Performance of Seismically Isolated Structures during the Mw 7.4, Hualien, Taiwan, Earthquake on April 3, 2024

1. M_W 7.4, April 3, 2024, Hualien Earthquake, Taiwan

The April 3, 2024, Hualien earthquake occurred at 07:58:09 AM Taiwan time, registering a moment magnitude (M_W) of 7.4. The epicenter was located 25 km south-southeast of the Hualien County Government. The eastern and northwestern regions of Taiwan bore the brunt of the earthquake impact. The highest seismic intensity was recorded in Heping Township, Hualien County, with a peak ground acceleration (PGA) of 363.7 gal (0.37 g) and a peak ground velocity of 65.7 cm/s, followed by Hualien City with a PGA of 458 gal (0.47 g)  and a PGV of 56.3 cm/s. Analyses and statistics from the National Center for Research on Earthquake Engineering (NCREE) [1] indicated that the maximum PGA, 1491 gal (1.52 g) in the north-south direction, was recorded at the Taroko station. Additionally, fourteen stations spanning from the northeast to the central western regions of Taiwan detected near-fault pulse-like ground motions using the algorithm proposed by Shahi and Baker [2].

2. Summary of Post-Earthquake On-Site Inspections

The eastern and northwestern regions of Taiwan bore the greatest impact of the April 3, 2024, Hualien earthquake. Drawing from historical data [3-5], it marked the most significant seismic event experienced by numerous seismically isolated buildings in these areas. Following the earthquake, the Chinese Society of Seismic Isolation (CSSI), in collaboration with NCREE, conducted comprehensive inspections across the Hualien and Greater Taipei areas. Nineteen examples of hospital, school, government, and private residential buildings employing seismic isolation technology were examined (Fig. 1). Among these buildings (Table 1), ten have isolation at the base, while the others are mid-story isolated buildings; thirteen utilize elastomeric bearings, such as lead rubber bearings (LRBs) and high-damping rubber bearings (HDRBs), four employ sliding bearings, such as friction pendulum bearings (FPBs), and two adopt a combination of elastomeric bearings and sliding bearings. The findings from these inspections are summarized in this paper.

Table 1. Statistics of isolated buildings inspected following the April 3, 2024, Hualien earthquake

Number	Building name	Location	Plane of isolation	Isolator type
a	He Xin Building of Hualien Tzu Chi Medical Center	Hualien County	base	elastomeric, sliding
b	Lee-Shinn Sky Vision	Yilan County	base	elastomeric, sliding
c	Taipei Laboratory of National Laboratory Animal Center	Taipei City	base	elastomeric
d	Oscar Tan Mei	Taipei City	mid-story	elastomeric
e	Sotai Di Tian Tai	Taipei City	mid-story	elastomeric
f	Civil Engineering Research Building of National Taiwan University	Taipei City	mid-story	elastomeric
g	Taipei Performing Arts Center	Taipei City	base	pendulum
h	Dahin Blue Ocean Residence	New Taipei City	base	elastomeric
i	Mercedes-Benz Guandu	New Taipei City	base	pendulum
j	Oscar Yong Ju Ren Ai	Taipei City	mid-story	elastomeric
k	Shang Guan Jin Dian	Taipei City	mid-story	pendulum
l	Zhong Zheng Yenjim	Taipei City	mid-story	elastomeric
m	Lian Lian Tai Da	Taipei City	mid-story	elastomeric
n	Taipei Far Eastern Telecom Park	New Taipei City	mid-story	elastomeric
o	Taipei Tzu Chi Hospital	New Taipei City	base	elastomeric
p	Taipei Emergency Operation Center	Taipei City	base	elastomeric
q	Hanhuang Wu Yue	New Taipei City	mid-story	elastomeric
r	Caihui Lai Yin	New Taipei City	base	elastomeric
s	Novotel Taipei Taoyuan International Airport	Taoyuan City	base	pendulum

1. None of the seismically isolated buildings exhibited structural damage, and the isolation bearings and damping devices showed no signs of damage.
2. In the superstructures of the seismically isolated buildings, there was almost no damage to non-structural components or contents.
3. Components spanning the isolation gaps (such as movable cover plates for walkways, driveways, and suspended elevators, as well as architectural finishes) experienced compression, deformation, or collision damage due to insufficient isolation gap allowances (Fig. 2).
4. Damage or extrusion of fireproof mortar or sealants used on the fire covers of the isolation units occurred due to the movement of the isolation systems.
5. Local damage to non-structural materials resulted from insufficient vertical space for isolation movement.
6. Waterproofing materials in the gaps between the superstructures and substructures were squeezed, or deformed due to the motion of the isolation systems.
7. Some structures equipped with instrumentation systems experienced electrical system failures and loss of sensor functionality due to inadequate maintenance.
8. Some minor permanent residual displacements occurred due to the functioning of the isolation systems, which is a normal phenomenon and does not impact future isolation system performance. 

For instance, the Lee-Shinn Sky Vision building (Fig. 3(a)) (designated as (b) in Fig. 1), located in Yilan County, experienced the second-largest isolation displacement during this earthquake event (the largest being the He Xin Building of the Hualien Tzu Chi Medical Center, which is extensively discussed in Section 4.). It is a steel-reinforced-concrete, base-isolated structure with fifteen above-ground floors and two basement levels, with the isolation system located at the first basement floor. During the earthquake, a camera captured the actual performance of one LRB. Based on preliminary assessment from the recorded video, the maximum isolation displacement is estimated to be approximately 250 mm (Fig.3(b)). While this building suffered no structural damage, minor and reparable damage to the movable cover plates between the superstructure and substructure in the lobby was observed (Fig. 3(c)).

Fig 2: Damages due to inadequate isolation gap allowances

Fig 3: Lee-Shinn Sky Vision buildings (Courtesy: https://www.twmz.com.tw)

It is imperative to emphasize the hazard associated with isolation gaps in seismically isolated buildings during earthquakes and promote safety awareness with building occupants and the general public. Through the investigation process, it was observed that users of seismically isolated buildings generally recognized the effectiveness of seismic isolation design and understood that the seismic isolation system would undergo significant movement during an earthquake. However, they lacked precise awareness of the extent of this movement. Thus, it is crucial to promptly evacuate from areas that may pose a danger during earthquakes. The following summarizes the observed hazards.

1. Stairways connecting the superstructure and substructure may undergo movement at specific locations during earthquakes, which can vary depending on the architectural design. Such movements pose a risk of accidental injuries or falls.
2. In buildings employing suspended elevator systems, the substructure often features movable cover plates spanning the isolation gap, typically hinged at the elevator side (moving with the superstructure) and free to move at the other side. During earthquakes, individuals in proximity to the free end of these cover plates may be at risk of severe injury.
3. Similarly, individuals should avoid being in close proximity to movable cover plates, extension plates, decorative details, or piping systems  that move with the superstructure while situated in the substructure or on the ground during earthquakes. However, due to the architectural design, isolation gaps may be difficult to discern or they may obscured by decorative elements, thereby increasing the risk of accidental injury during earthquake movement.

Therefore, in addition to ongoing safety promotion efforts, it is important to clearly mark all junctions and expansions between the superstructure and substructure. Caution zones should be indicated to alert individuals to avoid stepping into these areas during earthquakes. Additionally, all related decorative details should allow for sufficient horizontal and vertical isolation gaps to prevent collisions and compression, thereby reducing the risk of falling debris and accidental injury during earthquakes.

3. Civil Engineering Research Building of National Taiwan University

The Civil Engineering Research Building of the National Taiwan University (Fig. 4(a)) [6, 7] is a precast reinforced-concrete, mid-story seismically isolated structure, comprising one basement level, nine above-ground floors, and two penthouse levels. The isolation level is situated at the second floor. The building includes laboratories, professors’ research offices, administrative offices, student research rooms, and multipurpose classrooms. The isolation system (Fig. 4(b)) consists of nineteen LRBs with a diameter of 900 mm, along with two fluid viscous dampers (VDs) in the longitudinal direction and four in the transverse direction. The dampers have a stroke of ±500 mm, a maximum force of 100 tons, and a nonlinear exponent of 0.6.

Following the April 3, 2024, Hualien earthquake, an immediate inspection was conducted, confirming that neither the structure nor the isolation system sustained damage. However, some minor non-structural damage occurred due to the movement of the isolation system, which are easily repairable. As shown in Fig. 5(a), slight misalignment and compression deformation was observed in the movable cover plates for the suspended elevator below the isolation level. Additionally, the waterproof sealing strip between the superstructure and substructure was squeezed, and its fasteners loosened (Fig. 5(b)). Furthermore, the flexible fire-resistant material between the fireproof covers of the isolation units sustained damage, deformation, or extrusion due to the deformation of the isolation units (Fig. 5(c)).

Fig 4: NTU Civil Engineering Research Building

Fig 5: Non-structural damage, NTU Civil Engineering Research Building

The structure is equipped with twenty-seven accelerometers and four displacement transducers. The accelerometers are located on the basement floor (B1F), the second floor (2F, below the isolation layer), the third floor (3F, above the isolation layer), the sixth floor (6F), and the roof (RF). The displacement transducers are installed at the isolation level. The 5%-damped response spectra of the longitudinal and transverse acceleration records for the B1F during the April 3, 2024, Hualien earthquake are shown in Fig. 6. The recorded maximum displacements in the longitudinal and transverse directions were 21 mm and 16 mm, respectively (Fig. 7). According to the project production bearing test report [8], the yield displacement of the LRBs is between 10 mm and 30 mm, which indicates that the LRBs were in the initial stage of yielding during the earthquake. A summary of the recorded peak accelerations in the longitudinal and transverse directions for each floor (Table 2) revealed no significant dynamic amplification in the superstructure. The reduction efficiencies in the longitudinal and transverse directions during this earthquake event were 20% (=(1-0.8)×100%) and 36% (=(1-0.64)×100%), respectively.

Table 2. Peak accelerations records from NTU Civil Engineering Research Building

Floor	Peak acceleration (gal/g)
	Longitudinal	Transverse
RF	95.90 / 0.1	97.13 / 0.1
6F	61.05 / 0.06	64.10 / 0.07
3F	77.11 / 0.08	60.43 / 0.06
2F	96.76 / 0.1	94.72 / 0.1
B1F	75.62 / 0.08	89.56 / 0.09
	Amplification ratio
RF/2F	0.99	1.03
6F/2F	0.63	0.68
3F/2F	0.80	0.64
2F/B1F	1.28	1.06

4. He Xin Building of Hualien Tzu Chi Medical Center

The Hualien Tzu Chi Medical Center stands as the premier medical facility in eastern Taiwan (Fig. 8(a)). The He Xin Building of the medical center primarily houses emergency, operating rooms, and general and intensive care wards. It is a steel-reinforced-concrete, seismically base-isolated structure with eleven above-ground floors and one basement level. The isolation system is located beneath the first basement floor. This system, designed to align the stiffness center with the structural mass center, consists of seventy-four LRBs with diameters ranging from 800 mm to 1200 mm, along with fourteen flat sliding bearings.

An inspection conducted after the April 3, 2024, Hualien earthquake revealed no structural damage to the He Xin Building. However, some non-structural components suffered damage due to inadequate allowance for isolation movement (Fig. 9(a)-(c)). In contrast, the Xie Li Building, another traditional aseismic structure within the Hualien Tzu Chi Medical Center, experienced various structural and non-structural damage. The contrasting responses of the buildings demonstrates that seismic isolation not only effectively reduces structural seismic response but also significantly lowers the risk of damage to non-structural components and equipment.

Fig. 8. He Xin Building of Hualien Tzu Chi Medical Center

Fig. 9. Non-structural damages of He Xin Building

The He Xin Building is equipped with twenty-six accelerometers and four displacement transducers. The accelerometers are installed on the foundation (B2F, below the isolation level), the first basement floor (B1F, above the isolation level), the fourth floor (4F), the fifth floor (5F), and the roof floor (RF). Each floor is outfitted with both longitudinal and transverse accelerometers, installed at both the centers and corners of the floor plan. The displacement transducers are installed at the center and corner of the isolation level in the longitudinal and transverse directions. However, these displacement transducers were damaged prior to the April 3, 2024, Hualien earthquake. The 5%-damped response spectra of the longitudinal and transverse acceleration records at the center of B2F during this earthquake event are shown in Fig. 10. Analysis of the recorded data (Fig. 11), revealed no evidence of structural torsion during this earthquake event, as indicated by the closeness of the plan center and corner records. Moreover, the responses transmitted through the isolation system demonstrated reduced acceleration and an elongated vibration period. The peak acceleration values recorded from each floor during the earthquake, as presented in Table 3, indicate a reduction efficiency ranging between 20% and 30% in the longitudinal direction, with a less pronounced reduction in the transverse direction, albeit without significant signs of dynamic amplification. It is primarily because, as shown in Fig. 10, the long-period spectral acceleration responses in the transverse direction far exceed those in the longitudinal direction. The spectral acceleration at the corresponding effective period with the corresponding equivalent damping ratio is close to or even larger than the PGA value. Therefore, an insignificant reduction or even slight amplification in acceleration in the transverse direction is unsurprising.

Table 3. Peak accelerations records from He Xin Building

Floor	Peak acceleration (gal/g)
	Longitudinal	Transverse
RF	152.36 / 0.16	290.84 / 0.30
5F	148.58 / 0.15	234.87 / 0.24
4F	135.53 / 0.14	226.21 / 0.23
B1F	152.11 / 0.16	196.84 / 0.20
B2F	193.53 / 0.20	205.19 / 0.21
	Amplification ratio
RF/2F	0.79	1.42
6F/2F	0.77	1.14
3F/2F	0.70	1.10
2F/B1F	0.79	0.96

The acceleration history recorded at the center of B2F served as input for numerical analysis (Fig. 8(b)). Incorporating the stiffness contributed by non-structural walls, the predicted acceleration at the center of B1F is shown in Fig. 12, indicating the reliability of the numerical model and the accuracy of the predictions. Additionally, the estimated isolation displacement is shown in Fig. 13, with maximum displacements of 270 mm and 450 mm in the longitudinal and transverse directions, respectively. The value of 450 mm represents the largest isolation displacement observed during the April 3, 2024, Hualien earthquake, and is the largest recorded in an earthquake since the implementation of seismic isolation technology in Taiwan.

Summary

The April 3, 2024, Hualien earthquake induced significant structural responses in numerous seismically isolated buildings located in the eastern and northwestern regions of Taiwan. Thanks to the effective function of the isolation systems, buildings like the He Xin Building of the Hualien Tzu Chi Medical Center, even when exposed to a strong near-fault earthquake, sustained no structural damage. Nevertheless, concerning damage to non-structural components in seismically isolated buildings due to significant isolation displacement, two crucial issues arise: whether adequate isolation gap allowances have been made for isolation movement and whether building occupants can safely navigate hazards during isolation movement in an earthquake. These are important issues that warrant careful consideration. The instrumentation data of the seismically isolated buildings (not limited to those discussed herein) necessitates further analysis and examination. This includes the characteristics of input response spectra, structural frequency responses, and comparisons between actual responses and numerical predictions.

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