If you live in a large city — especially in the northern parts of the country — you probably walk over deteriorating sidewalk vaults often without giving it much thought.
Structural engineer’s perspective
Cuono: Unlike facades, sidewalk vaults are not typically subject to regular mandated inspections. One rarely hears about sidewalk failures in the news. The danger, however, is there.
The loads and elements of nature on sidewalk vaults can be varied and extreme. Often, the structural engineer is only called in when the framing is so badly deteriorated that the only recourse is complete reframing. One may find steel beams that are rusted to razor-sharp edges or even partially disintegrated; screw jacks or haphazardly installed shores that have been employed hastily to head off the potential disaster; a hatch door with no discernible means of support; or water regularly raining down on the space below.
Faced with the above conditions, localized repairs may not be feasible. In rebuilding the vault framing, here are some key issues to consider: temporary bracing of the side walls (which are often unreinforced masonry) from soil pressure and heavy surcharges along roadways and at the building; connecting the new framing to the existing building girders/columns, which may also be deteriorating; the permit process, which may involve local Department of Transportation officials; current live load requirements, which may be much heavier than the original design; and historic elements, which may need to be retained.
Under the right conditions, it may be possible to erect new beams under existing ones and save the owner the expense of rebuilding the entire vault. Here is an example: A vault is framed with steel beams that are deteriorating and has existing brick arches that are in fair condition spanning between the beams. The solution may be to design new beams and install them directly below the existing ones, then remove and replace the sidewalk slab above the brick arches, install new waterproofing, and add a new topping slab.
Other historical construction includes cinder concrete slabs with a draped wire mesh, large slabs of granite spanning between beams, and concrete slabs with glass inserts. On older vaults, one may even find wrought-iron beams. New beams could be used intermittently between the existing framing to reinforce the slabs or reduce the weight on existing beams that have lost some steel thickness. Testing of materials is often warranted to determine capacity and weldability.
Bates: Extending building basements below adjacent sidewalks was not uncommon in many large cities and was quite typical for industrial neighborhoods.
Vaults permitted immediate access to building utility/delivery areas without having to enter the building proper. A walk through many historic industrial city neighborhoods will reveal a lot of hints to and uses of open spaces below, including obsolete coal chutes (often filled in with mortar), manholes, diamond-patterned or “bullet” glass set in cast iron frames, steel diamond plate, sidewalk elevator hatches, and thick granite slabs spanning from curb to building line.
Building owners were expected to maintain the vaults and were taxed for their use by the city who owned the property under rules of revocable consent. As sidewalk delivery of coal waned, and coal bins (typically housed in vaults) fell into obsolescence, building owners often walled off or filled in the vaults to prove non-use and thus avoid being taxed.
In many cities, the vault tax was eventually repealed (New York in 1998, for example), and building owners began to realize the potential value of subterranean real estate — albeit not necessarily valued for its original purpose. Thus began a surge of vault reclamation projects with plans to convert them into storage bins, offices, and — in a few rare occasions — restaurant dining rooms. It became clear, however, that years of neglect and concealed water infiltration had caused extensive damage to the vault structures. There are many cases where streets were widened by local transportation authorities, and the curbs (and the gutters) were pushed back over the vault space. Massive steel beams and columns were introduced within the vault space to support the new curbs and, by default, vehicular traffic. Poor or non-maintainable waterproofing measures were taken, and the curb supports likely began to deteriorate soon after installation.
Vault reconstruction should include extending the integral waterproofing membrane out over the face of the street-retaining wall to a practical depth because the curb line is continually susceptible to water infiltration.
Ciro Cuono, P.E., LEED AP, is an associate at Hage Engineering. He can be reached at firstname.lastname@example.org. Robert C. Bates, RA, AIA, is a principal at Walter B. Melvin Architects, LLC, in New York. He can be reached at email@example.com.