Cast-in-place piers with formwork lifts

How bridges get built · Piers, pylons and abutments · Cast-in-place piers with formwork lifts

Also called: panel formwork, lift-by-lift pier construction, gang forms

How it is done

Cast a kicker (starter) about 75–150 mm high with the footing so the first lift of forms seats tightly on true concrete; leave the starter bars with laps or couplers for the first lift.
Fix the first-lift reinforcement, usually as a cage prefabricated on the ground and lifted in, with cover spacers, earthing, drainage and any cast-in sockets for the pier-head brackets.
Set the formwork panels on the kicker, tie and plumb them, brace them with push-pull props and seal the base; inspect reinforcement, cover and formwork before the pour (hold point).
Pour by pump or skip in layers of about 0.5 m, compacting each layer, without exceeding the rate of rise the formwork was designed for.
Strike the vertical forms after 12–24 hours, start curing at once (wet hessian, wrap or curing compound) and prepare the construction joint by water-jetting or a surface retarder.
Lift the forms by crane to the next lift, re-plumb them on the previous lift and check position, section and verticality by survey before each pour.
At the top, form the pier head on brackets or shoring; then clean, repair and re-oil the panels and move the set to the next pier.

Key numbers

Lift height3–6 m per pour, set by form pressure, crane capacity and access
Formwork pressureup to full hydrostatic, about 25 kN/m2 per metre of fresh concrete, for fast pours or self-compacting concrete
Cycle3–5 days per lift per form set; 2–3 sets in rotation on a viaduct
Economic pier heightup to about 20–25 m, above which climbing forms usually pay
Verticalitycommonly about 1/1,000 of the height (check the project specification)
Striking of vertical formstypically 12–24 hours after the pour

Where it fits

Piers up to 15-25 m in numbers of 10-40 on a viaduct; any shape the formwork can be built for.

Where it does not

Tall piers where each crane lift costs a day; use climbing forms above about 25 m.

Choosing it

Choose it over climbing formwork for piers under about 20–25 m in modest numbers: the crane is on site anyway and no climbing system has to be bought, assembled and dismantled.
Prefer climbing or self-climbing forms when piers exceed about 25–30 m, crane hook time per lift becomes the bottleneck, or wind makes lifting large panels unsafe.
Prefer slipform for tall, constant-section shafts where a continuous 24-hour operation can be supported.
Prefer precast pier segments when piers are many and identical and site time, traffic management or marine access dominate the cost.

Plant, pace and money

PlantFormwork sets (two or three in rotation), crane, concrete pump, access scaffold.

ProductivityA lift every three to five days per set; a 15 m pier in two to three weeks.

CostUSD 400-700 per m3 of pier concrete in place.

Risks and controls

What goes wrong

Formwork blowouts, cold joints, cover loss at lift joints, falls from height.

Quality assurance

Plumbness surveyed every lift, cover meter checks, cube tests, joint preparation between lifts.

Origins

Casting piers lift by lift in timber forms has been ordinary practice since the early reinforced concrete bridges; from the 1960s, reusable steel and proprietary modular panels handled as crane-lifted gang forms replaced most site-built timber.

Examples

The default pier construction in the game's schedule for piers under 15 m.

Case studies

I-35W Saint Anthony Falls Bridge, MinneapolisUSA · 2008Replacement for the bridge that collapsed in 2007, built in about 11 months; its curved, sculpted river piers were cast in place.