by Barbara Porada

June 6th, 2013

Original Source: archdaily

SOM has come up with a structural system for skyscrapers that uses mass timber as the main structural material and minimizes the embodied carbon footprint of the building. The firm believes that their proposal is technically feasible from the standpoint of structural engineering, architecture, interior layouts, and building services and would revolutionize the traditional skyscraper as we know it.

“Tall buildings pose a unique 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. These benefits come at the cost of emitting more carbon dioxide to produce the materials and to construct the building. A tall building’s embodied carbon footprint is significantly higher relative to low-rise buildings on a per square foot basis. This is because the structure is usually responsible for the majority of the building’s embodied carbon footprint, and tall buildings require far more structure to support their height. Structural systems that minimize embodied carbon for tall buildings allow the positive environmental aspects of tall buildings to be more pronounced.” – SOM

Although structural engineers currently have four major materials to choose from when designing buildings – steel, concrete, masonry and wood – tall buildings use steel or concrete almost exclusively because most building codes require non-combustible materials for buildings higher than four stories and because steel and concrete have higher material strengths than masonry or wood.

But designers such as Michael Green of Michael Green Architecture have recently become interested in a lesser known quality of wood – that it is a carbon sink, the physical result of photosynthesis. Combined with the knowledge that wood takes significantly less energy to produce than steel or concrete, the idea of a tall wooden structure can be very attractive.

SOM‘s Timber Tower Research Project applied its mainly wooden structure to a prototypical building based on an existing concrete benchmark for comparison. This benchmark building is the Dewitt-Chestnut Apartments; a 395 foot tall, 42 story building in Chicago designed by SOM and constructed in 1966. SOM’s solution to the tall wooden building problem is the “Concrete Jointed Timber Frame,” which relies on mass timber for main structural elements and supplementary reinforced concrete only at the highly stressed locations of the structure: the connecting joints.

The structural system proposed by SOM is the Concrete Jointed Timber Frame. It consists of solid mass timber products for the primary members such as the floor panels, columns and shear walls, which are connected with steel reinforcing through concrete joints.

The Gravity Framing System would utilize 8″ thick timber floor panels which would span between the shear wall core at the center of the building and columns at the perimeter of the building. The ends of the floor panels would be restrained from rotation by the concrete joints and vertical structure, allowing the floor system to be more economical.

Lastly, the Lateral Load Resisting System would utilize solid timber shear walls which would be coupled with reinforced concrete link beams, designed to efficiently resist net uplift due to wind overturning.

Playing to the strengths of both materials, the resulting structure could potentially compete with reinforced concrete and steel while reducing the carbon footprint by 60 – 75%. Although SOM believes the system to be feasible, additional research and physical testing would be necessary to verify the performance of the system, especially in cases of fire.