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January 25, 2024
The “Pandora’s Box” of Sidewalk Vaults

A recent Habitat post characterized sidewalk vaults as a “Pandora’s box” of potential problems for building owners.  But that’s not the right analogy.

In Greek mythology, Pandora was given a box as a gift from the gods but instructed not to open it. She couldn’t resist opening the box and out flew every evil that would henceforth plague humanity.

But unlike Pandora’s box, you should enter or open vaults periodically to monitor their condition before problems escalate.

NYC’s miles of sidewalk vaults are generally out of sight, so they can also be out of mind. Facades, retaining walls, and garages are all subject to New York City Local Laws governing their safe condition. We’re not proposing another local law (although we wouldn’t bet against one) but vaults also require regular inspection and repair to keep them structurally sound and watertight.

Sidewalk vaults are often impacted by water infiltration causing corrosion of their steel elements. If left unresolved, structural degradation can lead to collapse of the sidewalk supported by the vault.

The good news: in many cases, this damage is superficial, and steel can be salvaged instead of scrapped. At first glance, it may appear that the steel requires wholesale removal and replacement—but initial appearances can be deceiving. In our extensive experience restoring sidewalk vaults, we’ve developed various methods of rehabilitation that don’t require complete, costly reconstruction.

When probes are deployed to assess the condition of corroded steel in sidewalk vaults, it’s important to keep some chemistry in mind: steel expands up to eight times when it rusts, so an inch-thick layer of rust represents a loss in cross-section of only about an eighth of an inch of uncorroded steel.

So, when cleaned of corrosion and measured, if a steel beam still has sufficient thickness to bear the load required, it can remain in place and continue to serve its purpose.

In other cases, only specific surfaces of a steel beam have corroded beyond the point of no return. For example, if the beam’s bottom flange is corroded to the extent that it requires reinforcement, the beam’s structural capacity can be restored by introducing a plate to reinforce the flange.

If the desired strength requires greater reinforcement, an inverted T can be welded to the bottom flange to increase the beam’s capacity. An existing beam with compromised structural integrity can also be reinforced through the introduction of new support at mid-span.

Another scenario we’ve encountered is a vault with a defective structural slab but sound supporting steel structure. In that case, our design called for a reinforced steel deck, supported by brackets attached to the beams, to be installed at the underside of the defective slab. Holes are made in the existing slab through which self-consolidating concrete is poured to form a new slab beneath the defective one.

In every case of sidewalk vault showing signs of rusted steel, a crucial component of restoration is to stop water infiltration to keep corrosion from recurring. Steel elements must be coated, waterproofing membranes above the structural slab must be repaired or replaced, and cracks and open joints in sidewalk slabs must be sealed to keep water out of vault spaces.

In the original Greek myth, only “hope” remained in the box after Pandora finally shut it. But in restoration, hope is not a strategy.

SUPERSTRUCTURES Engineers + Architects

14 Wall Street, 25th Floor, New York, NY 10005
(212) 505 1133


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