Performance requirements for new designs In Eurocode and associated seismic hazard levels

As a European standard (EN), Part 1 of Eurocode 8 provides for a two-level seismic design Clause 2.1(1) with the following explicit performance objectives:

• No-(local-)collapse: protection of life under a rare seismic action, through prevention of collapse of the structure or its parts and retention of structural integrity and residual load capacity after the event. This implies that the structure is significantly damaged, and may have moderate permanent drifts, but retains its full vertical load-bearing capacity and sufficient residual lateral strength and stiffness to protect life even during strong aftershocks. However, its repair may be uneconomic.

• Damage limitation: reduction of property loss, through limitation of structural and non-structural damage in frequent earthquakes. The structure itself has no permanent drifts; its elements have no permanent deformations, retain fully their strength and stiffness, and do not need repair. Non-structural elements may suffer some damage, which can be easily and economically repaired at a later stage.

The no-(local-)collapse performance level is achieved by dimensioning and detailing structural elements for a combination of strength and ductility that provides a safety factor between 1.5 and 2 against substantial loss of lateral load resistance. The damage limitation performance level is achieved by limiting the overall deformations (lateral displacements) of the system to levels acceptable for the integrity of all its parts (including non-structural ones).

The two explicit performance levels - (local) collapse prevention and damage limitation -are pursued under two different seismic actions. The seismic action under which (local) collapse should be prevented is termed the design seismic action, whilst the one under which damage limitation is pursued is often termed the serviceability seismic action. Within the philosophy of national competence on issues of safety and economy, the hazard levels for these two seismic actions are left for national determination. For structures of ordinary importance the recommendation in EN 1998-1 is for:

8 a design seismic action (for local collapse prevention) with 10% exceedance probability in 50 years (mean return period: 475 years)

• a serviceability seismic action (for damage limitation) with 10% exceedance probability in 10 years (mean return period: 95 years).

The design seismic action for structures of ordinary importance is the reference seismic action; its mean return period is termed the reference return period, and denoted by rNCR. The ratio, v, of the serviceability seismic action (for damage limitation) to the design seismic action (for local collapse prevention) reflects the difference in hazard levels, and is a nationally determined parameter (NDP). Clauses 2.1(2), Enhanced performance of essential- or high-occupancy facilities is achieved not by 2.1 (3), 2.1 (4), upgrading the performance level, as often specified in US codes, but by modifying the hazard 4.2.5(1), level (the mean return period) for which local-collapse prevention or damage limitation is

4.2.5(2), pursued. For essential- or high-occupancy structures the seismic action should be increased,

4.2.5(3), by multiplying the reference seismic action by an importance factor, 7t. By definition, 7, = 1.0

4.2.5(4), for structures of ordinary importance (i.e. for the reference return period of the seismic

For buildings, the recommended value of the NDP importance factor 7, is 1.2, if collapse of the building may have unusually severe social or economic consequences (high-occupancy buildings, such as schools, or public assembly halls, facilities housing institutions of cultural importance, such as museums, etc.). These are termed buildings of Importance Class III. Buildings which are essential for civil protection in the immediate post-earthquake period, such as hospitals, fire or police stations and power plants, belong in Importance Class IV; the recommended value of the NDP importance factor for them is 7, = 1.4. A value of 7, equal to 0.8 is recommended for buildings of minor importance for public safety (Importance Class I: agricultural buildings, etc.). All other buildings are considered to be of ordinary importance, and are classified as Importance Class II. Clauses For buildings of ordinary or lower importance (Importance Classes I and II) a value of 0.5, is recommended for the ratio v of the serviceability seismic action (for damage limitation) to

2.2.3(2) the design seismic action (for local collapse prevention). For buildings of importance above ordinary (Importance Classes III and IV) a value of 0.4 is recommended for v. This gives about the same level of property protection for ordinary and high-occupancy buildings (Importance Classes II and III), 15-20% less property protection for buildings of low importance and 15% higher protection for essential facilities. This additional margin may allow help facilities important for civil protection to maintain a minimum level of operation of vital services during or immediately after a frequent event. Clause 2.1(4) Despite the fact that EN 1998-1 recommends specific values for the NDPs - the importance factor of structures of other than ordinary importance, jI} and the ratio of the serviceability seismic action to the design seismic action, v - the nationally or regionally used values should reflect, in addition to national choice regarding the levels of safety and protection of property, also the regional seismo-tectonic environment. Eurocode 8 gives in a note the approach that may be used to determine the ratio of the seismic action at two different hazard levels. More specifically, the usual approximation of the annual rate of exceedance, H(ag), of the peak ground acceleration ag as H(ag) ~ k0a~k is mentioned, with the value of the exponent k depending on seismicity, but being generally of the order of 3. Then, the Poisson assumption for earthquake occurrence gives a value of ~(TL/TLR)1/k for the value by which the reference seismic action needs to be multiplied to achieve the same probability of exceedance in TL years as in the 7', K years for which the reference seismic action is defined (here, the index L denotes 'lifetime'). This value is the importance factor 7I; or the conversion factor to the serviceability seismic action, v. Alternatively, the value of the multiplicative factor, or v, to be applied on the reference seismic action in order to achieve a value of the probability of exceedance of the seismic action, PL, in Th years other than the reference probability PLR, over the same TL years, may be estimated as ~(PLiJP[)[ik. For Importance Classes III and IV, TLR < T, andPLR > PL: then 7, > 1. For Importance Class I and for the serviceability seismic action, ThR > TL and PU{ < PL: then the importance factor 7j of low-importance facilities and the factor v result in values of less than 1. It is noted that the combination of 0.4 and 0.5 for the values recommended for the ratio v of a serviceability seismic action with a recommended mean return period of 95 years to the design seismic action with a recommended mean return period of 475 years is consistent with a value of the exponent k for the decay of the annual rate of exceedance of the peak ground acceleration, H(ag), with a value of k around 2.

Although not explicitly stated, an additional performance objective in buildings designed for energy dissipation is prevention of global collapse during a very strong and rare earthquake (with a mean return period in the order of 2000 years). Although structural elements can still carry their tributary gravity loads after such an event, the structure may be heavily damaged, have large permanent drifts, retain little residual lateral strength or stiffness and may collapse after a strong aftershock. Moreover, its repair may be unfeasible or economically prohibitive. This implicit performance objective is pursued through systematic and across-the-board application of the capacity design concept, which allows full control of the inelastic response mechanism.

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