Question 1
Please expand on your experience with the different structural options for office and Residential as described in the concept package.
Post-Tension Floor Framing Considerations
The concept structural narrative and 50% SD drawings included for post-tensioned (PT) concrete floor framing through the residential tower and in the lower portion of the commercial tower.
Figure 1 Excerpt from Thornton Tomasetti’s Structural Narrative for the Floor Framing.
Figure 2 50% SD Drawing, RS1.04, Typical Floor Plan Extract.
For such a high tolerance, thin floor plate, the design and means and methods will need to be carefully planned. The tensioning of the PT will also need to be performed carefully as overstressing or under-stressing can lead to creep (deformation) issues in the flooring. However, the use of vinyl tile is beneficial in this consideration as they have a degree of flexibility that more rigid finishes, such as tiles, do not (there are instances of overstressed, thin PT floors causing tiles to crack and delaminate over time as the concrete slabs bend upwards).
Although only noted generally for parking levels (see below), we expect most, if not all, of the columns in the typical floor plates require additional reinforcing to resist punching shear. Refer to following figures. We note that a relatively high strength floor concrete is specified, f’c = 8,750 psi, to transmit column forces through the slabs while avoiding “puddling” high strength concrete at each column and casting lower strength concrete in the remainder of the floor. We concur with this strategy as a potential means of saving time on the floor cycle (albeit with a cost premium for the floor framing concrete).
To achieve suitable soffit finishes and to achieve the schedule we expect to use table forms or a similar formwork system. Refer to following figure.
Commercial Tower
In the lower portion of the commercial tower (L12 and below), the floor framing is comprised of PT band-beams that are relatively flat and wide with depths in the range of 18” to 20” deep by widths varying from 4’ to 8’. One-way, PT slabs with thicknesses from 8” to 10” will span between the band-beams.
Figure 3 50% SD Drawing, RS1.04, Typical Floor Plan Extract, Pricing Notes Extract.
Figure 4 Studrails and PT Interface and Column (DECON® STUDRAIL® System Shown).
Figure 5 Table Form Example (PERI UNIPORTAL)
Figure 6 Excerpt from Thornton Tomasetti’s Structural Narrative for the Floor Framing
Residential Tower
In the residential tower, with the typical residential floors meant to be 9’-0” clear (in the interior living spaces) and being 9’-8” floor-to-floor, the structural engineers have optimized the PT flat plate floor system to be 7.5 inches. The floors will be topped with “plank-style luxury vinyl tile.” This allows less than one‑half inch (< ½”) for the skim coat, tolerances, and structural deflections, which means there will be a high degree of quality control the construction of the floor plates including the top surface of the and the soffit. No floor depressions (for bathrooms, etc.) are currently shown, so the floor construction is fundamentally a flat plate. If modular elements, such as bathroom pods are introduce, potential depressions and PT locations will need to be coordinated in detail.
This is a relatively common concrete framing system for office spaces and less sensitive than the residential floor plates. Form systems can be conventional since the soffit finish will not be exposed. The concrete grade comments related to puddling from the residential tower also apply to this tower; however, with beams framing to each column, added punching shear resistance in the form of stud rails are not anticipated to be need.
An option utilizing the same concrete framing in the upper portion of the tower is addressed in the sloped column and leaning tower considerations narrative and is not recommend.