Limitation of overstrength

As explained in Section 6.2, the complete design process may generate a structure with more strength than strictly needed for the resistance to the design earthquake. The excess in material may have several origins, for example:

• the limits of deflection in the design of beams under gravity loading may lead to sections larger than those needed for resistance to earthquakes

• the capacity design of columns to meet equation (D4.23).

If the design is such that the drift limits under the damage limitation SLS earthquake are verified, the designer can consider reducing the excess size of structural members. EN 1998-1 offers three possibilities for reducing the overstrength of beam and column sections:

• The design of beams under seismic loading is made considering a combination of gravity loading and seismic action effects which result in an unsymmetric bending moment diagram, in which the absolute values of maximum positive and negative bending moments can be quite different (Fig. 6.12). The usual approach is to select the beam section such that in the beam M , Rd > maxflMgJ, | Af¿"d |). But smaller sections can be justified via a redistribution of bending moments within the limits allowed by EN 1993-1. Changing the reference line of the moments reduces the maximum design moment and the beam section accordingly. 8 Due to the shape of the diagram of beam moments, the capacity design of columns (see equation (D4.23)) refers to the plastic resistance of the beam ends sections, Mpl Rd b, or to the plastic resistance of the connection, Mpl Rd c, whichever is smaller. If beam design produces significant overstrength with M , Rd » max(|Ms+d F|, E|), compliance to equation (D4.23) brings about a similar overstrength of columns. This can be avoided by a reduction of M, Rd b through a reduction in the beam section (reduced beam or 'dogbone') at the beam ends. 8 The column overstrength can also be avoided by making use of the beam-to-column partial strength connections designed to achieve Mpl Rd c

Of course, if these possibilities are used, the modified structure is more flexible than the first design, and its response is changed. A new analysis and a verification of the structure considering the modifications have to be performed.

6.! 0. Frames with concentric bracings

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