Chapter 3. Environmental Impacts
3.3 Quantification: Material efficiency
3.3.1 Proponents of the Environmental Credentials of Structural CLT in the Literature
There have been several recent articles and reports highlighting the potential of CLT in multi-storey construction. Lehmann advocates the use of solid wood panel construction systems like CLT aligned with Design-for-Disassembly principles to meet sustainably the need for new urban dwellings in the urban environments of Australia[22]. Residential buildings of between 4 and 10 storeys were suggested. Structures made purely of CLT do already exist up to 10-storeys (e.g. Forte Apartments) but there is a movement towards combining CLT panels with other construction materials to enable even taller structures and to reap the greatest efficiencies.
Chapman proposes a system using CLT panels in a tube formation for a central core (with inclined Universal Column stays giving assistance under wind loads, analogous to those of a yacht mast), aligned vertically for the outer columns, and as floor beams by cutting the panels along their length (see Figure 3.3). Reinforced concrete is used to form shear-transferring connections between CLT panels in the core, in outer “hoop” beams at the building perimeters, and compositely with the CLT beams as a Timber-concrete floor[131].
Skidmore, Owings & Merrill, renowned for designing some of the most recognisable tall structures in the world, produced a report investigating how structural timber can be used for tall buildings[19]. They found that using timber in composite systems was the best course of action, as
materials such as reinforced concrete and structural steel are better suited for critically stressed members, and using timber compositely produced more efficient and economic structures, which are therefore more likely to be specified by clients. It was also noted that although foundations for timber structures would be much smaller there is the potential requirement for tall timber structures to have uplift restraint in the foundations, which would be a complicating factor. A potential structural system was developed, using a concrete-jointed timber frame, and is shown in Figure 3.4. This timber tower, combined CLT floor panels, with glulam beams and columns, and a coupled-shear-wall lateral support system of CLT. Reinforced concrete spandrel beams were also included. From a sustainability perspective, in their comparative study, they concluded that this tower had a 60 to 75% reduction in Embodied Carbon footprint against a benchmark structure, though it must be noted that this included carbon sequestration within values for the wood products.
The characteristics of the proposed system fit well with the recommendations of future development of tall structures incorporating timber, and suggest that a construction system with significantly reduced environmental impact may be possible in this way. In the next section, this reduction will be quantified. Table 3.2 shows the depths of slabs required for floor systems under a 2.5kN/m2 imposed load (suitable for office loading) and various spans, and the dead loads that these floor slabs produce. The significant reduction in dead loads compared to the current concrete floor systems offers potential reductions in the size of members in a steel frame and large reductions in the loads going down to the foundations. This is because for equivalently spanning slabs, the CLT panels are of similar depths to precast concrete panels, but are of lower density.
Table 3.2: Comparison of dead loads associated with various floor systems
qk = 2.5kN/m2 Span = 4m Span = 6m Span = 8m Span = 10m
Floor Material Slab Depth (mm) Dead Load (kN/m2) Slab Depth (mm) Dead Load (kN/m2) Slab Depth (mm) Dead Load (kN/m2) Slab Depth (mm) Dead Load (kN/m2) CLT[13] 125 0.59 182 0.86 208 0.98 300 1.41 Concrete Flat Slab[132] 200 4.70 206 4.84 250 5.88 343 8.06 Pre- Cast Concrete[133] 200 3.6 200 3.6 250 4.2 300 4.5 Composite Deck[134] 215 3.1 215 3.1 305 7.3 N/A -
Foundations are often extensive, and carry significant embodied carbon and energy, both from their materials (Reinforced Concrete) and their construction. Previous work by the author[135] deduced that if the axial force sent to a pad footing was scaled by a given ratio, λ, then the volume of the pad
foundation would be scaled by the approximate ratio λ3/2. In the case of using a CLT floor rather than a composite deck to span 4m, the ratio between the axial loads to foundations will be approximately 0.58 – this translates to a pad footing volume ratio of 0.44 i.e. the volume of foundations could be more than halved by substituting a composite deck for CLT panels.
Reductions of this magnitude in the foundation volumes has a significant effect, not solely on the material quantities and embodied carbon and energy, but also the cost of foundations due to easier design and integration with existing ground features and increased speed of construction. Furthermore, the volumes of excavation required will be lower, which will allow construction on sites that may not have been viable with existing construction systems due to poor soil characteristics or because a prohibitively large amount of contaminated soil would need to be removed and treated.
3.3.2 Design for deconstruction
Low carbon design of structures will have a key role in meeting the carbon emissions targets outlined in Section 3.2.1. Reusing structural elements allows the embodied carbon and energy to be distributed between the working lives of the product[136], but one of the reasons element reuse is uncommon is that structures (and notably connections) are not designed to facilitate its dismantling at the end of life. Design-for-Deconstruction
(DfD) is a methodology that puts this idea at its core. Lehmann offers a
comprehensive study into the potential for CLT panels to form part of a construction system where elements can be reused[22], as well as outlining some of the other aspects of CLT's suitability in the context of residential housing.
Densley-Tingley & Davison summarise some of the strategies that need to be taken during structural design and decision making to enable and facilitate
deconstruction[136]. These include (but are not limited to): using connections that are
easy to remove, avoiding adhesives and coatings where possible; having deconstruction in mind from the outset of the design process; using standard structural grids to allow the maximum number of potential projects that members can be reused in; making sure connection points are easily accessible; using as few connection types as possible; making use of prefabrication and mass production; and choosing materials that are easy to separate and readily reusable.
Notable examples of structures designed for deconstruction could be seen at the Olympic Games. In the 2012 games in London, the basketball arena (Figure 3.5), capable of holding 12,000 spectators and spanning 100m, was not designed to be permanently in place at the Olympic Park, but rather to be sold and relocated or recycled[137]. Similarly, the handball arena for the Rio Olympics of 2016 will be
dismantled and reconfigured to create four separate schools. Parts of the steel structure, façade,
concrete circulation and disabled access ramps, visible in Figure 3.6, will be reused and evident in the buildings to serve a total of 2000 pupils[138].
According to representatives of the masterplanning team for both the London and Rio Olympics, the strategies exploited for these so called “nomadic venues” included standardised structural elements (steel beams/columns and concrete slabs) and prefabrication of modular parts that are now feasible due to advances in material technology[139].