Partial strength connections

Clause 6.7.3(9) Partial strength connections are not a familiar option in frames with concentric bracings, though several reasons justify the statement that concentric bracings are an excellent application field for such connections. These reasons are:

0 Frames with concentric bracings possess a high stiffness because of their topology, and easily fulfil deformation criteria. Thus, unlike moment-resisting frames, additional flexibility in the connections is not penalized by the need to increase the sections of structural elements whenever semi-rigid connections are used. • Partial strength connections can be designed so that their plastic strength is lower than the buckling strength of diagonals. In this case, the difficulties in dealing with systems

Partial Strength Connections
Fig. 6.14. Soft-storey development in truss braces

with evolutive stiffness and strength explained above for 'tension-diagonal-only' systems do not apply.

9 In practice, the use of partial strength connections means that all diagonals can be present in the model used for the analysis, which brings additional stiffness in comparison with the 'tension-diagonal-only' structural model discussed above. This positive contribution compensates for the flexibility possibly introduced by the semi-rigid connections.

• Partial strength connections can be developed as industrial products with 'calibrated strength', reducing the effect of uncertainties in the overstrength design of beams and columns; both Q and %,, can then be taken as equal to 1.0.

• It can be demonstrated that the behaviour factor q of frames with concentric bracings and partial strength connections for the diagonals is higher than without such connections, due to better control of the global plastic mechanism.

• If the structure is damaged by an earthquake, yielded and permanently deformed components only require replacement in localized areas of the dissipative connections.

The soundness of these reasons has been demonstrated in experimental tests on connections and frames and by numerical modelling of complete structures under earthquake action. Specific designs79 have demonstrated a potential in terms of strength, stiffness and elongation capacity, complying for instance to the requirement of 117 mm for two connections in the short example of Section 6.6 (see p. 148).

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