In practice, falsework and formwork must carry every load that can reasonably occur from erection until the permanent concrete structure becomes self-supporting and crews safely remove the temporary system. Therefore, the design must consider more than the weight of wet concrete: workers, equipment, stored materials, impact, uneven placement, wind, fresh-concrete pressure, prestressing effects, and support settlement can all govern.
Formwork, falsework, and the load path
First, formwork contains and shapes fresh concrete. In contrast, falsework and shores support the formwork and its loads. For example, on a suspended slab, gravity load normally travels from the deck sheathing to joists, bearers, shores, base supports, and finally a receiving slab, foundation, or the ground. Meanwhile, horizontal actions travel through connections and bracing to stable supports.
Load categories to consider
| Load or effect | Typical source | Acts mainly on | Design question |
|---|---|---|---|
| Fresh-concrete weight | Concrete volume, thickness, geometry, verified unit weight | Deck forms, joists, bearers, shores, receiving support | What is the actual tributary load at each stage, including drops, beams, upstands, and unbalanced placement? |
| Formwork and falsework self-weight | Sheathing, frames, timber/steel members, platforms, screens, accessories | The complete vertical and lateral system | Does the load inventory include every permanent part of the temporary system in both loaded and unloaded stages? |
| Reinforcement and embedded items | Bars, tendons, ducts, inserts, prefabricated cages | Deck and local support members | Does the design distinguish concentrated storage/landing loads from distributed installed weight? |
| Construction live load | Workers, tools, runways, hoses, finishing equipment, plant | Working deck and support system | Does the adopted standard define minimum distributed, concentrated, or impact allowances? |
| Material and equipment point loads | Rebar bundles, plywood stacks, buggies, pump-line supports, pallets | Local sheathing, joists, bearers, shores, receiving slab | Where do the drawings show approved landing zones and maximum point loads? |
| Placement and impact effects | Concrete discharge, skip/bucket, pump surge, dropped material, moving equipment | Forms, connections, bracing, shores | Do the design assumptions match the actual delivery and placing method? |
Pressure, environmental, and load-transfer effects
| Load or effect | Typical source | Acts mainly on | Design question |
|---|---|---|---|
| Fresh-concrete lateral pressure | Concrete against wall, column, inclined, or deep forms | Sheathing, studs, walers, ties, braces, anchors | What do placement rate, temperature, unit weight, mixture behavior, vibration, and form height require? |
| Wind and other environmental actions | Exposed forms, screens, supported members, rain/snow where applicable, temperature | Bracing, ties, anchors, connections, foundations | Which unloaded, partially loaded, loaded, and out-of-service stages govern? |
| Prestressing and restraint effects | Early stressing, inclined tendons, anchorage reactions, imposed movement | Permanent member, forms, shores, adjacent work | How do force and deformation redistribute through the temporary system? |
| Settlement and support movement | Ground bearing, receiving slab deflection, base rotation, vibration, water | Shores, towers, bracing, form level, permanent concrete | Are bearing, punching, settlement, rotation, and differential movement within the design assumptions? |
| Stripping and reshoring stages | Transfer of self-weight and construction loads among floors | Young slabs, shores, reshores, foundations | Has the engineer analyzed each removal/reshore stage and authorized release using adequate strength evidence? |
Vertical weight is not the same as lateral form pressure
First, fresh concrete on a horizontal deck produces a vertical area load based on unit weight and depth. In contrast, fresh concrete in a wall or column form produces pressure normal to the form face. However, although concrete may behave fluid-like during placement, design pressure is not a universal hydrostatic value.
For example, lateral pressure depends on the adopted design method and variables such as placement rate, form height, concrete unit weight, temperature, setting behavior, mixture proportions, admixtures, self-consolidating behavior, and vibration. Therefore, use the approved formwork standard and mixture information. For the detailed calculation intent, see the CEW page on calculating loads and pressures on concrete formwork.
Worked load-path example: concrete self-weight only
For example, assume a 0.20 m thick slab and a fresh-concrete unit weight of 24 kN/m3. In addition, verify the actual unit weight for the project.
Next, if the layout spaces interior joists at 0.60 m, the concrete-only line load delivered to one joist is:
However, this example only converts the concrete weight from a volume load to an area load and then to a tributary line load. Therefore, it is not a formwork design. In addition, the design still has to add form self-weight, reinforcement, workers, equipment, impact, stockpiles, unbalanced placement, environmental loads, factors, combinations, member continuity, and reactions through the remaining support system.
Load combinations and construction stages
However, loads do not all act at their maximum at the same time, and temporary works can have several governing stages:
- First, erected formwork before concrete, when wind or stability can govern;
- Next, partial placement with unbalanced concrete and moving equipment;
- Then, full placement with finishing loads and temporary stockpiles;
- After that, early-age concrete supported by shores and reshores;
- Also, prestressing, form release, stripping, and load transfer; and
- Finally, dismantling, when continuity or bracing is progressively removed.
Therefore, the adopted standard determines which actions to combine, what factors to apply, and which resistance/allowable-stress method governs. For this reason, state the load cases on the temporary-works drawings instead of leaving the site team to infer them.
What the design drawings should state
- Governing standards, material properties, design assumptions, and load combinations.
- Concrete unit weight, placement sequence and rate, equipment, and fresh-pressure method.
- Maximum distributed and concentrated construction loads and approved landing zones.
- Member sizes, spacing, connections, braces, ties, anchors, shores, reshores, and foundations.
- Receiving-slab/ground bearing assumptions, settlement limits, and required verification.
- Wind and environmental criteria for unloaded, partial, loaded, and dismantling stages.
- Inspection hold points, permitted tolerances, monitoring, emergency actions, and change control.
- Concrete strength or other conditions required before stripping, reshoring changes, or construction loading.
Legacy CEW material-weight image
References
- OSHA, 29 CFR 1926.703 – Requirements for Cast-in-Place Concrete.
- American Concrete Institute, ACI PRC-347-14(21): Guide to Formwork for Concrete.
- Safe Work Australia, General Guide for Formwork and Falsework.
- Safe Work Australia, Guide to Falsework.
- Caltrans, Falsework Manual.
- ASCE, ASCE 37 – Design Loads During Construction.
Professional note: in short, this article identifies actions and the load path. Therefore, the responsible temporary-works engineer must establish design values and capacities under the project-adopted standards.