Change of use and the loads that come with it

Change of use and the loads that come with it

Adaptive reuse is generally a good idea. Existing buildings are already built, already connected and often better located than anything new. The engineering question is narrow and unavoidable: can the structure carry the new use.

The building does not know the use changed. It only knows what is standing on it.

Imposed loads vary more than people expect

Design loads for floors are set by occupancy, and the differences between occupancies are substantial.

A residential floor is designed for a relatively modest imposed load. An office floor is higher. Retail and assembly areas higher again. Storage and archive uses can be several times the residential figure, and plant rooms higher still.

Converting a residential floor to an assembly or storage use is not a paperwork exercise. It can more than double the imposed load, and the structure was not designed for it.

The conversions that tend to work easily run the other way, from a heavier use to a lighter one. Warehouse to residential is usually straightforward structurally. Residential to archive storage is usually not.

Concentrated loads and plant

Distributed load is the obvious check. Concentrated load is the one that catches people.

A commercial kitchen puts significant point loads into a floor that was designed for furniture. Gym equipment does the same, with dynamic amplification on top. Rooftop plant for a new mechanical system lands on a roof structure that was designed for maintenance access and wind.

Each of those is a local check on the member directly beneath it, and each can require strengthening in one bay while the rest of the floor is fine.

The parts that are not structural but decide the project

A change of use frequently triggers requirements that cost more than the structure.

Fire separation and egress change with occupancy classification, and the building's classification under the Building Code may shift, which brings its own list. Accessibility requirements apply to the new use. Services capacity, particularly hydraulic and electrical, often needs upgrading.

None of that is structural engineering, but all of it interacts with structure, because penetrations, new stairs, lift shafts and plant platforms all put load into or take material out of the existing frame.

The assessment sequence

Establish the existing structure, since older buildings rarely match their drawings. Establish the new loading. Assess capacity, member by member, following the load path to the footings. Identify what needs strengthening and where.

Then, and only then, is there a reliable basis for a feasibility. A conversion priced before that assessment is priced on hope.

Where it usually lands

In my experience most conversions are feasible, and the strengthening required is localised rather than wholesale. The projects that fail are the ones where the assessment happened after the deal was done and the number turned out to be structural rather than cosmetic.

Doing the structural assessment during due diligence rather than after settlement is the entire difference.