ERIES-FREISUST

2023 - 2025

Seismic Eng.

Daniele Losanno

Filipe Ribeiro

Fulvio Parisi

Pio Medaglia

Shiv Prakash

Enrico Tubaldi

+2 more

GEM Taxonomy string

CR/LO

2023 - 2025

Seismic Eng.

GEM Taxonomy string

CR/LO

Fiber-Reinforced Elastomeric Isolators for seismic resilient structures with SUSTainable Solutions

ERIES-FREISUST

SEISMIC ISOLATION/ DISSIPATIVE DEVICES

RC STRUCTURES

Dataset Description

The ERIES-FREISUST project investigates the effectiveness of two unbonded fiber-reinforced elastomeric isolator (FREI) systems to enhance the seismic resilience of buildings: (1) a Virgin-rubber Carbon-fiber FREI (VC-FREI), and (2) a Reclaimed-rubber Polyester-fiber FREI (RP-FREI). VC-FREI installed in recessed configuration offers better standardization in terms of axial stiffness and material properties, while RP-FREI is a eco-friendlier and more cost-effective alternative, particularly suitable for developing countries due to its use of reclaimed rubber and polyester fabric.

The project focuses on the seismic performance of both structural and non-structural components of buildings subjected to increasing levels of horizontal and vertical ground motions. The FREISUST shake table testing program aims to validate FREIs as a sustainable, low-cost base isolation solution.

Tests were conducted at the National Laboratory for Civil Engineering (LNEC), Lisbon, Portugal, ST3D shake table on a 2-story full-scale reinforced concrete infilled frame specimen. Three configurations were tested: fixed-base, RP-FREI, and VC-FREI.

This dataset includes the output files associated with the shake table tests, which comprised 107 steps in between seismic tests, dynamic identification, sine-sweep tests and free vibration tests. Three different specimen configurations were tested (see Table below).

For the configurations Fixed-base, RP-FREI and VC-FREI, separate folders were created inside the “Output files” folder. Each contains the output processed data (“.xlsx” files) for seismic tests (GMs), dynamic identification and free vibration (CAT), as well as sine-sweep signal (SINESWEEP), according to the test sequence. Separate folders were created with videos of the seismic tests (separated by test stage).

Inside the “Output files” folder you can also find, for sake of reference, the target records associated with the three tested ground motions. The records included in this folder, which correspond to the 100% intensity level ground motions, were calibrated previously to the test with dead weights on the shake table. However, these records do not correspond to the input of the shake table for this intensity level since they were not subject to any adaptation for lower intensities. You can obtain the shake table inputs inside the folders of each configuration.

Only the output files corresponding to the final trials of each stage were processed. Different stages were processed, namely CAT (dynamic identification), Free Vibration, Sine-Sweep signal and seismic test (GM2 to GM4). All the data, sampled at 200Hz, is presented in separate “*.xlsx” files, aggregated in CAT, GMs and Sine-Sweep stages. In each “.xlsx” file the sheet name corresponds to the test stage unique identifier indicated in the “Test_Sequence.xlsx” file.

The raw data was processed differently depending on the type of measurement: (1) acceleration (in units of mg) – band pass Fourier filter between 0.5Hz and 40Hz (filter order 2); (ii) displacement (in units of mm) – low pass Fourier filter at 15Hz (filter order 2). Each displacement sensor forms a time continuum (in pseudo time) from the first test to the last test (initial offset of the first test was removed from all tests). All channels were cropped in 30 points at the beginning and 20 points at the end, with offset removal at the beginning of each stage but with cumulative displacements for all displacement channels.

As detailed below, instrumentation included accelerometers at beam-column joints, infill walls, and non-structural components; displacement transducers at key structural interfaces, beam/column joints and non-structural components; optical transducers for global floor displacements.

Ground motion records from the 1994 Northridge (NORTHR/CCN), 1989 Loma Prieta (LOMAP/CYC), 1995 Kobe ('KOBE/NIS) and 1997 Jiashi (NWCHINA3/J411N) earthquakes were used as seismic inputs. These records were selected combining different intensity measures and scaled according to the target displacement of the FREIs and in accordance with shake table limits, with final scale factors larger than 1.0 and up to 150% of the original record. The testing protocol involved applying the ground motions with increasing nominal intensity in X, XY, and XYZ directions, interspersed with dynamic identification tests. Additionally, a sine-sweep excitation (0.1 Hz to 10 Hz) was applied along the X-axis to identify the resonance frequencies of the RP-FREI and VC-FREI configurations.

The base-isolation systems demonstrated lateral stability and re-centering capability, as well as adequate vertical stiffness and load-bearing capacity under different earthquake excitation conditions.

Base isolation

Unbonded elastomeric isolators

FREI

Shake-table testing

Non-Structural Elements

Specimens

1. RC building

The prototype building (Figure 1) consists of a three-dimensional two-story one-bay square-plan RC frame. The frame is provided with masonry infill walls and plasterboard walls along the two main directions (X and Y, respectively) as shown in Figure 1, and tested on two base isolation systems. The prototype was mounted on four FREIs (RP-FREIs first, then VC-FREIs) installed in unbonded configuration beneath the corner columns. The replacement of the base isolation is executed through proper uplift at the foundation level.

With regard to the bearings’ characteristics, the VC-FREI system (Experiment 3) consists of alternated layers of virgin rubber and quadriaxial carbon fabric and is characterized by higher compression modulus compared to RP-FREI (Experiment 2), where biaxial polyester fabric is adopted. Due to different fabric thickness the two isolators measured different heights, namely 80 mm and 95 mm for RP-FREI and VC-FREI, respectively.

The RC frame had been designed as a non-dissipative structure according to EN 1992-1-1:2004 (CEN, 2004a) and EN 1998-1:2004 (CEN, 2004b) to be more representative of existing buildings. It has plan dimensions of 4.0m x 4.0m and a story height of 2.7m. The class of concrete is of medium compressive strength (C30/37). The columns have square sections of 35cm x 35cm and are reinforced with A500 steel rebars. The beams’ cross section is 30cm x 40cm in size, whereas the slab is 12cm-thick and is reinforced with a double-layer welded wire mesh. The total weight of the 2-story infilled frame is close to 40t including the foundation beam and nonstructural components. Foundation beams at the base (with 35cm x 60cm section) provided adequate stiffness under horizontal excitation as well as vertical stiffness under lifting and handling operations. The frame had no special foundation since it rests on four FREIs transferring horizontal force by friction. In this regard, a concrete-filled plate was installed on the shaking table providing realistic friction conditions of rubber-concrete type. Two different infill systems were tested along the two horizontal directions. In one direction (X), a single-leaf, around 20-cm thick, hollow-clay brick wall was considered, which is very common in Europe also for its thermal insulation properties. Hollow clay bricks with vertical and horizontal holes were adopted for the first and the second story, respectively. In the other direction (Y), a plasterboard partition system was installed as a lower-weight and more versatile internal partition alternative. The infills have been tested both in-plane and out-of-plane due to the biaxial horizontal excitation. Nonstructural components were installed at the second story of the building, consisting in common housing and office contents, including a closet, a desk with a computer, a lamp and a suspended pipe.

The specimen was constructed inside the Earthquake Engineering and Structural Dynamics laboratory at LNEC, but outside the shake table area, by an external construction company. The construction work required approximately two months in total. Construction began with the cast-in-place reinforced concrete structural system, which was carried out in five main phases: (1) foundation beams; (2) first-story columns; (3) first-story beams and slab; (4) second-story columns; and (5) second-story beams and slab. Each phase involved, in sequence: formwork assembly, rebar bending and placement, and concrete casting. Since the model is in real scale, no special requirements were made to the construction company, apart from the quality assurance and control that this type of tests demand.

At each concrete pouring stage, five cubic samples (150mm edges) of the concrete were stored and tested after: (i) 7 days (1 sample), (ii) 28 days (3 samples), and (iii) on the first day of the testing program (1 sample). A total of 25 samples were tested at LNEC in the scope of the project. The concrete compression strength tests, according to EN 12390-3:2019, were carried out at the Cementitious Materials Unit of LNEC. Until the testing date the specimens were preserved in a chamber at 20ºC ± 2ºC and relative humidity ≥ 95%. The average concrete compression strength on the test date was 48.9MPa, a value significantly higher than the one expected for concrete class C30/37, which was the one prescribed to the construction.

The orientation of the model on LNEC's 3D shake table is shown below. It should be noted that the X direction (West-East direction) is usually referred to as the transverse direction (T), while the Y direction (North-South) is usually referred to as the longitudinal direction (L). The Z direction is associated with the vertical direction (V). The model has brick masonry walls, which provide greater stiffness, oriented along the X direction. Along the Y direction are plasterboard walls, as well as openings for access to the interior of the model.

1. Fixed-base structure

The fixation system consists of steel beams anchored to the shake table that restrain the concrete foundations.

For the fixed-base configuration only dynamic identification was performed, firstly based on an input-output modal analysis (named here as CAT) and secondly based on free vibration (output-only) tests, where the structure response is recorded without any shake table input (only considering the ambient noise of the laboratory). The CAT tests were performed by applying a white noise input with a wide frequency range, up to 40Hz, and amplitude of 6mm, 3mm and 4mm for the X, Y and Z directions, respectively.

Instrumentation

The motion of the shake table was recorded using three uniaxial accelerometers and three linear variable displacement transducers (LVDTs). The vertical axis of the shake table was instrumented with a load cell. The specimen itself was extensively instrumented with multiple sensor types, including displacement transducers (wire potentiometers and LVDTs), optical sensors (HAMAMATSU) and accelerometers. The instrumentation and the sensor layout on the specimen are described next. A summary of all installed instrumentation is provided the Table below. The same instrumentation setup was adopted for all testing configurations.

To monitor the accelerations at each floor of the specimen, accelerometers were employed. Each device is capable of measuring acceleration at a single point along one direction only; therefore, more than one accelerometer was combined to record multi-axial accelerations on several points at each floor. To capture potential in-plane rotations of the foundation plane and the two upper diaphragms, at least two points were monitored at each floor. The accelerometers on the same floor were oriented along the two horizontal directions and placed at locations chosen to avoid alignment with the previous sensor along any axis-parallel direction.

On the foundation plane, two more vertical accelerometers were added, for a total of four devices positioned at the corners of the building’s footprint. This configuration enables the detection of rocking-induced rotations about the Y-axis and provides redundancy in the measurements. Another four accelerometers were placed horizontally at the centre of the infill panels, two in the X direction on the plasterboard infills, and two in the Y direction on the masonry brick infills. These accelerometers were used to monitor the out-of-plane accelerations of the infills and to estimate overturning actions caused by the seismic input.

Moreover, eight accelerometers were installed on the non-structural components located on the second floor of the building. Three of these sensors were installed on the desk, three on top of a closet, and two on a suspended pipe.

The following naming convention was adopted to identify each device: ACC_ID_Floor_Position_Direction. For example, the accelerometer placed on the ground floor, at the North-East corner, oriented vertically, is labelled: ACC04_GF_NE_Z.

To measure the displacements of the structure’s floors, three external reference frames rigidly connected to the laboratory floor were installed, outside the shaking table, aligned with it along the two horizontal directions, X and Y (two on the east side and one on the north side). The displacements of the prototype measured with respect to these frames are referred to as global displacements. Wire potentiometers and optical transducers, which are well suited for measuring displacements over distances up to 250mm, were used for this purpose. Structural response parameters will be derived later based on these absolute measurements.

The following naming convention was adopted to identify each device: DISP_ID_SLAB_Floor_Position_Direction. For example, the displacement at the foundation level in the X direction - NE corner is labelled: DISP06_SLAB_GF_NE_X.

In what concerns relative displacements, sensors were installed to monitor: (i) potential damage to the infill walls; (ii) uplift of the model due to rocking movement on the isolators; and (iii) displacements of non-structural components. Damage on the infill walls is monitored both by measuring crack openings along the interface with the reinforced concrete elements using LVDTs, and by monitoring the shear deformation of the first-floor frames through wire potentiometers placed along their diagonals.

Regarding the monitoring of rocking motion, four LVDTs were installed at the four corners of the structure base to measure vertical displacements relative to the shaking table.

Finally, four LVDTs were installed inside the second floor of the prototype to track the horizontal displacements of a desk and a closet.

The following naming convention was adopted to identify each device: DISP_ID_Element_Position_Direction. “DISP” is used in the case of elongation of infill walls measured by wire potentiometers. In case of relative displacements monitored through LVDTs, “DISP” is replaced by “LVDT” in the naming. For example, the elongation (West to East) of the masonry infill wall due to drift on 1st floor - South façade is labelled: DISP01_1FWALL_SW. Otherwise, the vertical relative displacement between infill wall and foundation beam - South façade is labelled: LVDT05_1FWALL_SE_Base.

2. RP-FREI Isolation System

The design, carried out by Losanno et al. (2024), of the base isolation system considered a superstructure mass of 40ton and resulted in 4 FREIs with 220mm diameter and 70mm total rubber height and a second shape factor around 3.0 as per stability requirements (Galano et al., 2021a, 2021b).

Assuming a soft compound (G=0.4MPa at a shear deformation  of 150%) and a 30% reduction of the contact area at ultimate limit state due to roll-over (Losanno et al., 2019), the equivalent isolation period Tis was set at 1.6s, corresponding to an equivalent stiffness of 600 kN/m of the isolation system (150 kN/m per unit). A target displacement of 105mm corresponding to  =150% shear strain (i.e. prior to full roll-over) was assumed.

In terms of hazard level, a spectral demand of 150mm would be obtained for Tis and a 5% equivalent damping ratio; consequently, the target displacement is consistent with this value accounting for damping reduction factor as per EN 1998-1:2004 (equal to 0.7 for 15% damping ratio). Such hazard level is deemed representative of high seismicity regions in Italy with a 5% probability of exceedance in 50 years. Full roll-over (at around 200% shear strain) could be achieved under very-rare events (e.g. maximum-considered earthquake, MCE) but in order to prevent damage this condition was tested only at the last stage of the testing protocol for VC-FREIs with recessed configuration only.

For preliminary characterization two specimens for each system were tested under compression and shear-compression protocols. Tests were performed at the University of Naples Federico II to evaluate axial stiffness and hysteretic behavior of FREIs providing useful data for bearing model calibration and ground motion selection.

For the RP-FREI (the same applies to VC-FREI) configuration, in addition to the CAT and free vibration tests, two other types of tests were performed: (i) ground motion tests and (ii) sine-sweep tests. Three ground motions were considered. The sine-sweep tests consist in the input of a signal whose frequency changes continuously over time, typically following a known pattern. In this test, a sine-sweep signal was used to assess the isolated structure sensitivity to different excitations, with varying frequency, thus identifying the resonance frequencies.

Testing with the RP-FREI configuration started by performing dynamic characterization, then the first seismic input ground motion (Loma Prieta) was applied. Tests were conducted at increasing intensity levels: 50%, 75%, and 100%. These percentages refer to the signal already scaled according to the previously calibrated scale factors. The second seismic input applied was the Jiashi earthquake, also tested at gradually increasing intensities. On the third day, Kobe earthquake was tested. At the end of the sequence, another random noise test was performed to detect any changes due to potential damage. Subsequently, the prototype was lifted again to replace the isolators.

Instrumentation

The instrumentation is the same as described for the Fixed-base structure.

3. VC-FREI Isolation system

The VC-FREI configuration included a recessed configuration (e.g. two additional steel plates for each bearing installed beneath the base beam and above the table with a central hole having the bearing diameter). Even if previous tests demonstrated that rubber-concrete friction would prevent any sliding of the isolators, these additional plates were adopted for VC-FREIs to comply with EN 15129 fixing methods for transmission of lateral force. For ease of installation and further reduction in construction cost, no recess was provided to the RP-FREIs (Figure 4) to assess the adequacy of a friction-only restraint for the isolators.

The same tests previously conducted with the RP-FREI system were repeated using the VC-FREI isolators. Two extra tests were performed at intensities above 100%: one using the Kobe record at 120% intensity and another using the Jiashi record at 125%.