Naturally the faster a project can be completed, the less environmental and societal disruption. Under this topic we need to separate the speed of construction of the structure from the exterior envelope and interior finishing systems as they are independent decisions. A more holistic adoption of prefabrication will have the benefit of overall time shortening (see 8-Prefabrication). When it comes to framed systems such as mass timber and steel, the number of individual elements to handle as well as the number and types of site connections will factor heavily into the time to complete. Maximizing component size and minimizing the number of lifts will reduce crane time.
Steel, reinforced concrete and mass timber systems proceed at distinctly inherent paces, which are also climate and weather dependent. The pace of construction can be negatively impacted by rain and snow in particular, requiring extra protection to prevent damage to materials or a complete delay due to freezing temperatures. The seasonal timing of a project as well as its geographic location may suggest different material choices to avert the excessive extension of the project timeline and its associated impact on the area.
Reinforced concrete is the most temperature dependent of the primary structural materials. It cannot proceed in freezing temperatures, requiring the installation of space heaters and a temporary enclosure system to retain the heat. Excessive heat will also disrupt the curing process and require coverings to prevent evaporation. Some of this can be mitigated by looking at the construction timeline to ensure that the bulk of the work takes place in more suitable weather.
The choice of material for the core will tend to govern the overall timeline for the erection of the structure in composite systems. If reinforced concrete is used, as it tends to be for tall buildings regardless of the material chosen for the floors and columns, it will slow down the overall process as concrete takes 28 days to reach its fully cured strength. As concrete placement is a wet process, it cannot benefit from prefabrication, and so also has the added burden of the construction and moving of the formwork, and often addition and removal of temporary shoring (Fig. 4).
Mass Timber systems that consist of glulam columns and CLT slabs can ordinarily be completed faster than other systems. This is largely due to the prefabrication of the materials prior to arrival at the site, the larger size of the components, and the use of specialized connection procedures. The Wood Innovation and Design Centre in Prince George, British Columbia, Canada, by Michael Green Architecture was completed in 2014. The 6 story commercial building was fully complete within a 16 month design build process. Critical to this timeline was the use of a CLT slab core versus reinforced concrete. Additionally a prefabricated system of exterior cladding was also employed, greatly reducing the time to complete.
Structural steel systems will typically take longer than mass timber to complete the column, beam and floor framing as the connection completion time for standard bolted and welded steel connections takes longer than for the Megant or Ricon type fasteners used in mass timber systems. The advantage of using a standardized fastening system lies in the consistency of the actions of the workers. Variations between connections tend to slow down the process. The steel floor system will require the installation of beams and joists to support the steel decking. As these components are more numerous to construct equal floor area, versus CLT or hollow core precast slabs, this increases installation time and crane time.
Both mass timber and structural steel will be impacted by the time to add the reinforced concrete that is used for the composite floor system. Both systems require the installation of metal connectors to bond the concrete to the floor system. This is a very manual process adding significant time to the build. Mass timber buildings will often require the added mass provided by the concrete floor to control vibrations as the overall system is quite light and can be difficult to provide movement damping. This also means that these sites will need to queue concrete trucks for the delivery of this material.
Reinforced concrete buildings will typically take longer to complete the structural framing due to the added time for the installation and removal of formwork as well as curing time. There are building types, such as residential towers, for which this structural system has become standard and so to make an alternate choice for terms of speed may not possible due to local customs, material availability and labor. A more regularized floor plan and the use of flying forms can be used to accelerate the process as the building of the formwork adds significantly to the time to complete.
Precast concrete structural systems are not globally common for the construction of tower type infill projects. Precast concrete hollow slab flooring systems have similar advantages to CLT slab systems in terms of speed of erection over stick built steel systems and can often be seen used in conjunction with steel frame buildings resulting in an overall shortening of construction time. In certain countries such as Japan, precast concrete systems are becoming standardized in the construction of residential towers as they provide a good level of mass over steel systems (which still tend to be more common for office construction there) and have added speed over standard reinforced concrete methods, though admittedly still employ significant amounts of wet placed concrete to connect members and top the slabs. They are being designed for seismic resistance. As these tend to be used on towers in excess of 20 stories, the dimensions of the columns tend to be much larger than would be found in a traditional reinforced concrete apartment tower using shear wall type construction (Fig. 5).
The reality of excessive column sizes as impacting the plan could also be seen to feed into aspects of livability in the associated residential spaces. This is a trait shared by the new precast concrete residential structural system being used in Japan and the extremely tall mass timber towers. Although not a construction related issue, the material selection does impact the use of space quite directly.