Design of buildings using EC8

Transcription

1 Design of buildings using EC8 Philosophy of seismic design (Performance requirements and compliance criteria) The next generation of seismic codes 1 Philosophy of seismic design The scope of EC8 EC8 applies to the design and construction of buildings and civil engineering works in seismic regions. Its purpose is to ensure, that in the event of earthquakes: human lives are protected damage is limited structures important for civil protection remain operational Special structures, such as nuclear power plants, off-shore structures and large dams, are beyond the scope of EC8 2 1

2 Philosophy of seismic design The scope of EC8 Is the target of EC8 to exclude any damage for any earthquake that could possibly struck the structures? NO!!! The cost of building structures that would resist any seismic action without damage if prohibitive The possibility that a very severe earthquake strucks a structure during its lifetime is generally small The uncertainties that slip into the earthquake design are so many that it is not realistic to exclude damage for any seismic action 3 Philosophy of seismic design The scope of EC8 Structures designed in accordance with the EC8 provisions should, in general, be able to: Resist a minor level of earthquake ground motion without damage Resist a moderate level of earthquake ground motion without structural damage, but possibly experience some nonstructural damage. Resist a major level of earthquake ground motion having an intensity equal to the strongest either experienced or forecast for the building site without collapse, but possibly with some structural as well as nonstructural damage. 4 2

3 Performance Requirements And Compliance Criteria Fundamental requirements of EC8 Structures in seismic regions shall be designed and constructed in such a way, that the following requirements are met, each with an adequate degree of reliability: No-collapse requirement for a severe earthquake, with a low probability of exceedance Damage limitation requirement for a minor to moderate earthquake, with a higher probability of exceedance - The design seismic action (using the EC8 design spectrum) corresponds to the no-collapse requirement 5 Performance Requirements And Compliance Criteria Fundamental requirements of EC8 No-collapse requirement: The structure shall be designed and constructed to withstand the design seismic action without local or global collapse, thus retaining its structural integrity and a residual load bearing capacity after the seismic events. The design seismic action is expressed in terms of: a) the reference seismic action associated with a reference probability of exceedance, P NCR, in 50 years or a reference return period, T NCR, and b) the importance factor γ I to take into account reliability differentiation. 6 3

4 Performance Requirements And Compliance Criteria Fundamental requirements of EC8 - The values to be ascribed to P NCR or to T NCR for use in a country may be found in its National Annex. The recommended values are P NCR =10% and T NCR = 475 years. This means that the No-collapse requirement corresponds to a seismic action with probability of exceedance 10% in 50 years - Reliability differentiation is implemented by classifying structures into different importance classes. To each importance class an importance factor γ I is assigned. This factor implicitly corresponds to a higher or lower value of the return period of the seismic event (with regard to the reference return period), as appropriate for the design of the specific category of structures. 7 Performance Requirements And Compliance Criteria Fundamental requirements of EC8 Damage limitation requirement: The structure shall be designed and constructed to withstand a seismic action having a larger probability of occurrence than the design seismic action, without the occurrence of damage and the associated limitations of use, the costs of which would be disproportionately high in comparison with the costs of the structure itself. The seismic action to be taken into account for the damage limitation requirement has a probability of exceedance, PDLR, in 10 years and a return period, T DLR. - The recommended values are P DLR =50% and T DLR = 95 years. - EC8 uses reduction factor v to take into account the lower return period of the seismic action associated with the damage limitation requirement (with a reference to the design seismic action). The recommended values are: ν=0,4 for importance classes I and II and ν=0,5 for importance classes III and IV. 8 4

5 The Next Generation Of Seismic Codes Current seismic codes Next generation codes Analysis type Elastic analysis Static or dynamic Non-linear analysis Static or dynamic Basis of design Forces Displacements Performance levels The trend for the next generation of seismic codes is to pass from the force-based approach of current seismic codes to a displacement or, even further, a performance based approach Several performance levels will be adopted to account for the different levels of seismic action Damage will be modeled using non-linear instead of elastic analysis 9 Performance-Based Design p1 P 50%/50 year 20%/50 year 10%/50 year 2%/50 year T 72 years 225 years 475 years 2475 years P : T : Probability of exceedance of the earthquake Mean return period (the average number of years between seismic events of similar severity) 10 5

6 Slide 10 p1 FEMA 389 chapter4 pg11 panagop; 21/4/2005

7 Performance-Based Design Building safety levels (FEMA) Immediate occupancy: The building receives a green tag (safe-to-occupy) rating from building officials immediately following an earthquake. Any repairs would be minor, and the facility can be reoccupied quickly. Life safety: The building remains stable, with reserve capacity, and the threat of hazardous debris is minimized. Overall risk of life-threatening injury is low. While it may be possible to repair the building, for economic reasons it may not be practical. Collapse prevention: The building barely remains standing and would be a total economic loss, although occupants would still be able to exit the building with some difficulty. 11 Performance-Based Design Detailed Target Building Performance Levels (FEMA) Level Operational Immediate Occupancy Overall Damage General Nonstructural components Life Safety Collapse Prevention Very Light Light Moderate Severe No permanent drift. Structure substantially retains original strength and stiffness. Minor cracking of facades, partitions, and ceilings as well as structural elements. All systems important to normal operation are functional Negligible damage occurs. Power and other utilities are available, possibly from standby sources No permanent drift. Structure substantially retains original strength and stiffness. Minor cracking of facades, partitions, and ceilings as well as structural elements. Elevators can be restarted. Fire protection operable Equipment and contents are generally secure, but may not operate due to mechanical failure or lack of utilities Some residual strength and stiffness left in all stories. Gravity-load bearing elements function. No out-of plane failure of walls or tipping of parapets. Some permanent drift. Damage to partitions. Building may be beyond economical repair Falling hazards mitigated but many architectural, mechanical, and electrical systems are damaged Little residual stiffness and strength, but load bearing columns and walls function. Large permanent drifts. Some exits blocked. Infills and unbraced parapets failed or at incipient failure. Building is near collapse Extensive damage 12 6

8 Compliance Criteria In order to satisfy the fundamental requirements the following limit states shall be checked Ultimate limit states are those associated with collapse or with other forms of structural failure which may endanger the safety of people. Damage limitation states are those associated with damage occurrence, corresponding to states beyond which specified service requirements are no longer met. In order to limit the uncertainties and to promote a good behaviour of structures under seismic actions more severe than the design one, a number of pertinent specific measures shall also be taken 13 Ultimate limit state The structural system shall be verified as having the resistance and energy dissipation capacity specified in the relevant Parts of EC8 (e.g. the q-factor) The structure as a whole shall be checked to be stable under the design seismic action. Both overturning and sliding stability shall be considered. It shall be verified that both the foundation elements and the foundation-soil are able to resist the action effects resulting from the response of the superstructure without substantial permanent deformations. It shall be verified that under the design seismic action the behaviour of nonstructural elements does not present risks to persons and does not have a detrimental effect on the response of the structural elements. 14 7

9 Damage limitation state An adequate degree of reliability against unacceptable damage shall be ensured by satisfying the deformation limits or other relevant limits defined in the relevant Parts of EN In structures important for civil protection the structural system shall be verified to possess sufficient resistance and stiffness to maintain the function of the vital services in the facilities for a seismic event associated with an appropriate return period. 15 Specific measures Design Structures should have simple and regular forms both in plan and elevation. If necessary this may be realised by subdividing the structure by joints into dynamically independent units. In order to ensure an overall dissipative and ductile behaviour, brittle failure or the premature formation of unstable mechanisms shall be avoided. Since the seismic performance of a structure is largely dependent on the behaviour of its critical regions or elements, the detailing of the structure in general and of these regions or elements in particular, shall be such as to maintain under cyclic conditions the capacity to transmit the necessary forces and to dissipate energy. 16 8