## 1 Introduction  
Load-bearing structural glass elements in buildings can take the form of simple members (columns, beams and plates) but also complex assemblies. While the research community is spending efforts for the development/refinement of safe and optimized rules for the design of glass structures (see for example [1; 2; 3], etc. ), most of the current issues are related to their intrinsic vulnerability. This is also the case of glass structures under dynamic/impact loads, or complex systems and glass stairs ([4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14], etc.) where--in addition to conventional safety design requirements ([1; 2] and Section 2)--excessive vibrations may involve unfavorable feelings and require dedicated interventions, at the design stage as well as during the service life. The dynamic performance of a given glass structure can be in fact affected by several parameters, like the loading features, the structural geometry,the properties of the involved materials and bonding layers or the presence of mechanical and adhesive restraints that could manifest a time/temperature/humidity-varying flexibility (see Figure 1 and [15; 16]). Even more attention is required for pedestrian glass structures that could be subjected to severe operational conditions, with pronounced human structure interaction (HSI) phenomena [17; 18]. This paper focuses on the vibration serviceability assessment of an in-service glass walkway in Italy. The pedestrian structure (see Figure 2 and Section 3) was built in the early 2000, in the monumental building of the Basilica of Aquileia (UNESCO World Heritage Site). The original design of the indoor suspension system resulted in a sandwich laminated glass (LG) slab, composed of a triple fully tempered (FT) section and polyvinyl butyral (PVB(r)) bonding layers. A sacrificial annealed (AN) layer was placed on the top of each LG panel, to obtain an LG + AN resisting section. A series of steel tendons and frame members were then used to support the glass plates, for up to \(\approx\)140 square meters of transparent walking surface. The research outcomes presented in this paper follow and extend the preliminary experimental investigation summarized in [16], where the vibration performance of monolithic glass samples under soft impact was assessed as a function of flexible mechanical supports (Figure 1b), including analytical calculations derived from classical dynamic theories [17] and Finite Element (FE) parametric studies (ABAQUS [19]). In addition, the current investigation aims at further exploring the vibration response of the glass walkway discussed in [20] for diagnostic purposes. More in detail, the attention is  
Figure 1: Dynamic performance of glass members: (**a**) Sensitivity of polyvinyl butyral (PVB®) stiffness to humidity/loading frequency (reproduced from [15] with permission from Elsevier, license n. 4585281507083, May 2019) and (**b**) flexible restraints (reproduced from [16] under CC BY 4.0 license). Figure 2: In-service pedestrian system object of analysis: (**a**) General/front view (photo by [PERSON], courtesy of So.Co.Ba) and (**b**) nominal cross-section of the glass slab. spent for the analysis of the vibration comfort levels that the system can actually offer, under several loading scenarios of technical interest. To this aim, original field experiments inclusive of 26 setup configurations and 20 volunteers are first presented. The sensitivity of fundamental dynamic estimates is hence explored, based on the experimental outcomes, with respect to several key parameters (i.e., walking features, number of occupants, ambient conditions, etc., see Section 4). Selected guideline documents for the human comfort assessment of pedestrian structures are then and applied to the case-study walkway, namely the Eurocode 0--Annex A2 [21] and ISO 10137 [22] methods, the SETRA approach [23] and the AISC Design Guide 11 [24], see Section 5. Finally, possible \"perception index\" and comfort scale evaluations are proposed in Section 6.