Design of dissipative zones

Clauses 6.6.4(2), As mentioned before, dissipative zones in moment-resisting frames should be plastic hinges 6.6.4(3), activated by bending moments. They appear at beam ends, due to the shape of the bending

6.6.4(5), moment diagram under the seismic action (see Fig. 6.8).

6.6.4(6), Plastic hinges can take place in the connections, in the case of partial-strength or semi-rigid

6.5.5(7) connection design. There are many possible designs, using connecting components of various types: flexible end plates, angles, etc. Such designs have to face the problem of assessing the deformation capacity - in this case the rotation capacity - under dynamic cyclic conditions. However, there does not yet exist a list of adequate designs, prescriptive or otherwise. The designer thus has to develop his or her own design, referring to the existing literature.76 Recourse can also be made to EN 1993-1-8, by using the 'components method' after selecting as dissipative components those listed in Section 6.4 above as 'favourable factors' for ductility. National Annexes to EN 1998-1 may provide explicit reference to other literature as guidance on connection design.

Plastic hinges in moment-resisting frames are typically selected in beams, not only because of the lack of data on partial strength connections but also because moment-resisting frames are flexible by nature and add flexibility to the connections, resulting in the need to compensate for the high flexibility by bigger steel sections, which may result in an uneconomic solution.

If rigid connections are used, two design options exist. The design can be such that:

(1) the plastic hinges occur in the beam sections adjacent to the column flanges, which is the standard situation with classical connection design, or

(2) the plastic hinges are displaced from the column flanges, in order to separate the stress concentration of the connection from the plastic strains of the plastic hinge. This can be achieved by:

strengthening the connection - weakening the beam by trimming the flanges (reduced beam sections or a 'dogbone' design, as shown in Fig. 6.7). This original idea77 has been developed further and incorporated in design recommendations.78

Examples of the strengthening and the weakening techniques are sketched in Fig. 6.11. Possibilities for displacing the plastic hinges from the column flanges have been widely developed in the USA as a result of the poor performance of moment connections in moment-resisting frames in the Northridge (1994) and Kobe (1995) earthquakes. These are justified design options when there is concern about the quality of the most stressed zone, which is that at the column face. The inquiry and research following these earthquakes have shown that several factors other than the connection design generated the poor performance observed:

• base material with low weldability and low elongation capacity

• improper weld material (of low toughness)

• weld preparation promoting stress concentration and defects (V preparation with a cope hole and welding on a backing bar - both made necessary by the fact that only one-sided welding from above was feasible for on-site welding)

8 improper welding techniques (site welding, gas protection).

However, there is extensive experimental evidence of more classical connections achieving the target plastic rotations without difficulty. Such observations have been made in tests on H and IPE profiles with a depth of up to 450 mm, using European standards for materials and fabrication with standard precaution, such as suitable weld preparation (K), weld metal and welding procedure (e.g. in the case of K preparation, welding from one side is followed

Cover plate

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(a)

a

^—-

r n

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Fig. 6.11. Examples of (a) strengthening and (b) weakening techniques in moment frame connections by welding from the other side), and base material of appropriate toughness and weldability. This justifies the position adopted in EN 1998-1, that detailed prescriptions and limitations on connection design are not yet required. Further details may be included in the National Annexes of EN 1998-1. Some uncertainties remain for sections larger than H and IPE profiles with a depth of 450 mm, for which research is still ongoing.

Whatever the type of connection, the demonstration of deformation capacity has to be provided in terms of absolute rotation capability 6 , as stated in clause 6.6.4(3). The wording corresponds to a definition of 0p related in the simplest possible manner to process test data, not to theoretical definition of elastic and plastic rotations. 0p is the deflection measured at the midspan of the beam, divided by half of the span length. If the test is made on a shorter length, the obtained results can be adjusted to this definition. 0p includes the following contributions:

• deformation of the connection, including column web panel deformation

• plastic hinge rotation

• elastic deformation of the beam.

6p should not include any elastic contribution of the column deformation outside the panel zone, because this deformation has nothing to do with the rotation capability of the plastic zones under characterization in a test, namely those in the beam and its connection to the column.

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Responses

  • hayley
    What is dissipative zone?
    1 year ago

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