Ringlock Beam Shoring Calculator (Under-Beam Densification)

How to work it out

1. Lay out the rows along the beam: clear span divided by the spacing along the beam, rounded down, plus one, with a row at each end. The spacing along the beam can only be a multiple of 0.3 m — 0.6, 0.9 or 1.2 — because the longitudinal ledgers tying those standards together are fixed-length components. The site habit of 750 mm under a beam comes from coupler scaffolding; ringlock has no such size.
2. Decide the number of rows across the beam. There is no code number for this: the codes only require the spacing to come out of the design, and the row count is a design outcome. Two rows for 300 to 500 mm beams, three above 600 mm, a single centred row for 200 to 250 mm. The tool does not decide, but it does show how much beam width each row carries.
3. Work out the pitch and snap it to a size. Even pitch is beam width divided by one less than the row count; snap that to the nearest B-SG size. A 300 mm beam on two rows lands exactly on 300 and fits B-SG-300. A 400 or 500 mm beam falls between 300 and 600 and fits neither: snapping to 300 keeps the standards inside the beam and cantilevers the bearers, snapping to 600 puts them outside with eccentric jacks. Either has to be drawn.
4. Set the standard length as for slab shoring: total between height minus 0.85 m and height minus 0.15 m, with head and base jacks taking the remainder. Note that the height here is floor to beam soffit, one beam depth lower than under the slab — do not carry over the slab figure.
5. Count ledgers. Transverse ledgers run one per standard position along the beam, per level, per pair of adjacent rows, tying the under-beam rows together. Longitudinal ledgers run one per row, per level, per bay, along the beam. Levels are counted with ceil plus one.
6. Brace both beam faces. One brace plane on each side, at the class from Table 6.2.2-1 as for slab shoring, continuous from base to top, so the count is two faces times planes times whole lifts. Axial force under a beam is normally higher than under the slab, so take the worst standard.

Where it goes wrong

• Laying out at the architectural beam width without checking the ledger size — transoms are fixed-length and the pitch can only land on 300, 600, 900 and so on. Lay out at 400 and order 400 mm members and no supplier can fill it.
• Entering 750 mm spacing along the beam — the B-SG series starts at 300 and steps by 300, with nothing at 750. Laying out at 0.75 m and ordering 0.9 m leaves 150 mm in every bay, 1.5 m over ten bays, and the last member will not fit. Only coupler scaffolding does 750 mm.
• Taking off the beam alone — when beam and slab are poured together the under-beam standards also carry half a slab bay each side. Sizing on beam self-weight alone understates the axial force badly and drops the brace class a step with it.
• Building the beam frame and slab frame independently — they differ in stiffness and settle differently, so the beam soffit drops first and the side forms crack first. Standards within 300 mm should be shared, and where they are further apart the two frames must at least be tied at every lift.
• Forgetting the second face of braces — an under-beam frame is a narrow strip with two exposed faces, both of which need vertical braces. Counting one face halves the brace order.
• Looking the brace class up from the self-weight figure — 2.36 kN against a design axial force of 15 to 30 kN is an order of magnitude out, before formwork weight, construction load, slab load and load factors. Use the N from the design.

Material and cost

Under-beam ringlock is rented by the tonne-day or square-metre-day, but in practice it is hired as one frame together with the slab shoring, because quoting the beam strip alone makes no sense for delivery and collection. One thing to watch when comparing quotes: densifying under beams raises the average quantity for the whole frame, which is captured when the depot prices by tonne and easily lost when it prices by area, so state the basis in the contract.

On site

• Beams deeper than 700 mm need through-ties in the side forms. Soffit shoring and side-form ties are two independent failure modes: a soffit collapse is a shoring shortfall, a blowout is a tie shortfall. Check both, because checking one is the same as checking neither.
• Bearers must sit centred on the head jack (clause 6.2.4); an eccentric jack tilts the U-head. Snapping the pitch to 600 with the standards outside the beam produces a large eccentricity, and double-channel bearers are the way to bring the load back.
• Never let the standard bear directly on the soffit sheeting: jack, bearer, joist and sheeting are four layers and all four are needed. Skip the joist and the sheeting deflects, leaving honeycomb on the soffit that has to be patched after striking.
• Where a standard clashes with column reinforcement, add a column clamp or shift the row pitch, but do not delete the standard — shear at the beam end is highest exactly where the standard matters most.
• Pour sequence changes what the shoring sees. Pouring the beam first loads the under-beam standards once and then adds the slab; pouring the slab first reverses that. The sequence in the method statement must match the assumption in the shoring check.
• Striking time follows the strength of companion-cured cubes, not the calendar. For beams of 8 m span or less the soffit may come down at 75 per cent of design strength; above 8 m it is 100 per cent.

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, Chapter 5 (shoring and reshoring of multistorey buildings)
• OSHA 29 CFR 1926.703 (requirements for cast-in-place concrete)