Favourable factors for local ductility

Steel is a ductile material, if a correct steel grade is selected: a material elongation over 20% and a material ductility ratio e rnaj/ey over 10 can provide highly ductile dissipative zones. If the designer makes good choices in the design, the plastic mechanism developed in a structural component, such as a beam or a diagonal bar in a truss, can be fairly ductile and dissipative.

Reliable energy dissipation at the element scale can be found in:

(1) Bars yielding in tension. This possibility directly relies on the use of an adequate material, in a design avoiding stress concentrations or excessive section reduction (see Section 6.7), in which only tension develops. For these reasons, high-strength bolts in tension should not be used as dissipative components, because they are not made of a very ductile material and may be subjected to effects other than pure tension, such as being bent when a connection deforms. The cyclic plastic strains involved can result in early low cycle fatigue failure.

(2) Bars yielding in compression, if early buckling is prevented. Yielding in compression inevitably ends in buckling. However, stocky elements with A < 0.2 can keep their strength and develop some plasticity in compression. Recently, designs in which the compressed bar is inserted in a tube only intended to provide lateral support against buckling have been tested and used successfully.71

(3) Plates yielding in bending.

(4) Profiles yielding in bending, if flange buckling takes place at sufficiently large enough deformation (see Table 6.3 of EN 1998-1). Buckling of flanges cannot be prevented totally, but is delayed when the slenderness ratio c/t of walls of sections is small Eurocode 8 uses Classes 1, 2, and 3 of sections of Eurocode 3 to characterize the ability of sections to develop a 'plastic hinge', meaning their ability to provide a stable full plastic moment resistance when cyclic plastic rotations are applied. The required ability is linked to the demand for plastic rotation. For instance, sections belonging to Class 1 are able to undergo cyclic plastic rotations over 35 mrad without losing significant strength.

(5) Plate yielding in shear. Yielding in shear is a ductile and stable mechanism. The plastic strength in shear is related to the slenderness of the plate; its reduction with slenderness has to be considered.

Some ductile local phenomena can also contribute to the ductility of connections:

(1) Ovalization of bolt holes takes place in plates of connections which are made of a ductile constructional steel, in contrast to the failure of bolts in shear or of welds. Even in a connection which is capacity ('overstrength') designed to the assembled bars, which in principle need not fulfil any further design condition, the designer can obtain more assurance of ductile behaviour by designing one or both assembled plates so that their bearing resistance is less than the bolt shear resistance. This possibility is recommended for the following reason: even if the bolted connection has been designed to be 'non-slip', experiments have shown that there is always a relative movement between two assembled plates, once the connected elements are subjected to plastic cycles. In practice, this means that after few cycles the bearing resistance becomes the true mode of resistance of the bolted connection. This justifies the requirement to check the bearing resistance and to have it as 'weak link' in the chain of resistances.

(2) Friction between plates: connections made with pre-tensioned bolts working in shear are a site of friction, once relative movement between assembled plates takes place. As friction dissipates energy, which is favourable, and also to avoid destructive shocks in bolts between loose parts of connections, pre-tensioning of bolts is prescribed. These two positive influences are not quantified in the energy dissipation of the structure. Category B bolted joints in shear (slip-resistant in the SLS but not in the ULS) and surface preparation class B (alkali-zinc painting applied to a prepared surface) are permitted by clause 6.5.5(4). In practice, this means that slippage is allowed under seismic conditions, because it is a ULS situation.

(3) Reliable energy dissipation can take place also in the joints, rather than in the members themselves, if the joints are designed to develop one or several of the dissipative mechanisms listed above. This concept raises the problem of knowledge of the cyclic behaviour of the components of a joint, in order to select the ductile joints (e.g. plates in bending) and to capacity design the 'brittle' ones (e.g. bolts in tension). Section 1.8 of EN 1993-1-8 on the design of joints may serve as a basis for ascertaining this information, though the cyclic behaviour aspect is not covered. As the reality of connections behaviour may be complex, this design possibility will widen when the results of ongoing and future studies become available.

6.4.2. Unfavourable factors for local ductility

If one of the following unfavourable circumstances is realized, local ductility will be small. Little energy dissipation is expected:

(1) In zones made totally or in part of brittle or low-ductility material. This rule first concerns the steel of sections and plates, which should comply with requirements on steel grade, toughness and weldability. It also concerns the weld material, the adjacent

heat-affected zone, the welding process and the quality of execution of welds, all of which may affect negatively the base material.

(2) If plastic strains take place in a too narrow zone; this is a 'localization of strains' or 'stress concentration' situation. Even if all conditions related to materials and execution are correct, the design of elements and, in particular, of connections can be such that plastic strains take place in a short zone. A high elongation in a short zone can correspond to a very low deformability of a component, which can be significantly below the expectations of the designer and the requirements of the code. Bad design generates this outcome. This issue is discussed further in Section 6.7. Clause 6.5.3(I), (3) If early local or global buckling affects the element or local zone. As recalled above, 6.5.3(2) rules define limits of global and local slenderness to control these effects. Clause 6.5.3(1)

and Table 6.3 in EN 1998-1 aim at prevention of local buckling. Global buckling checks are those of Eurocode 3, but several relationships in Eurocode 8 aim at a safe-side definition of design action effects in columns. This is the case in clauses 6.6.3(1) and 6.6.3(2) for columns in moment frames and clause 6.7.4(1) for beams in moment frames.

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  • Markus
    What is favourable factorr?
    8 years ago

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