Mezzanine floors in existing warehouses

Mezzanine floors in existing warehouses

Adding a mezzanine is the cheapest way to increase usable area in an industrial building, and it is the job most often started without engineering because the structure looks simple from below.

The steelwork is simple. Where it lands is not.

The columns land on a slab that was designed for a forklift

An industrial slab is designed for distributed load and for wheel loads moving across it. A mezzanine column concentrates a substantial load into a single point, often a few hundred millimetres square.

That slab was almost never designed for it. The column can punch through, or settle locally, or crack the slab around it.

The usual answer is a pad footing under each column, cut into the slab and taken down to competent material. That means breaking out the slab, excavating, pouring, and reinstating, and it is most of the ground works cost on the job.

Whether the existing slab can be used without pads depends on its thickness, its reinforcement and what is under it, and none of those can be assumed. A geotechnical assessment and a slab investigation give the answer.

The floor loading has to match the actual use

Storage, office, plant and manufacturing are all different loads, and the difference between them is large.

Specifying a mezzanine for storage and then using it for racking with a forklift on it is the most common way one of these gets overloaded. Racking concentrates load into legs in the same way the columns do, one level up.

The floor also has to be designed for the imposed load plus whatever the deck build-up weighs, and for deflection, because a bouncy mezzanine floor is a complaint even when it is structurally fine.

Lateral stability

A mezzanine is a freestanding frame. It has to resist horizontal forces from people, from equipment moving on it, and in some cases from the building itself.

Bracing, moment connections or tying back into the building structure all work. Tying into the building is often the neatest option and it means the existing portal frames now have a load applied to them that they did not have before, so they have to be checked.

Fire and egress decide the approval

This is where mezzanine projects most often stall, and it is not a structural question.

The floor area, the use, the travel distance to an exit, the number of exits and whether the building's existing fire systems cover the new level all get assessed. A mezzanine used as an office is treated differently to one used for storage, and crossing a floor area threshold can trigger fire rating of the structure, additional exits, or sprinkler coverage.

Those requirements can change the design substantially, so they get established before the steel is designed rather than after.

Approvals

Most mezzanines require development consent or, where the standards allow it, a complying development certificate, plus a construction certificate before work starts.

The certifier will want structural documentation, fire engineering input where the use requires it, and confirmation the existing building can carry the additions.

If the building is leased, the lease usually has its own approval requirement and a make-good obligation at the end of the term.

The sequence

Establish the use and the load. Check the fire and egress consequences of that use. Investigate the slab and the ground. Then design.

Doing it in that order means the mezzanine you design is the one you are allowed to build. We run this work through structural engineering for industrial clients across Sydney.