Cyclic loading test of wall damping system with steel dampers

Abstract

Steel dampers, as an energy dissipation device, have been widely used to provide good seismic performance for building structures in strong seismic zones. Steel dampers for building structures are required to have adequate plastic deformation capacities so that they would not reach failure during strong ground motions and the aftershocks. However, the hysteretic behaviors of steel dampers considering the aftershock effects still remain unclear. In this study, wall damping system with a steel damper is proposed and tested. Steel damper in the wall damping system is installed horizontally at the bottom slit of the reinforced concrete wall aiming to dissipate energy during earthquake and prevent damage of the building structures, which could not only provide good seismic performance but could also be easily repaired after an earthquake. Test was conducted cyclically to simulate the effects of aftershocks, which can cause additional damage to already weakened structures from a main shock. Based on the test results, the seismic behavior of damping system was discussed with emphasis on the dissipated energy.

Keywords

aftershocks energy dissipation device seismic behavior steel damper wall damping system

Introduction

During the strong earthquakes, while many buildings were designed to avoid collapse, thus saving human lives, a large number of structural buildings suffered severe damage, where structural functions were destroyed (Miller, 1998; Nakashima et al., 1998). In conventional seismic design, designing for the life-safety performance level is considered adequate for ordinary structures (Architectural Institute of Korea (AIK), 2009; International Code Council (ICC), 2006). However, just to avoid structural collapse is not sufficient for modern buildings. The cost of non-structural components is much higher than that the cost of the structure itself and must be protected. Since it is important to restore buildings and the functions of the effected urban area as quickly as possible after an earthquake, the conventional seismic design methods are not effective when attempting to solve the issues caused by recent disasters.

As a response to the shortcomings inherent in the philosophy of conventional seismic design, a number of innovative approaches have been developed. For about 30 years, the development of energy dissipation devices for response control has increased in order to reduce the damage caused by earthquakes. Response control can be classified into three great divisions: passive control, active control, and semi-active control (Soong and Dargush, 1997). Passive control, by energy dissipation, is diversely applied depending on the yielding of steel material, phase transformation of material, sliding friction, deformation of visco-elasticity material, and so on. Among them, steel damper, noted for economic and productive advantage, can achieve elasto-plastic mechanism in various ways. Added damping and stiffness (ADAS) damper (Bergman and Goel, 1987), triangular-plate added damping and stiffness (TADAS) damper (Tsai et al., 1993), and Benavent-Climent et al. (2011) absorb energy through out-of-plane deformation of the steel plate. Slit damper (Benavent-Climent et al., 1998) operates by in-plane shear deformation mechanism of the steel plate retaining multiple openings and absorbs energy by strut bending deformation between the openings. Kim et al. (2012) developed cantilever-type steel damper that dissipates energy through bending deformation and experimentally evaluated its energy dissipation capability.

While steel damper is economical and easily applicable on structures, its performance reduces after experiencing earthquake-induced plastic deformation. Examination of earthquake occurrence properties reveals that despite the interval difference, multiple aftershocks occur after the main shock. In the event that a structure experiences a severe earthquake and the steel damper has undergone plastic deformation, swift replacement of damper is necessary to secure the building’s seismic capacity. However, in cases with high probability of imminent aftershock occurrence, it is too difficult to replace steel damper. Accordingly, if a steel damper that has a prior experience of plasticity receives additional seismic load, there is a need to understand the steel damper’s capacity against aftershocks (Pall and Pall, 2004). Kani et al. (2012) reported that during and after the 2011 Great East Japan Earthquake, all isolators of seismically isolated buildings worked well, but hysteretic dampers made of steel or lead worked hard due to many aftershocks, and some of these dampers were exchanged after so much consumption of energy resulting from earthquake vibrations. This result, therefore, indicates that the hysteretic behaviors of steel dampers considering the aftershock effects still remain unclear. To solve these issues, Jiao et al. (2015) conducted the dynamic loading tests to evaluate the low-cycle fatigue and hysteretic behavior of U-shaped steel dampers. Ene et al. (2015) also studied the experimental research on the bidirectional inelastic deformation capacity of U-shaped steel dampers for seismically isolated buildings.

In this article, a series of cyclic loading tests for wall damping systems with steel dampers were conducted to seek the answers to the above-mentioned problems. Cantilever-type steel damper was used as a damping device (Kim et al., 2012). The experiment program consists of two-step loadings. The specimens were all designed to conduct the initial loading up to the target story drift (1%, 1.5%, and 2% story drift ratio) and repeatedly conducted the secondary loading with the damper in a plastic range with residual deformation to achieve aftershock effect. The effects of repeated loading up to various initial target drifts on the hysteretic behavior and dissipated energy of wall damping system and steel damper were discussed through the experimental data.

Wall damping system

Concept of wall damping system

Figure 1 presents the configuration and behavioral principle of the wall damping system. The proposed system is composed of concrete wall, steel damper at the bottom of the wall, and seismic isolation device. This system allows the steel damper installed at the bottom of the wall to dissipate the majority of plastic energy and lets the primary structural member, concrete wall to retain its elasticity, which makes damage control possible. The seismic isolation device (isolator), consisted of laminated rubber from the natural rubber, burdens the weight of the corresponding concrete wall so that it aids the damper to exclusively resist against horizontal force. This device is designed to have large deformation capacity that is corresponsive to the damper’s horizontal deformation.

Figure 1.

Schematic drawing of wall damping system.

The installation of the damper at the bottom part, as shown in Figure 1, is to allow constructability convenience considering the construction environment in Korea. In terms of force transfer mechanism, although it is optimal to install the damper at the middle region of the wall where the moment is zero and only sear force exists, because constructability affects the atmosphere of the building due to field conditions, the damper was planned out to be positioned at the bottom of the wall.

The damper’s behavior is important during an earthquake, but seismic retrofit after the earthquake is also needed with urgency. The wall damping structure system proposed in this research has all of its junctions composed of bolts and is designed to allow easy replacement of a damaged damper after an earthquake with a new damper.

Damper design

The wall damping system proposed in the research is applied to cantilever-type steel damper (Figure 2), which demonstrated stable hysteretic behavior through the simple behavior of cantilever beam (Kim et al., 2013). This damper is devised to grant even distribution stress by increasing section width toward the fixed end of the damper depending on the cantilever element’s moment gradient.

Figure 2.

Damper model: (a) damper deformation and (b) idealization (Kim et al., 2012).

The characteristics of importance in damper design are yield strength and yield displacement, and the damper component model is idealized in Figure 2 for designing convenience. For the idealization of the damper, the circular part of the fixed end was assumed as a straight line, and the equivalent length (L′) of the damper components was calculated by referring to the research of Benavent-Climent et al. (1998). The yield strength of the damper components obtained using the fundamental principle of mechanics in cantilever behavior is as follows

Q_{y} = \frac{t B^{2} F_{ye}}{4 L'}

(1)

where B and b represent the depth of fixed-end part and free-end part, respectively; t represents the thickness of the damper components; L_T, L, and L′ represent the total length of the damper element, pure length, and effective length (= L + r²/L_T), respectively, where r represents the radius of the fixed end; F_ye is the expected yield stress (= R_yF_y), where F_y is the specified minimum (nominal) yield stress of steel to be used in the member; and R_y is the ratio of the expected yield stress to the specified minimum yield stress, F_y, of that material (AIK, 2009; American Institute of Steel Construction (AISC), 2010).

Ahn et al.’s (2012) research proposed the depth ratio (B/b) of the fixed end’s depth (B) to the free-end’s depth (b) as 1.25, 1.50, and 1.75, respectively, but this research selectively utilized steel damper with superior deformation capability with the depth ratio of 1.50. Based on simple beam theory, the calculated value of yield displacement (δ_y) and initial stiffness (K₁) of the damper with the depth ratio of 1.50 is as follows

δ_{y} = \frac{2 t B^{2} {L'}^{2} F_{y}}{E I_{N}} (\ln \frac{3}{2} - \frac{7}{18})

(2)

K_{1} = \frac{E I_{N}}{8 {L'}^{3} (\ln \frac{3}{2} - \frac{7}{18})}

(3)

where E represents the elastic modulus, and I_N represents the damper free-end’s geometrical moment of inertia (refer to Figure 2(b)).

Test program

Specimens

Figure 3 shows the specifications of the specimen. As described above, wall damping system is composed of reinforced concrete wall, steel damper, and seismic isolation device. For the damping system to properly function until the vibration control damper reaches its ultimate resistance force, all pin, bolts, gusset plates, brace extensions, and other components connected to the damper must be robustly designed (ASCE 7-05, 2005). Accordingly, the concrete wall attached to the damper was designed to have sufficient strength in consideration with the damper’s capability.

Figure 3.

Specifications of test specimen.

Table 1 presents the list of specimens. The specimens are composed of seven specimens of four series in total. All the specimens possess identical details and capabilities, and the only variable in the experiment is the loading method. MEED01 specimen is designed to conduct the 2% loading of targeted story drift angle only once and serves as the reference specimen in this research. The remaining specimens were all designed to conduct the initial loading up to the targeted story drift angle and conduct the secondary loading with the damper in a residual deformation to achieve aftershock effect (Figure 4). In particular, MEED02, MEED03, and MEED04 series specimens’ targeted story drift angle of the first loading was set as 1%, 1.5%, and 2%, respectively, in order to investigate the damper capacity in aftershocks according to the plastic residual deformation of the steel dampers. Consequentially, MEED02, MEED03, and MEED04 series specimens are all of one specimen, designed to conduct two loadings. Figure 5 shows the test result of isolator. Figure 5(a) presents the test setup, and Figure 5(b) shows the hysteresis of the seismic isolation device at each side and deformation configuration. The isolators are installed at the bottom of the wall, and the upper and bottom parts of the columns were processed as pin points to minimize deformation resulted from wall rotation and induce only horizontal displacement. The graph well illustrates the linear hysteresis of laminated rubber, and its insignificant strength and stiffness proves its little influence on the damper’s hysteresis. As mentioned above, the seismic isolation device only bears the self-weight of the concrete wall, and it aids the damper to exclusively resist against horizontal force due to the low lateral strength and stiffness in horizontal resisting capacity.

Table 1.

List of specimens.

Specimen		Target story drift (%)	Loading type	Aftershock effect
MEED01	MEED01	2.0	–	N/A
MEED02 series	MEED02	1.0	First loading	N/A
MEED02 series	MEED02A	2.0	Second loading	Yes
MEED03 series	MEED03	1.5	First loading	N/A
MEED03 series	MEED03A	2.0	Second loading	Yes
MEED04 series	MEED04	2.0	First loading	N/A
MEED04 series	MEED04A	2.0	Second loading	Yes

Figure 4.

Loading plan on aftershock effect.

Figure 5.

Test result of isolator: (a) test setup of isolator and (b) hysteresis curve and deformation of seismic isolation device.

Test setup and loading method

Figure 6 presents the test setup of the specimen. The specimen is installed inside a strong frame, and the upper and bottom parts of the columns were processed as pin points to minimize deformation resulted from specimen rotation and induce only horizontal displacement in order that the horizontal resistance capability of the damping device can be efficiently evaluated. The loading was conducted horizontally using the actuator with the capacity of 500 kN. In addition, the lateral support was installed at the upper frame to avoid out-of-plane deformation of the specimen.

Figure 6.

Test setup.

Figure 7 shows the loading protocol of the specimen. Since the most important variable in this experiment is the loading method, the loading methods for each specimen were all rendered and indicated. As for the reference specimen MEED01, its story drift angle undertook displacement control in the increasing order of 0.25%, 05%, 0.75%, 1.00%, 1.5%, and 2% rad, as observed in Figure 7(a), and two loadings were imposed per each step. MEED02 specimen received identical displacement control as the reference specimen, as shown in Figure 7(b), and the loading with the targeted story drift angle of 1% rad was imposed. Afterwards, MEED02A specimen, in the presence of the damper’s residual deformation, received loading by re-implementing displacement control with the targeted story drift angle of 2% rad. Upon completing MEED02’s experiment, the residual story drift angle was revealed as approximately 0.34% rad. In the identical method, MEED03 and MEED04 specimens also received loadings with the targeted story drift angle of 1.5% and 2.0% rad, respectively, and by examining the experiment result in advance, it can be observed that the post-experiment residual deformation angles were 0.75% and 1.10% rad, respectively.

Figure 7.

Loading protocol: (a) MEED01, (b) MEED02 series, (c) MEED03 series, and (d) MEED04 series.

Test result

Load–deformation relationship

Figure 8 shows the applied load–story drift angle relationship of each specimen, and Figure 9 presents the deformation components decomposed by respective element based on deformation data from displacement transducers. As shown in Figure 9(a), the deformation component of the damping wall system can be divided into damper horizontal deformation (δ_D), lateral deformation of the concrete wall (δ_C), and deformation by rigid-body rotation (δ_R), and the total displacement (δ_T) of the system can be described by the following equation

δ_{T} = δ_{D} + δ_{C} + δ_{R}

(4)

Figure 8.

Load–story drift relationship: (a) MEED01, (b) MEED02 series, (c) MEED03 series, and (d) MEED04 series.

Figure 9.

Example of components of lateral displacement (MEED01): (a) decomposition of deformation component, (b) measurement position of displacement transducers, (c) load–displacement relationship.

Figure 9(b) presents the measurement position of the displacement transducers. Total displacement is measured by the displacement transducer DT.1; damper’s horizontal displacement is measured by the average value of the displacement transducers DT.6 and DT.7; deformation by rigid-body rotation is measured by the displacement transducers DT.4 and DT.5; and shear deformation of the concrete wall is calculated either by the displacement transducers DT.2, DT.3, DT.4, and DT.5 or by excluding factors such as horizontal displacement of the damper and rigid rotation displacement from the whole and measured each deformation component.

Figure 8(a) presents the hysteresis curve of MEED01 specimen, and Figure 8(b) to (d) presents two curves simultaneously by each series, and the hysteresis curve of the second loaded experiment is shown by setting residual deformation angle as a starting point following the first-load termination. The proposed specimens exhibited stable hysteretic behavior during the target story drift angle, which sustained plastic deformation only at the steel dampers without any signs of damage to wall.

In the case of MEED01 specimen, it is shown that it performed stable hysteretic behavior up to targeted deformation angle of 2% rad. Deformation of the damper when story drift angle is at 2% can be observed in Figure 10. By referring to Figure 9(c), it can be observed that the deformation component (δ_D) of the steel damper shows good hysteresis loop. In the case of concrete wall, it is shown that it remains in elastic state in accordance with its hysteretic characteristic through wall deformation curve shown in Figure 9(c). Figure 11 shows the strain profiles of the steel damper, concrete, and the re-bars of the test specimen. The lateral axis shows the accumulative displacement, and the longitudinal axis shows the strain value. As shown in Figure 11, the maximum strain value in the dampers was approximately 1% and 2% in positive and negative bending, respectively, while the maximum strain value in the concrete and steel re-bars were approximately 0.025% and 0.006%. This result was confirmed through strain profiles, which showed that the reinforced concrete wall remained in the elastic range until the steel damper reached its large deformation state.

Figure 10.

Damper deformation relation to story drifts.

Figure 11.

Strain distribution (MEED01).

In the case of rigid-body rotation component, it shows slip behavior, and this is resulted from the difference in hole and pin of the 4-column hinges (refer to Figure 6) and from the crack on the upper part of the wall as shown in Figure 12. As a result, pinching phenomena partially appeared on each graph in Figure 8.

Figure 12.

Micro-crack on the damper after test (MEED04A).

In MEED02 series, as illustrated in Figure 8(b), MEED02 specimen was first loaded to the story drift angle of 1% rad, and then further loaded up to 2% rad story drift angle, under the presence of residual deformation by the plastic deformation of steel damper. According to the experiment result, despite loading specimen MEED02 when residual drift angle is at −0.34% rad, test result showed very stable behavior and similar hysteresis loop to that of specimen MEED01 was observed.

Series MEED03 and MEED04 were each tested with the aim of 1.5% and 2% rad story drift angle after specimens MEED03A and MEED04A loaded up to the residual story drift angle of −0.75% and −1.10% rad. The graph of MEED03A in Figure 8(c) shows poor hysteretic behavior when compared to that of specimen MEED02A. In the case of specimen MEED04A, the graph indicates that it seems to significantly lower the performance when compared to that of specimen MEED03A, and this is the result of decreased performance of damaged damper. In addition, as represented by the graph in Figure 8(d), specimen MEED04A clearly resulted in pinching phenomena, resulting that it had poor dissipated energy.

Figure 10 shows the steel damper’s deformation revelation by story drift angle. Proposed steel damper well displayed behavioral characteristics of simple cantilever beam, and no serious damage was done even when the story drift angle exceeded 3% rad. Figure 12 presents the damper deformation revelation of specimen MEED04A, and only a micro-crack occurred even when the maximum displacement was reached. Figure 13 presents the crack patterns of the concrete wall of specimen MEED04A before and after the experiment, which undertook the most severe experimental procedure. Despite the fact that repeated loading was performed twice, only the crack propagation of the concrete wall was observed, and it was confirmed that it remained in an elastic state, as shown in Figure 11.

Figure 13.

Cracks on concrete wall.

Damper behavior

As aforesaid, the steel damper develops good hysteretic behavior due to its plastic deformation capacity, but it needs to be replaced due to damper’s performance being degraded if force is applied after it experienced plastic deformation. However, the steel damper needs to resist the next loading if aftershock is applied right after the primary earthquake. Such case is simulated and is represented in Figure 14. Figure 14 shows the load–shear deformation angle relationship of the steel damper, and the shear deformation angle is the value of the damper’s horizontal displacement divided by the damper element’s height. In addition, two horizontal lines in each graph represent the damper’s yield strength (100 kN), and when applying the steel’s predicted yield stress, it matched the experiment result well.

Figure 14.

Load–shear deformation angle relationships of steel dampers (a) MEED01; (b) MEED02; (c) MEED03; (d) MEED04.

Hysteresis curve of specimen MEED01 well represents the stable hysteretic characteristics of the damper. Even after experiencing 5% rad of residual shear deformation, specimen MEED02A showed similar repeated characteristics like that of specimen MEED01’s experiment result. MEED03A experienced 10% rad of residual shear deformation, and it developed poor damper performance when compared to that of specimens MEED01’s or MEED02A’s results. Thus, the graph shows that due to residual deformation, hysteretic characteristics are slightly asymmetric to the negative side of the deformation angle. MEED04A, which experienced 15% rad of the residual shear deformation angle, showed significantly poor hysteresis behavior of damper because it already experienced large plastic deformation. When compared to MEED03A, it was acknowledged that the graph’s asymmetric property increased significantly, causing deformation angle’s damper hysteresis to shift significantly to the negative side and resulted in dissipation energy by hysteretic loop decreasing significantly as well.

Dissipated energy

The concrete walls of this test specimen were designed to be relatively strong, and the elasticity was confirmed through the experiment. Unlike the almost elasto-plastic behavior of the steel dampers (Figure 14), the behavior of the wall of MEED01 specimen was characterized by almost elastic or extremely narrow hysteretic loops (Figure 9). From this result, it is worth noting that in contrast to conventional concrete shear wall, the plastic deformation in the proposed structural systems were concentrated only at the steel dampers rather than at the walls.

Figure 15 shows the energy shares of each test specimen. The energy of each test specimen was obtained through the area of the hysteretic loop. Total dissipated energy, when MEED01 is set as the standard, resulted in dissipated energy of each 109%, 84%, and 60%, respectively, to MEED02A, MEED03A, and MEED04A. The damper’s dissipation energy ratio to the total dissipation energy of each specimen is 82%, 85%, 78%, and 61%, respectively, from MEED01 to MEED04A. This result shows that as the dissipation energy capacity of steel damper degrades, the total dissipated energy degrades in parallel.

Figure 15.

Dissipated energy.

Conclusion

In this research, the wall damping system with the steel damper is proposed, and repeated loading test was performed to evaluate dissipated energy. In addition, in order to simulate the possible aftershock effects shortly after the primary earthquake, steel damper was additionally loaded after it had already been deformed plastically due to initial loading, and the dissipated energy was evaluated by loading variable. The key conclusions are noted below:

The proposed structural system with steel damper and seismic isolation device installed at the bottom part of the reinforced concrete wall exhibits stable hysteretic behavior under various loading patterns, in which most of the energy is dissipated by the steel damper, and its elasticity is retained despite the occurrence of partial cracks.

It is believed that the energy dissipation is concentrated only at the slit dampers rather than at the reinforced concrete wall. Thus, the slit dampers can be replaced after an earthquake more readily than can walls, beams, and columns.

By referring to the load–shear deformation angle relation curves, it is observed that although the damper demonstrates good dissipated energy under second loading when the residual shear deformation angle is only at 5% rad, its capacity by residual shear deformation quantity significantly reduces when the residual shear deformation angle is above 10% rad. Total dissipated energy of MEED04A, which had a damper that experienced 15% rad of residual shear deformation, resulted in dissipated energy of 60% comparing to the reference specimen MEED01. Although the experiment was restricted, it can be assumed that if the residual plastic deformation of the steel damper is not extremely large, the dissipated energy of the steel damper is not completely exhausted even with the presence of aftershock effects.

By comparing each specimen’s energy dissipation capability, it was resulted that as the plastic deformation of steel damper increases, the total dissipated energy decreased significantly, and such phenomenon was resulted to be proportional to the damper’s energy dissipation quantity.

Unlike the conventional shear wall system, the load-carrying mechanism of the proposed structural system is not governed by shear force and bending moment but only by shear force. This mechanism demonstrates that the proposed system is well suited for middle- or low-rise building. Further research is thus necessary to investigate the behavior of a structural system equipped with slit damper so that a more generalized evaluation of seismic capacity of the proposed system can be achieved.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by Konkuk University in 2014.

References

Ahn

Kim

Park

. (2012) Experimental study on a cantilever type metallic damper for seismic retrofit of building structures. Journal of Korean Society of Steel Construction 24(2): 149–161.

Architectural Institute of Korea (AIK) (2009) Korean Building Code and Commentary (KBC 2009). Seoul, Korea: AIK.

ASCE 7-05 (2005) Minimum Design Loads for buildings and other Structures (ASCE/SEI7-05). Reston, VA: American Society of Civil Engineers.

Benavent-Climent

Akiyama

(1998) Ultimate energy absorption capacity of slit-type steel plates subjected to shear deformations. Journal of Structural and Construction Engineering 503: 139–147.

Bergman

Goel

(1987) Evaluation of cyclic testing of steel plate devices for added damping and stiffness. Report UMCE87-10. Ann Arbor, MI: University of Michigan.

Ene

Kishiki

Yamada

. (2015) Experimental study on the bidirectional inelastic deformation capacity of U-shaped steel dampers for seismic isolated buildings. Earthquake Engineering & Structural Dynamics. Epub ahead of print 27 August. DOI: 10.1002/eqe.2621.

International Code Council (ICC) (2006) International Building Code (IBC2006). Washington, DC: ICC.

Jiao

Kishiki

Yamada

. (2015) Low cyclic fatigue and hysteretic behavior of U-shaped steel dampers for seismically isolated buildings under dynamic cyclic loadings. Earthquake Engineering & Structural Dynamics 44(10): 1523–1538.

Kani

Ogino

Kitamura

. (2012) Effects of seismically-isolated buildings during the huge 2011 earthquake in Japan. In: Proceedings of 15th world conference on earthquake engineering, Lisbon, 24–28 September.

10.

Kim

Ahn

Kim

. (2012) Development of new steel damper for seismic retrofit of existing structures. In: Proceedings of 15th world conference on earthquake engineering, Lisbon, 24–28 September.

11.

Miller

(1998) Lessons learned from the Northridge earthquake. Engineering Structures 20(4–6): 249–260.

12.

Nakashima

Inoue

Tada

(1998) Classification of damage to steel buildings observed in the 1995 Hyogoken-Nanbu earthquake. Engineering Structures 20(4–6): 271–281.

13.

Pall

(2004) Performance-based design using pall friction dampers-an economical design solution. In: Proceedings of 13th world conference on earthquake engineering, Vancouver, BC, Canada, 1–6 August, paper no. 1955.

14.

Soong

Dargush

(1997) Passive Energy Dissipation Systems in Structural Engineering. Chichester: John Wiley & Sons.

15.

Tsai

Chen

Hong

. (1993) Design of steel triangular plate energy absorbers for seismic-resistant construction. Earthquake Spectra 9(3): 505–528.