Restoring and maintaining archaic concrete floor systems

Restoration and conservation

Within this soaring vaulted ceiling lurks an archaic concrete system that has been covered over, making it challenging to access and repair. Note the exposed structural wire mesh where fasteners for the roof were added in the 1980s.

Thousands of buildings with these floor systems are still in service today, many more than a 100 years after they came into service. Overall, alternative concrete floor systems are best left undisturbed; if in good repair there is no reason to assume they require replacement. Any repairs or alterations should be made in consultation with a structural engineer who should undertake a detailed examination of the floor system and its condition.

Localized damage to a slab can simply be patched if supporting wires or other reinforcements are intact. This type of repair will not restore the concrete’s structural capacity, but this is not crucial as the wires are the structural elements.

If an area has been weakened through the corrosion or cutting of the tension elements, then restoration will require replacement of the entire span between the supporting beams. As the structure relies on continuous tension from beam to beam, the spanning area of the slab cannot be partially replaced and still function. Since tension elements may run continuously from one span to the next, the tops of the beams must be fully exposed, and any wires must be tack welded to the beams before they are cut to maintain the integrity of the adjacent spans.

Cinder concrete decks were favored in the 1920s to 1940s for their load capacity, fire protection, light weight, low cost, and ease of construction.

Alterations to alternative concrete slabs should be made carefully with attention to special details, including any modifications that involve cutting into or drilling through the floor slab. As cutting into a catenary support system will destroy its load capacity, alternative measures must be taken to support the load between the opening and the adjacent steel beams.

In the case of an alternative concrete system as a roof deck, the existing slab may be abandoned in place if found to be compromised and more limited repairs are impossible. For this procedure, the construction team carefully removes the topping slab and cinder fill, then pours a lightweight reinforced slab into the cavity, spanning between the existing steel beams. As the existing cinder layer can be quite thick and heavy, such a repair can have the ancillary benefit of removing up to 244 kg/m2 (50 psf) of dead load from the floor.

Unlike a regular slab, loads cannot be hung from an alternative concrete system without careful consideration, as the slab itself does not have strength to resist pullout. Existing hangers attached to wire reinforcement may be used with caution. If new hangers are installed, they should be attached directly to the steel floor beams. If an element must be placed between beams, a rod can be drilled through the entirety of the slab and the load distributed by a wide flange on the top. However, this should be avoided if possible as it risks damaging tension elements during drilling.

Still in service

At the SoHo, New York headquarters of Scholastic Inc. (built 1890), building envelope restoration included a terra cotta and cast-iron floor system.

As a part of the living history of construction, archaic floor systems exist in many buildings despite having been supplanted by modern construction methods. As a building manager or design professional, it is important to be aware these systems are in use today, and to recognize one in place before attempting repairs, alterations, or construction to avoid inadvertently damaging the integrity of a structure. However, these historic systems are proven safe and durable, and with knowledgeable stewardship will be able to prove themselves reliable into the future.






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