where cyclic plastic bending has to occur. To this aim, it is recommended that beam-to-column connections are designed with sufficient over-strength [10,29], with respect to the connected beams, accounting for random material variability [29] and the occurrence of strain-hardening to guarantee the full development of the ultimate flexural resistance of plastic hinges. In addition, aiming to promote the plastic engagement of the greatest number of dissipative zones by properly controlling the failure mode, modern seismic codes, such as Eurocode 8, requires the application of hierarchy criteria to promote the yielding of beam ends rather than column ends. To date, the classical design philosophy based on weak beam-strong column-strong joint hierarchy has been widely applied in practical seismic design [11, 22] and surely provides some advantages, such as the development of quite stable hysteresis loops of dissipative zones and the prevention of soft-storey mechanisms which, as well known, have to be absolutely avoided because of their poor energy dissipation capacity. However, on the other hand, the traditional design approach provides also several drawbacks [10].

With reference to severe seismic events, the main drawback of the traditional strategy is intrinsic in the strategy itself. In fact, on one hand, even if structural damage is essential to dissipate the earthquake input energy, on the other hand, such structural damage is the main source of direct and indirect losses which are becoming more and more intolerable in case of industrialised countries, as testified by the amount of losses occurred during recent seismic events.

Another drawback, which is specific to the case of steel MRFs, is that the use of full-strength beam-to- column joints with the code required over-strength is not cost effective and constitute an important burden when there is an ongoing competition with structural solutions adopting other materials such as, in particular, reinforced concrete. In fact, in order to guarantee the desired connection performance levels, a significant oversizing is needed leading to the use of supplementary web plates, additional reinforcing ribs or cover plates or, even, the use of haunched beams.

In order to reduce the main drawback of the traditional design strategy, i.e. the occurrence of structural damage, in past decades several strategies have been proposed. In particular, a strategy well suited for application to steel structures is the so-called strategy of supplementary energy dissipation, or passive control [30-34], where the earthquake input energy is dissipated by viscous damping or hysteretic damping

by means of the introduction of energy absorbers generally located between couple of points of the structure where high relative displacements or velocities are expected [31]. Among the different strategies included in the framework of passive control systems, friction dampers have been proposed in past research activities as damping devices aimed to reduce lateral displacements for serviceability limit state requirements and to reduce structural damage to fulfill ultimate limit state requirements.

Friction dampers present high potential at a low cost and they are easy to install and maintain. Therefore, in past, several friction damping devices have been experimentally tested [35-37] and some of these have been used in buildings around the world. The most widely adopted system for installing such friction dampers consists in the introduction of a bracing system which is integrated with friction dampers [35, 38- 39]. Several researchers worked on this structural configuration proposing design procedures to optimize the slip force of the bracing system.

With reference to the drawbacks deriving, in the traditional design strategy, from the need to design beam- to-column connections with high over-strength with respect to the connected beams, an alternative to the weak beam-strong connection-strong column approach has been proposed. The alternative philosophy of strong column-weak connection-strong beam can be applied, because Eurocode 8 has opened the door to the use of partial strength connections, provided that they are able to provide sufficient plastic rotation capacity (typically 0.035 rad for high seismicity zones) to be checked by means of experimental tests. With such design approach, even in the case of big beam sections like those occurring in case of long spans or high gravity loads, the adoption of partial strength joints allows to control the bending moment transferred to the column which, in this way, can be prevented from an excessive oversizing resulting from the application of beam-column hierarchy criterion. In addition, the structural detail of beam-to-column connections can be significantly simplified by improving the overall cost effectiveness of the structural scheme, by overcoming the economical drawbacks resulting in case of connections designed to attain high over-strength.

The growing interest of the scientific community to the design of dissipative semi-continuous frames with partial strength joints [10,13, 15-16, 20-21, 40-46] in MRFs is also reflected in last version of Eurocode 8, but the actual application of this technique is still strongly limited in practice, because the actual dissipative capacities of the joints have to be demonstrated by means of experimental testing which is generally out of the possibilities of common designers. In addition, even though the application of partial strength connections can lead, on one hand, to a more economical design, on the other hand, it can provide also some disadvantages such as the reduction of the frame lateral stiffness and, generally, of the energy dissipation capacity at the beam end.

Starting from the background briefly summarized above, in order  to overcome the drawbacks of  the described design strategies (the traditional one and the passive control one), the proposed research project is aimed at the practical development of a new design strategy whose goal is the design of connections able to withstand without any damage not only frequent and occasional seismic events, but also destructive earthquakes such as those corresponding to rare and very rare events. In other words, the goal of the research project is the design of “Free From Damage Connections”. The acronym of the project “FREEDAM” is aimed to underline such goal.

The basic idea of the research work is inspired to the strategy of supplementary energy dissipation, but it is based on the use of the damping devices under a new perspective. In fact, while passive control strategies have been commonly based on the integration of the energy dissipation capacity of the primary structure by means of a supplementary dissipation coming from damping devices; conversely, the new design strategy, which could be named “Free From Damage Design”, is based on the use of friction dampers conceived in such a way to substitute the traditional dissipative zones of MRFs, i.e. the beam ends.

From the technological point of view, the innovation regards the conception of beam-to-column connections. In fact, beam-to-column connections are equipped with friction dampers which can be located either at the bottom flange level or at the levels of the both flanges. Such friction dampers have to be designed to assure the transmission of the beam bending moment required to fulfill serviceability limit state requirements and to withstand without slippage the gravity loads. In addition, they have to be designed in order to assure the dissipation of the earthquake input energy, corresponding to the collapse prevention limit state, without any 

damage. To date, within a Italian research project (RELUIS project, founded by the Italian Department for Civil Protection) some preliminary analyses and experimental tests of the proposed connection typology have been already performed investigating some different friction materials and their application to full-scale external beam-to-column joints [47-49]. However, due to a significant number of design issues which needs to be accurately investigated before the practical application of the new technology, a wide research project is needed which can be carried out only within an European framework. The European dimension of the research activity is needed to investigate not only those design issues coming from earthquake resistant design, but also other design aspects involving structural robustness which have not been investigated. Moreover, the European dimension is indispensable to join researchers coming from different countries who can, in view of their specific competences, provide the contributions needed to solve the new design problems.

Beam to column joints with friction dampers

The typical (non-limitative) configuration of the tested joint represents a modification of the classical detail of a Double Split Tee connection (Fig.1). In particular, with the proposed approach, it is suggested to realize at the top beam flange level a fixed classical T-stub, preventing the concrete slab damage, and to provide a friction damper at the bottom flange level by realizing slotted holes on the beam flange (or on an additional haunch) and exploiting the stem plates of the angles, used to connect the beam to the column, in order to realize the friction damper. This strategy allows the development of beam-to-column connections with high energy dissipation due to the slippage of the friction material and able to accommodate the required displacements without any damage by simply governing the length of the slotted hole. In addition, other advantages of the proposed strategy are related to the reduction of the structural cost both of columns and of connections. In fact, by using the friction dampers as joint components, it is possible to calibrate, with a reasonable level of accuracy, the exact amount of force that is transmitted to the column. In this way, it is possible to design joints with a flexural capacity very close to the nominal bending resistance of the connected beam. In such a way, the beam section is fully exploited, but both the oversizing of the other joint components (usually requiring supplementary web plates, reinforcing ribs, cover plates, increased bolt diameter, etc.) and the column oversizing (because of beam-column hierarchy criterion) can be significantly reduced. However, the main advantage is that such connection works like a “Free From Damage Connection” up to plastic rotation demands compatible with the stroke of the friction dampers.

It is also important to underline that the stroke end limit state does not represent an ultimate limit state. In

fact, in case of destructive seismic events leading to a plastic rotation demand attaining or exceeding the stroke end limit state, a new resisting mechanism is activated with the bolts and the horizontal plates of the friction dampers subjected to shear and bearing, respectively. Such new resisting mechanism provide additional flexural strength to the joint and an additional plastic rotation capacity before the ultimate limit 

state is reached. This specific property of the proposed connection can be particularly effective also in view of exceptional loading conditions requiring specific structural robustness.

Methodology and description of the research work

The objectives of the project can be achieved by developing the six work packages herein summarized to present the logic structure of the research programme.

[1] Nogueiro P., Simões da Silva L., Bento R., Simões R., “Experimental  behaviour of standardised European end-plate beam-to-column steel joints under arbitrary cyclic loading”, Proc. of SDSS’06 –Int.l Colloquium on Stability and Ductility of Steel Structures, Portugal, 2006.

[2] Nogueiro P., Simões da Silva L., Bento R., Simões R., “Numerical implementation and calibration of a hysteretic model with pinching for the cyclic response of steel joints", International Journal of Advanced Steel Construction 3(1), pp. 128-153 (2007).

[3] Weynand K., Huter M., Kirby P., Simões da Silva L., Cruz P., “SERICON – A Databank for tests on semi-rigid joints”, in Proc. of the COST C1 Inter. Conf. on Control of the Semi-Rigid Behaviour of Civil Engineering Structural Connections, pp. 217-228, Liège, Belgium (1998).

[4] Dubina D., Ciutina A., Stratan A. “Cyclic tests of  double-sided beam-to-column joints”. Journal of Structural Engineering (ASCE) 2001;127(2):129–36.

[5] Jaspart J.P., Demonceau J.F. “Simple connections”, Publ. 126, ECCS Press, Brussels, 2009.

[6] Jaspart J.P., Demonceau J.F.. “European design recommendations for simple joints in steel structures”. Journal of Constructional Steel Research, 64/7-8 (2008) 822-832.

[7] Bursi O.S., Jaspart J.P. “Benchmarks for finite element modelling of bolted steel connections”. JCSR 43 (1997) 17–42

[8] Bursi  O.S.,  Jaspart  J.P.  “Calibration  of  a finite  element  model for  isolated  bolted  end-plate  steel connections”. JCSR 44 (1997) 225–262

[9] Braham M., Jaspart J.P. “Is it safe to design a building structure with simple joints, when they are known to exhibit a semi-rigid behaviour?”. JCSR 60 (2004) 713–723

[10] Faella C., Piluso V., Rizzano G. “Structural steel semirigid connections: theory design and software”, CRC Press LLC, 2000.

[11]  Mazzolani F.M. (Editor). “Moment Resistant Connections of Steel Frames in Seismic Areas, Design and Reliability”, E&FN Spoon, 2000.

[12] De Matteis G., Mazzolani F.M., Landolfo R., Milev J. “Q-factor evaluation of moment resisting steel frames with semi-rigid connections by applying different approaches" . Acta Polytechnica, Journal of Czech Technical University, Vol.39, No. 5, 1999, pp.183-194.

[13] Calado L., De Matteis G., Landolfo R., Mazzolani F.M. “Cyclic behaviour of steel beam-to-column connections: interpretation of experimental results”. Proc. of 6th Intern. Colloquium on Stability and Ductility of Steel Structures (SDSS’99), Timisoara (Romania), Sept. 9–11, 1999.

[14] Calado L., Landolfo R., De Matteis G. “Fracture resistance design of bolted joints”. Proc. of 2nd Conference Construcao Metàlica e Mista, Coimbra (Portugal), November, 1999.

[15]  Calado L., De Matteis G., Landolfo R.. “Experimental response of top and seat angle semi-rigid steel frame connections”. Material and Structures, Vol.33, 2000, pp.499-510.

[16] De Matteis G., Landolfo R., Calado L. “Cyclic Behaviour of Semi-Rigid Angle Connections: a Comparative Study of Tests and Modelling”. Proc. of Third International Conference "Behaviour of Steel Structures in Seismic Areas", Montreal (Canada), August, 2000, Balkema pp.165-174.

[17] De Matteis G., Landolfo R., Mazzolani F.M. “The behaviour of Connections in steel MR-frames under high-intensity earthquake loading”. (in "Abnormal Loading on Structures: Experimental and Numerical Modelling",eds K.S.Virdi et al., E & FN SPON, I, 2000, pp. 59-73).

[18] Della Corte G., De Matteis G., Landolfo R. “The influence of connection modelling on the seismic response of MR steel frames” (in “Moment Resistant Connections of Steel Frames in Seismic Areas: Design and Reliability”, Mazzolani ed., E & FN SPON, 2000, pp. 485-512).

[19] Della Corte, G., De Matteis, G., Landolfo, R., Mazzolani, F.M. “Seismic analysis of MR steel frames based on refined hysteretic models of connections”. JCSR 58 (2002) 1331–1345.

[20]  Elnashai, A. S. and Elghazouli, A. Y.  “Seismic Behaviour of Semi-Rigid Steel Frames: Experimental and Analytical Investigations”, JCSR, 29 (1994). 149-174.

[21]  Elnashai, A. S., Elghazouli, A. Y. and Danish-Ashtiani, F. A.  “Response of Semi-Rigid Steel Frames to Cyclic and Earthquake Loads”, JSTRENG, ASCE, 124(8), (1998). 857-867.

[22]  Elghazouli, A. Y. “Seismic Design of Steel Frames with Bolted Beam-to-Column Connections”, Elnashai, S. and Dowling, P. J. (Editors.), ICP (pubs.), (2000).

[23]  Shi G., Fan H., Bai Y., Yuan F., Shi Y., Wang Y. (2012). Improved measure of beam-to-column joint rotation in steel frames. JCSR, Volume 70, Pages 298-307

[24]  Díaz C., Victoria M., Martí P., Querin O.M. (2011). FE model of beam-to-column extended end-plate joints. JCSR, Volume 67, Issue 10, Pages 1578-1590.

[25]  Díaz C., Victoria M., Martí P., Querin O.M (2011). Review on the modelling of joint behaviour in steel frames. JCSR, Volume 67, Issue 5 , Pages 741-758.

[26]  Abidelah A., Bouchaïr A., Kerdal D.E.. (2012). Experimental and analytical behavior of bolted end-plate connections with or without stiffeners. JCSR, Volume 76, Pages 13-27

[27]  Girao Coelho A. M., Bijlaard F.S.K. (2012).Finite element evaluation of the rotation capacity of partial strength steel joints. CONNECTION VII Workshop, Timisoara 31 May-2 June.

[28]  Pop A., Grecea D., Ciutina A.(2012). Low cycle performance of T-stub components of bolted moment beam -to-column connections. CONNECTION VII Workshop, Timisoara 31 May-2 June.

[29]  Piluso, V. & Rizzano, G., 2007. Random Material Variability effects on Full-strength end-plate Beam-to- Column Joints. Journal of Constructional Steel Research, 63(5), pp.658-66.

[30]  Aiken I.D, Clark P.W., Kelly J.M., 1993. Design and Ultimate-Level Earthquake Tests of a 1/2.5 Scale Base-Isolated Reinforced-Concrete Building. Proceedings of ATC-17-1 Seminar on seismic Isolation, Passive Energy Dissipation and Active Control. San Francisco. California.

[31]  Constantinou  M.C.,  Soong  T.T.,  Dargush,  G.F.,  1998.  Passive  Energy  Dissipation  Systems for Structural Design and Retrofit. Multidisciplinary Center for Earthquake Engineering Research, University at Buffalo, State of New York.

[32]  Christopoulos  C.  Filiatrault  A.,  2000.  Principles  of  Passive  Supplemental  Damping  and  Seismic Isolation. IUSS PRESS. Pavia. Italy

[33]  Yang T-S., Popov E.P., 1995. Experimental and Analytical Studies of Steel Connections and Energy

Dissipators. Report No. UCB/EERC-95/13, University of California, Berkeley.

[34]  Kelly J.M., 1979. Aseismic Base Isolation: A review. Proceedings, 2nd U.S. National Conference on Earthquake Engineering, Stanford, CA, 823-837

[35]  Pall, A.S. and Marsh, C., (1982) "Response of Friction Damped Braced Frames", Journal of Structural Division, ASCE, Vol. 108, No. ST6, June, pp. 1313-1323.

[36] Marsh, C., Pall, A.S., (1981), "Friction Devices to Control Seismic Response", Proceedings Second ASCF,/EMD Specialty Conference on Dynamic Response of Structures, Atlanta, U.S.A., January, pp. 809-818.

[37]  Pall,  A.S.,  Marsh,  C.,  Fazio,  P.,  (1979)  "Limited  Slip  Bolted  Joints  for  Large  Panel  Structures", Proceedings,  Symposium  on  Behavior  of  Building  Systems  and  Building  Components,  Nashville,  U.S.A., March, pp. 385-494.

[38]  Mualla, I. & Belev, B., 2002. Seismic Response of Steel Frames Equiped with a New Friction Damper Device Under Earthquake Excitation. Engineering Structures, 24(3), pp.365-71.

[39]  Kelly, J., Skinner, R. & Heine, A., 1972. Mechanisms of Energy Absorption in Special Devices for Use in Earthquake Resistant Structures. Bullettin of the New Zealand Society for Earthquake Engineering, 5(3), pp.63-88.

[40]  Faella, C., Piluso, V. & Rizzano, G., 1998. Cyclic Behaviour of Bolted Joint Components. Journal of Constructional Steel Research, 46.

[41]  Iannone, F., Latour, M., Piluso, V. & Rizzano, G., 2011. Experimental Analysis of Bolted Steel Beam-to- Column Connections: Component Identification. Journal of Earthquake Engineering, 15(2), pp.214-44.

[42]  Latour, M., Piluso, V. & Rizzano , G., 2011a. Cyclic Modeling of Bolted Beam-to-Column Connections: Component Approach. Journal of Earthquake Engineering, 15(4), pp.537-63.

[43]  Piluso, V. & Rizzano, G., 2008. Experimental Analysis and modelling of bolted T-stubs under cyclic loads. Journal of Constructional Steel Research, 64, pp.655-69.

[44]  Latour, M. & Rizzano, G., 2012. Experimental Behavior and Mechanical Modeling of Dissipative T-Stub Connections. Journal of Structural Engineering, 138(2), pp.170-82.

[45]  Latour, M. & Rizzano, G., 2013. A Theoretical Model for Predicting the Rotational Capacity of Steel Base Joints. Journal of Constructional Steel Research, 91, pp.89-99.

User access

Submit to FacebookSubmit to Google PlusSubmit to TwitterSubmit to LinkedIn