Lift-slab is a building method in which all the floor slabs of a building are cast on top of each other at ground level and then jacked up along already-erected columns to their final height.
The method was developed in the United States and used from the 1950s onward, mainly for offices, car parks and apartment buildings with a regular column grid.
How it works
- The foundations and columns are built first. The columns stand the full height of the building, or are extended as work proceeds.
- The ground floor slab is cast on the ground and immediately serves as the flat form for the slabs above.
- All the upper floor slabs are cast on top of it as a stack, separated by a bond breaker so they do not adhere. Around each column sits a steel collar — the lifting collar — which later serves as the connection.
- Jacks on the column heads pull the slabs up one by one on threaded rods, starting with the topmost. Lifting is slow and synchronised; every jack must make the same stroke or the slab hangs out of level.
- On reaching its level, the lifting collar is permanently welded or pinned to the column, and the slab carries itself.
Why it looked attractive
- Almost no formwork. The slab below is the mould. That saves plant, labour and scaffolding.
- Working at ground level. Fixing reinforcement and pouring happen on the ground, which is faster and safer than the same work at height.
- Little space needed around the building, which counts on tight inner-city sites.
The risk
The weak point lies exactly in the phase that makes the system distinctive: as long as a slab hangs on the jacks, it is not resting on a permanent connection. In that temporary condition the structure is vulnerable to going out of level, to the slender columns buckling, and to the failure of a single suspension point — after which the slab can slip and take the slabs below with it.
That is what happened in 1987 at the collapse of L’Ambiance Plaza in Bridgeport, Connecticut, during the construction of an apartment building using lift-slab. Twenty-eight construction workers were killed. The disaster led in the United States to considerably stricter requirements on temporary stability and on the detailing of the slab-to-column connection, and the method largely fell out of favour.
Today
Lift-slab is now rarely used. For the aims it was devised for — speed and minimal formwork — there are alternatives with a better risk profile: precast hollow-core slabs, half-slab (filigree) floors and table formwork systems. The term survives in construction mainly as a case study: a method that worked on paper, but whose governing probability of failure turned out to lie in the temporary construction stage.
Related terms
- Flat slab
- Hollow-core slab floor
- Half-slab floor
- Formwork
- Column
- Punching shear
- Precast concrete
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