A warehouse is a building where a massive amount of goods can be stored before its large distribution to the consumers. Due to the increasing mass production of goods and increasing consumption levels, demand for storage space increases. As the land becomes every day more valuable in both economic and environmental terms, highly optimized, reliable, and safe warehouses are needed. Reduced running costs, automation, and larger sizes are the future trends of the warehouse sector. The need of bigger and optimized working spaces, together with the possibility to increase the market, led to the continuous development of the storage technology [1-2]. Automated Multi-Depth Shuttle Warehouses (AMSWs) are one of the most modern types of warehouses that provide a large surface occupation. They are a particular type of Automated Rack Supported Warehouses (ARSWs). At present, ARSW also known as Clad Rack warehouses are usually built by manufacturers specialized in structural systems for logistics with the same or similar cold-formed profiles used for Warehouse Storage Pallet Racks. However, in the case of ARSW, the rack forms the load-bearing structure of the whole building by itself. Figure 1 [3] shows this type of building during the construction.
There is no official specific reference document for the design of automated high-rise warehouses, which leads designers to adopt the rules and parameters conceived for steel buildings to these particular structures, without any control of the specific construction characteristics of ARSWs. To design this type of highly sophisticated non-building structures, designers usually refer to the EN 15512 [4] which provides principles for the structural design of pallet racking systems and EN 16681 [5] which indicates principles for the seismic design for pallet racking systems, applying them in a combination often more by their own experience and engineering practice rather than by well-established principles and rules supported by experimental evidence and theoretical research [6-7]. However, ARSWs are much larger, taller and complex systems with respect to usual pallet racks. As a consequence, the big question mark for producers, design offices, and experts is about the suitability of these norms to be used for high-tech massive, automated warehouse buildings.
The structural collapse of ARSWs may have large economic impacts not only for the owner but also, more in general, for the community, in terms of loss of vast amounts of goods/merchandise. Furthermore, despite the limited number of workers required in automated warehouses, the life of the employees working inside and around the warehouse might be at risk. Hence, the solution to the problems connected with the safe and reliable design of clad rack buildings has a huge economic and safety impact. Some ARSWs were damaged during the Emilia Earthquake. In particular, the ceramic warehouse in Sant’Agostino collapsed. Figure 2.a [8] shows that part of the structure collapsed, while another part of it was barely damaged. The main cause of the failure was inadequate lateral resistance in a longitudinal direction, where approximately 70% of the structure collapsed. The connection of the frames with the foundation was also very weak as the vertical elements were observed to fail at the vertical element-to-foundation interface [9] as indicated in Figure 2.b [9].
In this article, the focus is given to Automated Multi-Depth Shuttle Warehouses (AMSWs), as a specific ARSW type. AMSWs are compact systems providing large surface occupation and maximum storage density. In AMSWs, the “aisles” enable the navigation of machines (shuttle and satellites) to reach the storage positions. Semi-automatic pallet shuttle system minimizes operational time. Operators do not need to use forklifts to handle the goods, it is done by shuttle, which carries out the movements autonomously. By removing the need to drive forklifts into the lanes, storage capacity is increased in terms of depth, the risk of accidents and damage to the racks is negligible, operator movements are optimized, and warehouse operation is modernized and made more flexible [3]. The main elements of storage distribution in AMSWs are (Figure 3 [10]):
- Unit load – basic storage and transport unit. All the goods in AMSWs are stored in unit loads.
- Pallet – support for the unit load.
- Satellite – machine, that is controlled remotely. It distributes pallets to their position in the warehouse.
- Shuttle – machine that distributes satellites along the longitudinal direction of the warehouse.
In a typical AMSW satellite moves from the shuttle tunnel up to the rear of the single cell, transporting one unit load at a time. Shuttle moves from an elevator tower to the opposite one, transporting the satellite with the unit load on top of it.
Today, a limited number of studies about ARSWs and their structural behavior exists worldwide. Therefore, reference will also be made to the state of the art of storage rack systems, which represents the structural basis of all Rack Buildings. Different rack types were introduced by Pekoz (1973) [11]. Racking is constructed from steel components including upright frames, beams, and decking. Special beam to column (so-called upright) connections and bracing systems are used, in order to achieve a three-dimensional steel “sway” or “braced” structure. These structures mainly consist of cold-formed steel members. There are two principal directions of the structure – cross-aisle and down-aisle; they are indicated in Figure 4 [12].
In the cross-aisle direction, two (often perforated) upright sections linked together by a system of bracing members to provide lateral stability of the structure in this direction. Bracings are usually X, D, Z and K type. In the down-aisle direction, the main lateral force resisting system is provided by vertical bracing. Diagonal bracing members have pinned connection to the uprights. The beam is connected to upright through a specific connector, which behavior is semi-rigid in flexure. These joints can also provide lateral stability in the down-aisle direction in the absence of a vertical bracing system [12]. Numerical and experimental research performed on the seismic behavior of pallet rack structures were mainly focused on structural components. Such as beam-to-upright connection, base plate connection, compression and tension tests on structural elements [13-18]. Also, several full-scale shake table tests were performed on these structures [19-21]. Nowadays, there are only a few studies about the seismic behavior of clad rack warehouses in the literature. Kilar (2011) [22] performed research regarding high-rack steel structures (it is not an ARSW, but its structural behavior is close to clad racks). He concluded that the proposal of EN 1998-1 [23], regarding 5% of center of mass eccentricity for typical buildings is not applicable for high-rack structures where the eccentricity can reach up to 10% in specific loading conditions. Therefore, the critical condition during seismic event has been reached when rack was loaded between 55% and 80%. However, EN16681[5] states that the critical condition during the seismic event is reached when rack is 100% loaded. Haque (2012) [24] performed a specific research on clad rack structures. He investigated only frames in a down-aisle direction in moment resisting configuration using incremental dynamic time history analysis. Selected frames had 4, 6, 8 and 10 stories. It was concluded that moment-resisting frame configuration is not appropriate as a lateral load resisting system for high-rise clad racks. It has much higher flexibility comparing with traditional steel moment-resisting frames due to the limited energy dissipation capacity of the beam-to-upright connector. In addition, their inter-storey drift ratios were two times higher that of traditional steel structures. As a consequence, the bracing system in the down-aisle direction is needed. Caprili (2017, 2018) [25-26] concluded that ARSWs cannot be treated as storage rack systems and therefore, they have to be studied as a “building-like” structure. Numerical studies were performed using pushover analysis on not dissipative and dissipative concepts, in accordance with EN 1998-1 [23] and Italian building code NTC 2018 [27]. A comparison between two design concepts showed higher diffusion of the plastic hinges in the dissipative case. The main outcome of this research is that standards for typical buildings are not applicable for ARSWs and specific code for these structures is required.
AMSWs are complex structures, and their structural behavior is hard to predict analytically. The research made up to now is mainly limited to steel storage racks which are a much smaller scale of automated warehouses. Automated storage systems, which will probably be the future of the warehouse sector, have not been investigated to such an extent so far. In a few research works that were done on clad racks, some shortcomings of current design provisions have been noted. AMSW is treated as a structure in between typical storage racks and multi-story buildings without its own design code, which causes a lot of doubts for structural engineers all over the world during the design of these structures. Therefore, extensive research is needed to fully describe AMSW’s structural behavior and draft a new design code on it.
In our work, to quantify the structural performance of Automated Multi-Depth Shuttle Warehouses (AMSWs) in the context of low-to-moderate seismicity, we collected 5 different AMSW configurations designed by 5 European rack producers according to current standards (EN 15512 [4], EN 16681 [5]). As site location, Montopoli (Pisa, Italy) has been chosen, which is characterized by a peak ground acceleration (PGA) of 0.13g. We produced 15 accelerograms compatible with the chosen location and performed 150 time-history analysis by direct integration including P-Delta effects using a commercial numerical analysis software (SAP 2000 [28]). To limit the complexity and dimension of such structures, the efficiency of different yielding patterns has been assessed by performing numerical analyses with the simplified models (without warping effects). Based on the results:
- The most stressed members and corresponding failure mechanisms of AMSWs during low-to-moderate seismic actions have been obtained.
- Proposals to enhance the structural behavior of AMSWs during earthquake events in low-to-moderate seismicity regions have been made.