Steel mezzanine floors

What it is

A steel mezzanine is an intermediate floor built inside a building to add usable area without extending the footprint – for storage, picking, offices, plant rooms or production lines. It is a free-standing structure of columns, primary and secondary beams, bracing and a deck, with stairs, edge guardrails or balustrades, and pallet gates for loading.

World Steel supplies mezzanines both inside its pre-engineered buildings – see PEB mezzanine floor systems – and as stand-alone packages for existing warehouses and factories. The mezzanine is designed or checked, modelled in Tekla, fabricated, finished and shipped as one bolted kit.

Types and options

Column grids are typically chosen to balance beam depth against the number of columns obstructing the floor below; longer spans reduce columns but increase steel weight and headroom loss. Edge options include guardrails with toe boards, mesh or glass balustrades, up-and-over or sliding pallet gates, and stair access by straight or switchback stairs. Columns sit on base plates anchored to the slab, whose capacity under the concentrated column loads must be confirmed.

Design and compliance

United States. IBC Section 505 limits the aggregate area of a mezzanine to one-third of the room below (with exceptions), and IBC Chapter 10 governs egress, stairs and guards in occupied areas: guards at least 42 in (1,070 mm), stair risers 4–7 in and treads at least 11 in. Floor live loads come from ASCE 7 Table 4.3-1 – 50 psf (2.4 kPa) offices, 125 psf (6.0 kPa) light storage, 250 psf (12.0 kPa) heavy storage. Design is to AISC 360; where the mezzanine is part of a metal building delivery, World Steel's IAS AC472 accreditation supports the fabricator approval in IBC 1704.2.5.1.

Australia. Floor imposed actions come from AS/NZS 1170.1, and the steel is designed to AS 4100 (AS/NZS 4600 for cold-formed joists) and fabricated to AS/NZS 5131. Public or occupied areas follow NCC 2022 – stair risers 115–190 mm, goings 250–355 mm, barriers at least 1000 mm with a 125 mm sphere rule – while plant and storage mezzanines used only by workers may be accessed via AS 1657 stairs. See Australian standards.

Europe. Imposed loads follow EN 1991-1-1 by category (for example B offices, E1 storage at least 7.5 kN/m²), design follows EN 1993-1-1, and the structure is CE-marked under EN 1090-1 and executed to EN 1090-2 in the class set by the designer. Guard heights and fire requirements are set by national building regulations. See European standards.

Fire resistance, sprinklers below the deck and egress distances are often the deciding factors for a mezzanine. Check them with the local authority early – see structural fire protection.

Fabrication and finishing at World Steel

Columns and beams are CNC cut and drilled with end plates and fin plates welded by SAW, FCAW or GMAW to AWS D1.1, AS/NZS 1554.1 or EN ISO 3834-2 procedures, so site work is bolting only. Shear studs for composite decks are welded on site or in the shop as specified. Primary steel is normally blast-cleaned and painted to EN ISO 12944 or AS/NZS 2312.1 for indoor exposure, with intumescent coatings where a fire rating is needed; stairs, rails and grating are usually hot-dip galvanized by qualified galvanizers.

Each mezzanine is packed as a kit with marked members, bolt sets and a packing list, sized for container shipping.

Installation and specifying

• Survey the existing slab, columns, services and sprinklers before design; confirm slab capacity under column loads.
• Erect columns and primary beams, plumb and brace, then fit secondary beams, deck, stairs and guardrails.
• Specify the use and floor load, plan size and column grid limits, clear height below and above, deck type, access and fire rating.
• State whether the mezzanine is a workplace or public area – it changes stair and guard rules.