Ringlock Shoring Calculator (Slab Soffit)

How to work it out

1. Count the grid first: bays along the length are the footprint length divided by the standard spacing, rounded up, and bays across likewise; standards are (bays along + 1) x (bays across + 1). The plus one at each edge is because a full shoring frame has a row of standards on all four sides, not one fewer at the boundary.
2. Set the standard length. This is where most take-offs go wrong. The distance from the top of the standard to the soffit can only be absorbed by the head jack and the base jack, at up to 400 mm and 300 mm respectively, less about 150 mm for bearers, joists and sheeting. The standards must therefore total between height minus 0.85 m and height minus 0.15 m. At a 6 m clear height the standards can only be 5.15 to 5.85 m, so they make up 5.50 m from a 3000 and a 2500, with 350 mm of head jack taking the rest.
3. Ledger levels are the actual standard length divided by the lift, rounded up, plus one. A 5.50 m standard at 1.5 m lifts carries ledgers at 0, 1.5, 3.0, 4.5 and 5.5, five levels. Floor plus one gives four and leaves the top ledger at 4.5 m, asking the head jack to reach 1.5 m where the code allows 650 mm — the frame simply cannot be built that way. The closing length at the top needs a ledger to lock the standard head.
4. Look the vertical braces up rather than guessing. Table 6.2.2-1 of JGJ/T 231-2021 (standard type B, 1.5 m standard lift) is entered with the design axial force N and the height H. For N up to 25 kN use three-bay spacing up to 16 m, two-bay from 16 to 24 m and single-bay above 24 m. For 25 to 40 kN use two-bay up to 8 m and single-bay above. Above 40 kN at a height above 24 m every bay is braced. N is the maximum within one independent frame.
5. Braces run both ways: one plane in every row of standards along the length, one plane in every column across the width, each at the interval from the previous step. Every plane runs continuously from base to top, so the count is planes times whole lifts, with the closing length left unbraced because no brace is made for it. Above 16 m the top lift is braced in every bay, counted separately and never added on top of the normal arrangement twice.
6. Fit one head jack and one base jack per standard and check both projections. What remains is coupler tube work: a horizontal brace course every four to six standard lifts, and a tie to the existing structure every four to six lifts above 8 m. Neither is a ringlock component, so weigh them separately.

Where it goes wrong

• Applying the facade scaffold method to shoring — a facade scaffold braces the outer face only, whereas shoring is braced in both plan directions. On the same 24 x 18 m frame that is 320 braces against 714, a factor of 2.2; order on the facade figure and the site ends up without enough braces to make the frame geometrically stable.
• Counting ledger levels with floor plus one — that is the facade rule, and on shoring it drops the closing level at the top. At 6 m clear height and 1.5 m lifts one level is missed, which on 336 standards is 3,360 ledgers, about 12 tonnes.
• Treating the shoring height as the standard length — standards, head jacks and base jacks add up. Order 6 m standards for a 6 m clear height and the frame finishes half a metre above the soffit, leaving the site to cut standards down or force them in.
• Leaving out the head jacks — every standard of a shoring frame takes one, where a facade scaffold has bases only. On 336 standards that is 1,593 kg missing, and worse, nothing to adjust when the bearers go on.
• Inventing the brace class — the usual shortcut is four-bay spacing because that is the facade rule. Table 6.2.2-1 depends on N and H, and above 40 kN at over 24 m every bay is braced, which changes the whole brace order.
• Comparing kg/m² directly with the rule of thumb of 30 kg per square metre — that figure is usually per square metre of floor area at around 3 m storey height, while shoring quantity scales with height. A 6 m frame at 60 kg/m² is normal; measured against 30 it looks over-designed and the cutting starts. Compare kg/m³ instead, where shoring sits at 8 to 15.

Material and cost

Ringlock shoring is rented by the tonne-day or square-metre-day with wide regional variation and strong dependence on hire period and whether erection is included, so no unit rate is quoted. Pin down four things when comparing quotes: charged by tonne or by area, and if by area whether it is floor area or shoring footprint; whether delivery and collection are included; the loss-and-damage rate; and whether head and base jacks are priced separately. Labour is separate, and high formwork adds the cost of the method statement and expert review.

On site

• Height 5 m or above, span 10 m or above, total design load 10 kN/m² or above, or line load 15 kN/m or above makes it high-risk and requires a method statement; 8 m, 18 m, 15 kN/m² or 20 kN/m or above is above-scale and requires expert review. The conditions are alternatives. This tool tests height and total load only, so check span and line load yourself.
• Keep the height-to-width ratio within 3 (clause 6.2.1). Beyond that, tie rigidly to an existing structure or add anti-overturning measures, or the horizontal force at the pour will push the whole frame over — in these failures the members are usually strong enough and the frame loses on overall stability.
• Above 8 m with an existing structure adjacent, tie back every four to six lifts (clause 6.2.6). Ties are not optional: they are how horizontal force reaches the structure, and no amount of bracing substitutes for them.
• A horizontal brace course every four to six standard lifts (clause 6.2.7), built from coupler tube and never from ringlock ledgers. Ringlock quotations do not include this, and the shortfall appears on site when the frame is already up — list it separately at the take-off stage.
• Sole boards under every standard, at least 50 mm thick and spanning two bays, on compacted ground with drainage. Uneven settlement is the most common cause of shoring collapse, more common than a shortage of members and far easier to overlook.
• Inspect before the pour: spacing, lift, brace count, jack projection and whether the wedges are driven home. Ringlock wedge seating should be within 3 mm, and wedges set in place but not hammered are a routine failure — check them one by one.

Basis

• EN 12812:2008 Falsework — Performance requirements and general design
• EN 12813:2004 Temporary works equipment — Load bearing towers of prefabricated components: particular methods of structural design
• BS 5975:2019 Code of practice for temporary works procedures and the permissible stress design of falsework
• ACI 347R-14 Guide to Formwork for Concrete
• OSHA 29 CFR 1926.703 (requirements for cast-in-place concrete)