Question 1

Please expand on your experience with the different structural options for office and Residential as described in the concept package.

DELTABEAM Framing

The concept structural narrative included a third option for the office framing in the form of DELTABEAMs with cross laminated timber (CLT). See design brief excerpt below.

Figure 1 Excerpt from Thornton Tomasetti’s Structural Narrative.

Turner has contacted Peikko USA, DELTABEAM’s manufacturer. The DELTABEAM system also includes configurations with metal decking and hollow core, which we believe may be more suitable for the project needs (rather than CLT). Additionally, CLT use may not be acceptable per local fire regulations. Peikko USA confirmed that the majority of their U.S. applications are with hollow core slabs.

With the DELTABEAMs + hollow core or metal decking, the overall slab can be expected to be thicker, on the order of 10” to 18” including the topping for typical applications; however, without beams below the slab.

Figure 2 DELTABEAM Hollow Core and Custom Metal Deck Options

The DELTABEAM system itself ranges from $8 to $14 per SF including design transportation but excluding all other materials and erection.

This cost is comparable to the structural steel supply cost.
Hollow core, metal decking, or CLT are procured separately from the DELTABEAM systems.

For the current steel framing, where the longer spans vary between 43’ to 46’ by 30’ in the short direction, the DELTABEAM + hollow core system reaches deeper than conventional systems with bays limited to less than 40’.

For DELTABEAMS to span to ~45’, they would be 28” deep.
- This is comparable to the W21 + 6.5” slabs that currently shown for the steel framing option of the commercial tower.
- In this scenario, with the area between the conventional steel beam unimpeded and with web openings possible for services to pass through, the DELTABEAM solution is less efficient for ceiling clearance.
Alternatively, 20” DELTABEAMs spanning 30’ with 14” hollow core (+ topping slab) spanning up to 46’ feet could be implemented.
- This would be a much heavier floor framing systems and would increase column sizes and potentially the foundations.
Both of the above options appear to eliminate the beam clearance gained with smaller span solutions.
The size of the hollow core sections (e.g., up to 46’) presents additional challenges in terms of lifting and placement.
Alternatively, columns could be added to shorten the beam spans, but this would disrupt the office floor plate.
Refer to table extract below for conventional DELTABEAMS + hollow core. It appears that DELTABEAMS are optimized for spans in the range of 30’.

The DELTABEAM system option would need to be engineered by Peikko USA.

Peikko designs, draws, and seals all of its DELTABEAM designs. They work with local engineers where necessary; however, Peikko takes responsibility for the design of its products.

Typical lead times in the U.S, are around three months.

The DELTABEAM systems appear to be time saving for orthogonal applications building but they require specialty connections.

DELTABEAM systems are fire tested to ASTM E119 and UL 263 and do not require fireproofing.

This will need to be verified per local code / AHJ.

Given the shorter spans and more uniform footprint of the residential tower, DELTABEAM system could be considered a time saving option (assuming re-design, procurement, and lead times are sufficient)

This would likely increase the typical PT slab beyond the current 7.5” in the drawings
- E.g., an 8” hollow core option would require a 2” topping for a total of 10”

Concerns for the commercial tower:

The commercial tower has shifting and changing floor plates at every floor, so every perimeter interface would require specialized fabrication.
The building lean and inclined columns may likely lead to significant diaphragm forces in the plane of the slabs; design for these with a hollow core system will be very specialized.
The adaptability for Grade-A office space, e.g., for tenant modifications, may be more complicated that with conventional steel and metal deck framing.
Maintaining the current framing, would necessitate more heavy crane picks for long-span hollow core sections, potentially more rebar, and similar concreting timing. Therefore, an improvement in schedule would not be expected.

Conclusions

The most common application is utilizing hollow core sections, rather than CLT or custom metal decking. Without exposed timber ceilings specified (and approved by the AHJ), CLT is not recommended.

For the residential tower, use of the DELTABEAM + hollow core system could potentially save time, but would increase the overall floor system thickness versus the 7.5” thick PT floor plates that are currently shown in the drawings. Therefore, this is not currently recommended.

For the commercial tower, due to the spans up to 46’, use of the DELTABEAM + hollow core system appears to be less optimum than the proposed conventional composite steel and metal decking system in the drawings. Additionally, the varying floor plate dimensions and sloping columns, which will add forces along the plane of the slab into the beams. make this a less ideal solution. Therefore, this is not recommended.

Figure 4 Drawing CS1.21 Part-Plan Extract w/ Typical Spans

Figure 5 DELTABEAM Technical Manual Extract

Figure 6 DELTABEAM Technical Manual Extract

Figure 3 Typical Indicative DELTABEAM + Hollow Core Design Table Extract