Crowd loading is the one live load where people genuinely reach the code value, and the one where a static number can still be the wrong answer. StructLoads carries assembly areas at their full unreduced rate, usually 4.0 to 5.0 kPa, and flags the two conditions that a static rate does not cover: synchronised movement, which turns a crowd into a dynamic excitation, and lateral crowd force on barriers, which is a horizontal load on a structure sized for vertical ones. A dance floor that satisfies 5.0 kPa statically can still be unusable.
What a crowd actually weighs
Start with the physical limit. A densely packed standing crowd reaches roughly four people per square metre; beyond that, movement stops and the situation becomes a crowd safety problem rather than a loading one. At an average mass of 75 kg, four people per square metre is 300 kg/m2, about 2.9 kPa. Codes set assembly rates at 4.0 to 5.0 kPa, so the design value sits meaningfully above the physical packing limit, which is the right place for it given that people carry things and move.
This is unlike an office. An office designed for 2.4 kPa typically operates around 0.5 kPa, so the design value carries large hidden margin. An assembly area designed for 5.0 kPa can genuinely operate near 3.0 kPa on a busy night. The consequence for a load takedown is direct: live load reduction, which exists because large tributary areas are statistically unlikely to be fully loaded at once, is restricted or disallowed for assembly occupancies, because a full house is not a statistical outlier. It is Saturday.
The structural load categories that matter here are assembly with fixed seating, assembly with movable seating, stages and platforms, and stairs and exit routes, and they carry different rates. Movable seating attracts a higher rate than fixed, because it allows denser packing and it permits the crowd to move together.
How to carry assembly loads through a takedown
| Method | Best for | Why it works | Main limit | Verdict |
|---|---|---|---|---|
| StructLoads | Assembly floors inside a normal building takedown | Applies the unreduced assembly rate over the true occupancy footprint and keeps stairs, stages and balconies as their own areas rather than one blended figure | Does not run the dynamic response or footfall assessment | Best overall for the static takedown |
| Blended average over the whole floor | A very quick column estimate | Fast | Wrong for every local member, and it hides the balcony and stair concentrations | Not adequate for design |
| Dynamic footfall or crowd response analysis | Dance floors, gyms, grandstands, long spans | Assesses the frequency and damping that decide whether the floor is usable | Needs a mass and stiffness model, and its inputs are behavioural rather than structural | Required wherever synchronised movement is credible |
| Fixed seating layout from the architect | Establishing which rate applies where | Tells you where seats are fixed, where the standing areas are, and where the crowd will actually gather | Layouts change, and a fixed-seat hall gets used as a standing venue | Necessary input, treat as provisional |
The order that works is to map occupancy zones from the layout, apply the correct unreduced rate to each, add stairs and exit routes at their own rate, then ask separately whether anything in the room can produce synchronised movement.
Synchronised movement, and why frequency beats magnitude
A crowd jumping in time with music does not apply a larger static load. It applies a rhythmic one, and if its frequency lands near a natural frequency of the floor, the response amplifies. This is straightforward resonance, and it is why dance floors, aerobics studios, gyms and grandstands are treated as a separate problem rather than as an assembly area with a bigger number.
Human jumping happens at roughly 1.5 to 3.0 Hz, and the second and third harmonics reach 6 to 9 Hz. A floor with a natural frequency in that band can be driven by a crowd that weighs nothing unusual. The standard structural response is to keep the floor’s fundamental frequency above the excitation band, commonly above 8.4 Hz for rhythmic activities in some guidance and lower for general assembly, or to add mass and damping so the response is tolerable.
The practical trap is that a floor which passes the static check comfortably can fail this one badly, and stiffening it is not always the fix: adding stiffness raises frequency, adding mass lowers it, and the two are often changed together without checking which effect dominates. Long-span composite floors are the usual candidates because they are efficient in bending and light in mass, which is precisely the combination that produces a lively floor.
Barriers, balconies and horizontal crowd force
A crowd pushes sideways. Barriers to assembly areas are designed for a horizontal line load applied at handrail height, and the values are far above what a domestic guard rail carries: guidance for areas with crowd loading commonly requires a horizontal line load of 3.0 kN/m or more at the top of a barrier, against 0.36 to 0.74 kN/m for ordinary residential and office situations. The Approved Documents framework sets these by use, and the difference between categories is roughly a factor of five.
That load has to go somewhere. A barrier at the edge of a balcony applies a moment into the balcony structure, which is already a cantilever carrying its own vertical load. A barrier fixed to a slab edge applies a pull-out force into the edge detail. A glass balustrade transfers it through a channel fixing into the slab nib. In every case the horizontal action is the one that governs the fixing, and it is the one most commonly absent from a gravity takedown.
Grandstands add a further case. A grandstand with movable or demountable elements carries horizontal crowd surge along the seating as well as across it, and demountable stands are checked for stability under horizontal crowd action because the vertical load alone will not hold them down.
Stages, platforms and rigging
Stages are assembly areas with equipment on them. The floor rate applies, and then scenery, lighting towers, speaker stacks, and stored equipment concentrate load in ways the rate does not describe. A line array hanging from a rigging grid puts point loads into a roof structure at a handful of pick points, often several tonnes each, and those points are chosen by a touring production rather than by a designer.
The honest approach is to state a rigging capacity per point and a total, publish it, and design to it, rather than to design for one show. The same applies to floor point loads for scenery trucks and forklifts crossing a stage, which is the same category of concentrated load discussed in area load versus line load versus point load.
Stairs, landings and exit routes
Stairs in assembly buildings carry the assembly rate rather than a residential one, and their landings frequently carry more, because a landing is where a crowd stops moving. Exit routes are the same: a corridor serving an assembly space is an assembly area for loading purposes even though nobody sits in it.
Two details are routinely missed. Landings at the head of a stair from a large hall can be loaded to full packing density during an evacuation, and the same landing is often a transfer point in the structure where a beam picks up two flights. And the handrail on a stair in a crowd building carries the higher horizontal line load along its full length, delivering it into the stair stringers, which then deliver it into the stair and landing supports.
Fixed seating is not permanently fixed
The loading category depends on how the room is used, and rooms change use faster than structures change. A hall built with fixed seating at the lower assembly rate becomes a standing venue the first time somebody removes the seats for a wedding. A conference room becomes an exhibition space with equipment trolleys on it. A church becomes a community hall.
This is not a hypothetical risk, it is the ordinary life of an assembly building, and it argues for designing the general floor at the movable-seating rate even when the layout shows fixed seats. The cost difference on the floor plate is usually modest; the cost of discovering later that a room cannot legally hold a standing crowd is not. Where the higher rate genuinely is not affordable, the answer is to record the limit somewhere a building manager will find it, because a load assumption that lives only in a calculation file will not survive the building’s first change of use.
Balconies deserve particular caution here. A balcony designed for seated audience at a lower rate, later used for standing at a bar, has changed both its vertical load and its barrier load at once, and the barrier is the more dangerous of the two.
Where crowd loads land in the column takedown
Assembly loading changes the shape of a takedown, not just its magnitude. Because the rate is high and unreduced, the live component is a much larger share of the total than in an office, so the ratio between factored and service load shifts and the governing load combination can change from the dead-dominated case to the live-dominated one.
That matters at the foundations. A column under an office picks up a live share that reduces as tributary area grows; a column under a hall does not. On a multi-storey building with assembly at one level and offices above, the assembly floor can contribute more live load to the column than several office floors combined, and it does so at a single elevation. Keeping that load at its own level rather than folded into a running total is what makes the result checkable, which is the same discipline any multi-storey takedown requires.
Transfer structures under assembly spaces are the acute case. A hall over a car park, or a function room over an open foyer, concentrates a large unreduced live load onto transfer beams, and the pattern cases matter because a crowd can genuinely occupy half the room. Pattern loading on a transfer beam under an assembly floor is not a theoretical refinement.
A worked example: a first floor function room
Take a 20 m by 15 m function room on a 7.5 m grid, movable seating, with a 2 m deep balcony on one long side and a stage at one end.
Main floor: 300 m2 at 5.0 kPa unreduced is 1500 kN of live load, plus self-weight and finishes. An interior column with a 56 m2 tributary area picks up 280 kN of live load alone, against roughly 134 kN if the same area were an office at 2.4 kPa. The assembly rate more than doubles the live component of the column takedown.
Balcony: 40 m2 at 5.0 kPa is 200 kN vertical, plus a barrier line load of 3.0 kN/m over 20 m, so 60 kN of horizontal force applied about 1.1 m above the balcony floor. That is 66 kNm of moment into the balcony edge, on a structure whose vertical design load is 200 kN. The horizontal case is small in force and decisive in detail.
Stage: 8 m by 15 m at the platform rate, plus a stated rigging allowance of, say, 500 kg per point at eight points, is 40 kN of concentrated roof load that must be traced to the roof structure rather than the floor.
Key takeaways: assembly and crowd loads
Assembly rates are 4.0 to 5.0 kPa and are not reduced, because a full house is normal rather than exceptional. The static rate is only half the problem: synchronised movement is a frequency question, and a floor that passes statically can still be unusable. Barrier horizontal loads for crowd areas run around five times ordinary values and govern edge details. Stages carry stated rigging capacities rather than one production’s needs, and stairs, landings and exit routes carry the assembly rate too.
Quick answers
Assembly loading is the case where the code number and the real number are closest together. Treat it accordingly.