The heaviest load a concrete frame ever carries is usually applied before the building is finished. StructLoads treats construction loading as a real design case rather than a contractor problem, because a slab supporting fresh concrete, formwork, and the crew placing it can be carrying more than its finished service load, at an age when it has a fraction of its final strength. The governing question is not how strong the slab will be. It is how strong it is on the morning of the pour above.
What a freshly poured slab actually carries
Three loads arrive at once. Fresh concrete on the deck above, which for a 250 mm slab at 24 kN/m3 is 6 kPa. The formwork and falsework carrying it, typically 0.5 to 1.0 kPa for a table form system and more for timber. And a construction live load covering the placing crew, hoses, vibrators, and stacked materials, commonly taken at 1.5 to 2.5 kPa but reaching much higher under a stockpile.
That totals 8 to 9.5 kPa arriving on a slab that was designed for perhaps 2.4 kPa of office live load plus finishes. The finished slab has capacity for it, because its own self-weight is the dominant permanent load and construction load is not additive with a full service live load. The slab that is three days old does not.
Formwork and its supporting falsework are the visible half of this. The invisible half is where the props land. A prop under a fresh pour transmits its load down through the slab below, which transmits it further down through its own props, and the load distributes across however many levels are propped together. That distribution is the entire subject of reshoring, and it is routinely misjudged.
How to handle construction loads in a takedown
| Method | Best for | Why it works | Main limit | Verdict |
|---|---|---|---|---|
| StructLoads | Checking whether the permanent frame can take the temporary case | Lets the construction case run as its own load combination on the same frame, so column and foundation totals include it instead of assuming it away | Does not design the falsework, the props, or the pour sequence | Best overall for the permanent structure |
| Temporary works designer’s scheme | The props, frames and reshore layout | It is a designed system with its own calculations, checked by someone whose job it is | It assumes a slab strength at prop removal that somebody has to verify on site | Mandatory, and not the engineer of record’s substitute |
| Simplified two-level assumption | Small simple frames | Quick, and conservative if only two levels are ever propped | Wrong whenever three or more levels are connected, which is common on fast cycles | Use only when the sequence is genuinely simple |
| Cube or maturity testing on site | Deciding when to strike | Measures the actual strength of the actual concrete in the actual conditions | Only tells you the strength, not whether the load is acceptable | Essential input to the decision, not the decision |
The order that works is to establish the intended pour cycle, count how many levels will be propped together, distribute the construction load across those levels in proportion to their stiffness, compare each level’s share against its strength at that age, and only then agree a striking time.
Reshoring: where the load actually goes
When a slab is cast, its full weight goes into the props beneath it and down to the slab below, which is itself propped. If two levels are interconnected, roughly speaking the new load shares between them. With three levels, it shares three ways, but not evenly: the newest slab is the least stiff and the oldest is the stiffest, so the oldest takes the largest share. A frequent result is that the level three floors down, which everybody considers finished, is carrying more construction load than any level above it.
Two errors follow. The first is removing props too early because the slab immediately above looks strong enough, without noticing that removing them dumps its share onto fewer remaining levels. The second is reshoring with props tightened hard, which pre-loads the slab below and can be worse than leaving the original props in. Reshores are meant to be snug, not jacked.
The load path here is exactly the gravity load path from slab to foundation that the finished building will use, running through a temporary set of members at a temporary set of levels. Foundations feel it too: on a fast cycle with several levels propped, the foundation can see close to the full weight of the propped stack early in the programme, at a time when the ground has had less opportunity to consolidate.
Concentrated construction loads that outweigh everything
Uniform construction load is the easy part. The cases that damage slabs are concentrated.
Stacked material is the largest. A pallet of blockwork is commonly one to one and a half tonnes on roughly a square metre, which is 10 to 15 kPa on a slab designed for 2.4 kPa live load. Stacked bricks, plasterboard, and tile crates behave the same way, and they get placed wherever there is room, which is often mid-span. Two pallets side by side mid-span on a young slab is a genuine failure scenario, not a theoretical one.
Mobile plant is the second. A concrete pump outrigger, a scissor lift, a small excavator on a podium, or a rough terrain forklift concentrates several tonnes onto small patches. The same reasoning applies as for any point load rather than an area load, with punching shear as the check that governs.
Third is the pour itself, which is not static. Placing concrete produces a moving surcharge and the pump line delivers pulses. On long spans the practical control is the pour sequence: filling one end fully before starting the other produces a pattern load on a continuous slab that its design pattern cases may not cover, in the same way that pattern live loading governs service design.
Formwork pressure on vertical elements
Walls and columns have a different construction load problem: the fluid pressure of fresh concrete on the form face. Fresh concrete behaves as a fluid up to a certain height, so form pressure grows with pour rate, temperature, and the concrete’s setting behaviour. A fast pour in cold weather keeps the concrete fluid longer and produces the highest pressures, and a self-compacting mix behaves as a fluid for its full depth.
For the permanent structure this matters at the kicker and at the base restraint, where the form ties and the slab they anchor into carry real horizontal load. Formwork practice guidance treats pressure prediction as a function of pour rate and temperature for exactly this reason, and the number can be several times the equivalent hydrostatic value assumed by eye.
Wind on a part-built frame
An incomplete building is more vulnerable to wind than a finished one, and for a specific reason: the stability system is often the last thing completed. A frame with its cladding off has lower wind pressure on it, but a frame without its shear walls, cores, or bracing has no way to resist what it does get. Add the sail area of formwork panels, sheeting, and a hoist mast, and the temporary condition can control the bracing design.
Cranes make this concrete. A tower crane tied into a part-built frame delivers out-of-service wind loads into a structure that is not yet complete, at the tie levels, on top of everything else that crane and hoist loads on building structures already put into the frame. Those tie forces are a design case for the permanent frame, and they belong in the takedown alongside anything else the crane does.
Early age strength, and why age is the wrong variable
Striking decisions are often made on days, which is a proxy for the thing that actually matters. Concrete gains strength as a function of temperature and time together, so the same mix at seven days in July and seven days in January are different materials. A slab poured in cold weather can be at half the strength its programme assumed, and the programme rarely adjusts.
Two practices close this gap. Cube testing gives the strength of a sample cured alongside the pour, which is closer to reality than a standard-cured cube but still not the slab. Maturity monitoring, using temperature sensors embedded in the pour, tracks the actual thermal history of the actual element and is the better basis for a striking decision on a fast cycle. Neither one changes the load; both change the confidence with which the load can be allowed.
There is a serviceability tail to this as well. A slab loaded heavily at an early age deflects more and keeps deflecting, because creep is larger under early loading. A frame that was struck aggressively can end up with permanent deflections that were never in the deflection calculation, and the effect is not recoverable once the concrete has crept. The load takedown does not predict this, but it establishes how large the early load was, which is the input the deflection assessment needs.
Openings, edges and the slab that is not yet continuous
A slab under construction is frequently not the slab that was designed. Temporary openings for material hoists, pump lines, and stair access interrupt continuity exactly where props want to land. A prop placed next to a large temporary opening loads a slab strip that has lost its two-way action, and the local capacity can be well below the average.
Edges behave similarly. Perimeter zones carry edge formwork, safety systems, and often a working platform cantilevered beyond the slab line, so the edge strip picks up load it will never see again. Cantilevered loading platforms for landing materials are the acute version: they apply a downward load beyond the support and an uplift at the tie-back, and both reactions land on a young slab. The reaction pair is the same shape as any balcony or cantilever slab load, except that neither the load nor the concrete is at its design value.
Steel and timber frames have the same problem differently
Concrete gets most of the attention because its strength changes with time, but a steel or timber frame carries a temporary case too. A steel frame is at full material strength from the moment it is bolted, so the issue is stability rather than strength: an erected frame without its bracing or its composite slab is a mechanism, and erection sequences exist to keep a partial frame stable. Decking loaded with bundles of studs or with a stack of deck sheets before the concrete goes down is the concentrated case.
Timber and mass timber frames sit in between. The material is at strength immediately, but panels are vulnerable to moisture and to concentrated loads before the diaphragm connections are complete. Stacked panels on an installed floor plate are the same pallet problem in a different material, and the load path through a partially connected cross-laminated timber floor is not the one the design drawings show, which is one more reason a CLT and timber frame load takedown has to reach the connections rather than stopping at the members.
A worked example: a three-level propped stack
Take a 250 mm flat slab on a 7.5 m grid, cast on a seven day cycle, with three levels propped together.
The new pour delivers 6 kPa of wet concrete plus 0.8 kPa formwork plus 2 kPa construction live load, about 8.8 kPa. Shared across three interconnected levels by stiffness, with the oldest and stiffest taking the largest share, a reasonable distribution is roughly 45 percent to the oldest level, 35 percent to the middle, and 20 percent to the youngest. The oldest level therefore takes about 4 kPa on top of its own 6 kPa self-weight, so 10 kPa at an age of about 21 days.
The youngest level takes about 1.8 kPa on top of its self-weight at seven days, when the concrete may be at 60 to 70 percent of its characteristic strength. Neither number is alarming on its own. What is alarming is the version where props are struck from the oldest level to reuse them: the 4 kPa it was carrying redistributes onto two levels instead of three, and the middle level, still young, jumps by roughly half of that.
Key takeaways: construction and temporary loads
Construction load is often the largest load a slab ever sees, applied when the concrete is weakest. Count the propped levels and distribute by stiffness rather than assuming two. Watch stacked pallets and mobile plant, which turn a 2.4 kPa design into a 15 kPa reality on a small footprint. Form pressure governs wall and column formwork and delivers real horizontal load into kickers. And a part-built frame without its stability system is the wind case people forget.
Quick answers
The useful habit is to ask what this structure looks like on its worst morning, not on its finished day.