The code — IS 14687:1999
IS 14687:1999 is the Indian standard for falsework for construction works — guidelines. It covers design principles, load cases, material requirements, and safety provisions for temporary works. The word "guidelines" in the title is misleading — the loads and tolerances inside are hard numbers. Treat them as mandatory.
I see formwork treated as a trade craft rather than engineering on too many Indian sites. The carpenter decides tie spacing based on experience, the supervisor checks by eye, and nobody runs the pressure calculation. This works — until it doesn't. A blown formwork failure dumps cubic metres of wet concrete on the slab below, with people underneath. The math isn't hard. Let's walk through it.
Lateral pressure of fresh concrete
Fresh concrete behaves like a fluid before it sets. It pushes outward against your formwork, and the faster you pour, the higher the pressure climbs. IS 14687 Cl 7.3 lays out the factors: rate of pour (R in m/hr), temperature (T), concrete consistency (slump), and vibration depth.
The worst case is full hydrostatic pressure: P = density × height = 24 × H kN/m². This happens when the concrete stays fluid through the full pour height — slow-setting mixes, cold weather, or very rapid pours where the concrete hasn't started stiffening at the base by the time you've filled to the top.
CIRIA Report 108 (an international reference, since IS 14687 references general principles) gives the practical formula for walls:
Where C is a coefficient depending on temperature and cement type (lower temperature = higher C), Cw is the concrete density coefficient (1.0 for normal weight), and R is the pour rate in m/hr. For columns, the formula is fuller because the cross-section restrains the concrete differently.
The pressure envelope on the form is typically triangular — maximum at the base, decreasing upward as concrete starts setting. For a 3 m wall pour at 2 m/hr in Mumbai at 30°C, Pmax works out to roughly 40–50 kN/m² at the base. That's a lot of force on a plywood panel.
Dead loads, live loads, lateral loads
Self-weight of wet concrete at 24 kN/m³ + formwork self-weight (typically 0.5–1.0 kN/m² for timber/steel systems). Don't forget the weight of reinforcing steel — adds 1–1.5 kN/m³ to the wet density.
Workers, equipment, material storage: 2–3 kN/m² minimum. Add impact loads for pumped concrete placement — sudden surges when the pump pushes through a blockage.
Wind on the formwork (especially tall wall and column forms), equipment vibration (concrete pumps, vibrators), and the impact of concrete placement. Provide diagonal bracing for all vertical forms.
Deflection limits — what the eye can see
IS 14687 Cl 4.2 sets the deflection limit for formwork: span/270 or 6mm, whichever is less, for visible concrete surfaces. For architectural or premium finishes, the limit tightens further.
Why does this matter? Because formwork deflection is permanent. The concrete takes the shape of the form. If your plywood sags 8 mm between studs, you'll see an 8 mm ridge on the wall surface after stripping. You can't grind it out without cosmetic patching.
The rule of thumb: your formwork must be stiffer than the structure it's supporting. If a slab span deflects 10 mm at service load, your formwork deflection must be tighter — or the concrete will set with that deflection baked in.
Tolerances — IS 14687 Cl 4.3
Even well-designed formwork isn't perfect. IS 14687 Cl 4.3 specifies acceptable tolerances:
| Deviation | Tolerance |
|---|---|
| Positional (plan location) | ±25 mm |
| Cross-sectional dimensions | ±6 mm to ±12 mm (size dependent) |
| Verticality (per metre height) | ±6 mm per m, max ±25 mm total |
Reference: IS 14687:1999 Cl 4.3 — Tolerances
Common failures — the patterns I see
Wall and column forms blow out or tip over under wind or concrete placement impact. Diagonal bracing is the cheapest insurance against formwork collapse. Don't skip it.
Tie rods snap or pull through walers when spacing is too wide for the concrete pressure. Always check tie capacity against the pressure envelope — especially at the base where Pmax occurs.
Props punch through the slab below, or settle into soft ground. Use sole plates — timber or steel spreaders — under every prop base. Check bearing capacity of supporting slab.
The formwork was designed for 1.5 m/hr, but the pump operator is delivering 4 m/hr. Pressure exceeds design and the form blows. Communicate pour rate limits to the pump operator before starting.
Use the formwork pressure calculator to run the pressure envelope for your specific wall height, pour rate, and temperature.
Frequently Asked Questions
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Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in construction QA/QC, concrete technology, and RMC plant operations. NRMCA Certified Trainer (USA) and ISO Lead Auditor.