materials Article Highly Dissipative Materials for Damage Protection against Earthquake-Induced Structural Pounding Anna M. Str ˛ ek 1, *, Natalia Lasowicz 2 , Arkadiusz Kwiecie ´ n 1 , Boguslaw Zaj ˛ ac 1 and Robert Jankowski 2   Citation: Str˛ ek, A.M.; Lasowicz, N.; Kwiecie ´ n, A.; Zaj ˛ ac, B.; Jankowski, R. Highly Dissipative Materials for Damage Protection against Earthquake-Induced Structural Pounding. Materials 2021, 14, 3231. https://doi.org/10.3390/ma14123231 Academic Editor: Angelo Marcello Tarantino Received: 11 May 2021 Accepted: 8 June 2021 Published: 11 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Faculty of Civil Engineering, Cracow University of Technology, 31-155 Cracow, Poland; akwiecie@pk.edu.pl (A.K.); bozajac@pk.edu.pl (B.Z.) 2 Faculty of Civil and Environmental Engineering, Gdansk University of Technology, 80-283 Gdansk, Poland; natalia.lasowicz@pg.edu.pl (N.L.); jankowr@pg.edu.pl (R.J.) * Correspondence: anna.strek@pk.edu.pl Abstract: It is a common situation that seismic excitations may lead to collisions between adjacent civil engineering structures. This phenomenon, called earthquake-induced structural pounding, may result in serious damage or even the total collapse of the colliding structures. Filling the gap between two buildings erected close to one another by using visco-elastic materials can be considered to be one of the most effective methods to avoid seismic pounding. In this paper, a new polymer–metal composite material made of polyurethane and closed-cell aluminum foam is proposed as a pounding energy absorber for protection against earthquake hazards. The composite was created in two versions, with and without an adhesive interface. A series of experiments which reflect the conditions of seismic collision were performed: quasi-static compression, dynamic uniaxial compression and low-cycle dynamic compression with 10 loops of unloading at 10% strain. The composite material’s behavior was observed and compared with respect to uniform material specimens: polymer and metal foam. The experimental results showed that the maximum energy absorption efficiency in the case of the new material with the bonding layer was improved by 34% and 49% in quasi-static and dynamic conditions, respectively, in comparison to a sole polymer bumper. Furthermore, the newly proposed composites dissipated from 35% to 44% of the energy absorbed in the cyclic procedure, whereas the polymer specimen dissipated 25%. The capacity of the maintenance of the dissipative properties throughout the complete low-cycle loading was also satisfactory: it achieved an additional 100% to 300% of the energy dissipated in the first loading–unloading loop. Keywords: polyurethane; metal foam; polymer–metal composite; structural pounding; earthquakes 1. Introduction 1.1. The Issue of the Pounding of Structures due to Seismic Load, and the Existing Solutions Earthquakes are considered to be the most unpredictable and dangerous dynamic loads that can act on civil engineering structures [1–3]. Moreover, it is a very common situation that collisions between adjacent structures occur during seismic excitations due to their differences in structural dynamic parameters and the small gaps between them [4,5]. In most cases, the differences in mass or stiffness of neighboring buildings result in out- of-phase vibrations leading to collisions between them [6]. This phenomenon, called earthquake-induced structural pounding, appears when the relative structural displace- ment exceeds the in-between separation distance [7–9]. It was observed in the past that such situations may result in a large amount of damage or even collapses of neighboring structures [10–12]. During the earthquake in Mexico City in 1985, for example, structural pounding was one of the main causes of major damage [10]. Furthermore, after the Loma Prieta earthquake in 1989, the destructive influence of collisions was observed in more than 500 buildings that were located over 90 km from the epicenter [11]. Other exam- ples of earthquakes causing structural damage due to pounding were summarized by Anagnostopoulos [13]. Materials 2021, 14, 3231. https://doi.org/10.3390/ma14123231 https://www.mdpi.com/journal/materials