Temporary Works Engineering

A structured learning path from temporary works fundamentals through advanced design and management. Master scaffolding, formwork, falsework, shoring, and excavation support for safe construction.

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Level 1

Beginner — Scaffolding and Formwork

Start here if you are new to temporary works.

Scaffolding Design and Erection

Scaffolding provides temporary elevated platforms for workers and materials. Types: tube-and-coupler (traditional, versatile, using standard tubes 48.3mm dia, 4mm wall, right-angle/sleeve/swivel couplers, loading capacity 5-15 kN per standard), system scaffold (Cuplok, Ringlock, Kwikstage — prefabricated components with fixed geometry, faster erection, less loose components, higher safety), H-frame/access frame (mobile rolling towers, pre-assembled frames). Design loads: dead load (scaffold self-weight), live load (workers + materials — typically 1.5-3.0 kN/m2 for light duty, 5.0 kN/m2 for heavy duty), wind load. Maximum bay length: 2.0-2.7m. Maximum lift height: 1.5-2.0m. Ties to structure: minimum every 4m horizontally, 4m vertically, at free ends within 300mm.

Key components: base jacks (adjustable, with base plates on sole boards), standards (vertical tubes, 2m-3m lengths), ledgers (horizontal along building face), transoms (horizontal across bay width), bracing (longitudinal and cross — diagonal, plan), decking (timber or metal boards, 38-50mm thick, overlaid by 2 boards per bay minimum), guardrails and toe boards (top rail 950mm above platform, intermediate rail, toe board 150mm high), ladder access (internal or external ladder bays). Foundation requirements: sole boards (200x50mm timber) on compacted ground, minimum bearing capacity 50 kN/m2. Maximum scaffold height: independent tied scaffolds up to 50m height (with engineering design). Loading classes: Class 2 (1.5 kN/m2 — inspection/light), Class 3 (2.0 kN/m2 — general), Class 4 (3.0 kN/m2 — bricklaying), Class 5 (4.5-6.0 kN/m2 — heavy stone).

Formwork Systems for Concrete

Formwork molds wet concrete to the required shape until it gains sufficient strength to support itself. Materials: timber/plywood (traditional, 18-25mm marine-grade ply, lower cost, labor-intensive), aluminum (lightweight, modular panels for walls and slabs, 25-50 reuses), steel (heavy-duty prefabricated panels for large repetitive pours, 200+ reuses), plastic/composite (lightweight, dome formers for waffle slabs, limited reuse). Formwork classification: traditional (site-built), engineered (shop-fabricated modular systems), climbing/jump forms (self-climbing for core walls, slip forms (continuous pour for silos/ chimneys). Formwork design: lateral pressure from fresh concrete per CIRIA Report 108 or ACI 347: P = gamma * h + surcharge (placement rate and temperature dependent, typical 50-100 kN/m2 for walls, 20-50 kN/m2 for columns).

Slab formwork: traditional (props + timber beams + plywood decking, span up to 1.5-2m), table form/flying form (large prefabricated panels with integrated props, crane-handled, for repetitive floor plates), drop-head system (early striking — props remain while slab forms removed, allowing faster reuse). Column formwork: round (steel or cardboard), square/rectangular (timber or modular steel panels, clamped at heights). Wall formwork: twin-face panels with ties (through bolts with plastic cones), alignment jacks/brace, working platforms. Props: adjustable steel props (BS 4074), safe working load 15-40 kN depending on extension height (max safe height = 3.2m typical). Bracing: longitudinal and transverse bracing required for stability. Striking times: determined by concrete strength (cube tests or maturity method): slabs > 70% specified strength, beams/props may remain longer.

Falsework and Shoring

Falsework supports the permanent structure during construction until it becomes self-supporting. Types: birdcage (dense grid of props on sole plates with bracing, for slabs up to 8m height), table form/flying form (prop-supported tables for repetitive slabs), tunnel form (paired half-tunnels forming walls and slab monolithically). Falsework design per BS 5975 or similar standards: dead load (self-weight of falsework + wet concrete), live load (construction loads 0.75-1.5 kN/m2, concrete placement surcharge 1.5-2.0 kN/m2), wind load (during assembly and in-service). Prop grids: typically 1.2-1.8m spacing in both directions for slab thickness 200-500mm. Prop capacity decreases with height (second-order effects). Bracing: plan bracing at every lift (horizontal), vertical bracing at maximum 4.5m centers (transverse and longitudinal).

Shoring provides lateral support to excavations, trenches, and existing structures. Types: hydraulic shoring (aluminum hydraulic struts with steel rails, for trenches up to 6m deep, quick installation), timber shoring (traditional vertical sheeting with walers and struts), sheet piling (steel or vinyl interlocking piles, cantilever or anchored), soldier piles and lagging (steel H-piles with timber/concrete lagging between). Shoring design: lateral earth pressure (active pressure behind shoring: Pa = Ka * gamma * H, passive pressure in front: Pp = Kp * gamma * D, water pressure must be included). Factor of safety: minimum 1.5 for overall stability, 2.0 for heave and piping. Bracing: struts (steel pipe or H-section, preloaded to reduce deflection), rakers (inclined struts bearing on temporary footings or existing structure), tiebacks (ground anchors, post-tensioned, for deeper excavations where struts are impractical).

Level 2

Intermediate — Excavation Support and Temporary Bridges

Build on fundamentals with deep excavation and bridge temporary works.

Excavation Support Systems

Deep excavation support retains soil and groundwater adjacent to the excavation. Diaphragm walls (slurry walls): cast-in-place reinforced concrete walls (0.6-1.5m thick, up to 50m depth), excavated under bentonite slurry support, panels 2.5-6m long, provide structural wall and water barrier simultaneously. Secant pile walls: overlapping piles (hard/soft or hard/firm), 600-1200mm diameter, form continuous wall, used for groundwater cut-off and structural support. Contiguous pile walls: non-overlapping piles with gaps, for dry cohesive soils or where groundwater not an issue. Soldier pile and lagging: H-piles at 1.5-3m spacing, timber/ shotcrete lagging between piles, economical for temporary support up to 10m depth.

Groundwater control: dewatering (well points, deep wells, ejector wells), cut-off (sheet piles, slurry walls, secant piles, grout curtain), or combination. Dewatering design: flow rate Q = k * H * L / R (Dupuit equation for unconfined flow, Theim equation for confined), number of wells N = Q / q_single_well. Filter design: prevent migration of fines while allowing water flow (Terzaghi filter criteria: D15_filter / D85_soil < 5, D15_filter / D15_soil > 5). Surface settlement adjacent to excavation: empirical methods (Peck's chart, Clough and O'Rourke) relate settlement magnitude to soil type, wall stiffness, and support system stiffness. Protection of adjacent structures: compensation grouting (preventative grouting before excavation-induced settlement occurs), underpinning (extend existing foundation to deeper bearing stratum).

Temporary Bridges and Works for Infrastructure

Temporary bridges maintain traffic during permanent bridge construction. Bailey bridge: prefabricated steel truss panels (10ft x 5ft x 5ft 3in), assembled in various configurations (single/double/ triple storey, single/double/ triple truss), standard span 30-200 ft, capacity up to 70 tons (military loading). Panel bridging: Mabey Compact 200 (modular steel panel bridge, spans up to 60m, single lane width 4.2m, capacity full highway loading). Design: simply supported spans, transverse distribution (wheel loads distributed to panel points), wind loading on bridge and traffic, axial forces in chords and diagonals checked per structural steel code.

Cofferdams: temporary enclosures keeping water and soil out of excavations for bridge piers, dam construction, or river works. Types: cellular cofferdam (interlocking sheet piles in circular/straight diaphragm cells, filled with granular material, stability from gravity and interlock friction), single-wall cofferdam (sheet pile wall with internal bracing, for shallow water), crib cofferdam (timber cribs sunk into place, filled with rock). Design: external water pressure (hydrostatic + hydrodynamic from current), soil pressure (active on high side, passive on low side), uplift at base (balanced by weight + friction + seepage length), seepage analysis (flow net, piping safety factor > 3). Dewatering inside cofferdam: sump pumping, well points, or tremie concrete seal (1-3m thick) for cutoff at base of cofferdam.

Lifting Plans and Heavy Lifting

Lifting temporary works include the design and planning of heavy lifting operations. Roles: Appointed Person (AP — competent person who plans the lift), Crane Supervisor (oversees the lift), Slinger/Signaller (attaches load and directs the crane operator). Categories: basic lift (straightforward, < 75% crane capacity, documented in risk assessment), standard lift (75-90% capacity or complex, requires method statement and lift plan), complex/critical lift (> 90% capacity, tandem cranes, lifts over occupied areas, lifts near power lines — requires engineered lift plan, structural checks of crane supporting ground or structure).

Lift plan content: load details (weight, dimensions, center of gravity, lift points), crane selection (configuration, capacity at radius, ground bearing pressure), rigging (sling type, capacity, angle factor, spreader beam if needed), exclusion zone (radius of load + maximum radius of any part of crane), week-hour weather criteria (wind speed limit typically 8 m/s for general, 5 m/s for large area loads, no lightning). Ground conditions: crane outrigger pad size (minimum area = outrigger load / allowable bearing capacity), typically 1-2 m2 pads on compacted hardcore or crane mats. Common failure modes: overturning (center of gravity moves outside tipping axis), structural failure (boom collapse, jib failure), rigging failure (sling cut, shackle overloaded). Tandem lifts (two cranes): load shared 50:50 or unequal with engineered share calculation, slower lift speed, constant communication between operators.

Level 3

Advanced — Temporary Works Design and Management

For senior students and practicing engineers.

Design of Temporary Works

Temporary works design follows limit state principles similar to permanent works but with modified factors. Load factors: BS 5975 recommends 1.2 * dead load + 1.5 * imposed load (construction + environmental). Load combinations: construction stage with wind (wind considered as imposed load with factor 1.5), concrete placement with impact factor (1.5 * normal live load for concrete placement surcharge). Material partial factors: steel 1.05, aluminum 1.10, timber 1.3-1.5 (depending on grade and duration of load). Serviceability: deflection limits generally L/300 for formwork surfaces, L/250 for other temporary members. Stability checks: overturning (minimum restoring / overturning = 1.2), sliding (base friction resistance / horizontal force > 1.4), uplift (weight sufficient to resist wind uplift).

Structural analysis: prop design with second-order effects (P-delta, Euler buckling effective length factors), scaffold design as braced or unbraced frame (BS EN 12811), falsework grid bracing analysis as 3D frame. Connection design: scaffold coupler capacity (typical 6.25-10 kN slip resistance per BS EN 74), formwork tie capacity (15-80 kN working load), prop base plate bearing on timber sole plate (timber compression perpendicular to grain < 2.0 N/mm2 typical). Foundation design: temporary work foundation must be verified for bearing capacity (typically minimum 50 kN/m2 for standard scaffolds, higher for heavy falsework). Bearing pressure = (imposed load + self-weight) / base plate area. Drainage around temporary foundations essential to prevent softening in wet conditions.

Construction Stage Stability

Construction stage stability considers structural behavior during erection before the permanent stability system is complete. Steel frame erection: partial bracing (temporary guys, plan bracing required for each lift before releasing crane), column base fixing (anchor bolts fully torqued before lifting next tier), diaphragm action (metal decking acts as diaphragm when fastened but only after sufficient welding is complete). Bridge construction: cantilever erection requires temporary stay cables or temporary piers, stability against wind during erection (bare deck has low torsional stiffness). Concrete structures: backpropping (load transfer from freshly cast slab to lower slabs through props during construction), typically 2-3 levels of backprops required for multi-story buildings.

Wind loading during construction: BS EN 1991-1-6 considers reduced return period (10-year vs 50-year for permanent), partial factors for construction loads. Wind speed limit for crane operation: typically 8 m/s for general lifting, 5 m/s for large surface area loads, 14 m/s for out-of-service cranes with slew brake released. Formwork striking: concrete must reach minimum strength (typically 10 N/mm2 or 70% specified strength for slab striking, determined by cube tests, maturity method or in-situ pull-out tests). Progressive collapse during construction: robust design with alternative load paths or catenary action. Tie forces: minimum horizontal ties in floors (permanent design aids robustness during construction). Temporary works coordinator (TWC) role: design check, inspection before loading, monitoring during construction, authorization for striking/removal.

Temporary Works Management

Temporary works management systems (TWMS) ensure safety through a structured process. The Temporary Works Coordinator (TWC) role under BS 5975: competent person appointed by contractor, responsible for coordinating all temporary works activities. The 4-stage process: (1) Design brief — define requirements, loads, duration, ground conditions, interfaces with permanent works; (2) Design — by competent temporary works designer (typically chartered engineer with TW experience), using recognized standards (BS 5975, BS EN 12811, BS EN 1065 for props); (3) Check — independent design check by another competent engineer (category dependent: Category A — standard design checked by TWC, Category B — complex design checked by separate engineer, Category C — very complex design checked by independent specialist); (4) Implementation — permit-to-load system, inspection before loading, monitoring during use, permit-to-remove for dismantling.

Risk assessment: Hazard Identification and Risk Assessment (HIRA) for each temporary works activity, considering: failure of temporary works under load, collapse during erection/dismantling, falls from height (leading cause of temporary works fatalities), being struck by falling objects, ground collapse, and underground services strike. Method Statement: step-by-step procedure for erection, loading, monitoring, and dismantling. Permit to Load: formal authorization that temporary works have been inspected and are ready for loading. Inspection requirements: after installation before loading (check geometry, connections, bracing, supports, plumb/level), during loading (visual monitoring for deflections, distortion, cracking), before and after extreme events (storm, heavy rain). Register of temporary works: all TW installations recorded with design reference, check status, inspection records, and removal authorization.

Practice Exercises

Exercise 1: Slab Formwork Design

Design formwork for a 300mm thick flat slab at 4.5m height. Concrete density 25 kN/m3, construction live load 2.0 kN/m2, plywood decking 18mm thick, timber joists at 400mm spacing. Calculate: load on props, prop spacing in both directions, maximum prop extension (allowable load for 3.0m extension = 25 kN). Check bearing on timber sole plate (permissible stress 2.0 N/mm2).

Exercise 2: Excavation Support Design

Design cantilever sheet pile wall for 4m deep excavation in sandy soil (gamma=18 kN/m3, phi=32 deg). Calculate active and passive earth pressures. Determine minimum embedment depth for stability (factor of safety 2.0). Calculate maximum bending moment in sheet pile and select appropriate pile section modulus (steel yield strength 275 N/mm2).

Exercise 3: Scaffold Design Check

A freestanding independent tied scaffold is 30m high, 5 bays long (2.1m bay length), 1.2m lift height. Loading: Class 3 (2.0 kN/m2 general purpose). Wind load: basic wind speed 24 m/s, site exposed. Check the maximum load in a standard at base level. Determine tie spacing required. Verify base plate bearing on sole boards (timber bearing capacity 2.5 N/mm2).

Exercise 4: Temporary Bridge Span Check

A Bailey bridge is proposed to span 24m as single-storey single-truss configuration, carrying highway loading (HB 45 units = 180 kN axle, 12 kN/m UDL). The maximum moment capacity of the configuration is 1000 kNm and shear capacity is 250 kN per truss. Check the adequacy of the Bailey bridge for the proposed loading. Determine if a strengthened configuration is needed.

References

BS 5975:2019. Code of Practice for Temporary Works Procedures and the Permissible Stress Design of Falsework. BSI, 2019.
BS EN 12811-1:2003. Temporary Works Equipment — Scaffolds — Performance Requirements and General Design. BSI, 2003.
ACI 347-14. Guide to Formwork for Concrete. American Concrete Institute, 2014.
Pallett, P.F. Guide to Falsework. Concrete Society, 2003.
CIRIA C660. Early-age Strength Assessment of Concrete on Site. CIRIA, 2006.
Civil Engineering Handbook — Temporary works chapter.
Engineering Formula Library — Earth pressure and formwork design formulas.
Engineering Standards Reference — BS 5975, BS EN 12811, ACI 347 standards.
Engineering Glossary — Definitions of temporary works terms.