Changing the Use of a Commercial Building? When the Floor Needs a Structural Engineer

Commercial buildings in Melbourne change hands and change tenants all the time. A ground floor shop becomes a pilates studio, a first floor office becomes a records store, a warehouse becomes an indoor sports centre. The fit out drawings usually focus on partitions, lighting and services, and the structure is assumed to be fine because it has stood for years. That assumption is where problems start. The floor was designed for the loads of its original use, and a new use can ask for much more.

This article explains when a change of use in a commercial building needs a structural engineer, which loads change, how an existing floor is checked and what the building surveyor will ask for before issuing a permit. For the wider picture of commercial work, see Principal Built Engineering's structural engineer for commercial buildings in Melbourne page.

What counts as a change of use?

In building terms, a change of use happens when the way a space is occupied changes enough that a different set of rules applies to it. Under the National Construction Code, every building or part of a building has a class: Class 5 for offices, Class 6 for shops and cafes, Class 7b for warehouses and storage, Class 8 for factories and Class 9b for assembly buildings such as gyms, halls and function rooms. Moving from one class to another is a change of use, and in Victoria it normally needs a building permit.

Structurally, the question is a little different. Two uses can sit in the same building class and still place very different loads on the floor. A quiet office and a busy call centre are both offices, but a compactus filing room within that office is a storage load. The engineer looks at the actual activity, the equipment and the stored goods, not just the class on the permit.

Why does the new use change the floor load?

Australian floors are designed to AS/NZS 1170.1, which sets the imposed load a floor must carry for each type of activity. These loads cover people, furniture, equipment and stored goods. The table below shows typical uniformly distributed imposed actions from that standard for some common commercial uses. The engineer also checks concentrated loads, partition loads and any specific equipment, so these figures are a guide to the size of the change, not a design.

The pattern is clear. Moving to a use where people crowd, jump or move in rhythm, or where goods are stacked, can ask far more of the floor than its original use did. Gyms add a second issue: dropped weights and group classes create impact and vibration that a static load figure does not capture. Storage adds a third: racking and compactus units put heavy loads on small areas of floor.

Which changes of use most often need a floor check?

Some changes are low risk. Turning a shop into an office usually reduces the floor load. Others almost always need an engineer before the lease is signed or the fit out starts.

• Office or retail space becoming a gym, a martial arts studio, a dance studio or a pilates studio.
• Any suspended floor becoming storage, an archive, a library or a compactus room.
• A warehouse floor or mezzanine taking racking, heavy machinery or a forklift for the first time.
• Office or retail space becoming a function room, a church, a school or a childcare centre.
• A commercial kitchen with cool rooms, heavy cooking equipment or new roof plant.
• Medical or dental fit outs with imaging equipment, which can be both heavy and sensitive to vibration.
• New rooftop plant, solar arrays or signage added as part of the change.

Ground bearing concrete slabs in warehouses are often more forgiving than suspended floors, but they still need checking for racking legs, forklift wheels and point loads from machinery. The article on how industrial slabs and hardstands are designed explains why.

How does a structural engineer check an existing floor?

The check works out what the floor was built to carry and compares it with what the new use needs. A typical process looks like this.

Original drawings make a large difference. Where they exist, the engineer can read the design loads, the reinforcement and the member sizes directly. Where they do not, the engineer works from what can be seen and measured, and may recommend limited investigation such as locating reinforcement with a cover meter or taking concrete cores, so that the capacity is based on evidence rather than guesswork.

The result is usually set out in a structural engineer report or a structural adequacy statement that the building surveyor can rely on. The related article on structural adequacy reports for commercial buildings explains what these documents contain.

What if the floor is not strong enough?

A floor that falls short is not the end of the project. There are several ways forward, and the right one depends on how large the shortfall is and where it sits in the building.

Strengthening a floor in an occupied building takes planning. Access, noise, fire separation and the tenants below all need to be considered, and the building surveyor will want the strengthening design included in the permit documents. Mezzanines raise their own questions, covered on the mezzanine floor structural engineer page and in the article on how much weight a mezzanine floor can hold.

What does the building surveyor need?

When a change of use needs a building permit, the building surveyor has to be satisfied that the building is suitable for its new class, including its structure. The documents usually requested from the structural engineer are:

• A statement or report confirming the existing structure is adequate for the new use, with the design loads stated.
• Structural drawings and calculations for any strengthening, new supports or plant platforms.
• Any load limits or conditions of use, so they can be noted on the permit.
• A certificate of compliance for the structural design where the surveyor requires one.

Fire, access, egress, toilets and energy requirements also change with the building class. Those sit with the building surveyor and other consultants, but the structural answer often decides whether the change of use is practical at all, so it is worth getting first. For fit outs where the use is not changing, the article on whether a commercial fit out needs a structural engineer covers the common triggers.

When should the structural engineer be engaged?

The best time is before the lease or purchase is final. At that stage a short desktop review of the drawings, followed by a site inspection if needed, can confirm whether the floor suits the planned use and flag any strengthening early enough to be priced into the deal. Engaging the engineer after the fit out design is complete, or after the building surveyor has asked for evidence, tends to cause redesign and delay.

Principal Built Engineering provides structural engineering services in Melbourne for commercial and industrial buildings, including change of use assessments, strengthening design and documentation for building permits. Site inspections are carried out across Melbourne and Victoria, and design and certification are available in all Australian states.

Frequently asked questions

Do I need a structural engineer for a change of use?

You need one when the new use places more load on the structure than the old use, or when the building surveyor asks for evidence that the structure suits the new building class. Gyms, storage, assembly uses and heavy plant are the most common triggers. Moving to a lighter use, such as retail to office, often needs no structural work, although the surveyor may still ask for confirmation.

What floor load does an office need compared with a gym?

AS/NZS 1170.1 typically sets 3.0 kPa for offices for general use and 5.0 kPa for areas where people move in rhythm, such as gyms and dance studios. That is about two thirds more, before allowing for dropped weights, impact and vibration, which a gym floor also needs to handle.

Can I store goods on an office floor?

Light storage within normal office use is allowed for in the office load. Dedicated storage rooms, archives and compactus units are a different use, with loads that rise with the stacking height. A floor designed for offices often cannot take full height shelving or compactus units without a structural check and, sometimes, strengthening.

Planning a change of use for a commercial or industrial building? Get in touch with Principal Built Engineering with the address, the current and planned use, and any drawings you have, and the team will advise what checks the project needs.