Capacity Design

Seismic Capacity Design Chain

Figure 5.2 Ductility of chain with brittle and ductile links

Figure 5.2 Ductility of chain with brittle and ductile links

According to capacity design principles, to maximise the ductility of the chain, some links have to be chosen to have ductile behaviour and be designed with that purpose. The rest of the structure must be designed with excess strength in order to remain elastic during the plastic deformations of the ductile links. For this purpose the design force of the brittle links must be equal to the maximum resistance of the ductile links after yielding, that is, a force equal or above F . The ductile link behaves like a fuse, which does not allo the applied force acting on the brittle links to increase above their maximum resistance. Therefore the force applied on the chain can increase above Fy up to the value Fu, but cannot exceed this value. At this stage the chain collapses at a displacement much higher than the chain designed with the direct design methodology, as follows:

ence, the brittle links ust be designed for a force different fro the ductile link, hich is a function not of the notional applied load but of the capacity of the ductile link, in order to prevent the premature failure of the brittle links before the deformation capacity of the ductile links is exhausted. The fact that the design action effects in predefined 'protected' elements are a function of the resistance of other key elements is a basic characteristic of capacity design, and is an important difference to standard design procedures for quasi-static loading.

This highlights the fact that the indiscriminate provision of excess strength, hich is usually considered positive according to standard design procedures, ay adversely affect the non-linear behaviour of a structural system, as it may prevent an intended ductile link from acting like a fuse. Hence, if after designing a ductile frame, the flexural reinforcement of beams or of the base section of walls is increased, this is not necessarily a 'safe' change since it may increase the forces transmitted to other parts of the structure.

Whilst capacity design is an important concept for seismic design in all aterials, it is included here because it is particularly relevant to reinforced concrete structures, hich can potentially exhibit brittle failure odes unless attention is paid to suppressing these odes in the design and detailing.

In the case of reinforced concrete elements the best way to dissipate energy is by flexural yielding, as shear and axial forces tend to induce brittle behaviour. herefore, the ductility of a structure can generally be optimised by enforcing flexural yielding at specific locations (ductile links), called plastic hinge zones, avoiding any type of shear or axial compressive failure (brittle links) and designing the rest of the structure to remain elastic throughout the development of the plastic hinges.

The approach adopted by EC8 to promote capacity design of reinforced concrete structures is to choose critical regions of the structure (the plastic hinge zones referred to above) that are designed to yield in flexure hen subject to the design earthquake loading, modified by the q factor appropriate to the structural system. These critical regions are then detailed to undergo large, inelastic cyclic deformations and fulfil the role of structural 'fuses', limiting the inertial loads that can be transferred to the remaining 'protected' parts of the structure, which can then be designed to normal EC2 provisions.

The capacity design rules in EC8 are discussed in more detail later but primarily cover:

• Derivation of shear forces in members from the flexural capacity of their critical regions.

• Promotion of the strong column/weak beam hierarchy in frame structures, evaluating column moments as a function of the capacity of the beams framing into them.

In both cases, in the design of notionally elastic parts of the structure, an allowance for overstrength of the critical regions is made, a greater allowance being ade for than for structures.

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