Formwork and Shoring Design for Cast-in-Place Concrete

The permanent concrete structure gets all the engineering attention, but for the few days between pouring and stripping, a temporary structure made of plywood, lumber or aluminum forms, and shoring towers is doing all the work, and it's carrying a load case the finished structure will never see again: the full weight of wet, unset concrete plus workers, equipment, and the impact of the concrete being placed, all resting on a member that has zero strength of its own until it cures.

Wet Concrete Behaves Like a Fluid, Not a Solid

Freshly placed concrete pushes on formwork the same way any fluid pushes on a container, as lateral pressure that increases with depth, at least until the concrete begins to set and gain enough internal structure to stop transmitting that pressure hydrostatically. Wall and column forms are designed against this lateral pressure, which depends heavily on the rate of placement, how fast the pour rises up the form, since a faster pour keeps more concrete in its unset, fully fluid state simultaneously, and on temperature, since cooler concrete sets more slowly and can generate substantially higher peak lateral pressure than warm concrete placed at the same rate.

Slab and beam formwork instead sees vertical load, self-weight of the wet concrete plus the formwork itself, plus a specified construction live load allowance for workers and equipment moving across the deck during the pour, and this vertical formwork load path runs down through the form face into joists, then into stringers, then into the shoring posts or towers beneath, each layer sized for the accumulated tributary load from everything above it, the same layered load path logic that governs how load moves through the finished structure, covered in load paths and structural redundancy.

Shoring Towers Carry Load in Compression, and Buckling Governs

Shoring posts and tower systems are almost always governed by buckling rather than crushing, since they're slender compression members supporting a load path that stacks straight down through multiple stories if reshoring beneath isn't removed in the right sequence. Bracing between shoring legs, and full lateral bracing of the shoring tower system against the previously placed structure, is what keeps the unbraced length short enough that buckling capacity, not just axial crushing strength, governs the post spacing, following the same Euler buckling logic covered in structural buckling in columns but applied to temporary rather than permanent members.

Because shoring towers stack loads from multiple freshly poured levels simultaneously when reshoring is used, the cumulative load on the lowest level of shoring in a multi-story pour sequence can actually exceed the load any single floor was designed to carry once it's fully cured, which is why formwork engineers track load accumulation across levels explicitly rather than assuming each floor's shoring only ever sees that floor's own concrete weight.

Formwork Design Is a Separate Engineering Discipline from the Permanent Structure

In most jurisdictions, formwork and shoring design is the contractor's responsibility, often delegated to a specialty formwork engineer, and it is checked against its own load combinations and safety factors distinct from the permanent structure's design codes, since the loading, duration, and failure consequences are genuinely different problems even though both involve the same concrete. Formwork's own design life is measured in days or weeks rather than decades, so the material properties assumed for plywood, lumber, and aluminum forming systems account for repeated reuse cycles and the reduced capacity that comes with each successive stripping and re-erection, a degradation curve tracked separately from anything in the permanent structure's design basis.

Field modifications to a formwork or shoring drawing, adding an extra opening for a sleeve, skipping a shore because equipment was parked underneath it, or reusing a shoring tower rated for one loading condition under a heavier one, are a recurring cause of formwork failures precisely because those changes happen fast, on site, without routing back through the engineer who sized the original system. That gap between the engineered drawing and what actually gets built is why inspection of formwork and shoring before each pour is treated as a distinct, mandatory checkpoint rather than an optional courtesy, separate from any inspection the permanent concrete structure receives once it's cured and stripped.

ACI 347, Guide to Formwork for Concrete, is the primary reference most formwork engineers design against, published by the American Concrete Institute, alongside OSHA's construction standards governing concrete and masonry construction safety during pours and stripping operations.