A change of use, part of the wider practice of adaptive reuse, is one of the most common reasons to revisit an existing structure. Offices become apartments, warehouses become offices, ground floor shops become gyms or restaurants, and houses become clinics or nurseries. Each change shifts the imposed load the floors must carry, and sometimes the dead load too. StructLoads reruns the takedown with the new occupancy assigned floor by floor, so the comparison with the original design becomes a list of column and foundation loads, before and after, rather than a judgement made from the drawings.
Why a change of use is a structural question
Imposed loads are tied to occupancy. Codes such as Eurocode 1, Actions on structures assign a value to each category of use, based on how densely the space is likely to be occupied and what it is likely to hold. A building designed as offices was sized for office loads. If it becomes a library, its floors may need to carry far heavier book stacks. If it becomes housing, the imposed load drops, but new partitions, bathrooms and kitchens add dead load in places the original design did not expect.
Planning and building control treat these changes formally. In England, the Planning Portal guidance on change of use explains when a change needs planning permission, and a material change of use also brings the building regulations into play. Building control will expect the structure to be shown adequate for the new use, and the guidance in Approved Document A on structure sets the baseline the designer works to. Other jurisdictions have equivalent triggers. Whatever the route, the structural engineer usually ends up answering one question: can the existing frame carry the new loads?
How imposed loads change between common uses
The table below gives typical characteristic imposed floor loads under the UK national annex to EN 1991-1-1 and the matching ASCE 7 uniform live loads. Values vary by edition and annex, so confirm them for the project, and see typical floor live load values by occupancy for a wider list.
| Use | UK annex, EN 1991-1-1 | ASCE 7 | Change from office | Verdict for the takedown |
|---|---|---|---|---|
| Residential, within dwellings | 1.5 kPa | 1.92 kPa (40 psf) | About 40 percent lower | Spare capacity, check new dead loads |
| General office | 2.5 kPa | 2.40 kPa (50 psf) | Baseline | Reference case |
| Classroom | 3.0 kPa | 1.92 kPa (40 psf) | Similar or higher | Check corridor values too |
| Retail, general | 4.0 kPa | 4.79 kPa (100 psf) at ground floor | 60 to 100 percent higher | Rerun the takedown in full |
| Gymnasium, dance and physical activity | 5.0 kPa | 4.79 kPa (100 psf) | Roughly double | Add dynamic checks |
| Assembly without fixed seats | 5.0 kPa | 4.79 kPa (100 psf) | Roughly double | Check floor and barrier loads |
| General storage | 2.4 kPa per metre of storage height | 6.0 to 12.0 kPa | Two to five times | Usually needs strengthening |
The pattern is simple. Moving toward residential usually reduces the imposed load. Moving toward retail, assembly, gym or storage use usually increases it, often by a factor of two or more.
The sequence for rerunning the takedown
A change of use review follows the same steps as any structural load takedown, applied twice: once for the original use and once for the new one.
- Establish what the building was designed for. Original drawings and calculations are best. Where they are missing, infer the original design use from the building type, age and member sizes, and say clearly that it is inferred.
- Survey what is actually there. Slab thicknesses, screed depths, finishes, partitions, plant. Existing buildings often carry more dead load than drawn, especially after previous refurbishments.
- Run the original takedown. This is the reference: the loads the frame was designed to carry, column by column.
- Run the new takedown. Assign the new occupancy to each floor, with the new partitions, finishes, services and plant.
- Compare. For every slab panel, beam, column and footing, the new load divided by the original load gives a utilisation ratio. Anything above 1.0 needs checking against actual capacity, not just against the original load.
The comparison approach works because the original design load is a reasonable proxy for capacity when original calculations are unavailable. It is not a substitute for checking capacity where the ratio exceeds 1.0, but it narrows the search to the members that matter.
Live load reduction can change in both directions
Live load reduction depends on occupancy as well as area. Under ASCE 7, assembly uses, garages and live loads above 100 psf are generally not reducible or only partly reducible, while office and residential loads are. A change from office to assembly can therefore increase the column load at the base by more than the ratio of the floor loads suggests, because the new use loses the reduction the old one enjoyed.
The same logic applies to the Eurocode reduction factors for area and number of storeys, which apply to some categories and not others. A change of use should always revisit the reduction, not just the headline floor load.
Dead load changes that come with a new use
The imposed load gets the attention, but a conversion usually adds dead load too:
- New partitions. Converting an open office into apartments introduces many walls. Lightweight stud partitions can sit within a partition allowance; blockwork walls usually need to be treated as line loads.
- Wet areas. Bathrooms bring screeds laid to falls, tiles, tanking and fittings, and kitchens bring heavy worktops and appliances.
- Raised or acoustic floors. Residential conversions often need acoustic separation, which can mean a floating screed of 65 mm or more, adding about 1.5 kPa.
- Plant. New heat pumps, water tanks and ventilation units often end up on the roof, adding point loads that were never part of the original roof design.
It is common for an office to residential conversion to reduce the imposed load by 1.0 kPa and add 1.0 kPa or more of dead load, leaving the total roughly where it started. The factored load can even go up, because dead load and imposed load carry different partial factors.
Where the problems usually appear
When a change of use does increase load, the members fail the check in a fairly predictable order:
- Slabs and secondary members, because they carry the load on a small area with no reduction and little redistribution.
- Primary beams and transfer structures, especially a transfer beam carrying columns from above.
- Columns, mostly at the lower levels where the accumulated load is largest.
- Foundations, which are often the most expensive to strengthen and the hardest to inspect.
A local change, such as a single gym on the ground floor of an office block, may only affect the ground floor slab and the footings below it, if the floor is a ground bearing slab. The same gym on the third floor affects the slab, its beams, and every column and footing below it.
Changes of use that need special loads
Edges matter too: a floor that becomes retail or assembly space needs its guards rechecked for the higher handrail and balustrade loads of the new use.
Some new uses bring loads that are not captured by a single imposed value. Libraries and archives with compact shelving carry very heavy line loads, covered in library and archive storage loads. Gyms bring dynamic loads from jumping and dropped weights. Restaurants bring heavy kitchen equipment and walk-in cold rooms. Clinics and dental surgeries can bring imaging equipment, covered in hospital and imaging equipment loads. Each of these needs its own check on top of the headline floor load.
Foundations are the hardest part to strengthen
Slabs and beams can usually be strengthened from below or above with bonded plates, fibre wraps, new steel or added supports. Foundations are different. They are buried, often under ground floor slabs that the new use wants to keep, and strengthening them means excavation, underpinning or new piles, with the building occupied above. That makes the foundation reactions from the load takedown the number to settle early in any change of use review.
Two features help. Many older foundations were sized with generous bearing pressures and have some reserve, especially where the original design assumed a heavy use that never arrived. And ground conditions often improve with time under sustained load, as the soil consolidates beneath the footing. Neither can be assumed. Both are worth investigating with trial pits and a geotechnical review before the scheme commits to a use that loads the foundations beyond their original design.
Where the foundations cannot take the new load, the scheme can sometimes be rearranged so the heavy use sits where the foundations are strongest, typically over pile groups or on a ground bearing slab, while lighter uses go above. Arranging the uses to suit the structure is often cheaper than strengthening the structure to suit the uses.
Recording the comparison so it can be reviewed
A change of use often has several parties checking it: building control, the client’s surveyor, sometimes a lender or insurer. A comparison table that lists each column line, its original load, its new load and the ratio is far easier to review than a set of calculations. StructLoads produces both takedowns from the same model with only the occupancy changed, so the difference between them is traceable to the change of use and nothing else.
The same record helps the next change. A building that has been converted once is likely to be converted again, and a clear statement of what each floor was checked for is worth more to the next engineer than any amount of original drawings.
A worked example: an office floor converted to a gym
Take a four storey 1990s office with a 7.5 m by 7.5 m column grid and 250 mm flat slabs. The client wants to convert the first floor to a gym.
Original first floor loads: slab self-weight 6.0 kPa, finishes and services 1.0 kPa, office imposed 2.5 kPa plus 1.0 kPa partitions. Total characteristic load 10.5 kPa, of which 3.5 kPa is variable.
New first floor loads: same slab and services, a rubber gym floor adding 0.3 kPa, imposed load 5.0 kPa for physical activity, no partition allowance in the open gym area. Total characteristic load 12.3 kPa, of which 5.0 kPa is variable.
Factored under Eurocode, with 1.35 on permanent and 1.5 on variable, the original first floor load is 1.35 times 7.0 plus 1.5 times 3.5, or 14.7 kPa. The new load is 1.35 times 7.3 plus 1.5 times 5.0, or 17.4 kPa, an increase of about 18 percent on the slab. For an interior column over 56.25 m2, that adds about 152 kN at first floor level, which flows into the ground floor column and the footing.
The slab is the first check, because a flat slab designed for 14.7 kPa has to be shown to carry 17.4 kPa in bending and punching shear around the columns. If it has the reserve, the change may proceed with a dynamic check. If not, strengthening options include carbon fibre for bending and steel collars for punching, both of which are much cheaper before the gym fit out than after.
Key takeaways: load takedowns for a change of use
A change of use changes the imposed load, and the shift can be large: residential is about 40 percent lighter than office, while retail, gym, assembly and storage are two or more times heavier. Run the takedown twice, once for the original use and once for the new one, and compare member by member. Remember the dead load that comes with the new use, and recheck live load reduction because it depends on occupancy. Slabs fail the check first, then beams, columns and foundations. Record the comparison clearly, because it is the evidence building control and the next engineer will need.
Quick answers
Run the original takedown and the new one side by side. The ratio between them points to exactly which members need checking.