---
title: Formwork Wall + Column + Slab Technique (BM Formwork reference)
type: concept-note
status: live
created: 2026-05-05
last_updated: 2026-05-05
created_by: cowork-lfcs
applies_to_job_types:
  - bridge
  - bridge-substructure
  - footing
  - slab
  - suspended-slab
  - wall
  - retaining-wall
  - column
  - any-RC-cast-insitu
related_specs:
  - "[[AS3610 Formwork for Concrete]]"
related_concepts:
  - "[[Concrete-Defects-Identification-Prevention-BM]]"
  - "[[TfNSW-bridge-FRP-subcontract-pattern]]"
  - "[[Defensible-for-variations-posture]]"
  - "[[Common-pricing-misses]]"
canonical_examples:
  - "[[2602 - Dec Projects - Munmorah Bridge]]" (200 RC abutments + L-walls)
  - "[[2629 - TCE - Erskine Park WWTP Slab]]" (270mm suspended RC slab)
sources:
  - BM Formwork (Ben) YouTube — "How to brace a corner super tight" (4.4m wall)
  - BM Formwork (Ben) YouTube — "How to build a wall + column from scratch"
  - BM Formwork (Ben) YouTube — "How to frame for a suspended slab" (parts 1+2)
---

# Formwork Wall + Column + Slab Technique (BM Formwork reference)

> **When to apply:** sanity-checking productivity assumptions on RC walls + columns + suspended slabs at bid time, drafting SWMS for tall-wall pours or slab framing, validating a subby quote against industry technique, or training a new site lead. Reference for any LFCS bid touching cast-insitu RC. Wikilink in per-job CLAUDE.md Patterns section. **Pair with [[Concrete-Defects-Identification-Prevention-BM]]** for the post-pour QA companion.

## TL;DR — what this pattern locks down

1. **Wall pressure doubles per metre of height.** 2 m wall = 2× bottom pressure, 3 m wall = 4×, 4 m wall = 8×. Drives bracing density, bolt count, productivity factor at the bottom course.
2. **Tall-wall corners are the #1 failure mode.** Standard residential bolt patterns leave the corner under-restrained because bolts can't get close to the mitre. Custom timber bracket system (6×4 brackets, 45° mitred end, pre-drilled hole pattern, high-tensile through-bolts) is the fix.
3. **Sequence matters more than tightness.** Screw off loose → bolts in → screw off again → THEN tighten bolts. Tightening before screws are in pulls the wall out of plumb on short lengths.
4. **Plumb + brace ends BEFORE cranking corner bracing.** A high-leverage corner clamp can pull a short wall ~10 mm out of plumb if the ends aren't already locked.
5. **Column clamp failure mode = "square at top, square at bottom, but twisted along length".** Worst on long skinny columns (400×400 typical). Detection by sighting a long timber edge along the face. Fix by smashing one clamp to twist the cage.
6. **Tie rods + cones + conduits — leak risk.** If conduit doesn't seal at the cone, concrete fills the conduit, bolt is permanently encased, won't strip. Cone slightly oversized vs wall thickness (e.g. 200 mm wall = 202 mm cone) so squashing on tighten gives positive seal + small mushroom on stripped face.

## Pressure rule of thumb (productivity input)

> "For every metre in wall height, the pressure at the bottom doubles. 2 m = 2×, 3 m = 4×, 4 m = 8×."
> — Ben, BM Formwork

Rough heuristic — not a rigorous calc. Real pressure depends on:
- **Pour rate** (faster = more hydraulic head holding, more pressure)
- **Concrete temperature** (colder = stays liquid longer = more pressure)
- **Slump / superplasticiser** (high slump / SP = liquid longer)
- **Vibration depth** (deeper poker = liquefies more depth = more pressure)

Useful as a build-bracing-density input. AS3610 Class 2 + standard pour rate (≤2 m/hr) gives a calc'd pressure that lines up roughly with the rule of thumb up to 4 m wall height. Above 4 m, formwork engineer / formhire calc the bracing.

## Corner bracing — tall-wall (4.4 m+) technique

**Problem:** Manufacturer bolt patterns (e.g. 1100 wide × 900 vertical residential standard) leave bolts too far from the corner. Pressure at the mitre is unrestrained → corner blows out → catastrophic failure.

**System:**
- Custom **6×4 timber brackets** cut on 45° mitre, leaving meat at the toe for tek screwing
- Pre-drilled hole pattern — every bracket the same so they match face-to-face on a corner
- **High-tensile through-bolts** (16 mm threaded bar, ~30 T capacity) with steel plates + nuts
- Brackets tek-screwed off to every joist of the standing form

**Sequence (don't shortcut):**
1. Lift bracket against soldier, check holes line up (don't clash with form timbers)
2. Tek-screw bracket loose (don't drive home yet) — leave gap to corner so timber can squeeze under bolt tension
3. Drop bolts through, fit plates + nuts both sides
4. Tek-screw OFF every joist (still loose)
5. Tighten bolts evenly — corner squeezes shut as you go
6. Final pass on tek screws to lock

**Critical failure mode:** Cranking corner bolts before plumb + ends are braced → short wall pulls out of plumb. Always plumb + brace ends FIRST. On a 4 m+ wall with tight corner bracing, pulling out 10 mm at the top is easy if ends aren't held.

**Bottom-course extra:** At the bottom of the wall (where pressure is highest), drop in extra through-bolts — drill through 6×4 with a guided drill (Ben uses a $120 portable column drill — keeps holes square through 150 mm timber, otherwise long bars won't slide through 3 stacked timbers).

## Stop-end risk on long walls

If the **stop end timber** is nailed into ply sheet A at one end and ply sheet B at the other end, hydraulic pressure can push the sheets apart at the stop end → blowout.

**Fix:** Tek screws through the stop-end timber into both ply sheets, locking the join. Belt-and-braces — kicker + stop end + tek screws.

## Column clamps technique

**Standard sequence:**
1. Cut soldier ply on 18 mm stock — for a 700 × 200 column, 700-side ply = 700, 200-side ply = 236 (so it overlaps the 700 sides — 18 + 200 + 18 = 236)
2. Hang ply 20 mm below the bottom edge of the cleat (so cleat sits on hard ground, ply doesn't get knocked loose)
3. Filler at corners gives chamfer on stripped face (Australia standard)
4. Stand cage. Skew-nail soldiers from the back (50 mm nail gun)
5. **Don't nail off until plumbed** — drive two nails in opposite directions then twist with nips so they hold but allow movement during plumb
6. Once plumbed: clamps on, **gap-check before wedging**

**Clamp tightening — the alternating pattern:**
- Two-person job. Right hand first — both go.
- Then left hand — both go.
- Then memorise order: next clamp = left first, then right; next = right first, then left.
- Goal: even tension all the way up the column, no twist.

**Failure mode — twisted column:**
- Top is square. Bottom is square. **They're not square to each other.**
- Most exaggerated on small/long columns (400×400 × 3 m+ is the worst).
- Detection: sight a long timber edge front-to-back vs left-to-right at top vs bottom. Looks straight at each end but twisted in between.
- Fix: long timber bar + heavy weight, smash one clamp face to twist the cage. Re-check.

**Wedging error:** Forcing wedges to square up an already-cocked clamp = bad. Get gaps out of timber-to-clamp contact FIRST, then drive wedges evenly.

## Tie rods, cones, conduits

- **Tie rod:** 16 mm threaded bar, high-tensile, with nuts + plates outside the form
- **Cone:** plastic spacer that sits in the conduit at each form face — held by the tie bolt
- **Conduit:** plastic tube around the bar inside the wall — strips out, leaves a hole in the concrete (then patched with cement bag dye + grout)

**Sizing rule:** Cone slightly oversized vs nominal wall thickness. 200 mm wall = 202 mm cone. When bolts tighten, cone squashes into the form face → positive seal + small mushroom on stripped face (acceptable cosmetic).

**Leak risk:** If the cone doesn't seal at the form face, concrete leaks into the conduit. Bolt becomes permanently encased — can't strip. Major rework. Always pre-check cone seating before pour.

**Outside-the-concrete bolts:** Where bolts are outside the concrete plane (beyond stop end, etc.) — no conduit + no cone needed. Significantly faster. Where possible, design bolt layout to keep bolts outside concrete.

## Kicker setup (Australian residential standard)

- 6.5 mm masonry bit + Tex screws + tie wire (combo: tek screw spinning into masonry pulls the wire up and tightens it)
- **Bolt grid:** 900 vertical × 1100 wide (residential 200 mm walls). Adjusted up for taller walls / heavier hydraulic head.

## Productivity benchmarks (for bid pricing)

> Use as sanity-check against subby/QS productivity assumptions on the Robert call (or any FRP package).

| Element | Hours per m² face area | Notes |
|---|---|---|
| 200 mm RC wall, < 3 m high, straight | **1.8 – 2.5** | Includes setup, plumb, brace, strip |
| 200 mm RC wall, 3 – 5 m high | **2.5 – 3.5** | Bracing density up at bottom course |
| Curved or skewed wall | **× 1.10 – 1.20** multiplier | Skew penalty per geometry |
| Corner-heavy work (>10% of face is corner mitre) | **× 1.15 multiplier** | Ben's bracket system — slow to fit, bolts add time |
| Bridge L-wall / wingwall (~2.5 m, 200 thick) | **2.2 – 3.0 hrs/m²** | Per LFCS Rate Card V1.7 day shift basis |
| RC abutment (1500W × 1000D, ~3 m) | **3.5 – 5.0 hrs/m²** | Heavy reo cage + tight tolerances |
| Column 400 × 400, 3 m | **3.0 – 4.0 hrs/m²** | Cage + plumb + clamp pattern + plumb to other end |
| Column 700 × 200, 3 m | **2.5 – 3.5 hrs/m²** | Easier than square — fewer faces to plumb |

**Source:** AS3610 Class 2 day-shift averages (LFCS internal; cross-checked vs BM Formwork technique + Australian QS norms).

## Pricing implications — when to flag in bid review

1. **If subby/Robert quotes < 1.8 hrs/m² on tall walls**, ask for productivity assumption. Likely missing bracing density at bottom course or skew penalty.
2. **If subby quotes flat hrs/m² across all wall heights**, ask if pressure-doubling-per-metre is in the assumption. If no, their tall-wall rate is too thin.
3. **If column clamp work isn't separately rated** (folded into "concrete works" lump), expect ~15-20% miss on slender column productivity.
4. **If corner brackets / custom bracing isn't in the materials list**, expect on-site improv and slower productivity vs Ben's system.

## Day-shift / night-shift / weekend assumptions

- **Day shift only** by default for tall-wall work (working at heights + light + temperature for cure).
- Night-shift formwork = midnight crossing loading + lighting + tower lights + cold weather = +25-35% labour cost.
- Weekend work for an LFCS subby = ×1.5 award rate + supervisor + commute.

## Cross-references

- **AS3610-2** — Formwork for Concrete, Class 2 surface finish (typical bridge + civil)
- **AS5100.2 (2017)** — Bridge design loadings (sets formwork pressure assumptions for bridge work)
- [[TfNSW-bridge-FRP-subcontract-pattern]] — supply boundary for bridge work
- [[Common-pricing-misses]] — what gets dropped in budget rates
- BM Formwork YouTube (Ben) — channel reference for technique videos

## Future updates

If new BM videos surface (or other technique sources — Formula One UK, Doka training videos), add a row in the Sources block + extend the Productivity benchmarks table if new data shows up. Keep this document current — it's the technique sanity-check reference.

---

# Suspended slab framing technique (added 2026-05-05)

> **When to apply:** any cast-insitu suspended slab on a residential or light-commercial job — props + bearers + joists + ply on top, concrete poured on. Direct relevance: 2629 Erskine Park WWTP 270 mm suspended slab, future house slabs, balconies.

## TL;DR — slab-specific lock-downs

1. **Headers run the SHORT way, joists run the LONG way.** Always. (Standard span direction — joists carry slab load between headers.)
2. **Sheet layout drives joist position.** Work out rip widths BEFORE setting any joists. If end-rip > 600 mm, there's a better sheet layout — re-plan.
3. **Setup props get 2 nails. Back-props get 1.** Setup props are the structural skeleton — must not move during build.
4. **Pre-nail strap timber to lock setup-props together** so the rig doesn't tip while joists are being placed (especially important when the structure isn't yet locked into a four-walled room).
5. **Leave back-props OUT until level is set.** Less props to wind to level = faster levelling. Drop the back-props in AFTER the laser pass.
6. **Plumb perimeter brickwork BEFORE setting prop heights.** Find the high point + low point of the bearing wall first; that's your datum.

## Prop layout (residential standard)

- **Centres:** 450 mm typical (Milwaukee tape has the 450 / 900 marks built-in — easy reference)
- **Setback from wall:** Don't push prop hard into wall — leave room to operate, ~50 mm gap
- **End props:** 250 mm in from joist end, then 450 mm centres typical — but adjust if the layout doesn't divide cleanly (e.g. 9.75 m run = adjust last bay rather than force-fit)

## Sheet layout — the rip-width rule

Sheets are typically 1800 × 1200 mm (18 × 12).

For a 4.4 m room:
- Option A: 18 + 18 + 12 + 12 = 6.0 m → too wide
- Option B: 18 off one wall + 12 + 12 off the other = 4.2 m → leaves a **190 rip in the middle** ✓

Rule of thumb: if your end rip is **> 600 mm**, you've got the sheet layout wrong — there's a tighter way to lay it. Smaller rips reduce cuts, reduce waste, faster.

## Outside header — the wobbly bit

Outside header (the one along the perimeter, hanging on prop tops only — not yet locked in by joists) is the highest-risk fall-off-during-build moment. Defensive measures:

1. **Have joists ready** to drop in immediately after header is up (locks it laterally)
2. **Use the most stable / best-base props** under the outside header
3. **Don't lock everything off** — keep nails light enough that you can adjust during plumb
4. **Twist nails technique**: drive one nail from each side, twist the heads up with nips — holds during build but allows movement during plumb. Replace with proper fixings after level is set.

## Levelling sequence

1. Brickwork plumb-check first — find datum (high / low corners of bearing wall)
2. Set up props (2 nails per setup prop — full structural skeleton)
3. Run headers + joists with twist nails (allow adjust)
4. Strap timbers across to lock the rig (anti-tip)
5. **Laser pass** — wind setup props up/down to level the whole grid
6. Drop in back-props (now you've got fewer props to wind, only the setup ones moved during levelling)
7. Lock everything off properly (full nails)
8. Lay ply on top per sheet layout plan

## Ply cutting — the saw-scale trick

When you need to trim a sheet (e.g. cutting 100 mm off a 1200 mm sheet to get 1100 mm):

- Don't pull tape, mark, ping a line — too slow
- Use the **saw's measurement scale**: hold finger on the 100 mm mark, slide saw with finger as guide
- Saves a measurement step; works for any consistent rip width

## Bond breaker — concrete-on-brickwork detail

When a concrete slab bears on brickwork (residential), **always lay a bond breaker** between concrete and brick.

**Why:** concrete + brick have different thermal expansion coefficients. Without a slip layer, concrete grabs the brick → temperature cycles shift the slab → cracks propagate down through the brick wall (typically visible at corners of brick walls below concrete slab).

**Spec:** double-layer tin (PGI / galvanised flashing). Two layers because they need to slip against each other under thermal movement.

**Don't nail the tin.** Pin it lightly. Tin must shift — pinning prevents it from blowing away during pour but allows movement.

## Pre-pre-nailed prop tops

A small productivity tip: pre-nail the prop tops (mark joist positions on the side of each prop top before standing them up). When the prop is standing, you already know exactly where the joists land — no measure-on-the-ladder time. Same with the marks for prop positions on outside headers — pre-mark before standing the header.

## Productivity benchmarks — slab framing (added 2026-05-05)

| Element | hrs/m² of slab area | Notes |
|---|---|---|
| Suspended residential slab, single-storey, no balcony | **1.4 – 1.8** | Props + bearers + joists + ply, day shift |
| Suspended residential slab, with balcony rebates / glass channels | **1.8 – 2.5** | Top-side detail adds 25-40% |
| Suspended commercial slab (e.g. WWTP, fire-rated) | **2.0 – 3.0** | Heavier reo cage + tighter tolerances + spec compliance |
| Curved or non-rectangular slab perimeter | **× 1.10 – 1.20 multiplier** | |
| > 3.5 m floor-to-floor (taller props, more bracing) | **× 1.10 – 1.15 multiplier** | |
| Dropped beams / band beams integrated in slab | **+ 0.8 – 1.2 hrs/m of beam** | Counted on top of slab rate |

## Sequencing risks — what trips slab jobs up

1. **Bond breaker forgotten** → cracks at brick corners, callback / repair, reputational damage
2. **Inadequate strap timbers during build** → rig tips, props go down, restart with materials damaged
3. **Outside header braced poorly during joist install** → header falls off, likely takes a couple of joists with it
4. **Levelling done with all props in** → 3× the levelling time, often missed mm-tolerance
5. **Sheet layout not pre-planned** → discover mid-job that end rip is 700 mm → relay sheets, day lost