Live Loads And What A Floor Is Designed For

Every floor in every building was designed for a number, and the number came from what the floor was going to be used for.

Which means the question "can this floor take it" always has the same shape: what was it designed for, what is being put on it, and how do those compare.

Dead load and live load

Dead load is the weight of the structure and everything permanently attached: the slab, the screed, the finishes, the ceiling, the services, the partitions if they are fixed.

Live load, also called imposed load, is everything else: people, furniture, equipment, stored material, vehicles.

The distinction matters because they are treated differently in design. Dead load is known with reasonable accuracy and applied with a smaller factor. Live load is uncertain, so it carries a larger factor and it is applied in whichever pattern produces the worst effect.

The numbers follow the use

The standards give a design live load per square metre by occupancy, and the differences between categories are large.

Residential floors sit at the low end. Offices sit above them. Assembly areas, retail and classrooms higher again. Storage and plant areas higher still, and storage scales with what is stored and how high it is stacked. Vehicle areas have their own values, which is the territory of [loading docks and heavy vehicle slabs](/blog/loading-docks-and-heavy-vehicle-slabs).

Which means a floor designed for residential use can be carrying a fraction of what the same floor would need for storage, and converting one to the other is not a finishes exercise. That is the whole of [change of use structural assessment](/blog/change-of-use-structural-assessment).

Concentrated loads matter separately

Alongside the uniform load per square metre, floors are checked for a concentrated load applied over a small area.

Which frequently governs for thin slabs, for timber floors and for anything with a punching failure mode, and it is the check that catches a single heavy item: a safe, a compactus, a tank, a piece of plant, a pallet stacked on a point.

A floor that satisfies the uniform load can fail under a concentrated one, and a tenant moving in a heavy item on a small footprint is a different question from a tenant filling the floor with desks.

Where the load is matters as much as how much

Live load is patterned, not spread evenly, because the worst case for one part of a structure is not the worst case for another.

Full load everywhere is the worst case for a column.

Load on one span and not the next is the worst case for the mid span moment of the loaded one, and for the support moment of a continuous member.

Load on a cantilever only is the worst case for the cantilever and for the uplift at its back span, which is the mechanism behind many balcony problems, as in [cantilevered awnings and balconies](/blog/cantilevered-awnings-and-balconies).

Which means a change in where load sits can matter even when the total is unchanged. Moving a heavy store from mid span to adjacent to a column changes which member is critical.

Load reduction, and why a column is not a floor

A column supporting many floors is designed for less than the sum of the full live loads on all of them, because the probability of every floor being fully loaded at once is low.

That reduction is legitimate, it is in the standards, and it has a consequence worth understanding. A column designed with that reduction has a smaller reserve against the scenario where several floors are in fact all heavily loaded, which is exactly what happens when a building's use changes throughout rather than in one tenancy.

Which is why a change of use on several floors is a column and footing question, not only a slab question, and the load path thinking is in [understanding load paths in multi storey buildings](/blog/understanding-load-paths-multi-storey).

The additions that quietly add dead load

Dead load is supposed to be the predictable one and it is routinely increased without anybody recording it.

A new screed or a tile bed over the existing floor. A second layer of roofing. A green roof, which is the subject of [green roofs and roof terrace loading](/blog/green-roofs-and-roof-terrace-loading). Rooftop plant or solar, as in [rooftop solar and plant structural adequacy](/blog/rooftop-solar-and-plant-structural-adequacy). Heavier partitions than the allowance assumed. A suspended ceiling with services hung from the slab.

Each is modest and they accumulate, and every kilogram of added dead load is a kilogram less live load capacity.

Establishing what a floor was designed for

With drawings, the design load is usually noted or can be back calculated from the reinforcement and the section.

Without them, it is an investigation: scanning to find the reinforcement, coring for the slab thickness and the concrete strength, and a calculation from what is found. That is the exercise in [assessing an existing building with no drawings](/blog/assessing-an-existing-building-with-no-drawings) and it is the only honest way to answer the question.

What does not work is assuming the floor is adequate because nothing has happened yet. A floor at its limit looks identical to a floor with reserve, right up to the point where it does not.

Record it for the next person

The most useful thing a project can do for the building's future is write the design live load on the drawings and leave it where somebody will find it.

A floor with its capacity stated can be assessed in an afternoon. A floor without it needs an investigation every time somebody wants to put something heavy on it, which is the recurring cost described in [strengthening an existing slab for a new load](/blog/strengthening-an-existing-slab-for-a-new-load).

ACSES provides structural engineering and structural assessment across Sydney. Talk to us about a project.