Self-supporting steel staircase for an automated warehouse

Key data

Use	Access for maintenance and repair only to the warehouse levels
Overall height	over 23 m
Plan footprint	approx. 4.20 x 2.50 m
Typical module	Two opposing flights over a 3.350 m rise
Columns	HE 240 A
Beams	UNP 200 (transverse) and 80x80x5 hollow sections (longitudinal)
Stringers and bracing	250x10 flat bars and 28 mm round bar bracing
Seismic action	Ground type C, behaviour factor q = 1.5
Design code	Italian NTC 2018

The brief

To design and verify a self-supporting steel staircase serving the levels of an automated warehouse, intended for maintenance and repair access only. The structure stands outside the building and must be independent of the warehouse, without transferring its horizontal actions to it.

Structural scheme

The staircase is arranged as typical modules repeated up the height, each with two opposing flights covering a 3.350 m rise. In plan the structure measures approximately 4.20 x 2.50 m and is defined by four corner columns in HE 240 A, connected by UNP 200 transverse beams and 80x80x5 hollow section longitudinal beams. The flight stringers are 250x10 flat bars. Horizontal stability is provided by 28 mm round bar cross bracing on the free faces, while the face along which the flights develop necessarily remains open for passage.

The design issue: slenderness and horizontal actions

A structure over 23 m high on a base of just 4.20 x 2.50 m has a very high slenderness ratio: its behaviour is governed by horizontal actions and displacements rather than by vertical loads, which for a maintenance-only staircase are modest. Sizing is therefore driven by the stiffness of the bracing system and by the forces transmitted to the columns and foundations, with particular attention to second-order effects and to limiting interstorey drift.

Actions considered

Standing outdoors, the structure is also subject to wind action — significant precisely because of its slenderness and the exposed area of treads, landings and railings — and to snow action on the landings. Seismic action was assessed to the Italian code NTC 2018 with a design spectrum at the life-safety limit state, ground type C and behaviour factor q = 1.5, consistent with the low-dissipative structural behaviour appropriate to a braced service structure.

The result

Complete calculation report with three-dimensional modelling, verification of the structural members and connections, seismic analysis to NTC 2018 and verification of wind and snow actions, supporting construction and the filing of the documentation.