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By Sara Hart

At 7WTC, the two risers are interconnected at the top and bottom of the building, creating a loop. “If there were two completely independent risers, then a break in one riser would cut off water to every floor,” explains Galioto. “With a system of valve isolation, the loss of sprinkler protection could be limited to a section of the riser, and the water could still be supplied from above and below the break.” If the water supply is ruptured at a point along the loop, breach-control values automatically engage to prevent the system from draining out. Diversity is achieved by specifying, rather than a single tank, multiple tanks in various locations throughout the building. In addition, whereas the code requires a 30-minute supply of water, the 7WTC system doubles the output to an hour.

 

Click images to view them larger The steel members are sprayed with a robust, medium-density, portland-cement-based fireproofing that has a bond strength of 2,000 psf and a minimum density of 22 pcf, resulting in a compressive strength many times greater than that which is required by New York City building codes. Photography: Courtesy Grace Construction Products

The second ring of defense addresses protection of the structure. 7WTC is a steel structure with a concrete core. Spray-applied fire-resistant materials (SFRM) are the standard method of passive fire protection for steel. It goes without saying that fireproofing is only effective if it stays adhered to the structure. Unfortunately, building-performance analysis of the World Trade Center towers on 9/11 showed that the impact of the aircrafts dislodged the fireproofing materials on the steel columns, exposing them to extreme heat.

In response, the 7WTC team carefully considered the properties of existing SFRM products. There are two factors determining performance of SFRM—density and bond strength. Density refers to the amount of mass per unit volume. Bond strength refers to the amount of force required to separate the fireproofing from the steel. SOM sought a material with strength and adhesion superior to that required by the New York City code, and selected a medium-density, portland-cement-based product called Monokote Z-106/HY from Grace Construction Products. Galioto is confident that manufacturer’s claims that the product’s bond and compressive strengths are five times higher than the building-code requirements for steel structures are correct.