Sustainable Structures Steel vs Timber

A sustainable building begins with the choice of structure. I'm currently choosing between a steel or timber frame as I believe they both have different qualities that will lead to the success of my design. Below is a pro's and con's list for each material to help me to decide. 

Also to be considered is the logistics of construction. The site I have chosen is busy, with shops, residencies and faces onto two roads. To cause the least amount of destruction in the area through deliveries of materials, noise, extra pollution of the machinery it is necessary that the frame come's pre-fabricated. There will be less room for human error, more precision, less machinery required due to the construction time being reduced. Both steel and timber have the option of coming pre-fabricated, however there are limits such as lorry size, source of material eg importing from America will increase the carbon footprint therefore making it less sustainable.

STEEL

Pro's:

• The only material that can be 100% re-used and recycled. Almost no steel ends up in a landfill.
• Steel can be melted down over and over again into shapes for different aesthetics and never looses its strength.
• Pre-fabrication = quick construction = safer and faster construction, minimal mistakes, less community disruption, less pollution from onsite machinery. 
• Need's little to no maintenance and lasts for years and years. It is also entirely weather resistant with some treatment, but sometimes doesn't need treating at all (depending on the climate).
• If some steel within in a building does need to be replaced, it is much simpler to do compared to other materials. It is also much easier to redesign a space when there is a steel structure involved.

Con's:

• Steel does not retain heat well, so the thermal comfort of the buildings users could be at risk. It will also mean the building will need to install heating mechanism to use throughout the year. There also needs to be more focus on the installation, which can have an impact to the cost.
• The steel measurement requirements need to be exact in order for quick construction to take place. If a measurement is wrong there is no way for framework to be amended on site and will therefore need to be sent back to the factory to be amended.

TIMBER (specifically CLT- Cross Laminated Timber)

Pro's:

• CLT can also be pre-fabricated for quick construction on site with minimal human error mistakes. However, if there is an error it is possible to amend timber on site with the correct tools. 
• Buildings can be modified and extended easily with CLT.
• The production of CLT produces much less pollution than steel or concrete, it has a small carbon footprint and sequesters carbon. Meaning it absorbs carbon dioxide that is in the atmosphere. It is also now widely available, so can be more locally produced. 
• CLT is 5x lighter than concrete however performs at the same strength. This benefits the weight on the foundations, as well as benefits the construction. 

Con's:

• Due to CLT being a relatively new construction method, there are still some queries as to it is fire resistance. Following the Grenfell Tower disaster I feel any worries about this would be unsafe to use. 
• The structure must be maintained due to it rotting when wet. 
• Extra insulation and noise protection would need to be added to ensure the thermal comfort of users. 

Having looked into the pro's and con's of each material, I believe a steel frame will be the most sustainable choice of material of my hybrid housing. It is the most secure and reliable option and will work most effectively on the site. As the design with include accomodation, steel provides more fire resistance and safety as well as durability and strength for the residents of the building. Its longevity makes it a sustainable choice of material and finding a locally sourced supplier will help reduce the carbon footprint of the area. 

There will also be other ways to help increase the sustainability of the building such as adding more green space, natural lighting, natural insulation such as hemp crete and so on.