SPECTRAL PUSHOVER VERSUS STATIC AND MODAL PUSHOVER ANALYSIS

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1 SPECTRAL PUSHOVER VERSUS STATIC AND MODAL PUSHOVER ANALYSIS V.S. Tsiggelis 1, A.M. Athanatopoulou 2 and I.E. Avraidis 3 1 PhD Candidate, Dept. of Civil Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece 2 Assistant Professor, Dept. of Civil Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece 3 Professor, Dept. of Civil Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece Eail: avra@civil.auth.gr ABSTRACT : In this paper a newly developed siplified procedure for estiating seisic deands which is called Spectral Pushover Analysis [SPA] is briefly presented and evaluated. First, the theoretical background is presented, the underlying assuptions and approxiations are identified and the successive steps of the ethod are outlined. Then, in order to evaluate the seisic deands produced by SPA, a paraetric study of planar and spatial systes is carried out using: (a) the Nonlinear Static Procedure [NSP] as described in EC8 and ATC55 (=FEMA440), (b) the Modal Pushover Analysis [MPA], (c) the SPA and (d) the Nonlinear Response History Analysis [NLRHA]. A 9-story non-syetric planar frae as well as a set of single story onosyetric buildings with different inertial characteristics and eccentricity have been analyzed for 20 recorded ground otions using all the above ethods. The coparative evaluation of the results shows that, as far as the planar frae is concerned, the agnitude of errors is siilar for all approxiate procedures, whereas for the single story buildings SPA produces results that are closer to the "exact" ones produced by NLRHA. KEYWORDS : Seisic deands, siplified procedures, nonlinear static procedure, odal pushover analysis. 1. INTRODUCTION Estiating seisic deands at low perforance levels, such as life safety and collapse prevention, requires explicit consideration of the inelastic behavior of the structure. While nonlinear response history analysis is the ost rigorous procedure to copute seisic deands, it is ipractical for routine use. It is now coon to estiate seisic deands in a siplified anner by nonlinear static analysis (pushover analysis), which sees to be the preferred ethod in structural engineering practice. However, this ethod has soe shortcoings and liitations as a result of its underlying assuptions [Chopra and Goel (2001)]. In fact, the static pushover analysis can be reliably applied only to two-diensional low- and ediu-rise buildings that vibrate priarily in the fundaental ode [Krawinkler and Seneviratna (1998)]. During the last ten years uch research work has been done in order to iprove the results produced by the different siplified nonlinear static procedures suggested by codes [ATC40 (1997), ATC55 (2005), FEMA274 (1997), FEMA356 (2000), EC8 (2002)], as well as to develop new ore accurate ones. Concerning the latter aspect, Chopra and Goel developed the so-called Modal Pushover Analysis [MPA] [Chopra and Goel (2001)] in order to take into account the higher-ode contributions. However, the application of this procedure leads in any cases to anoalous capacity curves due to the fact that the higher odes roof displaceents often change their signs. Another siplified nonlinear procedure, the Spectral Pushover Analysis [SPA], has been developed by Anastasiadis [Anastasiadis (2001)]. SPA consists in a utatis utandis "translation" of the classical pushover analysis fro the static analysis doain to the response spectru analysis doain. Further iproved siplified nonlinear procedures have been proposed by several researchers and their pros and cons have been the subject of any papers. Here, the focus is on the SPA and its coparative perforance in refer to NLRHA, to MPA and to the procedures in ATC55 and EC8. First, the theoretical background of SPA is presented, the assuptions and approxiations are identified and the successive steps of the ethod are outlined (paragraph 2). Then, in order to check the practical applicability, the relative ease of use and the accuracy of the results produced by SPA as well as by the aforeentioned code prescribed nonlinear static procedures, a RC planar frae and a set of single story onosyetric buildings with different inertial characteristics and eccentricity are studied (see paragraph 3). The results produced by SPA are copared with the associated ones produced by NLRHA.

2 2. SPECTRAL PUSHOVER ANALYSIS PROCEDURE [SPA] The basic idea of this procedure consists in the direct "translation" of the classical pushover analysis fro the Static Analysis doain to the Response Spectru Analysis doain. In this doain "loading" is expressed through an elastic design or response acceleration spectru S a (T,ζ), depending on the natural period T and the daping ratio ζ. In the context of a pushover analysis, such a "loading" is applied increentally, as a sall portion of the spectru. For each spectral increent, the structure under consideration is analyzed using iteratively the traditional linear response spectru ethod. Figure 1 shows the spectru portions for n increents, where λ k (k=1, 2,, n) is the proportional spectru partition. Obviously, the su of all λ k is unity: n λ k = 1 (k=1, 2,, n) (2.1) 1 S a (T) (/sec 2 ) λ n S a λ n-1 S a T 1,n-1 T 1,n Λ n S a Λ n-1 S a λ2 Sa λ 1 S a T 1,1 Λ 2 S a T 1,2 Figure 1 Spectru portions Λ 1 S a T (sec) By analogy with the classical pushover analysis, the partitioned spectru is applied increentally to the structure, starting fro the first spectral increent λ 1 S a and adding succesively a new spectral increent λ k S a (k=2,, n) each tie one structural eleent section(s) yields (foration of plastic hinge(s)). The new spectral increent is applied to a revised odel of the structure which differs fro the previous one Π k-1, as a different stiffness atrix is used for the structural eleent(s) with the plastic hinge(s). For each spectral increent λ k S a, a linear response spectru analysis of odel Π k-1 is perfored and the peak seisic response is coputed and added to the su of those fro the previous increents. Thus, the total response of the nonlinear syste is estiated by successive contributions of responses of the linear systes Π k Flowchart of SPA The SPA procedure consists of a series of step-by-step coputations with systeatic updates being perfored at the end of each step. The successive steps are presented in Figure 2 in the for of flowchart. The following notations are used: k : Step nuber of spectral increent with k= 1, 2,, n. Π k : Model of the structure at step k (Π 0 is the initial odel). {F y } k,i : Available strength at critical cross-section i of odel Π k-1 ({F y } 0,i is the yield strength at critical cross-section i in the unloaded structure of odel Π 0 and {F y } 1,i is the available strength at critical cross-section i of odel Π 0 after the action of gravity loads). {F} i, : Response quantity (bending oent, axial or shear force) at critical cross-section i. {R} : Response quantity (force or deforation). {F g } i, {R g } : Gravity load effects in odel Π 0 ({F g } 0,i ={F g } i, {R g } 0 ={R g }).

3 Create the initial (unstressed) odel of the structure, Π 0, and define the initial yield strengths {F y } 0,i in all critical cross-sections i Perfor a static analysis of the structural odel Π 0 due to gravity loads and copute {F g } i, {R g } Select a spectru S a (response or design) with daping ζ k = 1 Copute the available yield strength: {F y } k,i = in({f y } 0,i -{F g } i ±{F s } k-1,i ) Perfor a response spectru analysis of the structural odel Π k-1 using the entire spectru S a and copute the peak seisic responses {F s } k,i, {R s } k k = k +1 Calculate the scale factor λ k : λ k = in(±{f y } k,i /{F s } k,i ) Copute the peak seisic responses { F s } k,i, { R s } k for the spectral increent (λ k x S a ) to the odel Π k-1 : { F s } k,i = λ k x {F s } k,i and { R s } k =λ k x {R s } k Copute the current peak seisic responses: {F s } k,i ={F s } k-1,i +{ F s } k,i and {R s } k ={R s } k-1 +{ R s } k Copute the current total responses under seisic and gravity loads: {F} k,i ={F s } k,i +{F g } i and {R} k ={R s } k +{R g } Revise the odel to account for the yielded cross sections (plastic hinges) and define the new odel Π k Check if: (1) Σλ k 1., (2) u u ultiate No Is one of these criteria fulfilled? Yes End Figure 2 Flowchart of SPA

4 {F s } k,i, {R s } k : Peak seisic responses of odel Π k-1 under the action of the entire spectru S a ({F s } 0,i =0, {R s } 0 =0). { F s } k,i, { R s } k : Increase of peak seisic responses under the action of spectral increent λ k S a on the odel Π k-1 ({ F s } 0,i =0, { R s } 0 =0). {F s } k,i, {R s } k : Su of { F s } k,i, { R s } k fro step 1 till step k ({F s } 0,i =0, {R s } 0 =0). {F} k,i, {R} k : Current total responses under seisic and gravity loads Significant assuptions of SPA Initially, the following assuptions are ade concerning the inelastic behavior of structural eleents: a) inelastic bea and colun ebers are odeled as elastic eleents with plastic hinges foring only at their ends, b) all plastic hinges are of flexural type, c) the plastic hinges are described by elastic-perfectly plastic oent-rotation relationships, d) no liit value is assued for the sections plastic rotations, e) P-δ effects are neglected and f) the effect of cyclic earthquake loading on the inelastic response of cross-sections is neglected. Except the assuptions concerning the inelastic behavior of structural eleents, soe additional assuptions are adopted in order to apply SPA. Firstly, the direct addition of the peak responses of the successive structural odels is accepted as valid. Secondly, we ake the particular assuption that the peak responses at different cross sections and joints of odel Π k are concurrent. This is a conservative approxiation leading to a cuulative agnification of responses, because in reality these peak responses are not concurrent. Moreover, one ore assuption concerns the position of the new plastic hinges in coluns of the structure. As the behavior of a bea eleent is controlled by one paraeter -bending oent- a new plastic hinge is fored at the cross section where {M} k,i becoes equal to yield strength (oent) {M,y } 0,i. However, the foration of a plastic hinge at a colun eleent is ore coplicated due to the bending oent-axial force interaction and, in addition, due to the indefinability of algebraic signs for bending oents and axial forces in the odal cobination. Plastic hinge at a colun cross section is assued to be fored only if either the two points defined by (+M, +N) and (+M, -N) or the two points (-M, +N) and (-M, -N) are outside the bending oent-axial force interaction surface. A last assuption concerns the question: When ust the SPA procedure be terinated? At first, it sees to be rational to terinate the procedure only if the su of factors λ k for all steps becoes equal to unity. In spite of that, the analyses results of several structural systes produced by SPA have indicated that the errors are strongly dependent on ground otion intensity. This fact has led to the prescription of an additional liit concerning the total displaceent: u u ultiate. However, the choice of u ultiate is based inevitably on engineer s judgeent (for exaple, u ultiate can be the value of the top-floor displaceent deterined by a response spectru analysis of the initial structural odel). 3. APPLICATIONS A critical and coprehensive evaluation of the ethodology is perfored by eans of analyzing a variety of structural systes using the proposed SPA as well as the conventional static pushover procedures. The effectiveness of this procedures to capture the inelastic response of structures is evaluated on the basis of results produced by nonlinear tie-history analysis, which is used here as a reference solution Structural Models, Ground Motions And Methods of Analyses Several planar fraes are analyzed in order to cover a wide range of fundaental periods. In this paper, an non-syetric, 3-bay, 9-story, RC frae with constant story height is selected to be presented (Fig 3a). The seisic excitation is represented by the response spectrus of 20 recorded ground otions listed in Table 3.1. For the sae ground otions and the corresponding response spectrus a set of single story onosyetric buildings with different inertial characteristics and eccentricity are analyzed (Fig 3b).

5 G, Q 35 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x 100 G = 20 kn/ Q = 10 kn/ E = kn/² v = 0.2 = 60 t C20/S500 (a) y 2,50 1,50 1,00 0,50 E = kn/² v = 0.2 My,T1 = 150 kn My,T2 = 325 kn My,T3 = 150 kn 6.00 T1 20x T2 20x130 T3 20x80 x ugo (b) Fundaental natural period Τ1 = 0.2 sec Τ1 = 0.5 sec Σηµείο 0: = t J = t ² = t J = t ² Σηµείο 1: = t J = t ² = t J = t ² Σηµείο 2: = t J = t ² = t J = t ² Σηµείο 3: = t J = t ² = t J = t ² Σηµείο 4: = t J = t ² = t J = t ² Figure 3 Geoetric and echanic characteristics of the investigated systes: (a) non-syetric planar 9-story RC frae, (b) single story onosyetric buildings (ten different cases)

6 Each syste is analyzed using all ethods listed in Table 3.2. The results derived by LRSA are used to define the u ultiate. Non-Linear Response History Analysis (NLRHA) is perfored in order to obtain the "exact" values of the response. The target roof displaceent for NSP according to FEMA is deterined by: (i) Equivalent Linearization and (ii) Displaceent Modification. For NSP according to EC8 the target displaceent is deterined by NLRHA of an "equivalent" single-degree-of-freedo-syste (SDOF). For the application of MPA to the planar frae three "odes" are taken into account and the peak "odal" displaceent of each inelastic SDOF syste is deterined by NLRHA. The single-story spatial buildings are analyzed for unidirectional excitation along the y-axis (Fig. 3b). Table 3.1 List of ground otions Earthquake Nae and Location ü go (c/sec 2 ) Loa Prieta (Agnews State Hospital) Loa Prieta (Capitola) Loa Prieta (Gilroy Array #3) Loa Prieta (Gilroy Array #4) Loa Prieta (Gilroy Array #7) Loa Prieta (Hollister City Hall) Loa Prieta (Hollister Diff. Array) Loa Prieta (Sunnyvale - Colton Ave.) Northridge (Canoga Park) Northridge (LA - N Faring Rd) Northridge (LA - Fletcher Dr) Northridge (Glendale - Las Palas) Northridge (LA - Hollywood Stor FF) Northridge (La Crescenta - New York) Northridge (Northridge - Saticoy St) San Fernando (LA - Hollywood Stor Lot) Superstition Hills (Brawley) Superstition Hills (El Centro Ip. Co. Center) Superstition Hills (Plaster City) Superstition Hills (Westorland Fire Station) 169 Table 3.2 Methods of analyses Procedure 9-storey planar Single-storey frae onosyetric building 1 Linear Response History Analysis (LRHA) 2 Linear Response Spectru Analysis (LRSA) 3 NonLinear Response History Analysis (NLRHA) 4 Nonlinear Static Procedure-Equiv. Lin. (NSP- ATC55) 5 Nonlinear Static Procedure-Disp. Mod. (NSP- FEMA440) NSP s 6 Nonlinear Static Procedure EC8 (NSP- EC8) 7 Modal Pushover Analysis (MPA) 8 Spectral Pushover Analysis (SPA) 3.2. Analyses results The edian error of a response value deterined by each one of the applied ethods with respect to the corresponding "exact" value deterined by NLRHA is calculated using the following forula: 20 1 u u error (%) ip id = u (3.1) 1 id

7 where u ip is the displaceent at level i deterined by the approxiate ethod P (P: ethods no. 4 to 8 of Table 3.2) and u id is the corresponding displaceent derived by NLRHA. The closer the edian error to zero, the closer the response derived fro the respective approxiate ethod to the "exact" value. The approxiate ethod underestiates the edian response if the ratio (Eq. 3.1) is less than zero, and provides an overestiate if the ratio exceeds zero. The present study is liited to the evaluation of displaceents. The edian errors for the floor displaceents of planar frae are illustrated in Figure 4, while the edian errors for the displaceents of CM and the left side of the spatial odels are shown in Tables 3.3 and 3.4, respectively EC8 ATC55 FEMA440 MPA SPA NSP: odal distribution NSP: unifor distribution Figure 4 Median error of roof displaceents of 9-storey frae Table 3.3 Median error of displaceents of CM Model CM NSP- ATC55 NSP- FEMA440 NSP- EC8 MPA SPA T 1 =0.2sec T 1 =0.5sec Table 3.4 Median error of displaceents of left (stiff) side Model CM NSP- ATC55 NSP- FEMA440 NSP- EC8 MPA SPA T 1 =0.2sec T 1 =0.5sec

8 4. CONCLUSIONS The presented SPA of structures is a step-wise linear ethod in which the traditional LRSA is applied iteratively. Fro the coparative study of the investigated wide-spread approxiate nonlinear ethods the following conclusions are drawn: a) Planar frae: The agnitude of errors is siilar for all approxiate procedures, when the "odal" load distribution is used in the NSP s (NSP- ATC55, NSP- FEMA440, NSP- EC8), while MPA and SPA are ore accurate than the NSP s when the "unifor" load distribution is used. Furtherore, both MPA and SPA provide good estiates for floor displaceents. However, the coputational effort required for MPA is larger than for SPA. b) Spatial odels with fundaental period T 1 =0.2sec (Fig. 3b): MPA and SPA underestiate in general the displaceents of the Center of Mass (CM) and of the building s right side (~-20% till +5%), while NSP s overestiates the. The opposite occurs for the left side displaceent and the diaphrag rotation: MPA and SPA overestiate these values, while NSP s practically fails to estiate the (underestiation of ~-50% to -90%). c) Spatial odels with fundaental period T 1 =0.5sec (Fig. 3b): MPA and SPA ay overestiate or underestiate the displaceents of CM and of the building s right side (~±20%), while NSP s overestiates these quantities (~+50%). Moreover, NSP s practically fails to predict the deands of the left side (errors are -90%). Both MPA and SPA overestiate the, but results obtained by MPA are less accurate than SPA (errors range between +160% and +240% in MPA and between +25% and 75% in SPA). d) As a general conclusion it can be said that the presented spectral pushover analysis (SPA) possesses soe iportant advantages with regard to the conventional approxiate nonlinear procedures: It perits in a straight forward way to account for (a) the step-wise change of structural characteristics (natural periods, odes), (b) the higher ode effects as well as (c) the torsional effects in non-syetric structures. Moreover SPA does not require neither the copilation of a pushover curve, nor the application of several height-wise distributions of lateral loads. Obviously, further investigation is needed in order to derive conclusive results. However, the presented preliinary evaluation gave very encouraging and proising results. REFERENCES ATC40. (1997). Seisic Evaluation and Retrofit of Concrete Buildings, Applied Technology Council, Redwood City, California. ATC55 (FEMA440) Iproveent of Nonlinear Static Seisic Analysis Procedures, Applied Technology Council, Redwood City, California. Anastassiadis, Κ.Κ. (2001). Spectral pushover analysis for assessent of seisic behavior of buildings. 2 nd Greek Conference on Earthquake Engineering and Seisology, Salonika (in Greek). Chopra, Α.Κ, and Goel, R.K. (2001). Α Modal Pushover Analysis Procedure to Estiate Seisic Deands for Buildings: Theory and Preliinary Evaluation, PEER Report 2001/03, Pacific Earthquake Engineering Research Center, University of California, Berkeley. EC8, CEN Techn. Co. 250 / SC8. Eurocode 8: Earthquake Resistant Design of Structures-Part 1: General rules. (ENV1998-1, Parts 1,2,3), CEN. Brussels, 1994, FEMA274 (NEHRP). (1997). Coentary on the Guidelines for the Seisic Rehabilitation of Buildings. Prepared by ATC, Washington, D.C. FEMA356. (2000). Prestandard and Coentary for the Seisic Rehabilitation of Buildings. Prepared by ASCE, Washington, D.C. Krawinkler H. and Seneviratna G.D.P.K Pros and Cons of a Pushover Analysis of Seisic Perforance Evaluation. Engineering Structures, Vol. 20, No. 4-6, pp