The majority of steel-framed buildings constructed in the first half of the 20th century utilized a particular type of concrete floor system known as “draped mesh concrete slabs”. This system proved highly effective and economical. Unfortunately, many buildings experience significant periods of deferred maintenance, especially when It comes to maintaining their roofs. It Is important to understand these historic building systems. There Is a significant hazard to occupants when this system Is exposed to water Infiltration, either from the roof or Interior plumbing Isuses.
Draped -mesh slabs utilized lightweight cinder-concrete and wire mesh draped over a steel or concrete superstructure for tensile strength. Replacing traditional stone aggregate made this type of concrete booth lightweight and economical cinders were a readily available waste product of coal combustion. Throughout New York City and other older urban areas, coal was frequently burned to provide heat and electricity. Iconic Manhattan buildings such as Empire State Building, Chrysler Building, and Rockefeller Center, are just a few to use draped mesh cinder-concrete.
Once coal burning in cities was phased out in the mid-1960s, concrete fill on metal deck started to replace draped mesh concrete slabs as the preferred method for steel high-rise floor framing. Because so many buildings in NYC were previously constructed with this system, knowledge of how the failure mechanisms and how to repair / reinforce them is is critical.
Quote from Dan Connolly here:
“Our first project involving draped mesh slabs was in 1999. Significant spalling and ceiling deflection was discovered throughout over 80% of the underside of the roof slab. We estimated that some of the spalled areas, if dislodged, would weigh more than 150 pounds!”
DRAPED MESH CONCRETE SLABS: THE BASICS.
Supported by hot rolled steel beams and girders, the one-way system, is made up of draped, welded wire reinforcement embedded in cinder-concrete (occasionally stone-concrete) that span between closely spaced steel beams. The wire mesh aka “draped mesh” is placed over the tops of steel floor beams and then hung down at the mid-span between the beams. The “catenary” or “hung chain,” ultimately provided optimal geometry for creating a tension cable system. This system was extremely popular for engineers and builders because of its high load capacity, superior fire proofing properties, light weight, and ease of construction.
IS THERE A STRUCTURAL PROBLEM IN THESE CITY BUILDINGS?
Testing has proven these concrete slabs to be quite strong. However, with time and natural deterioration due to things such as corrosion of the reinforcement, exposure to water/moisture, physical damage, eventual repair to the draped mesh concrete system is inevitable. Owing to the fact that cinder-concrete is very porous, it is extremely susceptible to damage from water infiltration. In addition, low quality coal cinders (the basis of the draped mesh concrete system) contain sulfur, when combined with moisture/water, can cause an acidic corrosion of the embedded steel reinforcing and the wire mesh. The strength of steel is reinforcing the whole concrete slab, and if that starts crumbling, there is risk to the entire slab.
Quote from Dan Connolly here:
“All building systems are susceptible to damage / deterioration if deferred maintenance occurs over an extended period of time. This system has performed extremely well over an extended period of time. However, we must be aware of the buildings that have not been properly maintained. ”
HOW TO REPAIR THESE SYSTEMS.
With decades of experience in repairing draped mesh concrete systems, some solutions include:
- PARTIAL-DEPTH REPAIR: For minor, shallow and isolated spalls our office utilizes a partial depth repair. This has been successful in slabs where the damage is not extensive or systemic through the structure.
- TOPPING SLAB REPAIR/REINFORCMEENT: In these instances, steel framing supporting the slabs should be inspected. Topping slab repairs will be either bonded (act compositely with the existing draped mesh cinder concrete slab) or unbonded (where a separate supported slab is installed over the existing slab).
- FULL-DEPTH REPAIR/REINFORCMENET: When the entire concrete slab has severely deteriorated to the point it can no longer support the required loading, full-depth repairs are needed. This may involve the installation of temporary shoring in adjacent bays, anchoring steel beams at the adjacent spans, and extensive demolition. One of the major considerations on a roof slab Is providing protection, an enclosure, to protect the top floor units while the slab Is being removed / replaced. This can be a significant portion of the cost.
- SUPPLEMENTAL SUPPORT REINFORCEMENT: In many cases, we encounter roof slabs subjected to water infiltration for extended periods of time. However, what do you do if the client has just installed a brand new $600k roof. This a major consideration in determining how to address the problem. We typically recommend supplemental support from below so we can re-support the roof slab without disturbing the new watertight roof system. In rooms where there are intermediate steel beams, steel angles and steel decking are installed between the existing steel beams. This reduces the spans and subsequent load capacity required. In rooms where there are no intermediate beams, intermediate steel beams are installed between the existing steel girders. These beams support steel decking spanning between them. In both cases, heavy gage Type B roof decking is used.
These solutions, however, can be extremely difficult to plan and execute. The mesh is extremely sensitive to work with, and sometimes, repairing one section of the reinforcing effects the way loads are transferred to the structure, which may lead to further damage to the building. Each building and its condition are extremely unique, and thus, a one-size-fits-all solution does not apply.
CONNOLLY ENGINEERING: YOUR TRUSTED PARTNER IN CONCRETE SLAB REPAIR.
Though this construction will eventually be necessary in most pre-war buildings, not all engineers and contractors are skilled enough to do so. Modern techniques for construction cannot be applied to the modification of systems as complicated as draped mesh concrete slabs. To provide sound structural support, an understanding and knowledge of material properties and the complex nature of the system is a must.
Successful construction that stands the test of time calls for engineers who have the experience and knowledge.
Trust our team to provide you with the highest level of service and a wide range of expertise to meet your needs. We are one of the few
NYC and Hudson Valley engineering firms in the industry that provides comprehensive both civil and structural engineering services and consultation.
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