Timber Tower Research: Concrete Jointed Timber Frame

Abstract

Tall buildings pose a challenge to the sustainability movement because they offer both positive and negative environmental impacts. Positive impacts include reducing urban sprawl, promoting alternative transportation, and allowing efficient energy use on a district scale. However, these benefits come with the cost of greater carbon emissions associated with both material production and building construction compared to a low-rise building. The goal of this research was to develop a structural system for tall buildings using mass-timber as the main structural material that reduces the carbon dioxide emissions associated with the structure. The structural system research was applied to a prototypical building based on an existing concrete benchmark for comparison. The selected concrete benchmark building is the Dewitt-Chestnut Apartments; a 120m tall, 42-story building in Chicago designed by Skidmore Owings and Merrill and built in 1966. This building was chosen as the benchmark because the geometry is a rectangular extrusion, the lease depths are consistent with contemporary residential buildings, and the concrete structural system is efficient in material usage providing a lower bound for comparison with the prototypical building. This paper discusses key design issues associated with tall mass-timber buildings along with potential solutions. Specific challenges include low structural weight and associated net uplift due to lateral loads, long term differential shortening, floor vibrations, and fire performance. It is believed that the system proposed in the research and discussed in the paper could mitigate many of these design issues. The proposed system, the “Concrete Jointed Timber Frame”, relies primarily on solid mass-timber for the main structural elements such as the floor panels, shear walls, and columns. The main structural mass-timber elements are connected by steel reinforcing through cast-in-place concrete at the connection joints. This system plays to the strengths of both materials and allows the designer to apply sound tall building engineering fundamentals. The result is believed to be an efficient structure that could compete with reinforced concrete and structural steel while reducing the associated carbon emissions by 60 to 75%.