Prof. A. Meher Prasad. Department of Civil Engineering Indian Institute of Technology Madras

Transcription

1 Prof. A. Meher Prasad Department of Civil Engineering Indian Institute of Technology Madras

2 Dynamic - Loads change with time Nonlinear - Loaded beyond Elastic Limit Type Usual Name Dynamic Effects Material Nonlinearity Linear Static Equivalent Static No No Linear Dynamic Response Spectrum Yes No Nonlinear Static Nonlinear Dynamic Pushover Analysis No Yes Time History Yes Yes

3 Overview What is pushover analysis? What are its fundamental techniques? What tools can be used? Common pitfalls in pushover analysis Example of pushover analysis application

4 Why Push-Over Analysis? Static Nonlinear Analysis technique, also known as sequential yield analysis, or simply "push-over" analysis has gained significant importance during the past few years. It is one of the three analysis techniques recommended by FEMA 273/274 and a main component of the Capacity Spectrum Method (ATC-40). Proper application can provide valuable insights into the expected performance of structural systems and components Misuse can lead to an erroneous understanding of the performance characteristics.

5 What is Push-Over Analysis? Push-over analysis is a technique by which a computer model of the building is subjected to a lateral load of a certain shape (i.e., parabolic, inverted triangular or uniform). The intensity of the lateral load is slowly increased and the sequence of cracks, yielding, plastic hinge formations, and failure of various structural components is recorded. Push-over analysis can provide a significant insight into the weak links in seismic performance of a structure.

6 What is Push-Over Analysis? A series of iterations are usually required during which, the structural deficiencies observed in one iteration, are rectified and followed by another. This iterative analysis and design process continues until the design satisfies a pre-established performance criteria. The performance criteria for push-over analysis is generally established as the desired state of the building given a roof-top or spectral displacement amplitude.

7 Objectives of Push-Over Analysis To obtain the maximum shear strength of the structure, V b, and the mechanism of collapse. To evaluate if the structure can achieve the collapse mechanism without exhausting the plastic rotation capacity of the members. To obtain the monotonic displacement and global ductility capacity of the structure. To estimate the concentration of damage and IDI (Interstorey Drift Index) that can be expected during the nonlinear seismic response.

8 Push-over Curve or Capacity Spectrum V/W (Acceleration) Using simple modal analysis equations spectral displacement and roof-top displacement may be converted to each other. High-Strength; High-Stiffness; Brittle Moderate Strength and Stiffness; Ductile Low-Strength; Low-Stiffness; Brittle Roof-top Displacement

9 Design Spectra Representation Ordinary Design Push-Over Analysis - Composite or ADRS Plot V/W (Acceleration) V/W (Acceleration) Constant Period Lines Period DESIGN SPECTRUM Spectral or Roof-top Displacement ELASTIC DEMAND SPECTRUM

10 What Tools Can Be Used? Nonlinear Analysis software with built-in push-over analysis capabilities DRAIN Spread Plasticity IDARC Spread and Point Plasticity SAP2000NL Point Plasticity ETABS Point Plasticity ANSYS Spread Plasticity SAVE Point Plasticity (Public version) Spread Plasticity (Research version) Sequential application of linear analysis software

11 Spread and Point Plasticity 1. Nonlinearity is assumed to be distributed along the length of the plastic hinge. 2. It provides a more accurate representation of the actual non-linear behaviour of the element Length of plastic hinge 1. Plasticity is assumed to be concentrated at the critical locations. In addition to usual moment hinges, there can be axial hinges and shear hinges. 2. Plastification of the section is assumed to occur suddenly, and not gradually or fibre-by-fibre. Plastic Hinge Curvature diagram along the length of the member

12 Establishing the Performance Point No building can be pushed to infinity without failure. Performance point is where the Seismic Capacity and the Seismic Demand curves meet. If the performance point exists and damage state at that point is acceptable, we have a building that satisfies the push-over criterion.

13 ATC-40 Method V/W (Acceleration) T 0 This is an iterative procedure involving several analyses. For each analysis an effective period for an equivalent elastic system and a corresponding elastic displacement are calculated. This displacement is then divided by a damping factor to obtain an estimate of real displacement at that step of analysis. T eff e /B 5% damped elastic spectrum δ e Roof-top Displacement β eff SR SR A V = κβ ln( βeff ) = ln( βeff ) = 1.65

14 ATC-40 Nonlinear Static Procedure 1. Develop the Pushover Curve

15 ATC-40 Nonlinear Static Procedure 2. Convert Pushover Curve to capacity diagram

16 ATC-40 Nonlinear Static Procedure 3. Plot elastic design spectrum in A-D A D format

17 ATC-40 Nonlinear Static Procedure 4. Plot the demand diagram and capacity diagram together Intersection point gives the displacement demand Avoids nonlinear RHA; instead analyse equivalent linear systems

18 ATC-40 Nonlinear Static Procedure 5. Convert displacement demand to roof displacement and component deformation. 6. Compare to limiting values for specified performance goals.

19 Points to be taken care.. 1. Do not underestimate the importance of the loading or displacement shape function. 2. Know your performance objectives before you push the building. 3. If it is not designed, it cannot be pushed. 4. Do not ignore gravity loads. 5. Do not push beyond failure unless otherwise you can model failure. 6. Pay attention to rebar development and lap lengths. 7. Do not ignore shear failure mechanisms 8. P-Delta P effects may be more important than you think. 9. Do not confuse the Push-over with the real earthquake loading. 10. Three-dimensional buildings may require more than a planar push.

20 1. Do not underestimate the importance of the loading shape function. The loading or deformation shape function is selected to represent the predominant dynamic mode shape of the building. It is most common to keep the load shape constant during the push. Loading shape importance increases for tall buildings whose earthquake response is not dominated by a single mode shape. For these buildings, a loading shape function based on the first mode shape may seriously underestimate the seismic demand on the intermediate floor levels.

21 1. Do not underestimate the importance of the loading shape function. Vb/W /H(%) Inverted Triangle Uniform parabola

22 Adapting Load Patterns So called higher mode effects as the load distribution changes Limit base moment increases adapts for maximum shear force Limit base shear increases adapts for maximum bending moment Not apparent from linear analysis

23 2. Know your performance objectives before you push the building. No building can be displaced to infinity without damage. It is of paramount importance to understand the specific performance objectives desired for the building. Performance objectives such as collapse prevention, life safety, or immediate occupancy have to be translated into technical terms such as: (a) a given set of design spectra, and (b) specific limit states acceptable for various structural components A push-over analysis without a clearly defined performance objectives is of little use.

24 BUILDING PERFORMANCE LEVELS Structural Performance Levels and Ranges Nonstructural Performance Levels S-1 Immediate Occupancy S-2 Damage Control S-3 Life Safety S-4 Limited Safety S-5 Collapse Prevention S-6 Not Considered N-A Operational 1-A Operational 2-A NR NR NR NR N-B Immediate Occupancy 1-B Immediate Occupancy 2-B 3-B NR NR NR N-C Life Safety 1-C 2-C 3-C Life Safety 4-C 5-C 6-C N-D Hazards Reduced NR 2-D 3-D 4-D 5-D 6-D N-E Not Considered NR NR 3-E 4-E 5-E Collapse Prevention No rehabilitation Ref: FEMA 356

25 Earthquake Levels (FEMA356) Earthquake levels p t N Approximate N Remarks years years years Serviceability earthquake % Frequent Serviceability earthquake % Occasional Design basis earthquake (DBE) 10% Rare Maximum considered (MCE) earthquake -1 (alternate) 5% % Very rare Maximum considered (MCE) earthquake -2 (alternate) 2% % Extremely rare

26 Performance Objectives (FEMA 356) Earthquake levels Probability of Exceedance in a period Opera- tional Target building performance level Immediate Occupancy Life Safety Collapse Prevention Serviceability earthquake % in 50 years a b c d Serviceability earthquake % in 50 years e f g h Design basis earthquake (DBE) 10% in 50 years i j k l Maximum considered Earthquake (MCE) 2% in 50 years m n Basic Safety Objective o p

27 3. If it is not designed, it cannot be pushed. E, I, and A are not sufficient. Push-over characteristics are strong functions of force-displacement characteristics of individual members and their connections. If detailed characteristics are not known, the pushover analysis will be an exercise in futility.

28 4. Do not ignore gravity loads. Inclusion or exclusion of the gravity loads can have a pronounced effect on the shape of the push-over curve and the member yielding and failure sequence. Example: Due to the unsymmetric distribution of + and - reinforcements in R/C beams, gravity load delays the onset of yielding and cracking in the beams, resulting in a stiffer structure at lower magnitudes of base shear. The ultimate capacity of the structure, is usually reduced with increasing gravity load.

29 5. Do not push beyond failure unless otherwise you can model failure Ultimate Capacity Lateral Force Actual Modeled with failures ignored Force or Moment Displacement Displacement or Curvature

30 6. Pay attention to rebar development and lap lengths. For R/C members of existing structures, it is very important to note the development lengths when calculating member capacities. If inadequate development lengths are present, as they are in most of the older buildings, the contributing steel area should be reduced to account for this inadequacy. Failure to do so will result in overestimating the actual capacity of the members and results in an inaccurate push-over curve.

31 Joint Detailing Such reinforcement detailing should not be used

32 7. Do not ignore shear failure mechanisms If the shear capacity of structural members is not sufficient to permit the formation of flexural plastic hinges, shear failure will precede the formation of plastic hinges at the end of the member. In R/C members, even if the shear capacity is sufficient, but lateral reinforcement is not spaced close enough at the plastic hinge zones, the concrete may crush in the absence of sufficient confinement. If this happens, the plastic capacity is suddenly dropped to what can be provided by the longitudinal steel alone.

33 Shear Failure

34 Short Column Failure This failure can be avoided by providing special confining reinforcement over entire column length

35 8. P- effects may be more important than you think. The P- effects become increasingly significant with larger lateral displacements and larger axial column forces. Strong column - weak beam design strategy commonly deals with the moment capacity of columns in the undeformed state. In a substantially deformed state, the moment capacity of columns may be sufficiently reduced to counteract the strong column - weak beam behaviour envisioned by the design. Cases of plastic hinge formations during a push-over analysis in columns "designed" to be stronger than the beams are not rare.

36 9. Do not confuse the Push-over with the real earthquake loading. The push-over load is monotonically increased The earthquake generated forces continually change in amplitude and direction during the duration of earthquake ground motion. Push-over loads and structural response are in phase Earthquake excitations and building response are not necessarily in phase. This is particularly true for near-fault ground motions which tend to concentrate the damage on the lower floors, an effect which is difficult to model by the push-over loads.

37 9. Do not confuse the Push-over with the real earthquake loading Vb/W IDARC SAP 0.16g 0.25g 0.3g 0.35g /H

38 10. Three-dimensional buildings may require more than a planar push. For building with strong asymmetry in plan, or with numerous non-orthogonal elements, a planar (two dimensional) push-over analysis may not suffice. For such cases a 3D model of the building must be constructed and subjected to push-over analysis. Three dimensional buildings may be pushed in the principal directions independently, or pushed simultaneously in orthogonal directions.

39 Analysis Procedure SAP2000 NL

40 Pushover Analysis Procedure Create 3D Model Assign end offsets Gravity Pushover (Force controlled) DL+0.25LL Lateral Pushover (Displacement controlled) Define Load case (Lateral Load at centre of mass) Define Hinge properties Run Static analysis Assign Hinge properties Beams Default M3 Columns Default PMM Run static pushover analysis Define Static Pushover Cases Establish Performance point

41 Material Properties Concrete Properties Cube compressive strength, f ck Modulus of Elasticity of concrete ( E = 5000 f ) c ck Reinforcing Steel Properties Yield strength of steel Modulus of Elasticity of steel E Modulus of Elasticity of steel E s

42 Modification Factors Factors to estimate the expected strength 1.5 times the Concrete compressive strength (f( ck ) Steel yield stress (f( y ) (Factor of 1.25 used for capacity estimation considering strain hardening of steel) Knowledge Factors, m k No Description of available information m k 1 Original construction documents, including material testing report 2 Documentation as in (1) but no material testing undertaken Documentation as in (2) and minor deteriorations of original condition 4 Incomplete but usable original construction documents Documentation as in (4) and limited inspection and material test results with large variation. 6 Little knowledge about the details of components

43 Material Properties Frame Elements Infill (struts)

44 Modeling of Structural elements Beams and columns 3D Frame elements Slab Flat slabs Diaphragm action (ignore the out of plane stiffness) Plate elements Beam column joints End offsets (Rigid zone factor 1) Asymmetric Structures Centre of mass (add non structural mass to corresponding beams) Centre of stiffness Inclusion of appendages Include water tanks, cantilever slabs

45 Modeling of Structural elements Stairway slabs Equivalent frame elements Shear Walls Wide Column Elements Infill walls Equivalent strut method Foundation Isolated footings Single pile Multiple piles Plinth beams Hinged at the bottom of foundation Fixed at five times the diameter of pile Fixity of columns at top of pile cap Frame elements

46 Modeling of Beams and Columns 3D Frame Elements Cross Sectional dimensions, reinforcement details, material type Effective moment of inertia Beams Rectangular 0.5 I g T-Beam 0.7 I g L-Beam 0.6 I g Columns 0.7 I g

47 Modeling of Beams

48 Modeling of Columns

49 Modeling of Beam Column Joints End offsets (Rigid zone factor 1)

50 Modeling of Slab

51 Modeling of Infill Equivalent Strut Approach Step 1. Equivalent Strut Properties Smith and Carter Model a) Strength of infill, P b) Initial modulus of elasticity of infill, E i c) Equivalent strut width (when force in the strut = R), w d) Thickness of infill, t Step 2. Stress Strain Values Stress = P/AE i,where A = wt Strain = P/A

52 Modeling of Shear Wall (Lift Core ) Type I Model - Single Lift Core Column Equivalent Wide Column Elements connected to the frame through rigid links y BEAM x MASTER NODE t Beam elements with rigid ends L

53 Modeling of Shear Wall (Lift Core ) Type II Model - Single Lift Core Column The lift core can be treated as a single column with master node defined at the centroid and the beams connected by rigid links BEAM y x CORE MASTER NODE SLAVE NODE

54 Modeling of Shear Wall (Lift Core Column Properties) For axial and torsional rigidity, the full cross-sectional sectional area should be used y BEAM x CORE MASTER NODE FOR A, J SLAVE NODE

55 Modeling of Shear Wall (Lift Core Column Properties) For shear along y axis and bending about x-axis x (ground motion along y-axis), y the walls in the direction of ground motion should be considered as two parallel elements y BEAM x CORE MASTER NODE FOR A y, I xx SLAVE NODE

56 Modeling of Shear Wall (Lift Core Column Properties) For shear along x axis and bending about y-axis y (ground motion along x-axis), x the walls in the direction of ground motion should be considered as three parallel elements y BEAM x CORE MASTER NODE FOR A x, I yy SLAVE NODE

57 Beam Hinge Properties - Flexural hinge (M 3 )

58 Hinge Properties for Beams Lateral Load 1.0 c A b a C B D E y Lateral Deformation Generalized Load Deformation Relations * ATC 40 Volume 1

59 Beam Hinge Properties - Shear hinge

60 Beam Hinge Properties - Shear hinge Shear capacity V = f A sy y sv d 0.6s Total Shear Capacity, V y = V c + V sy v = 0 Shear strength (V) V y 0.2 V y V u = 1.05V y Residual Shear Strength y m =15 y 1.5 y Shear deformation ( ) Refer Clause of IS13920

61 Column Hinge Properties- Flexural hinge (PM 2 M 3 )

62 Hinge Properties for Columns * ATC 40 Volume 1 Lateral Load 1.0 c A b a C B D E y Lateral Deformation

63 Column Hinge Properties- Shear hinge

64 Column Hinge Properties- Shear hinge Shear capacity τ c = where 0.8 f ( 1+ 5β 1) ck 6 β fckbd β = A 3Pu δ = A f V c g ck = δτ bd V = f A c sy y sv st d 0.6s Total Shear Capacity, V y = V c + V sy v Note: For moderate and high ductility of the column section δ = 3 P u A f g c k is taken in calculation (ATC 40) 0.5

65 Column Hinge Properties- Shear hinge Yield deformation ( y ) is to be calculated using the following formula. y = Yield shear strength Shear stiffness = R GA l eff = G R l 0.75 A g Where G = Shear modulus of the reinforced concrete section A g = Gross area of the section l = Length of member

66 Column Hinge Properties- Shear hinge The ultimate shear strength (V u ) is taken as 5% more than yield shear strength (V y ) and residual shear strength is taken as 20% of the yield shear strength for modelling of the shear hinges as shown in Figure. Shear strength (V) V y V u = 1.05V y 0.2 V y y m=15 y 1.5 Shear deformation ( ) y Residual Shear Strength Similarly maximum shear deformation is taken as 15 times the yield deformation. The values were taken as per SAP 2000 manual recommendations.

67 Infill Properties - Axial hinge (P)

68 Static Pushover Case Data (Gravity Pushover Force Controlled)

69 Lateral Load Pattern Determination of the Load pattern: (IS 1893 (part 1) : 2002 ) Fundamental natural period T a = d h Q3 Design Base Shear V B = A h W Q2 Design Lateral Force Q 2 Wihi i = VB 2 Wjhj Q1

70 Static Pushover Case Data (Lateral pushover Displacement controlled)

71 Seismic Evaluation of a Typical RC Building

72 Building Data Building frame system RC OMRF Usage Residential Built in 1999 Zone V Number of stories Footing Symmetry Material used Plan dimensions G+4 Multiple Piles About Y-axis M15 & Fe m X 13.95m Building height 15.7m Soil Type (assumed) Type-II (Medium)

73 Plan- Beam Locations n Storey number Beams (only in 1 to 4 floor)

74 Plan - Column and Equivalent Strut Locations Infill wall Location Storey number n

75 Comments Visual inspection did not reveal concrete deterioration. Knowledge factor was not applied. Architectural drawings were not available. Location of infill walls w was postulated. Geotechnical data was not available. Rebar detailing was not complete in the available structural drawings. Building considered to be noncompliant with IS 13920: 1993 (R = 3). Fixity considered at pile cap. Soil-structure interaction neglected. Elevator walls not considered as lateral load resisting elements.

76 Plan Frames along X-directionX

77 Plan Frames along Y-directionY

78 Elevation along line A-AA

79 Typical Beam Section (Ground Floor)

80 Typical Column Sections (Ground Floor) Tie spacing 100 mm c/c near beam-to-column joints

81 Detailed Structural Analysis Gravity Load Analysis Lateral Load Analysis Linear static analysis (Equivalent Static Method, IS 1893 (Part 1): 2002) Response Spectrum Method (IS 1893 (Part 1): 2002) Non-linear Static Analysis (Pushover Analysis, ATC 40)

82 Structural Parameters Floor Seismic Weight (kn) Lumped Mass (Ton) Center of Mass (m) X- direction Y- direction Center of rigidity (m) X- direction Y- direction Static Eccentricity, e si ( m) X- direction Y- direction Design Eccentricity, e di (m) X- direction Y- direction e = 1.5e b di si i e = e 0.05b di si i

83 Location of Centre of Mass

84 Calculation of Base Shear IS 1893(Part 1):2002 Base shear, V B = A h W A h ZI S 2R g a = A h = 0.15 W = Total seismic weight of the building Z = 0.36 (for Zone V) I = 1 (for normal building) R = 3 (for OMRF) S a /g = 2.5 corresponding to both the time period in with-infill case. V B = kn = 3039 kn

85 Comparison of Base Shear Analysis methods Without infill stiffness With infill stiffness V x (kn) V y (kn) V x (kn) V y (kn) Equivalent Static Method EQX EQY Response Spectrum Analysis EQ

86 Comparison of Fundamental Periods Empirical Formulae Computational Model With infill stiffness Without infill stiffness With infill stiffness Without infill stiffness Time Period (s) T ax = 0.28 T ay = S a /g

87 First five modes and their participation Without infill With infill Mode T (s) Mass Participation (%) T (s) Mass Participation (%) U X U y U X U y

88 Mode Shapes First Mode T=0.83s (U X =92.91%) Second Mode T=0.76s (U Y =90.51%)

89 Mode Shapes Third Mode T=0.39s (RZ) (U X =0.11% U Y =0.52%) Fourth Mode T=0.25s (U X =5.39% U Y =0.04%)

90 Mode Shapes Fifth Mode T=0.24s (U X =0.03% U Y =7.07%)

91 Demand and Capacity for Columns - Moment (Equivalent static method) Section Absolute Capacities Absolute Demand (Without Infill stiffness) Absolute Demand (With Infill stiffness) x z y Pu X Y θ ey ex X Puz Pu P (kn) M2 (knm) M3 (knm) P (kn) M2 (knm) M3 (knm) DCR P (kn) M2 (knm) M3 (knm) DCR Mux1 PuR θ MuR,x load contour A Muy1 MuR,y Y 0 Muy = Pu ey 1C Mux = Pu ex M = M + M 2 2 ur ux uy 1C C C C C

92 Demand and Capacity for Columns Shear (Equivalent Static Method) Sections Absolute Capacities V u (kn) Absolute Demand (With infill stiffness) V d (kn) DCR Absolute Demand (Without infill stiffness) V d (kn) DCR 1C C C C C C V u is higher of the shear from analysis and the shear corresponding to the flexural capacity M u (V u = M u / L s )

93 Maximum displacement response in X-direction (Equivalent Static Method) Storey Level 3 2 Storey Level Displacement (mm) Displacement (mm) With Infill Without Infill

94 Inter-storey Drift in X-direction X Equivalent Static Method Storey level (m) Storey level (m) Inter-storey drift ( X 10-2 %) Inter-storey drift ( X 10-2 %) With infill Without infill

95 Performance Objective 1. Design Basis Earthquake + Life Safety (2% total drift) 2. Maximum Considered Earthquake + Collapse Prevention (4% total drift)

96 Distribution of Lateral Force at each Storey Level for Lateral Pushover Q 5 = Q 4 = Q 3 = Q 2 = Q 1 =

97 Moment Rotation Curve for a Typical Element Hinge Property Moment/SF C B IO LS CP D E A Rotation/SF B Yield state IO Immediate Occupancy LS Life Safety CP Collapse Prevention C Ultimate state

98 Demand Spectrum Seismic Coefficient, C A Soil Zone II Zone III Zone IV Zone V (0.10) (0.16) (0.24) (0.36) Type I Type II Type III Seismic Coefficient, C V Type I Type II Type III

99 Base Shear Vs. Roof Displacement Push X V B µ = 2.41 /h = 0.49% Base Shear (kn) µ = 1.46 /h = 0.34% Roof Displacement (m) Without infill stiffness With infill stiffness

100 Base Shear Vs. Roof Displacement Push Y V B 3000 Base Shear (kn) Roof Displacement (m) Without infill stiffness With infill stiffness

101 Capacity and Demand Spectra (With infill stiffness) Spectral Accelaration Coefficient (Sa/g) Spectral Accelaration Coefficient (Sa/g) Spectral Displacement (m) Spectral Displacement (m) Lateral Push along X Lateral Push along Y

102 Capacity and Demand Spectra (Without infill stiffness) Spectral Accelaration Coefficient (Sa/g) Spectral Accelaration Coefficient (Sa/g) Spectral Displacement (m) Spectral Displacement (m) Lateral Push along X Lateral Push along Y

103 Retrofitting Scheme 1. Continuing infill walls only at a few locations. 2. Strengthening of the ground floor columns. Ground Floor Plan

104 Capacity Curve Push X C D A B /h=0.48% /h=0.75% Base Shear (kn) /h=0.28% V B /h = 1 % Roof Displacement (m)

105 State of the Hinge at A and B in Lateral load A, /h=0.28% B, /h=0.48%

106 State of the Hinge at C and D in Lateral load C, /h=0.75% D, /h=1%

107 Performance Point ( Demand spectrum- Z ) Spectral Accelaration Coefficient (Sa/g) % 15% 17.3% Demand Spectrum Capacity Spectrum Effective Period Performance Point Spectral Displacement (m) T eff = 1.224s β eff = 24.9% V = 7682 kn D = m = 0.93% of H S a = 0.29 m/s 2 S d = 0.11 m/s

108 Storey Displacements A B C D 12 H(m) Displacement (m)

109 IDI 5 4 A B C D 3 H(m) IDI

110 What if Performance Point Does Not Exist? F E ADD STRENGTH OR STIFFNESS OR BOTH V/W (Acceleration) F I Inelastic demand spectrum capacity spectrum 5% damped elastic spectrum Roof-top Displacement

111 What if Performance Point Does Not Exist? F E ENHANCE SYSTEM DUCTILITY V/W (Acceleration) F I Inelastic demand spectrum capacity spectrum 5% damped elastic spectrum Roof-top Displacement

112 What if Performance Point Does Not Exist? F E V/W (Acceleration) REDUCE SEISMIC DEMAND BY: ADDING DAMPING OR ISOLATION F I New demand spectrum 5% damped elastic spectrum Roof-top Displacement