University of Naples Federico II School of Engineering. 2 years Master in Structural and Geotechnical Engineering

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1 University of Naples Federico II School of Engineering Department of Structures for Engineering and Architecture 2 years Master in Structural and Geotechnical Engineering SEISMIC ACTION ACCORDING TO EUROCODE 8 Earthquake Engineering gand Structural Control Course a.y Giorgio Serino, Full Professor of Structural Engineering ing. Ivana Marino, Ph.D. in Structural Engineering 1

2 PERFORMANCE REQUIREMENTS AND COMPLIANCE CRITERIA 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 Damage limitation requirement In order to satisfy the fundamental requirements, the following limit states shall be checked: Ulti t li it t t Ultimate limit states Damage limitation states

3 PERFORMANCE REQUIREMENTS AND DESIGN LEVEL EARTHQUAKES The performance criteria are intended to achieve two objectives: Protect human lives Minimize repair costs The first objective is achieved primarily by the provision of adequate strength and ductility. This ensures that a building is protected from full or partial collapse during large earthquakes that occur infrequently during design life of the structures. The second objective limits building damage during lesser, more frequently occurring earthquakes during design life of structures, in order to minimize economic losses including loss of building functionality.

4 PERFORMANCE OBJECTIVES (VISION 2000)

5 Performance Level Fully Functional Operational Life Safe Near Collapse DEFINITIONS OF STRUCTURAL PERFORMANCE Description No significant damages has occurred to structural and non structural components. Building is suitable for normal intended occupancy and use. No significant damage has occurred to structure, which retains nearly all of its pre earthquake strength and stiffness. Nonstructural components are secure and most would function, if utilities available. Building may be used for intended purpose, albeit in an impaired mode. Significant damage to structural elements, with substantial reduction in stiffness, however, margin remains against collapse. Nonstructural elements are secured but may not function. Occupancy may be prevented until repairs can be instituted. Substantial structural and nonstructural damage. Structural strenght and stiffness substantially degraded. Little margin against collapse. Some falling debris hazards may have occurred.

6 ACCEPTANCE CRITERIA FOR LIFE SAFETY AND COLLAPSE PREVENTION Perfomance Level Primary Component Second Component Life safety 75% of the deformation at which significant loss of lateral force resisting strength occurs Collapse 75% of the deformation at Prevention which h loss of vertical load bearing capacity occurs, but not more than the deformation at which significant loss of lateral force resisting strength occurs 100% of the deformation at which significant loss of lateral force resisting strength occurs 100% of the deformation at which h loss of vertical load bearing capacity occurs

7 NO COLLAPSE REQUIREMENT EFFECTS Casa dello Studente Hotel Duca degli Abruzzi Earthquake L Aquila 2009 Collapse of a reinforced concrete (R.C.) building

8 NO COLLAPSE REQUIREMENT EFFECTS Soft storey mechanism on a three storey R.C. RC building Ptti Pettino (AQ) Earthquake L Aquila 2009

9 NO COLLAPSE REQUIREMENT EFFECTS JOINT DAMAGES Earthquake L Aquila 2009

10 NO COLLAPSE REQUIREMENT EFFECTS Earthquake L Aquila 2009 COLUMN DAMAGES

11 MINIMIZE REPAIR COSTS EFFECTS Reinforced concrete building Infills damage: four storey R.C. building Earthquake L Aquila 2009 INFILL DAMAGES

12 GROUND CONDITIONS AND SEISMIC ACTION Appropriate investigations shall be carried out in order to identify the ground conditions. The construction site and the nature of the supporting ground should normally be free from risks of: ground rupture slope instability permanent settlements liquefaction in the event of an earthquake d h l f h d h Depending on the importance class of the structure and the particular conditions of the project, ground investigations and/or geological studies should be performed to determine the seismic action.

13 IDENTIFICATION OF GROUND TYPES The site should be classified according to the value of the average shear wave velocity, v s,30, if this is available. Otherwise the value of N SPT should be used. The average shear wave velocity v s,30 should be computed in accordance with the following expression: For sites with ground conditions matching either one of the two special ground types S1 or S2, special studies for the definition of the seismic action are required.

14 SEISMIC ZONATION National territories shall be subdivided by the National Authorities into seismic zones, depending on the local hazard. By definition, the hazard within each zone is assumed to beconstant. The reference peak ground acceleration on type A ground, a gr,foruseina country or parts of the country, may be derived d from zonation maps found in its National Annex. The reference peak ground acceleration, chosen by the National Authorities for each seismic zone, corresponds to the reference return period T NCR of the seismic action for the no collapse requirement (or equivalently the reference probability of exceedance in 50 years, P NCR ) chosen by the National Authorities. In cases of low seismicity, reduced or simplified seismic design procedures for certain types or categories of structures may be used. It is recommended to consider as low seismicity cases either those in which the design ground acceleration on type A ground, a g,isnotgreaterthan0,08g(0,78m/s 2 ), or those where the product a g S is not greater than 0,1 g (0,98 m/s 2 ).

15 SEISMIC ZONATION IN ITALY Interactive Sesmic Hazard Maps gis.mi.ingv.it/s1_en.php Seismic Hazard point to point 4/consultazione_005.html

16 THE ELASTIC RESPONSE SPECTRUM The earthquake motion at a given point on the surface is represented by an elastic ground acceleration response spectrum, henceforth called an elastic response spectrum. The shape of the elastic response spectrum is taken as being the same for the two levels of seismic action for the no collapse requirement (ultimate limit state design seismic action) and for the damage limitation requirement. The horizontal seismic action is described by two orthogonal components assumed as being independent and represented by the same response spectrum. For the three components of the seismic action, one or more alternative ti shapes of response spectra may be adopted, d dependingdi on the seismic sources and the earthquake magnitudes generated from them.

17 HORIZONTAL ELASTIC RESPONSE SPECTRUM

18 THE SHAPE OF THE SPECTRUM The recommended choice is the use of two types of spectra: Type 1 and Type 2. If the earthquakes that contribute most to the seismic hazard defined for the site for the purpose p of probabilistic hazard assessment have a surface wave magnitude, Ms, not greater than 5,5 it is recommended that the Type 2 spectrum is adopted. d Otherwise, the use of Type I spectra is recommended.

19 HORIZONTAL ELASTIC RESPONSE a G S ae (T)/a SPECTRUM (TYPE 1) Soil A Soil B Soil C Soil D Soil E T [seconds]

20 HORIZONTAL ELASTIC RESPONSE a G S ae (T)/a SPECTRUM (TYPE 2) Soil A Soil B Soil C Soil D Soil E T [seconds]

21 HORIZONTAL ELASTIC RESPONSE SPECTRUM The value of the damping correction factor η may be determined by the expression: where ξ is the viscous damping ratio of the structure, expressed as a percentage. The elastic displacement response spectrum, S De (T), shall be obtained using the following expression: This should normally be applied for vibration periods not exceeding 4,0 s.

22 VERTICAL ELASTIC RESPONSE SPECTRUM

23 (T)/a G VERTICAL ELASTIC RESPONSE 3 2 SPECTRUM Type 1 Type 2 S ve T [seconds]

24 DESIGN SG GROUND DISPLACEMENT The design ground displacement d g, corresponding to the design ground acceleration, can be estimated as: with a g, S, T C and T D as defined for the elastic horizontal response spectra.

25 DESIGN SPECTRUM FOR ELASTIC ANALYSIS The capacityof structural t systems to resist itseismic i actions in the non linear range generally permits their design for resistance to seismic forces smaller than those corresponding to a linear elastic response. To avoid explicit inelastic structural analysis in design, the capacity of the structure to dissipate energy, through mainly ductile behaviour of its elements and/orother mechanisms, istaken into account byperformingan elastic analysis based on a response spectrum reduced with respect to the elastic one, henceforth called a ''design spectrum''. This reduction is accomplished by introducing the behaviour factor q. The behaviour factor q is an approximation of the ratio of the seismic forces that the structure would experience if its response was completely elastic with 5% viscous damping, to the seismic forces that may be used in the design, with a conventional elastic analysis model, still ensuring a satisfactory response of the structure.

26 DESIGN SPECTRUM FOR ELASTIC ANALYSIS

27 THE BEHAVIOR FACTOR q Spectr ral Acc celerat tion (T T, ξ) F e F y Design Spectrum q = F F e y Elastic Spectrum Spectral Displacement

28 THE BEHAVIOR FACTOR q the overstrength thfactor αu/α1/

29 VERTICAL DESIGN SPECTRUM FOR ELASTIC ANALYSIS For the vertical component of the seismic i action the design spectrum is given by the design ground acceleration in the vertical direction, a vg replacing a g, S taken as being equal to 1,0. For the vertical component of the seismic action a behaviour factor q up to 1,5 should generally be adopted for all materials and structural systems. The adoption of values for q greater than 1,5 in the vertical direction should be justified through an appropriate analysis. The design spectrum as defined above is not sufficient for the design of structures with base isolation or energy dissipation systems.

30 ALTERNATIVE REPRESENTATIONS OF THE SEISMIC ACTION Artificial accelerograms Recorded or simulated accelerograms Spatial model of ground motion

31 CAPACITY DESIGN A ductile structure is designed using the Capacity Design approach. This involves the following three steps: Choose how the structure is to deflect in a seismic overload situation so that the structure is able to adsorb sufficient earthquake energy before it deflects to its limit; Provide a hierarchy of strenght between and within structural members to allow structural plastic hinge only in non critical members and so prevent brittle failure occuring anywhere; Detail structural areas that are intended to act as plastic hinges so they avoid severe damage and excessive loss of stiffness and strength.

32 DUCTILITY Ductility is a measure of how far a structure can safely displace horizontally after its first element has been overstressed to the extent its steel yields. The degree of ductility indicates the extent to which earthquake energy is absorbed by the structure. Those areas of structuresdesigned to absorb or dissipate energy by steel yielding arecalled plastic hinges.

33 CAPACITY DESIGN Brittle mechanism Ductile mechanism The only ductile and desirable overload mechanism occurs if the other is suppressed by making them stronger than the force to cause.

34 CAPACITY DESIGN Shear Fil Failure If theshearh strength thof thecolumn is less than the strength at which any of the other damage occur, a sudden brittle diagonal shear crack forms. The crack creates an inclined sliding plane which greatly reduces the ability of the column to support vertical loads. Ductile Damage Mechanism The bending moment at the column base causes cracks in the concrete and increas the tension stress in reinforcement steel until it begins to yield. The maximum horizontal force the column can sustain has been reached. The area at the base of column with its wide cracks and where the steel has been strained plastically is a plastic hinge zone.

35 THE PRINCIPLES OF THE CONCEPTUAL DESIGN The guiding principles governing the conceptual design are: AVOID UNNECESSARY MASS; structural simplicity; uniformity, symmetry and redundancy; bi directional resistance and stiffness; torsional resistance and stiffness; adequate foundation.

36 VERTICAL STRUCTURES The vertical structure required for seismic resistance is often different form that resisting gravity forces: Post and beam structures with pins top and bottom of the columns are completely unstable. Frames designed to resist gravity forces only, where joints between columns and beams are rigid enough to form moment frames, may be more stable against horizontal forces depending on the slenderness of the columns. As for load bearing walls, they are usually weak at their hi base with ihrespect to out of plane forces, overturning easily when loaded in that direction.

37 VERTICAL STRUCTURES The three basicseismic i i force resistingframes: iti Shear wall A shear wall resists horizontal forces acting in its plane. Providing seismic resistance it should be continuous from foundation to roof. Braced frames In their most basic form they consist of columns, beams and one or more diagonal bracing members per storey. Essentially, braced frames are vertical trussesand all the joints can be pinned. Moment frames Required rigid connectivity between beams and columns. Horizontal forces are resisted mainly by bending and shear forces in the beams and columns.

38 STRUCTURAL SIMPLICITY Ground floor plans of a four and an eight storey building showing the p g y g g structural footprints first for gravity forces only, and then for seismic forces where resisted by shear walls and moment frames

39 UNIFORMITY, SYMMETRY AND REDUNDANCY Ground floor plans showing different shear wall layout options that provide symmetrical resistance for x direction seismic forces (Y direction and gravity structure not drawn). This is because shear walls resist forces only parallel lto their lenghts. Of the four options suggested for a two wall layout, all are equally effective although the lesser lever arm of option (d) reduces resistance against any in plan torsion that may occur.

40 HORIZONTAL CONFIGURATIONS As the Europeanseismic code remind us: To extent possible, structures should have simple and regular forms both in plan and elevation. If necessary, this may be realized by subdividing the structure into dynamically independent units. Every buildings, no matter how symmetrically configured in plan, requires torsion resistance. The horizontal lever arm between each line of structure should be as large as possible to maximize both the torsion resisting strength and stiffness. When twisting occurs about the CoR, the shear walls acting in y direction deflect in opposite directions a small amount Δy. The value of these reactions multiplied by the lever arm between them represents a moment couple that partially resists the torsional moment.

41 TORSIONAL RESISTANCE AND STIFFNESS HORIZONTAL CONFIGURATION An example of a torsionally unbalanced system: The structural configuration is therefore twice as torsionally flexible. This structure might still be structurally adequate especially if the perimeter gravity only columns can sustain the horizontal movements without damage.

42 TORSIONAL RESISTANCE AND STIFFNESS HORIZONTAL CONFIGURATION Poor configuration of a typical corner building with shear walls as boundary walls Undesirable configuration caused by an eccentric structural core Eccentric core, but regular horizontal configuration Torsional eccentricity are avoided in the case of eccentric structural core by making the core non structural

43 UNIFORMITY, SYMMETRY AND REDUNDANCY Regularity in plan Regularity in elevation

44 IRREGULARITY IN PLAN RE ENTRANT CORNER A typical definition of an irregular re entrant configuration The dynamic response of a re entrant tconfiguration and potential floor diaphragm damage area Irregular plan configurations improved by seismic separation gaps

45 VERTICAL CONFIGURATIONS The best possible seismic performance is achieved where both 3 D massing and vertical structure of a building are regular. This means an absence of the following vertical irregularities generally observed after earthquakes to have initiated severe damage: a floor significantly heavier than an adjacent floor; vertical structure of one storey more flexible and/or weaker than that above it; short columns; discontinuous and off set structural walls; an abrupt change of floor plan dimension up the height of a building.

46 VERTICAL CONFIGURATIONS SOFT STOREY Soft storey configuration describes structure where one storey of a building is more flexible and/or weaker than the one above it from the perspective seismic force. Stiff and strong upper The columns in one Soft storey caused floors due to storey longer than by discontinuous masonry infills those above column

47 VERTICAL CONFIGURATIONS SOFT STOREY In the figure there are two example methods of avoiding a soft storey where one storey is higher than others. The first approach introduces beams without floor slabs by restoring the regularity of the moment frame. The second approach, when the idea of inserting beams is unattractive, considers a mega frame solution. An alternate storeys floor beams are pinned at thier ends to prevent participating as moment frame elements. The disavantadge is that the frame member sizes are considerably larger than normal.

48 VERTICAL CONFIGURATIONS SHORT COLUMNS There are two types of short column problems: where some columns are shorter than others in a moment frame; wherecolumns are so shortthey are inherentlybrittle. The short columns of the second group are usually normal length columns that are prevented from flexing and undergoing horizontal drift over most of their height by partialheight infill walls or very deep spandrel beams. Northridge, 1994

49 VERTICAL CONFIGURATIONS SHORT COLUMNS The stiffness of a column is extremely sensitive to its lenght and it is inversely proportional to the column lenght cubed (L 3 ). If two columns together, one that is half the height of the other, resist a horizontal force, the shorter column is eight times stiffer than the other and so it tries to resist almost eight times as much force as the longer column. It is unlikely to be strong enough to resist such a large proportion of the seismic force and may fail.

50 VERTICAL CONFIGURATIONS SETBACKS A setback is where a plan dimension of a storey above a certin level in a multi storey building reduces. Seismic codes categorize buildings with abrupt setbacks as irregular Typical setback configurations Different approaches to the Different approaches to the configuration of a tower and podium building

51 STRUCTURAL REGULARITY & IMPORTANCE FACTOR 51

52 METHODS OF ANALYSIS Lateral Force Method Modal Response Spectrum Analysis Non linear Static Pushover Analysis Non linear Dynamic Time history Analysis

53 LATERAL FORCE METHOD OF ANALYSIS This type of analysis may be applied to buildings whose response is not significantly affected by contributions from modes of vibration higher than the fundamental mode in each principal direction. a)they have fundamental periods of vibration ato T 1 in the two main directions which are smaller than the following values. b) they meet the criteria for regularity in elevation The above two requirements generally coincide with the The above two requirements generally coincide with the following:

54 THE FUNDAMENTAL PERIOD OF THE BUILDING

55 BASE SHEAR FORCE

56 DISTRIBUTION OF THE LATERAL FORCES The fundamental mode shapes in the horizontal directions of analysis of the building may be calculated using methods of structural dynamics or may be approximated by horizontal displacements increasing linearly along the height of the building. The seismic action effects shall be determined by applying, to the two planar models, dl horizontal lforces Fi to all storeys. The horizontal forces Fi determined in accordance with this clause shall be distributed to the lateral load resisting system assuming the floors are rigid in their plane.

57 TORSIONAL EFFECTS If the lt laterall stiffness and mass are symmetrically ti distributed ib t d in plan and unless the accidental eccentricity is taken into account by a more exact method the accidental torsional effects may be accounted for by multiplying the action effects in the individual load resisting elements resulting from the application of afactorδ given by:

58 MODAL RESPONSE SPECTRUM ANALYSIS This type of analysis shall be applied to buildings which do not satisfy the conditions for applying li the lateral lt lforce method of analysis. The response of all modes of vibration contributing significantly to the global response shall be taken into account. the sum of the effective modal masses for the modes taken into account amounts to at least 90% of the total mass of the structure; all modes with effective modal masses greater than 5% of the total mass are taken into account. When using a spatial model, the above conditions should be verified for each relevant direction.

59 COMBINATION OF MODAL RESPONSES The response in two vibration modes i and j (including both translational and torsional modes) may be taken as independent of each other, if their periods Ti and Tj satisfy (with Tj Ti) the following condition: Whenever all relevant modal responses can be regarded as independent of each other, the maximum m value EE of a seismic action effect (Square Root of Sum of Squares or SRSS) may be taken as:

60 NONLINEAR PUSHOVER ANALYSIS The mathematical model used for elastic analysis shallbe extended to include the strength of structural elements and their post elastic behavior. Pushover analysis is a non linear static analysis carried out under conditions of constant gravityloads and monotonically increasing horizontal loads. to verify or revise the over strength ratio values αu/α1/ 1 to estimate the expected plastic mechanisms and the distribution of damage; g; to assess the structural performance of existing or retrofitted buildings; as an alternative e to the design based on linear elastic elastic analysis which uses the behavior factor q. Buildings not conforming to the regularity criteria shall be analyzed using a spatial model.

61 LATERAL LOAD PATTERNS At least two vertical distributions of the lateral loads should be applied: a uniform pattern, based on lateral forces that are proportional to mass regardless of elevation (uniform response acceleration); a modal pattern, proportional to lateral forces consistent withthe the lateralforce distribution in the direction under consideration determined in elastic analysis. Lateral loads shall be applied at the location of the masses in the model. Accidental eccentricity shall be taken intoaccount account.

62 CAPACITY CURVE The relation lti bt between base shear force and the control displacement (the capacity curve ) should be determined by pushover analysis for values of the control displacement ranging between zero and the value corresponding to 150% of the target displacement. Thecontroldisplacementmaybetakenatthecentreof mass of the roof of the building. The top of a penthouse should not be considered asthe roof.

63 PERFORMANCE POINT By correlating the capacity curve to the seismic demand generated by any specific earthquake or ground shaking intensity, a performance point is found on the capacity curve. The performance point estimates t the maximum displacement of the building caused by the specific earthquake.