Solutions for Openings in Hollow Core Floors

Aug 15, 2019 by David Vail, P.E., Project Engineer

Many of STI’s existing UL systems for firestops within concrete floors contain an option that allows their use in assemblies constructed of hollow-core precast concrete units. These listings will limit the opening diameter to 7 inches or less for round openings or to a maximum 7 inch by 7 inch square opening. UL offers a bit of commentary on hollow-core precast concrete units within their Guide Information for Through-penetration Firestop Systems if readers would like additional information. We are often asked what this caveat regarding opening sizes is all about
and what should occur when larger openings are called for by the design drawings.

Understanding how hollow-core precast concrete units are produced and function is important to unraveling the mystery of opening size limits. The reinforcement method used for hollow-core units is called prestressing, and starts by embedding steel strands, which are installed under tension, within the concrete as it cures in its formwork. After the concrete has cured long enough to reach a specific design strength the steel strands can be cut. The strands are bonded to the concrete and as they attempt to return to their original length they create a compressive force along the hollow-core precast unit. This compression strengthens the hollow-core unit and allows it to extend along greater spans.

The risk of larger openings in hollow-core floors is the increased potential for cutting one or more of the prestressed strands embedded within the concrete, which could serve to weaken the structure. UL and STI are not able to comment on the structural aspects of a floor design, so UL limits the opening sizes in their systems and STI will place a similar note on any engineering judgements (EJs) issued for use in hollow-core. However, this does not mean that larger opening sizes should necessarily be prohibited! If a floor design is calling for relatively large openings in a hollow-core floor then in all likelihood a structural engineer has reviewed this procedure and is qualified through their knowledge of the precast unit to use proper safety tolerances in allowing the opening.

When larger openings are prescribed by design professionals STI is able to provide guidance on firestopping penetrations contained within these
openings. General rules of thumb suggest the use of firestop pillows or mortar for larger sized openings in precast. When the pillows are installed lengthwise through the opening they will typically project past both surfaces of the floor, which ensures that there are no exposed cores for flames to enter. Similarly, mortar can also be used to fill the cores completely and is a great solution since it is a cement-based product and easily bonds to the precast units. Please contact our Engineering Services for project specific solutions as needed.

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