All buildings are modelled for inelastic pushover and time history analysis as plane frames, using an extended version of the computer program Drain-2DX [36].Vertical loads and corresponding masses are estimated from the dead loads plus 30% of the live load. Beams and columns are modelled using the two component lumped plasticity beam column element (type 02) with the hysteretic characteristics shown in Fig. (2a). The slab reinforcement within the effective width is included in the calculation of the flexural inelastic characteristics of the beams in negative bending. Effective slab widths of 1.0 m and 0.5 m are adopted for the 3.5m spans, while 1.30 m and 0.65 m are used for the 6.0m spans, for internal and external frame beams respectively.

Inelastic moment-curvature diagrams are calculated for each critical region using average material properties, taking into account the effect of axial load in the columns. The mean concrete strength is assumed to be equal to 16 MPa and 22.5 MPa, for concrete grades B160 and B225, respectively. Separate confined core and cover concrete constitutive models are considered. The average yield stress of the reinforcement is equal to 310 MPa and 430 MPa for S220 and S400, respectively, while the ultimate tensile strength is assumed to be 420 MPa and 630 MPa. The steel is modelled as having trilinear stress-strain characteristics.

In this study the perimeter infill walls are modelled with diagonal struts, resisting loads only in compression. An element type has been developed for this purpose in the base program Drain-2DX by Tassiou [37], as an extension of the simple inelastic asymmetric truss bar type 01, having the cyclic characteristics given in Fig. (2b). The parent trilinear envelope curve follows the formulation suggested by Zarnic et al. [38] and consists of zero (or small) tensile response, followed by an elastic and a post-cracking part in compression with positive stiffness, and a post-peak softening part with negative stiffness. The mechanical properties of the masonry are evaluated using the expressions described in [18]. According to standard practice all exterior infill walls are double leaf with a total thickness of 25 cm. The compression strength f_m of the masonry infill was assumed to be 2.5 MPa and the corresponding modulus of elasticity E_w being equal to 750 ·f_m [39]. Maximum resistance of the infills is reached at an interstorey drift of 0.5%. The main parameters of the infill resistance-deformation envelope for building T60 are summarised in Table 1.

Fig. (2). Hysteretic behaviour of (a) line elements and (b) infill walls

The inelastic performance of existing RC frames has been investigated using a static pushover analysis methodology described thoroughly in [18]. The overstrength of the structure is evaluated from pushover analysis as the ratio of base shear at failure to ultimate limit state (ULS) reference base shear, equal to the allowable stress level design base shear multiplied by a material strength correction factor [18]. Ductility capacity is evaluated as the ratio of failure to yield displacement, for an equivalent bilinear system. Subsequently, the behaviour factor is evaluated from ductility ratio, as described thoroughly in [18]. Finally, maximum lateral roof deformation capacity of the buildings is compared with target displacement demands, determined from simplified nonlinear static procedures. In this study, the Capacity Spectrum Method (CSM) proposed by ATC-40 [21] and the N2 Method by Fajfar [23] are used for the evaluation of the target displacement demand imposed on the buildings examined.

Fig. (3). Incremental dynamic capacity curve.

Incremental dynamic analysis (IDA) [40] or Dynamic Pushover Procedure [41] is a parametric method for the estimation of structural response under seismic loads. The objective of an IDA study is the understanding of structural behaviour under different levels of seismic intensity. A structural model is subjected to multiple level of seismic intensity using one or more ground motion records. Presently, IDA is a state-of-the art method to determine the global collapse capacity of a structure. The use of an IDA approach was one of the two alternative methods proposed in the Background Document of EC8 [24] for the estimation of the behaviour factor of frame buildings and had been used in [42, 43] to evaluate the available q factor of modern irregular RC buildings designed according to this code. Later, the method has been generalised for seismic hazard analysis in a probabilistic context, directly relating the earthquake magnitude to a critical damage index [44].

The results of the pushover study are compared herein using equivalent performance index predictions from plane frame IDAs, using the same set of limit criteria (LC). A large number of actual base excitations are used, as described later on. For each record, nonlinear dynamic analyses are performed for different intensities, until all yield and failure criteria are satisfied. Maximum base shear, spectral acceleration or pga vs top displacement from each analysis is plotted to show the IDA curve (Fig. 3).

Similar to pushover, the quantification of structural performance is made both at the global and the local level. The behaviour factor q is evaluated as the ratio of the elastic spectral acceleration of the collapse record (S_a)^el_c and yield record, (S_a)^el_v [45]. Considering that spectral amplification is constant, this can be evaluated as the ratio of the peak ground acceleration (pga) of the collapse earthquake to the pga of the yield earthquake. Yield record is the record with the minimum pga that causes yield in any element of the structure, while collapse record is the record with the minimum pga signifying conventional collapse. Similarly, ductility is evaluated as the ratio of the collapse δ_c to yield displacement δ_y, where displacement is the maximum roof displacement resulted from dynamic analysis of these two records. The building q and ductility capacity μ are hence obtained from Eq. (1)

(1)

For the estimation of conventional collapse under pushover and dynamic excitation the same local and global LC are adopted, both at the member and at the structural level. The analytical assumptions of the LC considered were presented in detail in [18]. Step by step checks are performed for pushover and time history analysis, evaluating if:

1. The plastic rotation of the columns exceeds the corresponding plastic rotation capacity of the section at the critical region (LC designated θ_pl). This is defined as the product of the ultimate curvature of the critical section and the plastic hinge length, equal to half the effective depth or according to an empirical expression proposed in [39], whichever governs;
2. The shear strength exceeds the strength calculated according to the currently enforced design Code, using mean material properties (LC designated V);
3. The interstorey drift under dynamic excitation is less than 1.25% for all frames (LC designated dr) This value is adopted for existing RC frames based on experimentally determined failure limits of typical existing RC columns [46];
4. Theinfill strut compressive strength is not exceeded by the demand, as described in more detail in [18] (LC designated Inf).

A computer program, DrainExplorer [47], was developed to post process the nonlinear analysis results and monitor in a step-by-step manner the state of the structure during nonlinear static pushover and/or time history analyses. The program reads the frame geometry and member reinforcement details, load profiles and inelastic analysis results from Drain-2DX [36]. Subsequently, it calculates the cross-section inelastic characteristics of all the structural members, generates the pushover or time history curve, evaluates and checks all limit criteria step by step and plots the corresponding points on the curve where these limits are exceeded. For static pushover analysis it compares the collapse displacement with the target displacement evaluated using two alternative capacity spectrum methods proposed in the literature, Capacity Spectrum Method proposed by ATC-40 [21] and N2 Method by Fajfar [23]. Furthermore, in each step of pushover or time history analysis, vertical interstorey drift distribution, plastic hinge distributions, magnitude of the inelastic rotational demands in all the elements and local energy absorption per storey among the beams, columns and walls, if any, of the structure are provided. Furthermore, the condition of the infill walls, where they exist, is established also in each step.

IDAs were also performed with the help of this program. It post processes the results of each analysis of Drain-2dx, prepares the input file for the next dynamic analysis of the IDA. The algorithm performs successive refinements around first yield and collapse pga, to establish the structure’s q and ductility capacity. Finally the program plots the IDA curve, as shown in Fig. (4). For each intensity level, the failure criteria is identified and shown in the results. Finally the program calculates the available ductility and behaviour factor.

Fig. (4). Maximum base shear vs maximum roof drift obtained from IDA, with indication of the pga excitation levels at yield and collapse.

For each structure analysed herein it is examined whether the weak beam/strong column criterion is satisfied. To this purpose, the ratio of the sum of the capacity moments of the columns at a joint to the sum of the capacity moments of the beams adjoining this joint (ΣM_RC / ΣM_Rb) is calculated. Values of this ratio smaller than one indicate that the capacity check is not satisfied at the joint and the beams are stronger than the columns. The post processing program also calculates the ratio of the shear resistance to the shear demand resulting from the capacity design in each member end, for each step of the analysis and the results are plotted on the frames. Values less than one indicate that the capacity design for shear is not satisfied for the specific member’s end.

Static inelastic pushover analyses are performed with uniform and triangular distribution of lateral loads for both buildings. Regarding the local collapse criteria of a structural member, it is assumed that failure of the structure occurs only if a limit is exceeded in a column, assuming that the building does not collapse if a beam fails.

The capacity curve, base shear vs. roof displacement is shown in Fig. (5) for the typical building of the 60s with and without infill walls, and for the typical building of the 80s, for uniform and triangular lateral load patterns, together with an equal area bilinear approximation for the estimation of the available ductility and behaviour factor. These curves give important properties of the structures, such as the initial stiffness, the maximum strength and yield global displacement. At the same graph, the roof deformations at which the limit state criteria considered are exceeded in any of the members are also shown, together with the corresponding performance point demands following two performance point estimation methods, CSM and N2. Furthermore, the design base shear V_d and the ultimate limit state (ULS) reference base shear V_u (see 4.1) for all frames are also shown, in order to quantify the overstrength of the structure. Interstorey drift limiting criterion is observed at large deformations and it never seems to be critical for existing buildings, as also presented for more building forms in a previous study too [4]. For all the frames, it is observed that the critical limit state criterion is the plastic hinge rotation capacity. Shear failure is observed in columns of the infilled frame but in larger displacement. Failure of infill walls occurs but is not critical too.

The results of the static pushover analyses are presented in Table 2. These are the maximum base shear V_max attained by the structure under uniform and triangular lateral load profile, the overstrength Ω, the available ductility μ and the behaviour factor q of the equivalent bilinear system, evaluated using the methodology described thoroughly elsewhere [18], the roof drift at failure δ_u, the target point demands δ_CSM and δ_N2 following [21, 23] and the controlling LC based on which δ_u has been estimated. For all frames the fundamental period is shown, and for the infilled frames, the compressive strength of the masonry infill walls f_m is also shown.

Fig. (5). Inelastic pushover characteristics of buildings of the 60s and 80s, with and without infill walls.

Pushover curves with uniform load pattern have larger shear forces for the same displacement values. Similar results are shown by other researchers [48]. In Fig. (5) and Table. 2 it can be seen that building of the 80s (K80) exhibits better performance than the building of the 60s (K60). From pushover analysis with triangular lateral load pattern it can be seen that the building of the 80s has higher ovestrength and behaviour factor. The overstrength and the ductility of building K60 are 160% and 1.65, respectively. Building K80 has higher overstrength and ductility (173% and 1.61) from building K60, because the former structure is designed according to MOD84 [35], which introduced end member confinement and the requirement for a joint capacity criterion (less strict than the one applied in the present codes, EAK [49] and EC8 [24]), thereby increasing the structure’s lateral resistance.

The target displacement demands are higher than the maximum deformation capacity for the building of the 60s, while demand and capacity values are closer for the building of the 80s. It can be seen that the plastic rotation capacity criterion, which dominates in the building of the 60s improves in the 80s frame as a result of local detailing measures in introduced in 1984 (Fig. 5).

This behaviour changes when infill walls are taken into account (building T60) and the target displacement is higher than failure displacement. The critical limit state remains that of plastic hinge rotation capacity of columns. As expected, the presence of the infills causes a significant increase of the initial global stiffness and failure displacement is reduced by 25% for the infilled structure. The shear capacity of columns is exceeded earlier than in the bare frames due to frame-infill interaction. However, this failure is not critical in these analyses because plastic rotation capacity is exceeded first. Infills in the lower part of fully infilled frames do reach their maximum strength, at a drift which is close to, but higher than this limit. The ductility of the infilled structure is lower than that of the bare frame; however, its target deformation demand is lower as well. After failure of infills occurs, the lateral resistance of the structure approaches that of the bare frame.

In Fig. (6) shear capacity ratio values, calculated as described above, are shown for columns (values for beams are not shown for clarity) at failure deformation. Shear capacity ratio is satisfied for most of the beams and columns of the building of the 60s. Furthermore, for building of the 80s, characterised by longer bay sizes, for which however, the joint capacity check increases the overall column reinforcement, the shear capacity ratio is satisfied in both beams and columns (Fig. 6). This could indicate the reason why shear failure is not critical in most analyses.

Fig. (6). Shear capacity ratio of the columns.

In Fig. (7) the plastic hinge distribution of the buildings considered under uniform and inverted triangular profile of lateral forces is presented, at the point when the first collapse limit state criterion is exceeded. Both the exterior and the interior frames are shown. The values indicated at each joint correspond to the joint capacity ratios estimated at the same deformation. These ratios become less than one when the capacity check is no longer satisfied at the joint and the beams are stronger than the columns. In the same figure, the inelastic energy absorption at each floor, evaluated as the area under the bending moment - inelastic rotation curves of the hinged members and the area under the force - deformation curves of the equivalent infill struts are also shown, as a percentage of the total energy absorbed at the building during the same roof deformation.

In Fig. (7) it can be seen that in building K60 the joint capacity check is not satisfied (index smaller to 1.0 in most of the joints). On the contrary, for building K80 this check is satisfied in most joints, except for the top floor, where there is no requirement from the design code. The plastic hinge distribution observed at this point is in agreement with the joint capacity ratios. The exterior frames exhibit higher inelastic demands, since their columns are subject to lower axial load, compared to the interior frames. Furthermore, for building K60, relatively higher bottom reinforcement in the exterior beams results in high flexural resistance of these members, thereby concentrating all hinging primarily to the columns. The opposite holds true for the interior frames, whose beams are relatively weaker and, moreover, the columns have higher strength reserve as they bear higher axial loads than their exterior counterparts.

Regarding the energy distribution it can be seen that for the building of the 60s designed according to past generation of Codes, a significant amount of energy is absorbed by the columns. It is also shown that the inelastic energy absorption in the case of the triangular load profile results into a shift and concentration of damage towards the middle third of the building. On the other hand, building K80, designed with a simple form of joint capacity design, exhibits a concentration of its plastic hinges mainly in the beams and absorbs the energy mainly in the beams. A weak beam - strong column behaviour is observed, while the inelastic energy absorption is concentrated to the beams and it is better distributed along the height of the structure, as shown in Fig. (7). All the inelastic energy absorbed by the columns concentrates at the base of the columns of the first storey.

Fig. (7). Distribution of plastic hinges and local inelastic energy absorption with height (%) for pushover analysis. Values at joints indicate the joint capacity ratios (Σ MRC / Σ MRb). Infills plotted in “dashed” line when cracked.

In case of the fully infilled frame (T60) energy is mainly absorbed by the infills, while columns in first floor still absorb a significant portion. Plastic hinges are observed mainly at lower stories. Infills cracked are plotted with a dashed line.

Pushover results are subsequently compared with nonlinear dynamic analysis. In particular, Incremental Dynamic Analysis is performed using 15 strong motion records for a more realistic evaluation of the expected range of performance. For each IDA 25 nonlinear dynamic analyses were performed for different seismic intensities and a total of more than 1000 nonlinear dynamic analyses were finally carried out for all buildings and records. For consistency with the design assumptions, each record is also scaled according to the Velocity Spectrum Intensity (VSI) [50] to the zone I design spectrum in the Greek Design Code [49] (similar to the EC8 [24] design spectrum) which has a pga of 0.16g. The VSI is evaluated by Eq. (2)

(2)

where S_v is the spectral velocity, ξ is the dampingand T is the period.

In each IDA curve, four points are marked. These are (i) the point where yield happens to a member for the 1^st time (Yield), (ii) the point where failure happens to a member for the 1^st time (Failure), (iii) the point for dynamic analysis with the natural record (Unscaled) and (iv) the point for dynamic analysis with the record scaled to design spectrum, as mentioned above. The last point is compared with the target displacement using nonlinear static methods in order to evaluate these methods with nonlinear dynamic analysis whose results are considered more accurate.

Fig. (8). Elastic response spectra of the records.

The elastic response spectra of the records used, along with the Elastic Design Response Spectrum currently enforced by EAK 2000 [49] for zone I are shown in Fig. (8). The period of the three buildings examined is pointed out. The recorded pga, the peak ground velocity (pgv), the Velocity Spectrum Intensity (VSI), the Arias Intensity and the significant duration of the ground excitation (considered to be the time bounded by the 3% and 97% limits of the Arias Intensity) of the unscaled records are shown in Table 3.

The maximum predicted total displacement during each nonlinear time history analysis is plotted against the spectral acceleration in Fig. (9) for bare frame of the 60s. Likewise, IDA response between different excitations is compared in the same figure for the fully infilled frame of the 60s (T60) and the bare frame of the 80s (K80). In these plots, the mean IDA curves of all the records are shown too.

Conventional collapse is checked using the same set of LC for each record. The global IDA results are shown in Table 4 and plotted in Fig. (10) for each record together with the pushover predictions. Average, maximum and minimum predictions for q and ductility capacity (q_min, μ_min, ), together with the minimum predicted roof displacement at collapse from IDA are compared to the pushover results. Critical limit state is also presented in Table 4.

Fig. (9). IDA curves for bare frame of the 60s (K60), fully infilled frame (T60) and bare frame of the 80s (K80).

Fig. (10). Behaviour factor, ductility and displacement at failure evaluated from Pushover and IDA analyses.

From Table 4 and Fig. (10) it can be seen that the q factor, ductility capacity and roof displacement at failure predicted by the dynamic analyses exhibit a wide variability around the mean values. In general, mean values for the behaviour factor and ductility capacity evaluated with IDAs are higher compared to the values evaluated with pushover analysis, with an exception of the mean behaviour factor of the infilled frame of the 60s. However, the values evaluated from pushover analysis are included between the minimum and maximum values from IDAs. On the other hand, failure displacement values evaluated with the two methods are in good agreement. Behaviour factor and ductility capacity are larger for building K80, as expected. Failure displacement is also larger than building K60. Failure displacement of infilled frame T60 is smaller than the one of bare frame K60. On the other hand, behaviour factor and ductility capacity is smaller or larger, depending on the record.

Critical limit state for IDAs in most cases is that of plastic hinge rotation capacity of columns, the same with pushover analyses, with an exception for three cases were shear failure was critical, like the bare frame of the 60s (K60) under A299-1Tran record. However it should be noted that in some cases critical limit state may differ according to the seismic intensity of a record for the same building, while in other remains the same. In Fig. (11) IDA curve is plotted for K60 building for the KOZ19501TRAN record. Each point represents the maximum results from dynamic analysis with a record with different intensity. Different marks at each point show the different critical limit state at each intensity. At the 3^rd point 1^st yield occurs at a member. At the 6^th point, 1^st failure occurs. Plastic hinge rotation capacity is the critical limit state. The same limit state is critical up to intensity of Sa = 0.305g. At the next analysis, shear capacity is the critical limit state, up to intensity of 0.52g. Critical limit state switches to plastic hinge rotation capacity up to intensity of 0.78g and then back to shear, up to maximum intensity of 1.0g.

For every record, time histories for displacement, interstorey drift and shear forces at each storey are evaluated. Profiles of these are also presented at every step of the analysis. In Fig. (12) time histories for top displacement, interstorey drift of the 3^rd storey and base shear are presented for building K60 for the 1986 Kalamata earthquake (KAL186-1Long). Profiles of these values are presented in the same figure at time of maximum displacement and maximum base shear. It can be seen that for this record, interstorey drift is maximum at the 4^th storey.

Fig. (11). IDA curve for K60 under KOZ19501TRAN record.Critical limit state for each nonlinear dynamic analysis with different seismic intensity.

Fig. (12). Time-histories and profiles for the KAL186-1Long record (K60 building).

In Fig. (13) profiles of storey displacements, interstorey drifts and shear forces are presented for all the methods. Mean values from nonlinear dynamic analysis are compared with the values from pushover analysis at target displacement using CSM and N2 method. Nonlinear dynamic analyses were performed with the records scaled to the design spectrum. Results are plotted for the three examined structures. It can be seen that nonlinear static methods seems to overestimate the displacements, intestorey drifts and shear forces. There are differences among the two static evaluation techniques in the distribution of storey displacements and interstorey drifts, while their results are almost similar for the distribution of shear forces. The results of N2 method seem to be more close to the nonlinear dynamic analysis but with significant differences.

In Fig. (14) the plastic hinge distribution is shown for dynamic analysis of building K60 for the unscaled KAL186-1Long record. Results are shown for two different steps of the analysis, the step at maximum interstorey drift and the final step of analysis. In the same figure ductility rotation demands are shown near each plastic hinge. Furthermore, energy absorption for beams and columns along the height of the building is plotted near the frames. At step of maximum drift, energy absorption is maximum at the 4^th storey, where interstorey drift is also maximum. On the contrary, on the final step energy absorption is maximum at the 3^rd storey. Values of ductility rotation demands are larger in the 4^th storey at the step of maximum drift. From the plastic hinge distribution it can be seen that a soft storey mechanism is created in the 3^rd and 4^th storey and energy is mainly absorbed by the columns of these two storeys similar to the results obtained from pushover analysis. Plastic hinges in beams are more at interior beam, since capacity design check is satisfied at exterior beams.

Fig. (13). Profiles of storey displacements, interstorey drifts and shear forces for pushover and IDA.

Fig. (14). Plastic hinge distribution, ductility rotation demands and energy absorption for (a) the step at maximum drift and (b) for the final step of analysis. K60 building and KAL186-1Long record. Red are plotted the failed plastic hinges.

Fig. (15). Displacement of the K60 building from IDA analysis for 15 records. Values for yield, failure, the record scaled to the design spectrum and the unscaled record.

Fig. (16). Sa of the K60 building from IDA analysis for 15 records. Values for yield, failure, the record scaled to the design spectrum and the unscaled record.

In Fig. (15) displacements of K60 building subjected to 15 records are presented. Displacements correspond to 4 levels of intensity for each record. The values represent (i) the displacement from dynamic analysis with the record under minimum intensity that causes 1^st yield to any member of the structure, (ii) the displacement from analysis that causes 1^st failure, (iii) the displacement with the record scaled to design spectrum and (iv) the displacement with the natural (unscaled) record. Similar results are plotted for spectral acceleration Sa for these 4 different cases (Fig. 16).

For K60 building, in all IDAs, the maximum displacement with the minimum record that causes failure is smaller than the maximum displacement obtained from dynamic analysis with the scaled record (Fig. 15). Similarly, spectral acceleration at failure is smaller than Sa for the scaled record (Fig. 16). This indicates that K60 fails for all dynamic analyses with the scaled records. However, if the frame is fully infilled (T60) failure displacement is larger, in most cases, than displacement of dynamic analysis with the scaled record, indicating a better seismic behaviour of the structure (Fig. 17). Infilled frame T60 does not fail when subjected to records scaled to design spectrum in most of the cases. Furthermore, building of the 80s (K80) shows a better behaviour compared to building of the 60s (K60) since it doesn’t fail for every scaled record and when it fails it is with a smaller margin (Fig. 18). Mean value for deformation capacity for this building is larger than the mean deformation demand of the scaled record, as also shown in Fig. (19).

Fig. (17). Displacement of the T60 building from IDA analysis for 15 records. Values for yield, failure, the record scaled to the design spectrum and the unscaled record.

Fig. (18). Displacement of the K80 building from IDA analysis for 15 records. Values for yield, failure, the record scaled to the design spectrum and the unscaled record.

Fig. (19). Mean IDA curves for frames K60, T60 and K80.

Fig. (20). Pushover prediction compared with mean IDA prediction.

Fig. (21). Local energy absorption over total energy with height (%) for pushover analysis and nonlinear dynamic analysis with the records scaled to design spectrum.

In Fig. (19) the mean IDA curves of all the records are compared for the three buildings. In the same figure, the mean points from dynamic analysis of record at yield, failure, record scaled to design spectrum and unscaled record are also shown. Spectral accelerations for building K80 are smaller, since spectral amplification is smaller for its fundamental period. On the contrary, base shear is larger. Maximum deformation capacity of K80 is significantly larger than the one of building K60.

In Fig. (20) pushover predictions are compared to IDA predictions. Mean IDA curves in terms of maximum base shear vs maximum roof deformation are shown in Fig. (20), with indication of the mean level at failure and excitation with the scaled record. Similarly, in pushover curves, target and failure displacements are also depicted.

The target displacement for K60 varies from 6.5 to 9.2 cm, evaluated from pushover analysis with uniform or triangular distribution of lateral loads, evaluated with CSM and N2 method. Displacement from nonlinear dynamic analyses using records scaled to the design spectrum varies from 2.8 cm to 10 cm (Figs. 15, 20), with a mean value of 6.0 cm. It seems that for this building, nonlinear static methods overestimate the target displacement compared to mean value from IDA. However, target displacements are within the extreme values obtained from dynamic analyses. Similar conclusions are made for buildings T60 and K80. The target displacement for T60 varies from 2.6 to 3.4 cm, while displacement from dynamic analyses varies from 1.6 to 4.3 with a mean value of 3.0 cm (Figs. 17, 20). Finally, target displacement of K80 varies from 12.4 to 14.6 cm, while displacement from dynamic analyses varies from 6.8 to 14.0 with a mean value of 9.6 cm (Figs. 18, 20).

In Fig. (21) the inelastic energy absorption at each floor is evaluated as a percentage of the total energy absorbed at the building during the same roof deformation. A comparison is made between the energy absorbed from pushover analysis and from nonlinear dynamic analysis with records scaled to the design spectrum. Similar to the conclusions from pushover analysis, a significant amount of energy is absorbed by the columns for building of the 60s. On the other hand, building of the 80s (K80), designed with a simple form of joint capacity design, absorbs the energy mainly in the beams and energy absorption is better distributed along the height of the structure. In case of the fully infilled frame of the 60s (T60) energy is mainly absorbed by the infills, while columns still absorb a significant portion. In case of the I-ELC180 record, the amount of energy absorption is larger for the beams and columns of the 1^st storey than the infills of the structure.