Life-cycle analysis factors
The study’s main objective was to evaluate the advantages of different structural systems in terms of embodied energy and life-cycle costs. It is a commonly held belief that mass timber is more sustainable than most other structural systems because it is made from renewable wood grown from trees as opposed to extractive materials like ore and lime. But what does the data say?
The students analyzed the Hacker-designed office buildings and structural systems using Tally, software that conducts whole-building, life-cycle analysis using BIM (building information modeling). The program scans the 3D BIM model and generates inputs based on scientific life-cycle data collected for various materials. In order to directly compare structural systems, only the structural components were analyzed.
Life-cycle analysis measures the energy used to extract, manufacture, transport, and assemble a given material. It also accounts for the end-of-life impact of that material – assessing, for instance, whether a material can be re-used or recycled or if it will end up in a landfill. Tally generates life-cycle analysis comparisons between materials based on multiple factors:
Primary Energy Demand: The total amount of primary energy extracted from the earth, expressed in energy demand from renewable and non-renewable resources. This measures buildings’ dependence on fossil-carbon energy sources.
Acidification Potential: Excess CO2 that affects ocean pH levels and thus impacts marine life. For example, metal or plastic particulates entering large bodies of water due to runoff from from industrial processing, resulting in a loss of fish mortality.
Eutrophication: The richness of nutrients delivered to water sources, such as streams and lakes. An influx of nutrients can cause excessive growth of plants (e.g. algal blooms in rivers) that deteriorate water quality and harm aquatic life.
Global Warming Potential: Greenhouse gases entering the atmosphere, trapping radiant heat at the earth’s surface.
Smog Formation Potential: Ground-level ozone gases and particulates produced by vehicles, utilities, and industrial facilities.
Ozone Depletion Potential: Chemicals released into the atmosphere that deteriorate the ozone layer, increasing UV radiation at the earth’s surface. A major contributor of ozone depletion is Chlorofluorocarbons, commonly used in refrigerant air-conditioning systems.
The problem with assumptions
Based on wood’s reputation for having a more limited environmental impact, Tally’s material life-cycle analysis of the wood, steel, and concrete structural systems used in First Tech, Field Office, and 9North, respectively, surprised us at first. Contrary to our initial assumptions, the wood structure seemed to perform worse from a life-cycle perspective compared to the steel and concrete structures.
However, when we looked more closely into Tally’s assumptions, we learned that the program assumes a high amount of wood is incinerated at its end-of-life, which releases much of the carbon the wood would have originally sequestered. The analysis also did not account for regeneration (tree regrowth once a tree is cut down). These assumptions significantly raised the primary energy demand of wood.
Building design also presented end-of-life variables. Recycling of structural materials is a common practice in construction: steel is recycled into new steel; wood is reused as structure or furniture; concrete is broken down into paving materials. What determines whether or not structural materials can be recycled is its original use and design; you can design a building with disassembly and material reuse in mind or construct it in ways that prevent material reuse.
Ultimately, we proceeded with the investigation of the life-cycle impact of our three buildings and structure types without taking into account the end-of-life metric or assumptions about recycling. This allowed for a pure life-cycle comparison of the impact of construction, occupation, and operation for wood, steel, and concrete structures of similar scale and type.
Life-cycle analysis takeaways
With the scope of Tally’s analysis narrowed to exclude the end-of-life metric, the wood structural system did appear to have less of a life-cycle impact compared to steel and concrete.
Concrete had a significantly worse impact on smog formation and also had the highest global warming potential of the three, followed by steel. Steel was a major contributor to ozone depletion. Wood showed dramatically less impact in all of the other factors.
