River Beach Tower / Perkins + Will / United States

River Beach Tower

• Architects:  Perkins + Will
• Location:  Chicago, United States
• Structural Engineer:  Thornton Tomasetti
• Structural Material:  Timber
• Type:  Residential
• Status:  Vision

The Chicago River Beach Tower is a collaborative research effort with the goal of identifying challenges and opportunities associated with designing increasingly tall mass timber structures. A key objective of the project is to explore new design potential with timber buildings, rather than substituting timber in the familiar forms of conventional construction in steel and concrete.

Designed by the Chicago offices of Perkins + Will and Thornton Tomasetti in collaboration with researchers at the University of Cambridge's Centre for Natural Material Innovation, the River Beach Tower follows the program of an 80-story, 244 meter tall timber residential tower situated along the South Branch of the Chicago River. Exposed mass timber would offer a unique experience within the residential market by connecting occupants with natural materials. The program includes 300 units, a development density in keeping with current construction along the Chicago River. The 80-story program of the tower forces the design team to consider many of the issues typical to designing tall buildings, while staying within the established 70-80 story height range for tall residential towers in Chicago.

Design Strategies -

The overarching goal of their study is to identify areas of mass timber research that could help advance the use of timber in tall building structures. Approaching this from a practitioner's perspective, the team placed focus on developing an all-timber superstructure and allowed the planning and architectural to adopt in response. Using a residential floor plate as a planning framework, the team set out to examine how design professionals could use current timber technology to make the design of a tall timber tower feasible.

1. Proportion the tower footprint to make a timber superstructure feasible.
2. Maximize the participation of all vertical members of the tower's lateral system.
3. Arrange the timber material in plan for maximum effectiveness.
4. Use the material in its most naturally effective way.
5. Use CLT for walls and Nail Laminated Timber (NLT) for slabs.
6. Expect to use more material with timber than if using steel or concrete.
7. Prefabrication can leverage the adjacent Chicago River for large off-site assembly and shipment to the site.

Analysis -

The timber superstructure performs in a similar way to residential towers of similar heights and size constructed using concrete or steel. The tower was structurally analyzed, from which a number of observations can be made.

• The interconnection of the structure using cross-bracing, shear walls, and diagrids successfully activates the vertical members for resisting lateral loads.
• Under gravity load, there is good load balance across the diagrid system.
• The tower coupled together with cross-bracing show good lateral load distribution across the system.
• The diagrid network is highly engaged for load resistance, without dependence on moment connections.
• Timber material proportions are larger than if using steel or concrete. However, because of the above design strategies, the added timber material is minimized.

This research supports the feasibility of using timber as a tower's primary superstructure material. In addition, it has also successfully identified next-step topics of research for advancing the use of timber in tall building structures incorporate:

• Wind Engineering to assess unique responses of a tower with low-mass density and low-dynamic periods.
• Further develop connection details in the diagrid, shear walls, and atrium braces.
• Establish vibration control criteria for CLT floor planks.
• Conduct performance testing for rationalization of seismic loading.
• Coordinate fire engineering solutions and structural design.
• Research material modifications that may improve member stiffness including genetic improvements, and hybrid composites with steel and carbon fiber.

Each module would be glazed with utilized curtain walls to the extent possible. Pieces between modules would be glazed in the field. Fireproofing between floor panel and curtain wall follows a conventional two-hour UL-rated solution.

The skin of the atrium facade is a lightweight Ethylene-tetraflouroethylene (ETFE) system. In all tower areas, the timber is protected within the exterior skin. On balconies, where sealant can be easily accessed for maintenance, the timber is exposed to the elements.

The River Beach team however, use a different system entirely, and credit this innovation as their key to new wooden heights. The system engages an exterior diagrid system, taking advantage of the natural axial strength of timber. The building's vertical and lateral loads are resisted by connecting the outer diagrids with the internal cross bracing that skirts the central atrium, allowing for efficient load distribution across all timber elements.

At 80 stories tall, the conceptual brief caters for 300 duplex units and multi-story penetrations that form communal spaces. The play between private and public within a carbon-neutral structure ties together the team's vision of their "social and sustainable adaptation to high-rise development."