GfoInvestigation of the Applicability and Effectiveness of Base Isolation in Retrofitting an...

Editorial Office tel: +385 (0)31 540 082 fax: +385 (0)31 540 071 e-mail: [email protected]

Faculty of Civil Engineering Osijek Electronic Journal e-GFOS

http://e-gfos.gfos.hr/

1 of 18

Investigation of the Applicability and Effectiveness of Base Isolation in Retrofitting an Interwar RC Building Dimitris K. Zimos1, Petros K. Petridis1 1Civil Engineering Student, Greece

Corresponded author: Petros K. Petridis, [email protected]




2 of 18

Investigation of the Applicability and Effectiveness of Base Isolation in Retrofitting of an Interwar RC Building Abstract In many Mediterranean and European cities, including Thessaloniki, there is a considerable stock of reinforced concrete buildings -still in use today- which were constructed during the interwar period without any seismic consideration. In response to the need for reducing the vulnerability of these structures to future earthquake events, this paper aims at investigating the effectiveness of base isolation on such flexible RC buildings. This is accomplished by simulating a RC building of this period in a 3D model and conducting comparative analyses between the existing building and the alternative retrofit solutions considering low and high damping rubber bearings. It is proposed that the structure is isolated below the ground floor slab. Spectral analyses and non-linear time-history analyses are performed and the results are compared in terms of fundamental modes, interstorey drifts, and base shear. It is shown that the base isolation provides a viable retrofitting solution by improving the seismic response of the structure. Finally, the low damping bearings are proven preferable. Keywords: Earthquake Engineering, Seismic Base Isolation, Reinforced Concrete Building

Retrofit, Response Spectrum Analysis, Nonlinear Time-history Analysis

1 Introduction Reinforced concrete was first introduced as a building material in the Mediterranean region during the interwar period. Its potential for constructing more storeys with smaller vertical elements compared to the -predominant until then- load bearing masonry was quickly




3 of 18

understood and exploited. The additional advantages concerning shorter construction time, further architectural liberty and cost effectiveness gradually established the new material in multi-storey buildings. However, the scientific knowledge regarding reinforced concrete design was limited. At that era few codes were available (i.e. the German DIN 1916) and no aseismic considerations were taken. In this context, Thessaloniki experienced an intensive reconstruction phase, fuelled by the great fire of 1917, which levelled the entire city centre, as well as by the extensive housing needs caused by the influx of refugees from Asia Minor (1922-1923). Whilst most of them were replaced during the postwar era reconstruction, many of the interwar multi-storey RC buildings are still in use, reflecting the architectural trends of those times (neo-classicism, modernism). The fact that many of these building have never been retrofitted bears a concern over the level of seismic protection that they can offer, thus creating the need for assessment and probably retrofitting. There are plenty of intervention methods that can be considered for retrofitting such a building, ranging from conventional strengthening methods, such as RC jacketing of beams and columns, to using innovative materials, such as FRPs, and various control systems, such as base isolation or energy dissipation devices. In this specific study, the seismic base isolation is investigated, which has been proven to be a very effective method for protecting buildings and other structures against seismic hazard [1]. The approach followed in this study comprises a comparative response-spectrum analysis for base isolation with both Low- (LDRB) and High-Damping Rubber Bearings (HDRB). To obtain a better insight on the effect of base isolation on the buildings seismic behaviour, non-linear time-history analyses were conducted as well for the three different models based on real records of earthquakes from Greece.




4 of 18

2 The building under consideration 2.1 General Characteristics

The building studied is a seven-storey (basement and six storeys) RC frame with the average floor covering 180 m2, and could be considered a typical example of this eras building stock. It is not uniform in elevation, since the basement is 2.8 m high, the 1st and 2nd stories are 3.0 m and 3.2 m high respectively, while the upper floors (3rd to 6th) are 3.6 m high. Neither is it symmetrical in floor plan as it can be seen in Figure 1(b) but the eccentricities are not such as to deem it torsionally sensitive. The 0.10 m slab combined with the dense grid of beams form an adequately rigid diaphragm in each level. The masonry infill walls of the perimeter are of the flemish type and 0.20 m wide, whereas the interior masonry is of the running type around 0.10 m. 2.2 Materials

Information relative to the material characteristics of the buildings of the interwar period is scarce, the most reliable of which being recent experimental studies. The concrete compressive strength is anticipated to be low by todays standards since it was prepared in situ with the use of materials of unknown quality and it has probably degraded in time to some extend; each of these factors introduce considerable uncertainties [2]. Mild Stahl I (according to DIN1916) was used for both the longitudinal and transverse reinforcement [2,3]. It has been observed that in general the compressive strength of the bricks used during the interwar era is relatively high whereas the mortars strength ranges largely depending on the ingredients (lime, cement or both). 2.3 Sectional Characteristics: Dimensions and Detailing

The columns dimensions and reinforcement differ from one storey to another, decreasing upwards, as they were designed only in consideration of the vertical loads they were to carry.




5 of 18

Therefore, in the basement, first and second floor the ratio of columns to the floor surface is 1.92% with an average longitudinal reinforcement of 8.8, in the 3rd, 4th, 5th and 6th floor the percentage drops to 1.23%, 1.11%, 1.00% and 0.77% respectively, while the average relative longitudinal reinforcement increases from 7.2 on the 3rd storey to 8.3, 9.1 and 11.7 on the 4th, 5th and 6th. It is worth mentioning that all the columns sections are rectangular ranging from 0.45 x 0.45 m in the lower storeys to 0.25 x 0.25 m in the top storey and their transverse reinforcement is considered to be 6/300 mm based on reconnaissance after the 1978 Thessaloniki earthquake. The dimensions and detailing of the beams are more scattered than those of the columns, since they were designed for bending and shear forces. The larger range of beam dimensions is also a result of the fact that some beams are supported by other beams, as seen on Figure 1b.

Figure 1 (a) The buildings faade on Tsimiski str., Thessaloniki. (b) The typical floor plan.

(c) The 3D model created in SAP2000 15.




6 of 18

3 Base isolation system description

In order to improve the buildings performance under a seismic event, the solution of seismic base isolation is explored. The Increase of the structures flexibility lengthens its fundamental and not only period, thus reducing the accelerations and, consequently, the forces acting on it, while increasing its design displacements. Furthermore, it modifies the shape of the structures modes and in one of the two cases increases its damping, decreasing the displacements as well as the acting forces.

The isolation interface is situated under the ground floor slab, as depicted in Figure 2(a). This solution was preferred compared to others the most common one being under the basement slab, above the foundation level , because this location is much easier to access, which makes the whole retrofitting project much more effective time- and cost-wise. Since the project will have a smaller duration than in other solutions and the specific floor serves merely as a storage space, it will cause the least possible disruption to the users of the building. Moreover, it leads to the isolation of most of the buildings mass, which maximises the desirable effect.

It is suggested that a retrofitting scheme including reinforced concrete jacketing is applied to the basement columns. This will allow for the dimensions of all the columns to be normalized to 0.45x0.45 m and their bending and shear strength to be upgraded to the desired level. The circular bearings will be located on the basement columns, having a diameter slightly smaller than the columns dimensions, namely 0.40 m, and the same centre, as shown in Figure 2(b).




7 of 18

Figure 2 - (a) Bearings position in cross-section, just below the 1st floors slab. (b) Bearings position in plan, on the retrofitted basement columns

A parametric investigation of two types of bearings was conducted, regarding the damping coefficient being 5% and 15% in the Low-Damping Rubber Bearing (LDRB) and the High-Damping Rubber Bearing (HDRB), respectively. In both cases, the dimensions and shear modulus considered were the same and were based on products available on the market. Each bearing consists of rubber layers reinforced by integrally bonded steel plates endowed with external plates of fit thickness with holes for smooth anchorage bars (see Figure 3), which ensure the shear force transfer between the bearing and the structure.

Figure 3 Type of bearing used in the base isolation system, both LDRB and HDRB.

(a) (b)




8 of 18

The characteristic value of the shear modulus of the rubber is G = 0.9 MPa, while the bulk modulus has a typical value of K = 2000 MPa. Following an iterative process, the final dimensions were selected, so that it does not fail to compression from the superstructure as well as displacements and rotations resulting from seismic loading.[4] The used bearings are of 400 mm diameter and 121 mm height, having a shape factor of S = 8,333.

4 Analytical Modelling 4.1 Modeling Assumptions and Parametets

A three-dimensional model is created in a finite elements programme (SAP2000 15) in order for the linear and nonlinear analyses to be performed. The materials and their parameters are the first to be defined. Taking into consideration the nominal compressive strength of the concrete suggested by the interwar periods design codes and the uncertainties mentioned above the compressive strength is assumed 7.0 MPa.[5] The required yielding and ultimate stress of the reinforcement is taken 220.0 MPa and 330.0 MPa respectively as suggested for Stahl I.[2] Based on the experience from masonry of this period, the brick compressive strength is assumed 5.0 MPa and the mortar 1.5 MPa, thus calculating the masonrys strength under diagonal loading 1.85 MPa, according to the Greek Retrofitting Code [6]. The linear elements, i.e. the columns and beams, are modeled with the use of frame elements, whereas shell elements are utilized for the concrete slabs in order to achieve the best possible simulation of the loads distribution to the vertical elements and the in plane relative displacements. Taking into account the great influence of the masonry infill walls in the buildings lateral response, especially for the case of flexible frame as the one examined herein, not only increases substantially the buildings stiffness, but it also alters the response shape shifting the focus of damage to different bearing elements depending on its distribution




9 of 18

[5,7]. In the current study, the interior running masonry is neglected and the flemish masonry of the perimeter is modeled in spans with small or no openings, according to the provisions of the Greek Retrofitting Code [6]. For modeling of the infill walls in a given span, an equivalent diagonal strut is used; it is modeled as a frame element with moment releases on both ends, having the same width as the masonry and varying height according to the diagonal distance [6]. As suggested by Eurocode 8 [4], cracked sections were considered for the linear elements with 50% reduced stiffness in order to conduct linear analyses. The full stiffness is considered for the time-history analyses, since the non-linear behavior of the sections is modeled with plastic hinges as follows. 4.2 Modelling of the non-linear behaviour

4.2.1 Columns and beams

In the case of linear elements, the well-established model of lumped plasticity on both ends is assumed in the form of plastic hinges. Automatic plastic hinges considering the interaction of axial force and biaxial moment are assigned on all columns near both ends, while similar plastic hinges considering only the bending moment are assigned near both ends of all beams. The moment-rotation curves for defining the properties of plastic hinges are computed automatically by the software based on the dimensions and provided detailing of each section, according to the provisions of the respective FEMA tables. The plastic hinges are assigned at the ends of the vertical and horizontal members where yielding is expected to occur: in particular, below the beams for a column head, over the slab for a column foot and beside the joint for the beams. It is noted that brittle modes of failure (i.e. shear failure, joint shear failure) are not considered, even though they can be critical in poorly reinforced elements [5].




10 of 18

4.2.2 Masonry infills

A multi-linear approach of the strut behaviour is rendered necessary by the cracking of the masonry which happens in relatively low displacements as well as by the considerable post-cracking strength that it develops. The cracking is considered to happen at 60% of the maximum strength, the post-elastic displacement at maximum strength being 5 times the one at cracking [7]. This was modeled as an axial stress hinge at the middle of the strut. 4.2.3 Rubber bearings

The rubber bearings are modelled using a special type of element available in SAP2000, namely the NLlink element (Non-linear link) and more specifically the Rubber Isolator NLlink, which corresponds to the behaviour of an elastomeric bearing like the ones considered in this study. They are modelled at the proposed position of application and with a length equal to their real height.

For the linear analyses, effective stifnesses are defined for all the bearings degrees of freedom [8], thus functioning as a set of linear springs. These are the same for both LDRB and HDRB. The equivalent viscous damping, 5% and 15% respectively, is assigned as uniform modal damping in the structure when defining each load case.

For the non-linear analyses, the bearings behaviour in the transverse direction is defined more accurately, through a bilinear model [8]. Consequently, its damping results from the hysteresis loop and it is not introduced in any other way in the structure, whose modal damping is in this case assumed to be 3%. 4.3 Loads

The vertical loading considered in the model corresponds to the seismic loading combination as suggested by Eurocode 1 (1.0G+0.3Q). The self-weight of the bearing elements is




11 of 18

computed automatically and the additional loads that were assigned are 1.5 kN/m2 dead load distributed on every slab, 2.0 kN/m2 live load also on every slab, the masonry and balcony weight assigned to the beams as distributed loads accordingly.

Figure 4 Design spectrum of Eurocode 8, Thessaloniki and Athens records spectra in both directions. The structures fundamental period before and after the retrofitting is included.

As mentioned in the introduction, both linear and non-linear analyses are performed. The linear analysis, besides the vertical load case, is a spectral analysis based on the Eurocode 8 elastic response spectrum type I (used in Greece) for q=1.5 and soil type B (see Figure 4).[4] The spectrum was based on a ground acceleration level of 0.16g, as such is the provision of the Greek National Annex to Eurocode 8 for Thessaloniki.

The nonlinear analyses are performed for the records of two seismic events of Greece: Thessaloniki 1978 and Athens 1999. It is noted that the records first underwent baseline correction and filtering and then they were scaled to 0.16g with the use of appropriate software (Seismosignal). The strong components of both records are inserted along the X




12 of 18

direction, in which the building is more flexible, as the unfavourable scenario. The analyses are supposed to be an indicative tool for the depiction of the effect of the applied form of retrofitting.

5 Analyses results 5.1 Eigenvalue analysis

An Eigenvalue Analysis is conducted for both the existing and the retrofitted structures. As expected, the periods of the structure were lengthened considerably: the fundamental period in the X direction increased from 1.33s to 2.32s (74%), while in the Y direction from 0.88s to 1.93s (120%). This leads to much smaller accelerations and forces during a seismic event, as shown in the spectra of Figure 4.

Another important influence of the seismic isolation system is the important shifting of the mode shapes of the structure. In Figure 5 the first two modes are depicted. It is characteristic that in the second model most of the deformation takes place at the level of the bearings, which leads to a more uniform shape of the superstructures deformations, which will decrease inter-storey drifts. Moreover, the excited mass of the isolated model at the fundamental mode of each direction is approximately 90%. On the contrary, in the model of the existing structure the same percentage of mass is accumulated by the first 7 modes. It is noted here that Eurocode 8 requires the consideration of such a number of modes that at least 90% of the mass is accumulatively excited [4].




13 of 18

Figure 5 Mode shapes of the structure before and after the retrofit

5.2 Drifts

5.2.1 Inter-storey drifts (IDs)

(a)

(b)




14 of 18

Figure 6 Inter-storey drifts (ID) of the various building floors. (a) Response-Spectrum analysis, (b) Thessaloniki 1978 record anal, (c) Athens 1999 record.

According to the Figure 6(a), it seems that bearings constitute a very effective way of minimising the IDs of the floors, thus leading to less hinges being created in the various structural members which are created when the ID exceeds the y of the respective member. It is noted that the column rotation at yielding (y) is calculated for every column according to

the equation , where y is the curvature at yielding and Ls the shear span.

Considering the average y, which is depicted on the diagram, the building retrofitted with HDRBs does not run the risk of yielding or failure whatsoever. The one with LDRBs seems to exceed it slightly in the x direction in some of the floors, which means that there are some hinges expected locally, without, of course, leading neither to section nor to global failure.

Nonetheless, Figures 6(b) and 6(c) do not project a similar behaviour. On the one hand, the IDs are close to those of the Existing model and way below the limitation of y, which is fairly explained by the discrepancy between the EC8 design spectrum and the spectra of the two records (Figure 4). On the other hand, what is way more important is that the IDs of the two retrofitted models are almost identical in some cases HDRBs produce even higher IDs than

(c)




15 of 18

the LDRBs. This can be explained by the fact that the extra damping of the HDRB case as opposed to the LDRB case is of course local, exactly above the isolation interface and it does not influence the whole structure uniformly. So, even if some floor displacements decrease in absolute value, it doesnt necessarily mean that their relative value will be decreased and that is actually the case in these diagrams. Nevertheless, the spectral analysis considers a uniform distribution of the damping in the structure (given as modal damping) and this is why its predictions are partially incorrect, in cases where the real damping cannot be regarded as uniformly distributed [9]. 5.2.2 Bearing displacements

The assumption that the rubber bearings isolate the structure by decoupling the displacements of the substructure and the superstructure is confirmed by the Figure 7, where the displacements of the points above and below two bearings (a LDRB and a HDRB) are shown. It is observed that the point below follows the same displacement pattern, but the decrease of its absolute value in comparison to the point above is vast, ranging from 80% to 95%.

Figure 7 Displacements of the points above and below a bearing versus time, for the Thessaloniki earthquake, for the two types of bearings, (a) HDRB and (b) LDRB.

(a) (b)




16 of 18

Comparing the Figures 7(a) and 7(b), it is clear that the point below the bearing moves the same way in both cases. Nevertheless, the point above, although following a similar pattern, has somewhat lower displacements in the case of the HDRB, due to the increased hysteretic damping of HDRBs compared to LDRBs. 5.3 Base shear

In Figure 8 it is shown that the usage of bearings mitigates the shear force at the base of the structure in every case. In the spectral analysis, it is quite clear that HDRBs have a much higher effect on this, while in the time-history analyses the difference becomes more obscure and in some cases they even result to higher base shear than the LDRBs. This could be attributed to the same phenomenon mentioned in Section 5.2.1.

Figure 8 Base shear as a percentage of the vertical load of the structure in the various analyses, directions and models.




17 of 18

6 Conclusions

To sum up, the main outcomes of this study are hereby highlighted:

The retrofit of the existing building with the use of elastomeric bearings, both Low-Damping (LDRB) and High-Damping (HDRB), provides an essential improvement of the seismic behaviour of the structure. The inter-storey drifts of the structure are considerably lower after retrofitting, due to the influence of the isolation system, leading to the introduction of bearable rotations to the low ductility vertical elements.

Base isolation is proven effective in such a building, the superstructure of which is a flexible RC frame, although more suitable for rigid structures.

Comparing the two solutions, low and high damping bearings, the former appear to be more appropriate a choice than the latter, since the cost of the HDRBs is higher, while the analyses showed more or less the same performance under seismic events.

The assumption of uniform distribution of the damping in the structure, in cases like this one where the real damping cannot be regarded as such, leads to incorrect results and, thus, analysis methods that adopt it should be cautiously applied.

Acknowledgements

Finally, we would like to wholeheartedly thank Dr. Georgia E. Thermou, Lecturer at Aristotle University of Thessaloniki, for her help and advice along the course of this project.




18 of 18

REFERENCES [1] IST Group, MIT, 2004. Methods for Seismic Retrofitting of Structures. [2] Ignatakis Ch. 2008. 30 years after the Thessaloniki earthquake memories and

perspective, Faculty of Engineering, Aristotle University of Thessaloniki [3] Karavezyroglou M., 2009. Strength of Concrete and Steel according to the requirements

throughout the years, 16th Concrete Conference, TEE-ETEK, Paphos, Cyprus [4] Eurocode 8 Part 1: General rules, seismic actions and rules for buildings, December

2003 [5] Petridis P. 2013. Assessment of the seismic behaviour of interwar RC buildings with

rapid assessment criteria and non-linear analyses, Diploma-thesis, Aristotle University of Thessaloniki

[6] Greek Retrofitting Code (KANEPE 2012), Organization for Antiseismic Design and Protection (OASP)

[7] Dolek M.; Fajfar P. 2008. The effect of masonry infills on the seismic response of a four-storey reinforced concrete frame-a deterministic assessment, Engineering Structures 30

[8] Kelly J.M.; Farzad N. 1999. Design of Seismic Isolated Structures, John Wiley and Sons, Inc.

[9] Zimos D. 2013. Assessment of the seismic behavior of a 12-span bridge with passive control system, Diploma-thesis, Aristotle University of Thessaloniki

[10] Kilar V.; Koren D. 2008. Seismic behaviour of asymmetric base isolated structures with various distributions of isolators, Engineering Structures, Elsevier Ltd.

GfoInvestigation of the Applicability and Effectiveness of Base Isolation in Retrofitting an...

Documents

Transcript of GfoInvestigation of the Applicability and Effectiveness of Base Isolation in Retrofitting an...