Since joining Tocci three years ago, I have been involved in multiple projects with varying types of construction. I have worked with steel podiums, concrete podiums, steel framing, wood framing, and load-bearing stud and Girder-Slab’s. As an engineering student, I studied the typical steel, wood, and concrete structures that you generally see covered in a college curriculum, however, never had I been introduced to the concept of precast hollow core plank structures.
Precast hollow core plank is prestressed concrete, primarily used for floor and roof decks in multi-family or hotel buildings. The continuous hollow cores help to add structural stability while also reducing the amount of material, which ultimately reduces weight and overall cost.
Precast hollow core plank construction can take many forms. Typically, it’s either a girder-slab system or it’s supported by a load-bearing metal-stud wall.
Both of these systems are an effective way to lower floor-to-floor heights because of the way plank is attached in the field. A girder-slab system uses structural members that resemble an upside down ‘T’ (D-beam) to adjoin side-by-side plank. The planks will rest on the D-beam, then they will be connected by inserting reinforcing bar into the core and grouting the two cores solid (FIGURE 1). Used in combination with a steel frame, the D-beam system allows for the floor-to-floor heights to be limited since there are no longer typical W-beams disrupting the above ceiling space. Designers are able to reduce heights while still allowing for the necessary space to run mechanical equipment and conduit above.
Figure 1 – Girder slab system
Figure 1.1 – Drawing of girder slab system
Load-bearing wall systems utilize structural metal wall studs to support the precast plank, in lieu of the steel framing seen with the girder-slab system. This concept was most interesting to me because I learned that steel and concrete structures are framed with column and beam members. This steel and concrete design is constructed similarly to a wood structure, in the sense that the bearing walls need to be built first, then the floor plank is laid on top of the framing (FIGURE 2). This process repeats floor to floor, without the need for structural steel columns. This process is advantageous because both the hollow core plank and the structural bearing walls can be prefabricated off site, which helps to significantly reduce the construction duration.
Figure 2 – Load bearing metal wall stud
To better understand this fabrication process, I had the opportunity to visit a hollow core plank factory in New York. Joe Cavallaro and I received a tour from their project manager to demonstrate how plank is constructed and to better understand its limitations. The plank is fabricated on a platform that is typically 4ft or 6ft wide and approximately the length of a football field (FIGURE 3).
Figure 3 – The prestressed plank has the capability of long spans and typically comes in 4ft to 8ft widths
In this case, the plant was fabricating plank at 4ft widths. Since the plank is also prestressed, strands of wire are installed on the platform and stressed using a wire pulling machine. At this stage, any additional embed requirements are added to the reinforcement. To place the concrete, there is a special slipformer machine that gradually moves down the platform and molds the concrete onto the platform. Once completed, the plank is cut using a cutting saw into the required lengths for the project. This process allowed me to understand tolerance of embed locations and the importance of not compromising the prestressed strand/wires within the plank. Understanding these characteristics is important for properly coordinating penetration and opening needs with other trades.
To better understand how the precast concrete construction comes together check out this video from the CTSR Group.