Formwork Loads, Shoring, Reshoring & Stripping

Formwork Loads, Shoring, Reshoring & Stripping

Quick Answer: Formwork carries dead load (concrete at about 150 pcf plus the forms themselves) and construction live load (workers, tools, buggies). ACI 347 sets minimums of 50 psf live load — 75 psf with motorized carts — and a 100 psf total design load (125 psf with carts). OSHA 1926.703 requires formwork to be designed and maintained for all vertical and lateral loads. Shores support fresh concrete; reshores are re-installed after the slab has taken its own weight; backshores never let the slab deflect fully. Strip only when the specified strength is verified — never on a calendar.

Foundations and Formwork is a 10-item domain, and formwork collapse is one of the highest-consequence failures in commercial construction. The previous section covered the components of vertical and horizontal systems. This section covers the loads and the sequence — the reasoning that decides whether those components are adequate and when they can safely come out.

Where formwork loads come from

Slab dead-load example. How much dead load does a 6-inch normal-weight elevated slab put on the deck forms?

• Thickness in feet: 6 / 12 = 0.5 ft
• Dead load: 0.5 x 150 = 75 psf from the concrete alone

Add formwork self-weight and the 50 psf minimum live load and the deck is designing to roughly 130-140 psf — comfortably above the 100 psf ACI minimum, which is exactly why the minimum is a floor, not a target.

ACI 347 minimums to remember:

• Construction live load: not less than 50 psf
• With motorized carts: not less than 75 psf
• Total design load (dead + live): not less than 100 psf
• Total design load with motorized carts: not less than 125 psf

Lateral pressure on vertical forms

Fresh concrete behaves like a heavy fluid until it stiffens. Lateral pressure against wall and column forms increases when:

• The rate of placement is faster (concrete stays plastic deeper into the pour)
• The pour height is greater
• Temperatures are cooler (slower set keeps concrete fluid longer)
• Superplasticizers or high-slump mixes delay stiffening
• Vibration is applied deeper than the current lift

It decreases with warmer temperatures, slower placement, and mixes that stiffen quickly. The practical control is the rate of placement specified in the form design — pouring faster than the forms were designed for is a leading cause of blowouts, and it is a decision made by the crew, not the engineer.

Never compute a pressure from memory on the job. ACI 347 publishes formulas with limits of applicability; the form design drawing states the assumed rate and temperature. Your responsibility is to place within those assumptions and to stop if conditions change.

Shoring, reshoring, and backshoring

These three terms are frequently confused and are a reliable source of exam items.

The distinction matters structurally. A reshore is installed snug but not preloaded, so the slab below it is not asked to carry the new slab's dead load — the reshore only picks up subsequent construction loads. A backshore keeps the slab supported continuously, which is used when a young slab must not be allowed to take its own weight.

Multi-story load path. When a new deck is poured, its weight travels down through shores into the slab below, then through reshores into the slabs below that, until enough levels share the load that no single young slab is overstressed. The number of levels of shoring and reshoring is an engineering decision shown on the shoring drawings. Removing a level early because it is in the way of a delivery is how a progressive collapse starts.

What OSHA requires — 29 CFR 1926 Subpart Q

Subpart Q covers concrete and masonry construction; 1926.703 addresses formwork specifically.

• Formwork must be designed, fabricated, erected, supported, braced, and maintained to support all vertical and lateral loads that may reasonably be anticipated.
• Drawings or plans for the jack layout, formwork, shoring, working decks, and scaffolds must be available at the jobsite.
• Shoring equipment must be inspected before, during, and immediately after the pour; damaged or weakened equipment is removed from service.
• Reshoring must be erected as the original forms and shores are removed, and must be capable of supporting the loads imposed.
• Forms and shores may not be removed until the employer determines the concrete has gained sufficient strength to support its own weight and superimposed loads — that determination is based on compliance with the design drawings/specifications or on tests (field-cured cylinders or in-place testing) showing the concrete has reached the required strength.
• Single-post shores in more than one tier must be designed, spliced to prevent misalignment, and laterally braced.
• Vertical slip forms, lift-slab operations, and precast erection carry their own specific requirements.

Stripping: criteria before sequence

The criterion is strength, not time. "We always strip at three days" is not an acceptable answer on an assessment or in an incident investigation. Acceptable bases include:

1. The specification or engineer's stated stripping strength or time, when the design provides one.
2. Field-cured test cylinders broken to confirm the required compressive strength — field-cured, because they experience the same conditions as the structure, unlike standard lab-cured cylinders used for acceptance.
3. In-place methods such as maturity or pullout testing where specified and calibrated.

Cold weather, poor curing, low cement content, or admixture problems all delay strength gain. The concrete does not know what the schedule promised.

Stripping sequence

Cantilevers are the classic trap. A cantilevered slab or balcony has no backspan support once the shores are gone, and its critical stress is at the support, not midspan. Cantilever shores stay in longer and are removed strictly per the engineer's direction.

Additional stripping discipline:

• Do not drop or pry recklessly — damaged panels become tomorrow's failure, and falling forms are a struck-by hazard.
• Apply release agent properly on the previous cycle so panels come away cleanly instead of tearing the concrete face.
• Protect green concrete edges and corners; arrises chip easily at early ages.
• Keep exclusion zones below any stripping operation.
• Clean, inspect, and repair panels, ties, and shores before the next cycle; bent shores and elongated tie holes are rejected, not reused.

Failure modes carpenters control

Formwork is temporary structure that carries permanent-structure loads at the moment the permanent structure is weakest. That is why the Foundations and Formwork domain rewards candidates who can explain why a shore stays in — not just what it is called.