The present invention relates to a new dissipative device. It also relates to a frame and a metal structure comprising the new dissipative device.

Earthquake resistant metallic frames are usually designed so that they exhibit a dissipative structural behaviour. In such a case, parts of the structure (dissipative zones) exhibit inelastic deformations during strong seismic motions.

Steel buildings are generally assigned to one of the following structural types according to the behaviour of their primary resisting structure under seismic actions: Frames with concentric bracings, Frames with eccentric bracings, Moment resisting frames, Moment resisting frames combined with concentric bracings, Moment resisting frames combined with infills and the like.

In the conventional design of frames with concentric bracings (X-bracings, V- or inverted V bracings, dingle diagonals etc), the dissipative zones should be mainly located in the tensile diagonals, for frames with eccentric bracings in the links that are the beam parts between braces or between column and brace. The non-dissipative parts and the connections of the dissipative parts to the rest of the structure shall have sufficient over-strength to allow the development of cyclic yielding in the dissipative parts only.

The main structural typologies, the correspondent performance characteristics of the dissipative zones are listed in Table 1. It may be seen that conventional frames have certain disadvantages in respect to stiffness or ductility. Additionally in such frames, following problems arise after strong earthquakes due to the position of the dissipative zones, where damage is expected to concentrate: a) the need for strengthening or replacement of damaged and buckled braces which have a certain length and are difficult to handle, b) the need for strengthening and repair of the links or the beams that are part of the main system that supports gravity loading.

Such works require therefore considerable skill and are associated with high material and labor costs.

Table 1. Structural typologies and main characteristics for known Steel Frames

Damages in steel framed structures after recent strong earthquakes indicate the need for improvement of existing structural typologies and for introduction of innovative systems.

There is a need for systems having a) High stiffness in order to limit drifts during moderate seismic motions, b) high ductility in order to dissipate energy during strong motions and c) possibility for easy inexpensive repair, if required.

We have now found a new dissipative device for metal building structure which performs better during earthquakes, limits the damage to localized areas and is easier and more effective to repair after strong earthquakes .

The present invention relates to a new dissipative device comprising at least two external eye-bars welded or bolted to an adjacent member (column or beam), at least one internal eye-bar welded to a brace and a pin framing through the eyes of each internal and external eye-bars wherein the pin is simply supported by the eye-bar at its place of contact in the eye of the eye-bar. When the pin deforms plastically in such a dissipative device, a bending moment is developed in the

pin and in the eye-bars at its contact in the eye of the eye-bar.

The dissipative device according to the invention may also comprise several pins framing through the eyes of the corresponding eye-bars. The adjacent member may be a column or a beam. The internal eye-bar may have one or several eyes and may be made of one or several plates connected together by conventional means.

The pin has preferably a rectangular plain cross section, most preferably with rounded corners.

In a preferred dissipative device the pin is at soft contact into the aforementioned eye-bars. By soft contact one means that the gap between the pin and the eye-bar is minimum.

Dissipative device and particularly pin connection are preferably made of metal, most preferably of steel.

Dissipative devices may be used in a metallic frame and a metallic building structure.

They are preferably applied to a concentric or an eccentric braced frame.

Dissipative devices may also be used in a reinforced concrete frame as connection between structural elements , particularly in precasts concrete elements.

Brief Description of Figures:

The invention will become clearer with the following description, reference being made to the accompanying drawings showing several preferred embodiments.

Figure 1. Illustration of one dissipative device with a single internal eye-bar. A dissipative device comprising two external eye-bars welded or bolted to an adjacent column, one internal eye-bar welded to a brace and a pin connection running through the eyes of each internal and external eye-bars.

Figure 2. Details of a pin-shape dissipative device with two internal eye bar and a corresponding frame configuration. A Frame comprising four dissipative devices; each dissipative device comprises two external eye-bars welded or bolted to an adjacent column , two internal

eye-bars welded to a brace and a pin connection running through the eyes of each internal and external eye-bars.

Figure 3: Test set-up for the pin connection and cyclic force-displacement diagram. Figure 4: Experimental result of frame test with pin connections. Figure 5. Energy absorption for frames with pin connections

Figure 6. Experimental versus numerical results for pin connections

Dissipative device is hereafter also called INERD connection . Contrary to connections in conventional Braced Frames which shall be stronger than the connected members and remain elastic, INERD connections shall be weaker than the connected members, exhibiting inelastic deformations and dissipating energy during seismic loading. An example in which INERD connections do connect braces to adjacent members is presented at Figure 2.

The dissipative device consist here of two external eye-bars welded or bolted to the adjacent member (column or beam), two internal eye-bars welded to the brace and a pin running through the eye-bars, as indicative shown in Figure 2. In this type of connection the pin exhibits inelastic bending deformations and dissipated energy is due to the fact that the eye-bars are placed at some distance between each other. The advantage of these connections is that, by appropriate sizing, inelastic deformations are limited within exactly predetermined zones, the pins , whereas the adjacent parts remain elastic. Consequently, damage is restricted to the pins that are small parts that may be easily replaced, if they are largely deformed after a strong earthquake.

The study of the performance of the new system included experimental and theoretical investigations, as following:

• Full-scale tests on INERD connection details • Full-scale tests on frames with INERD connections

• Theoretical investigations on connection details and frames with

dissipative connections

Tests on INERD connection details

For pin-shape dissipative device a series of lull-scale tests was performed . For each examined connection configuration following types of tests were included:

• Two monotonic tests for compression and tension

• One cyclic test according to the ECCS Recommendations (ECCS 1986)

• Fatigue tests with constant amplitude deformations

In the monotonic and cyclic tests the load-deformation curves were recorded, while in the fatigue tests the number of cycles to failure was established.

A tested INERD connection is shown at Figure 3. The pin-shape dissipative device is subjected to the brace force. The test parameters were the cross section of the pins (rectangular 40x60 and circular 050) and the clear distance between eye-bars (50 and 70 mm), while the thicknesses of the eye-bars were kept constant (30 and 15 mm for external and internal eye-bars). In total 24 tests on pin connections were performed. Figure 3 shows a pin at large deformations and an indicative cyclic curve. The pin-shape dissipative device exhibited large strength and small deformations. The tests indicate that the pin connections are very ductile. Figure 3 also shows the resulting curve for the cyclic ECCS test . Similar curves were derived for other plate dimensions. The capability of the connection to exhibit large inelastic deformations (up to 75 mm) without considerable reduction in stiffness and strength is well demonstrated. Fatigue tests were also performed at constant deformations of 20, 40, 60 and 80 mm, providing the number of cycles to failure and accordingly the relevant Wδhler curves.

Tests on frames with INERD connections

Cyclic full scale tests on complete X-braced frames with INERD connections were performed . Each frame was subjected to two loading histories, as following: • History according to the ECCS Recommendations

• History according to the expected deformations for real earthquakes The ECCS loading history includes 3 cycles at deformations 2, 4, 6, 8, ... times the "yield" displacement and is very, actually too, severe. More realistic results in respect to the expected response of the connections are derived from the second type of loading history, which is more probable to happen during natural earthquakes.

The overall frame dimensions were the same for all tests (length 3,4 m, height 3,0 m). All member sections were European sections, the strong axis for columns and braces being out-of plane, while for the beam in-plane. The column bases and the beam-to-column connections were nominally pinned, although, as will be seen later, the frame itself had some lateral rigidity. The frame was supported against lateral deformations. The horizontal load was applied at the top of the frame. The overall force, the lateral displacements and the brace deformations were recorded during testing. Additionally, strain gage measurements were made in the diagonals to record the actual forces in the braces.

Frame tests with pin connections

The overall response of frames in which diagonal bars were connected to beams and columns by means of INERD was similar for all tests. An example of such response is shown in Figure 4. The tests were stopped due to the limitation of the capacity of the actuator and not the capacity of the frame.

As was the design intention, the braces were protected from buckling due to the fact that the yielding in the ESfERD connection preceded any such buckling.

Accordingly, both braces participated equally by tension and compression to the load transfer and contributed to the overall lateral stiffness. As the braces were protected also from yielding, the energy was overwhelmingly dissipated by the connections. After unloading, the pins for the tests with the ECCS loading history exhibited permanent deformations. However, it was always easy to dismantle the braces and proceed to the next experiment.

Theoretical investigations on INERD connections

In order to accurately describe the connection response a series of theoretical investigations were performed . They included FEM analyses and development of simple engineering models. FEM analyses were performed on the pin connections for monotonic and cyclic loading, using different general purpose programs (ABAQUS, NASTRAN, Straus 7, etc.).

By means of simple engineering models, simple formulae appropriate for practical use should be derived that allow the correct prediction of the connection response.

For the pin connections, the model of a beam subjected to four-point bending describes the behavior only up to the yield load. Subsequently, the external eye-bars provide some "clamping" to the pin that allows further increase of the load. In addition, friction effects at higher loads have to be considered. Finally, taking all effects into account a tri-linear relationship that describes accurate enough, the connection response could be derived. The formulae for the determination of the relevant points are given in Table 2.

Table 2. Forces and deflections of pin connection

a f _y = yield stress of pin f _u = tensile strength of pin

Wp ₁ = plastic modulus of pin cross section

Theoretical investigations on braced frames with INERD connections The response of braced frames with INERD connections was studied by means of pushover and nonlinear dynamic analyses. In the latter, the connection response was modelled by adequate cyclic laws representing closely the behaviour observed during the tests. Parametric studies were performed in order to study the influence of the connection behaviour (elastic and post-elastic stiffness etc.) to the overall frame response.

The following observations for the frames with INERD connections indicate an advantageous behaviour: a) The elastic frame stiffness is unaffected by the flexibility of the connections. This indicates that there will be no serviceability problems for moderate earthquakes. b) The reduction in overall strength for frames with INERD connections starts at much larger displacements compared to conventional frames. This indicates that the influence of P-Δ- effects will appear at much higher earthquake intensities. c) The distribution of drifts between floors is much more uniform for frames with INERD connections, indicating that inelastic action will be distributed within the entire structure. d) The overall post-ultimate stiffness is much higher compared with conventional frames. This indicates that buckling of the braces was largely avoided.

Non-linear dynamic analyses were performed for a large number of braced frames with INERD connections. The frames were subjected to various seismic records with different spectral responses. The frames were pre-designed for a PGA =

0,24g and a value of the behaviour factor q = 3,5, valid for conventional braced frames according to the provisions of the European seismic Code (EN 1998, 2004). Drifts were recorded for increasing PGAs. The results show that maximal drifts don't exceed 2,0 % when the frame is subjected to the design PGA. For a target drift of 2,5 % corresponding to life safety performance criterion and appropriate selection of the connection characteristics, PGAs almost twice as large could be achieved. This indicates the positive influence of the INERD connections to the performance of braced frames.