Momentresisting frames

6.9.1. Design objective

Clause 6.6.1 (I) Like moment-resisting frames made of other materials, the design objective for steel moment frames is that plastic hinges form in the beams and not in the columns. This requirement is waived at the base of the frame, at the top floor of multi-storey buildings and for one-storey buildings. This requirement is assumed to be fulfilled if equation (D4.23) is satisfied.

Equation (D4.23) expresses a local hierarchy criterion between plastic resistances of beams and columns intersecting at one node, in line with the capacity design concept. This criterion has the advantage of simplicity. However, parametric studies using dynamic non-linear or pushover analysis show that it does not guarantee that plastic hinges occur exclusively in beams. Absolute prevention of plastic hinging in columns would require that the overstrength factor 1.3 in equation (D4.23) be raised to higher values. In fact, in order to ensure the global plastic mechanism defined as the design objective, it has been demonstrated73 that the use of more sophisticated design methods, such as that proposed by Mazzolani and Piluso,74 are needed. However, as the plastic mechanisms obtained by fulfilling equation (D4.23) are always global ones, despite involving some plastic hinging in columns, the decision has been taken to keep equation (D4.23) as the design criterion in EN 1998-1 to achieve the strong column-weak beam design of moment-resisting frames.

6.9.2. Analysis issues in moment-resisting frames

Moment-resisting frames are sensitive to P—A effects. However, if the sensitivity coefficient defined in equation (D4.20) (see Section 4.6.5) is less than 0.1 at every storey, the action effects may be determined using a first-order theory considering the initial geometry of the structure. As the damage limitation requirements on drift (see Section 4.11.1) are demanding, the sensitivity coefficient is in practice always less than 0.1.

Background studies74,75 have shown that overstrength in moment-resisting frames can be high, in particular in design for moderate earthquake regions. By undertaking pushover analysis of a trial design, it is possible to better evaluate the value of the parameter ajal and to increase the value of q from the default value of 1.1-1.3 to a value up to q - 1.6, as allowed in clause 6.3.2(5). This effort in analysis may generate a significant economical impact in structures which are not governed by limits of deformation under vertical and horizontal actions (damage limitation earthquake included). Heavy low-rise industrial frames are most likely to correspond to this situation.

Under a combination of gravity loading and seismic action effects, the values of the maximum positive and negative bending moments in beams can be very different. To be adequate, sections must be related to the absolute maximum values. However, following the general statement in clause 4.4.2.2(1), redistribution of bending moments according to

Steel Beam Lateral Buckling
Fig. 6.7. Support of the bottom flange to prevent lateral buckling of beam ends under hogging moments. (Courtesy of Chia-Ming Uang, University of California, San Diego)

EN 1993-1 for steel structures is allowed, which brings a reduction in the design moment of beams. The practical interest of this redistribution is explained further later in this chapter.

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