Special Structures
Mezzanine Floors in Pre-Engineered Steel Buildings
Definition: A mezzanine floor is an intermediate structural floor built inside a pre-engineered building, partly or fully between the ground slab and the roof, supported on its own columns and beams or on the main frame columns, and used for offices, storage, plant rooms or production space.
Key facts
- Can be designed as a free-standing structure or integrated with the main building frame.
- Floor finishes are commonly a composite concrete slab on steel deck, or steel plate or grating for industrial use.
- Design live load depends on the intended use and is set by the governing building code.
- Vibration and deflection often govern beam sizes rather than strength alone.
Role in the structure
The mezzanine collects floor loads from the deck or slab and passes them through joists and main beams to mezzanine columns, or to brackets on the main frame columns, and then down to the foundations. Where it connects to the main frame it also adds vertical and lateral load to those columns, and it can act as a horizontal diaphragm that must be tied into the building bracing system or braced independently.
What is a mezzanine floor used for in a metal building?
Mezzanines add usable floor area without enlarging the building footprint. In warehouses they commonly carry offices, amenities or light storage above loading areas, while in factories they support plant rooms, control rooms, conveyor transfers or packing lines.
Because the space is already enclosed by the building envelope, a mezzanine is usually quicker and more economical than a separate structure. The trade-off is that its loads, column positions and headroom must be agreed early, since they influence the main frame, the foundations and the location of doors and services.
Structural arrangement: beams, joists and columns
A typical mezzanine uses primary beams running between columns, with secondary joists or cold-formed sections spanning between the beams to support the deck. Primary beams may be hot-rolled sections or built-up plate girders, and joists may be rolled sections, open-web joists or deep cold-formed members depending on span and load.
Columns are usually rolled or tubular sections on their own base plates and anchor bolts. Grid spacing is chosen to balance the cost of beams against the number of columns that interrupt the floor below, and is often aligned with the main frame bay spacing to simplify connections.
Free-standing versus frame-connected mezzanines
A free-standing mezzanine carries all of its own gravity and lateral loads, so it can be added, removed or relocated with little effect on the main building. It needs its own bracing or moment connections to resist sway.
A frame-connected mezzanine uses the main frame columns for part of its support and may rely on the building bracing for stability. This can save steel, but the main frame must be designed for the additional loads from the start. Adding a connected mezzanine to an existing building normally requires a structural check of the frames and foundations by the project engineer.
Floor decks and finishes
Composite steel deck with a reinforced concrete topping gives a stiff, quiet floor that suits offices and areas with wheeled traffic. Where loads are light or the floor must be dismantled later, steel deck with board finishes, chequer plate or open grating may be used instead.
The choice of deck affects the self-weight, fire performance, vibration behavior and construction sequence. Concrete toppings also require temporary propping or deck design for wet concrete, edge forms at openings and coordination of embedded items such as conduits and floor boxes.
Materials & coatings
- Hot-rolled or built-up structural steel beams and columns to ASTM, CSA, AS/NZS or EN grade families as specified
- Galvanized steel floor deck, composite or non-composite, with concrete topping where required
- Steel chequer plate or welded or pressed steel grating for industrial platforms
Design considerations
- Define the occupancy and design live load, plus any concentrated loads from racking, equipment or forklifts.
- Check floor vibration for offices and areas with walking traffic, not only static deflection.
- Decide whether the mezzanine is free-standing or tied to the main frame and design the frame accordingly.
- Confirm clear headroom below beams and above the finished floor against the code and the user needs.
- Provide lateral stability through bracing, moment frames or a diaphragm connected to braced lines.
- Coordinate fire resistance, means of escape, stair locations and guarding with the building code.
Fabrication
- Beams and columns are cut, drilled and fitted with end plates, cleats or seat connections in the shop.
- Shear studs for composite floors are usually welded on site through the deck, or in the shop where deck is not continuous.
- Surface preparation and coatings are applied to the specified system before dispatch.
Erection
- Set mezzanine columns on leveled base plates and secure anchor bolts before placing beams.
- Install primary beams, then joists, and complete bracing before loading the floor.
- Lay and fasten deck to the supporting steel in the pattern shown on the drawings.
- Prop deck where required and place concrete only after the steel and deck are inspected.
Inspection & quality
- Check column plumb, beam levels and grid dimensions against the erection drawings.
- Verify bolt grades, tightening and weld quality at all main connections.
- Inspect deck fastening, side-lap stitching and shear stud placement before concreting.
- Confirm handrails, toe boards and stair connections are complete before the floor is used.
What to include in your RFQ
- Mezzanine area, location in the building and finished floor level.
- Intended use, design live load and any equipment, racking or vehicle loads.
- Preferred floor finish: concrete on deck, plate, grating or board.
- Required column-free zones and clear headroom below the mezzanine.
- Stair, handrail, gate and loading-bay requirements.
- Fire resistance rating and applicable building code.
3D model & IFC of this component
Rotate the model, isolate the component and download its IFC4 file for your BIM coordination model.
IFC entity: IfcColumn + IfcBeam + IfcSlab. Illustrative geometry for coordination and learning. Final member sizes come from the project design.
Frequently Asked Questions
Can a mezzanine be added to an existing pre-engineered building?
Often yes, especially as a free-standing structure on new foundations. If any load is transferred to the existing frames, the frames and footings must first be checked by a qualified engineer against the governing code.
What floor finish is best for an office mezzanine?
A concrete topping on steel deck is the usual choice because it is stiff, reduces footfall noise and vibration, and suits floor coverings. The final choice depends on loads, fire requirements and the construction program.
Does a mezzanine need its own bracing?
A free-standing mezzanine needs its own lateral system, such as bracing or moment connections. A frame-connected mezzanine may share the building bracing, but only if the building was designed for it.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
