In a multi-storey concrete building under construction, the newest slab is supported by shores bearing on the slab below, which is itself often shored to the one below that. The construction load from a fresh pour therefore distributes through several levels of partially cured concrete, and the capacity of the whole arrangement depends on the in-place strength of slabs that are days old. Removing shores from a level transfers its share of that load onto concrete that may not yet be able to carry it, and the decision about when to strip is among the most consequential on the site.
Construction loads can exceed service loads
A slab in service carries its own weight plus the occupancy it was designed for. The same slab during construction may carry its own weight plus the weight of a fresh pour above transmitted through shores, plus the shoring, plus materials and equipment. That total frequently exceeds the design service load, and it is applied to concrete that has not reached its specified strength. The construction condition, not the finished condition, governs — and it is analysed less often than it should be.
How load shares between levels
When a slab is poured on shores bearing on slabs below, the load distributes among those levels according to their relative stiffness. Analysing this properly requires treating the shored levels as an interconnected system rather than checking each slab in isolation, and the distribution changes every time a level is stripped or reshored. Simplified approaches exist and are widely used; where the sequence departs from the assumptions those approaches rest on, the actual distribution can differ substantially.
Strength is a function of maturity, not of days
Concrete gains strength as a function of both time and temperature, so a fixed number of days is not a safe criterion. The same mix reaches a given strength far more slowly in cold weather, and a schedule built around calendar days can strip slabs in winter that would have been ready on the same day in summer. Cold-weather pours, unheated enclosures and overnight temperature drops are recurring factors in early-stripping failures.
What field-cured cylinders actually tell you
Standard-cured cylinders are used for acceptance and represent the mix's potential, not the slab. For stripping decisions, the relevant question is the strength of the concrete in the structure, which depends on the conditions it actually experienced. Field-cured cylinders stored alongside the slab are closer, though they still differ in thermal mass. The maturity method, calibrated for the specific mix and using temperature sensors embedded in the slab, tracks in-place strength more directly, as do pull-out and other in-place tests.
Reshoring is not the same as leaving shores in place
Reshoring means removing shores and reinstalling them, which allows the slab to deflect and take up its own weight before the reshores are set. The reshores then carry only subsequently applied load. Backshoring, where supports are removed and replaced one at a time without letting the slab deflect, behaves differently. Confusing the two, or installing reshores with a preload they were not meant to carry, changes the load distribution in ways the analysis did not assume.
Punching shear is the usual failure mode
Flat plate slabs under concentrated shore loads are vulnerable to punching shear at the shore locations and at columns, and punching failures are sudden and offer no warning. Because the mechanism is local and brittle, a single overloaded location can initiate a progressive collapse through several floors as debris from each level loads the one below. This is why early-stripping failures in concrete frames tend to be severe rather than localised.
Who decides, and on what basis
Stripping and reshoring are normally the contractor's means and methods, with a shoring design prepared by a qualified engineer and a strength criterion specified. The investigation asks what criterion applied, what testing supported the decision, who made it, and whether the sequence followed the design. A stripping decision made on schedule pressure without strength verification is a specific, documentable finding rather than a general observation about site practice.
Reconstructing the sequence
The analysis needs the actual pour dates for each level, the shoring and reshoring configuration at the time of failure, when each level was stripped, the strength test results available at each decision point, and the temperature history. Daily reports, pour logs, testing laboratory reports and site photographs together usually establish this, and photographs are often the most reliable record of what was actually standing where.
What to preserve
The shoring components and their arrangement as found, level by level, with spacing recorded. Concrete from the failed slabs for in-place strength testing, taken from locations of known original position. Testing records including field and standard cured cylinder results and any maturity data. The shoring design and calculations, the specified stripping criterion, daily reports, pour logs, temperature records and site photographs covering the days before the collapse.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.