Weiyi Chen

Disclaimer—This paper partially fulfills a writing requirement for first year (freshman) engineering students at the University of Pittsburgh Swanson School of Engineering. This paper is a student, not a professional, paper. This paper is based on publicly available information and may not provide complete analyses of all relevant data. If this paper is used for any purpose other than these authors’ partial fulfillment of a writing requirement for first year (freshman) engineering students at the University of Pittsburgh Swanson School of Engineering, the user does so at his or her own risk.

GREEN BUILDINGS: CROSS-LAMINATED TIMBER

Weiyi Chen ()

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Weiyi Chen

THE NEW SKYSCRAPER BUILDING MATERIAL: WOOD

The demand for housing in cities grows as the world urban population rapidly increases. Concrete and steel are the two most common infrastructure building materials. Civil engineers combine these two materials to produce reinforced concrete, which consists ofsteel bars enwrapped in concrete. Reinforced concrete can resist compression and can be formed into almost every desired shape [1]. Despite the highly recognized performance of the material, large amount of coal is burned to heat up iron and limestone during the production of concrete and steel, emitting large amounts of CO2, which contributes to the green-house effect and climate change [2]. Thus, architects and engineers came up with the idea of wood construction, which can become a green alternative building material for large infrastructure constructions. Large wood building are not made by traditional wood. Cross-laminated timber (CLT), a material which is made by gluing layers of wood together to form enormous wood planks, is used in large wood constructions. The idea sounded appealing to me because as a freshman who is intended to major in civil engineering, I would probably get in touch with and use this technology during my career. People in this field have already applied cross-laminated timber in various constructions, including Stadthaus, a nine-story building which opened in 2009, and Forté, a ten-story apartment building in Melbourne which opened later in 2012 [3]. Engineers and architects are trying to build higher and higher wood buildings. This technology have a lot of potential in the future due to its eco-friendly, low-priced, fire safety, and light-weighted characteristics.

BUILDING WITH WOOD: WHY IS WOOD A GOOD ALTERNATIVE?

Wood vs. Reinforced Concrete

At first glance, wood buildings seems to harm the environment more than reinforced concrete buildings. But actually, building buildings with wood is more eco-friendly than using reinforced concrete. First of all, emitting CO2 is inevitable during the production of concrete and steel. Currently, there are several ways to reduce the amount of CO2 in the atmosphere, including “high-tech machines or materials that scrub CO2 from the air, or eco-friendly farming practices that trap carbon in the soil” [2], but yet the most efficient way to remove CO2 is by tree.According to a research from RASTRA, a construction firm, 11,779 pounds of CO2 are released to the atmosphere during the construction of walls of a 2,400 square foot house as fossil fuels are burnt during the process of the materials. In contrast, building a house of the same size out of wood, sequesters 11,818 pounds of CO2, resulting a difference of 23,597 pounds of CO2 less in the atmosphere [4]. Wood buildings storage CO2 rather than producing it.

Some people would say that removing trees is always going to be harmful to our environment, but with a technique called “thinning”, removing trees smartly can improve the growth of the forests. “Thinning” consists of cutting off trees in high density areas to lower the competition for soil, light, and nutrients to increase the growth of the remaining trees. It is proven that thinning can be used to gain financial benefits for the timber industry while adjusting the growth of forests, producing more high quality trees. Also, thinning can reduce fire risk in forests by varying the original uniform forest density [5]. This is another reason why wood buildings have a great potential to come back and become the main construction material.

The New Wood: Cross-laminated Timber (CLT)

Traditional wood was not used in the construction of tall buildings due to the weaker resistance to wind or any other forces compared to reinforced concrete.

However, the combination of wood and adhesives provided the opportunity to build wooden skyscrapers. Manufactures are able to produce larger wood panels from small pieces of wood using cross-lamination. These panels have the resistance and lastingness of concrete and steel but only weight a fifth of the same size concrete panel. While traditional wood is only strong in the direction of the grain, similar to reinforced concrete, CLT is strong in all directions. The lighter weight allows buildings to have a smaller concrete base, allowing construction on soil with relatively poor conditions, saving materials and reducing CO2 emissions. Also, buildings with lighter weight can lead to faster constructions, satisfying the huge demand for expansion in urban areas [2]. Construction time is a crucial factor as cities develop rapidly.

Furthermore, unexpected for most people, CLT has better fire resistance than reinforced concrete. When catch on fire, a thick panel of CLT is able to seal the inside wood from damage. On the other hand, steel melts in fire, and the concrete it is embedded in breaks down to pieces [3]. Buildings built with CLT increases fire safety, lowers the risk of fire and reduces the difficulty of repair after a fire.

Moreover, CLT is “five times more insulative than concrete and 350 times more so than steel [3]”, meaning far less energy is required to heat up or cool down a CLT building. People would be able to save electricity on air-condition, reducing their electricity bills while having the same accommodations.

CURRENT AND FUTURE APPLICATIONS OF CLT

Stadthaus is an all wood nine-story building located in Hackney, London. Construction was finished in 2009 and was the world tallest modern timber building for 3 years until 2012, when a 10-story CLT building was built in Melbourne, Australia. After the construction of Stadthaus, people started to recognize the advantages of CLT buildings. According to Waugh Thistleton, the architect who designed Stadthaus, it only took 27 days for four men working three days a week to build the timber portion of the building – 30% faster than an equivalent-size concrete-steel building. “Building Stadthaus was more like assembling a piece of furniture”, Waught stated after the construction of the building [3]. Besides having a short construction time, aremarkable feature of the building is its sequestration of CO2. It has been estimated that the wood used to build Stadthaus stores around 186 tons of CO2 [3]. CLT buildings not only have a reduced amount of CO2 emission, they are actually carbon negative and can help fight the greenhouse effect. If one CLT building can already reduce tons of CO2 released to the atmosphere, a city built up of CLT building could have a huge positive effect on climate change.

Engineers and architects are working on building higher structures with CLT. “Skyscrapers” made out of CLT are usually called “plyscrapers” [6]. In 2015, the University of British Columbia gave approval to the construction of an 18-story building mostly made of CLT, which will serve as a student residence. Once constructed, it will be the tallest wooden structured building in North America [2]. As CLT buildings gain popularity within the construction industry, people would put more effort on it and be able to build green cities combining plyscrapers with sustainable and clean energy, achieving CO2-free communities.

CLT IS A NEW WAY TO FIGHT CLIMATE CHANGE AND HAS A BRIGHT FUTURE

In conclusion, CLT buildings’ popularity will increase in the future. Its characteristics of producing less pollution, having a light weight that can help accelerate construction, high fire safety, and contribution to the climate change makes it an alternative building material that can substitute steel and concrete. As a future civil engineer specializing in sustainable engineering, I would love to do further research about CLT and its applications. CLT seems to be a suitable material that can be used in future green building designs and constructions.

SOURCES

[1] K. Aggeliki. “Fundamentals of Reinforced Concrete.” 2.22.2011. Accessed 10.28.2016.

[2] J. Deaton.“Wood and Glue Skyscrapers Are on the Rise: And they could help fight climate change.”4.26.2016. Accessed 10.28.2016.

[3] C. Risen. The World's Most Advanced Building Material isWood and It's Going to Remake the Skyline.” 2.6.2014. Accessed 10.28.2016.

[4]Rastra: CO2 Impact: Wood vs. Concrete. 9.12.2014. Accessed 10.28.2016.

[5] F. Roth. “Thinning to Improve Pine Timber”. 5.7.2012. Accessed 10.28.2016.

[6]Time Magazines. “Rise of ‘Plyscrapers’.” 4.15.2016. Accessed 10.29.2016.

ACKNOWLEDGEMENTS

I would like to thank my parents, Mr. Zhirong Chen and Mrs. Meiweng Chin to always supporting me and giving me the opportunity to study abroad in one of the best universities in the US.I would also like to show gratitude to the faculty members of the University library for providing excellent resources that helped me understand my research topic and the specific technology.

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