Nonstructural elements (NEs) of a building consist of (building) elements/components and contents that are not included in the structural system of the facility. Nonstructural elements can be either single elements or distributed systems (e.g., networks). They are typically not intended to be load-bearing elements, and are often attached/connected to structural/building elements. Nonstructural elements can also be not connected to the structure/building and can be moved or relocated during their lifetime (e.g., furniture). The dynamic behavior of NEs is typically associated with high seismic risk, since NEs are often highly vulnerable and exposed to damage caused by seismic actions. The critical seismic response of NEs was highlighted by several recent earthquake events, e.g., the 2010 Darfield, 2011 Christchurch, and 2016 Kaikōura New Zealand earthquakes ^[1][2][3][1,2,3], the 2010 offshore Maule earthquake in Chile [4], the 2009 L’Aquila (Italy) earthquake [5], the 2012 Emilia (Italy) earthquake [6], the 2016 Central Italy earthquake [7], and the 2020 Petrinja (Croatia) earthquake [8]. In particular, these and other seismic events proved that damage to NEs can be significant even within buildings that exhibit minor or negligible structural damage, potentially resulting in major economic losses and casualties.

In most buildings, the biggest contributor to economic losses resulting from earthquakes is damage to NEs. In particular, they represent a large percentage of the total construction cost [9]. Figure 1 presents the cost distribution of four sample buildings including reinforced concrete (RC) residential buildings, hotels, office buildings, and hospitals, in the United States ^[9][10][9,10]. The cost of NEs is highest in hospitals, where approximately 92% of total construction costs are NEs; this number reduces to 87% for hotels, 82% for office buildings, and 60% for RC residential buildings. Furthermore, NEs are likely to exhibit moderate to severe damage even under relatively frequent earthquakes. Accordingly, the combination of major exposure and vulnerability makes NEs extremely critical in terms of seismic risk even over low to moderate seismicity areas.

Over the last twenty years, the research community has been investigating the seismic assessment of NEs, and copious amounts of the literature have been developed accordingly. As a result, the current European and international codes and standards (e.g., ^{[11][12][13][14][15][16]}[11,12,13,14,15,16]) refer to the Performance-Based Earthquake Engineering (PBEE) approach and methods for the design and assessment of NEs. Significant attention has been recently focused on the estimation of (building) floor accelerations for the assessment of the seismic demands on NEs in buildings ^{[17][18][19][20][21][22][23]}[17,18,19,20,21,22,23]. Existing codes establish rules and criteria for the seismic design and verification of NEs considering the effects of the earthquake in terms of forces (i.e., accelerations) and displacements (i.e., deformations). The dynamic proprieties of NEs should be typically known, in order to apply criteria and methods established by the codes and standards for the determination of seismic demand measures, whereas the seismic capacities of NEs should be estimated in order to carry out the design and assessment safety verifications. The evaluation of the seismic capacities shall be carried out via one of the following methods: (a) analysis, (b) (experimental) testing, (c) experience data, (d) a combination of methods (a), (b), and (c) ^[13][24][25][13,24,25]. The seismic assessment of NEs, aiming at producing robust standard capacity estimations and safety evaluations, is often referred to as seismic qualification and, in some cases, seismic certification ^[13][24][26][13,24,26].

Several analytical/numerical models have been developed in the literature for the seismic evaluation of NEs. However, each study is typically associated with a specific NE and is not generally extendable to different NE properties and characteristics ^{[27][28][29][30][31][32][33]}[27,28,29,30,31,32,33]. Experimental testing is the most common method used to assess NEs, as this is typically considered to be the most reliable and robust option. Many research activities are focused on the seismic assessment/qualification of NEs by experimental tests, such as on plasterboard partition walls ^{[34][35][36][37][38][39][40][41][42][43][44]}[34,35,36,37,38,39,40,41,42,43,44]; masonry infill walls ^{[45][46][47][48][49]}[45,46,47,48,49]; innovative partition walls ^{[50][51][52][53]}[50,51,52,53]; bracing systems for suspended ceilings [54] and suspended ceiling systems ^{[55][56][57][58][59][60]}[55,56,57,58,59,60]; curtain walls ^{[61][62][63][64]}[61,62,63,64]; technical equipment and systems: motor control centers (MCC) [65], cooling machines [66], riser pipe systems [67], hospital piping assemblies [68], pressurized fire sprinkler piping systems [69], medical gas and fire-protection pipeline joints [70]; hospital components ^[71][72][73][71,72,73]; museum artifacts and art objects ^[74][75][76][74,75,76], among many others. Seismic assessment and qualification through experience data might be theoretically more representative than other methods, since the NE response is estimated according to actual seismic events, but this is limited to specific characteristics of NEs, buildings, and seismic events. Regarding experimental test methods, testing protocols available in the literature generally vary depending on the type of NEs involved and the reference code. Finally, it is possible to develop hybrid methods to optimize the best characteristics of the single methods. Therefore, experimental testing is generally favored and considered to be more generally applicable and efficient, especially for critical and complex NEs.

2. Nonstructural Damage and Losses

Seismic events generally have four main effects on building NEs: (1) inertial or dynamic effects, (2) distortions imposed on NEs when the building structure deforms, (3) interaction at the interface between adjacent structures, and (4) interaction between adjacent NEs [77]. Seismic damage to NEs is associated with several parameters and features, including the dynamic properties of the NEs, NE location within the building (e.g., along height), NE vicinity to structures or other NEs, NE vicinity to structure anchorage or connection systems and properties. Nonstructural element damage may result in human losses and casualties, costly property damage to buildings and their contents, and functioning disruptions. In particular, the potential consequences of earthquake damage to NEs are typically divided into three types of risk referred to as the 3Ds: Deaths, Dollars, and Downtime [77]. The death type of risk is associated with the human losses caused by damage to or the response of NEs. Furthermore, life safety can also be affected in cases where damaged NEs obstruct safety pathways or exits [78]. Damage to life safety systems such as fire protection components or devices can also pose a safety concern in cases of fire after an earthquake or prior to fully restoring the system. Representative examples of critical and possibly hazardous NE damage include but are not limited to glass breaking, cabinets overturning, the collapse of ceilings and lighting system fixtures, the rupture of gas lines and other piping containing hazardous materials, damage to friable asbestos materials, the collapse of decorative molding parts, the failure of masonry infill walls, parapets, and chimneys (e.g., ^[9][79][9,79]). The property losses (Dollars) may be the result of direct damage to a NE or the consequences produced by its damage [77]. The loss can be associated with private or public properties and might be particularly critical (even priceless) in cases of damage to museums/historical monuments/facilities/objects or archives/storage facilities. For example, the 2016 Central Italy earthquake caused major damage to the stuccoes and decorations of monumental churches and historical palaces [7], and the 2009 L’Aquila earthquake caused damage and even destruction of historically valuable sculptures located in the Spanish Fortress (L’Aquila) and damage to furniture and artifacts in a church [1]. Several earthquake events have highlighted that the functioning of buildings can be typically disrupted by damage to NEs even though structural damage is not exhibited (e.g., they were designed according to modern codes). For example, severe damage to masonry infills and partitions was observed after the 2009 L’Aquila earthquake, involving many buildings that were designed and constructed not much earlier than the earthquake; this caused significant losses in terms of Downtime [5]. Post-earthquake downtime is highly critical when the building/facility function is considered vital, especially in the aftermath of the event, e.g., hospital and healthcare facilities, fire and police stations, manufacturing facilities, and government offices. Furthermore, downtime has a huge impact in situations where seismic and other emergencies/crises (e.g., sanitary emergencies) are combined. For instance, in 2020, in Croatia, the combination of the COVID-19 pandemic and two destructive earthquakes had an extremely critical impact on the population in terms of physical, economical, and social/psychological burden [8].

3. Nonstructural Element (NE) Classifications

Nonstructural elements can be divided into three broad categories according to their service and function, namely: (1) architectural elements, such as infill and partition walls, curtain walls, ceiling systems, and architectural ornamentations; (2) mechanical, electrical, and plumbing elements for example pumps, chillers, fans, air-handling units, motor control centers, electrical cabinets, distribution panels, transformers, and piping; (3) furniture, fixtures and equipment, and contents such as shelving and bookcases, industrial storage racks, medical records, computer and desktop equipment, wall- and ceiling-mounted TVs and monitors, industrial chemicals and hazardous substances, historical and cultural objects [77]. Nonstructural elements are also generally classified in relation to one or more response parameters of the structure, namely engineering demand parameters (EDPs), and the related damage sensitivity of NEs [9]. Seismic actions potentially cause damage to (building) NEs according to four main modalities, as depicted in Figure 2: (a) inertial or shaking effects, causing component motion/oscillation, sliding, rocking or overturning; (b) building deformations, damaging interconnected NEs; (c) building separations damaging NEs at the seismic building joints due to differential displacements or at the boundaries of the facility; (d) interaction between adjacent NEs having relative displacement/response [77]. According to the above-mentioned classification, NEs can be grouped into three classes: (a) force-sensitive or acceleration-sensitive NEs, where inertial forces or accelerations can be considered as main EDPs, (b) displacement-sensitive or interstory-drift-sensitive NEs, where relative displacements/deformations can be assumed as main EDPs, (c) combined force/displacement-sensitive NEs, where both inertial forces/acceleration and relative displacements/deformations can be considered as main EDPs. There might be cases in which NEs are particularly sensitive to alternative or additional EDPs, such as velocity, as can be observed with regard to rocking-dominated NEs [80]; therefore, these classifications represent the basic reference and might not be exhaustive over the wide range of NE scenarios, especially for components that exhibit complex seismic response, with regard to multiple response and damage mechanisms. Nonstructural elements can also be classified as a function of their dynamic parameters. In particular, ASCE 7-16 establishes in relation to the fundamental period of the NEs two types: flexible NEs, with a fundamental period larger than 0.06 s; and rigid NEs, with a fundamental period lower than or equal to 0.06 s. Finally, NEs can be defined according to the way they are built/assembled, e.g., the Italian code [12] defines two categories: built-on-site NEs and assembled/mounted-on-site NEs.

4. Testing Protocols

4.1. Outline

Seismic tests are typically classified as quasi-static or dynamic according to the testing procedure and protocol. Single-floor dynamic tests define the traditional and most common dynamic tests, which are often carried out by means of shake tables. More recently, multi-floor dynamic testing has been developed to replicate the actual seismic demands on NEs in a more accurate manner; in particular, this novel testing approach takes into account the response of the hosting buildings/facilities (in terms of both acceleration and deformation inputs) for the more representative and consistent seismic testing of NEs.

4.2. Quasi-Static Testing Protocols

Quasi-static tests are often performed to test displacement-sensitive NEs, or more generally, to test NEs that are sensitive to deformations. Nonstructural elements that can be tested according to this protocol include but are not limited to infill/cladding/partition panels (along their in-plane directions), piping and electrical network systems, fixed ladder systems, technical equipment fixed to multiple stories, and other systems that are connected to multiple building parts that might exhibit relative displacement under earthquake actions. Quasi-static tests are typically carried out by implementing cyclic loading procedures, i.e., slow load/deformation cycles, which can be conducted in force or deformation control. Overall, it is preferred to adopt a deformation-controlled testing protocol, except when forces govern the response of the NE or the deformation parameter is difficult to control. Quasi-static protocols should not be considered for NEs whose behavior is significantly conditioned by dynamic effects or is velocity-sensitive, including strain–rate-sensitive NEs. An example of the application of a quasi-static testing protocol is described in Pali et al. [37]. In this study, the authors designed a specific test setup for performing the in-plane cyclic tests on full-scale wall configurations according to the testing protocol. The test setup was built and assembled at the Test Laboratory of the Department of Structures for Engineering and Architecture at the University of Naples “Federico II”, and a schematic of the testing arrangement/setup and instrumentation is depicted in Figure 3. The test setup was a bidimensional frame made of S350JR steel-grade hot-rolled steel profiles, which reproduced the behavior of a typical story of a building structure. It was made of a bottom beam, a top beam, and 2 hinged columns. In particular, the bottom beam was fixed to the laboratory floor, whereas the top beam was connected to the hydraulic loading actuator by means of a sliding hinge used for avoiding vertical load components. The loading actuator had a stroke of 500 mm and a load capacity of 500 kN. The bottom and top beams were connected to each other by means of 2 vertical columns having rectangular hollow sections. The connections between columns and beams were uniaxial hinges with axes of rotation perpendicular to the plane of the testing frame. The out-of-plane displacements of the testing frame were avoided by 2 steel portal frames, having HEA140 vertical profiles equipped with roller wheels that allowed the top beam of the testing frame to slide in its plane. Therefore, the testing frame had negligible lateral in-plane stiffness. The indoor partition wall was infilled in the testing frame in such a way that it was surrounded along its perimeter by the top beam, bottom beam, and columns. Moreover, the setup was designed to allow the interposing of C25/30 strength-class 50 mm thick concrete blocks between specimens and testing frame, in such a way as to simulate the connections with a reinforced concrete building structure. In particular, concrete blocks were placed on the faces of beams (horizontal connections) and columns (vertical connections) of the testing frame facing the indoor partition wall.

4.3. Single-Floor Dynamic Testing Protocols

Single-floor dynamic tests are frequently performed to test acceleration-sensitive NEs. Examples of NEs that may be tested in accordance with this protocol include cabinets, storage racks, bookcases and shelves, appliances (refrigerators, washing machines, diesel generators), hoardings anchored on rooftops, antennas (communication towers on rooftops), horizontal projections (sunshades, canopies, and marquees), storage vessels, mechanical equipment (boilers and furnaces, HVAC equipment), hospital cabinets, and museum artifacts. Single-floor dynamic tests are generally performed through a shake table or earthquake simulator, which is usually characterized by one or more degrees of freedom (translational and rotational). Tests are conducted by assigning input signals to the earthquake simulator, which depending on different protocols may be generated differently (artificial or real earthquakes). Earthquake simulators often have performance limitations in terms of acceleration, velocity, displacement, and frequency range, so the assigned input signals must be adjusted/modified to be compatible with these limitations. However, the input signals must meet the spectral compatibility conditions required by the testing protocols. An example of the application of a single-floor dynamic testing protocol is described in Truong et al. ^[82][85]. In this paper, the authors carried out shaking table tests for seismic performance evaluation of a base-isolated Uninterruptible Power Supply (UPS). The tests were performed according to the protocol described later. Figure 4 presents the shaking table test setup for the UPS test specimen. The shake table used for the triaxial test has the following primary characteristics: 4.0 × 4.0 m plan dimensions, six degrees of freedom, a maximum payload of 300 kN, peak accelerations of 1.5, 1.5, and 1.0 g in the X, Y, and Z directions, respectively, for the maximum payload, and a maximum overturning moment of 1200 kNm. The shake table reproduces earthquake input ground motions through a system of eight hydraulic actuators. The UPS test specimen was anchored to an RC slab using pins or through isolator systems with anchor bolts. The concrete slab was connected to the shaking table using anchor bolts.