Over the last few decades, there has been growing interest in the use of low-carbon materials to reduce the environmental impacts of the construction industry. The advent of mass timber panels (MTP), such as cross laminated timber (CLT), has allowed structural engineers to specify a low-carbon material for a variety of floor design considerations. However, serviceability issues such as vibration and deflection are limiting the construction of longer span timber-only floor systems and have encouraged the development of timber-concrete composite (TCC) systems. The use of concrete would negatively impact on the carbon footprint of the TCC floor system and should be minimized. The purpose of this study was to study the impact on embodied carbon in the TCC system, when the ratio of timber and concrete was varied for specific floor spans. Two MTP products were considered, CLT and glued laminated timber (GLT). The floors were designed to satisfy structural, acoustic, and vibration criteria, and the results were presented in the form of span tables. It was found that using thicker MTP instead of adding concrete thickness to meet a specific span requirement can lead to lower embodied carbon values. Increasing concrete thickness for long-span floor systems led to a reduction in allowable floor span due to the vibration criterion being the controlling design parameter. Increasing timber thickness also resulted in higher strength and stiffness to weight ratios, which would contribute toward reducing the size of lateral load resisting systems and foundations, resulting in further reductions in the embodied carbon of the entire structure.
In recent years, the use of timber in multistorey residential and commercial buildings has increased worldwide, primarily inspired by consciousness surrounding the sustainability of timber as the primary structural material [
Construction with mass timber is approximately 25% faster than similar on-site concrete construction. It also requires 90% less construction traffic and 75% fewer workers which yields a much quieter job site [
(a) Longitudinal section of a TCC system with dowel, notched, or proprietary connectors; cross section of a TCC floor (b) with timber beam and (c) with MTP.
In TCC construction, the concrete slab resists compressive stress while timber primarily resists tensile stress generated by an out-of-plane bending action. In reinforced concrete design, the tensile strength of concrete is often neglected, and steel reinforcement is installed to resist the tensile stresses caused by bending. In the ultimate limit state design, the concrete is assumed to crack to about 2/3 of its depth under bending [
Embodied carbon (EC) is defined as the carbon footprint of a material. It considers the amount of greenhouse gas emissions that is released throughout the supply chain of a material or product, including all extraction, transport, processing, and fabrication activities of a material or product via cradle-to-gate, or cradle-to-site. Cradle-to-gate refers to a partial product life cycle associated with embodied carbon which considers all activities from resource extraction (cradle) to the factory gate (i.e., before it is transported to the consumer). Cradle-to-site extends the cradle-to-gate results to include transportation of the material or product to its site of use [
Despite the ongoing research in the field of timber structures, most of the timber standards around the globe including North America do not have a standardized method for designing TCC floor systems. Among several proposed methods, the Gamma method based on Annex B of Eurocode 5 [
As the structural integrity of the TCC system primarily depends on interlayer mechanical connectors, analytical models have also been developed for directly calculating the strength [
It is common practice in TCC floor design to choose the thickness of the concrete layer based on several requirements of concrete design standard such as anchorage of the fastener, limits for concrete crushing, minimum cover to reinforcement, sound transmission, and a minimum thickness for diaphragm action. Generally, concrete thicknesses of 75 mm to 100 mm are used in practice, though past studies have considered thicknesses as low as 30 mm (plus 20 mm timber interlayer) with self-tapping screws [
Research has shown that suitable indicators of human response to floor vibration are the fundamental natural frequency of the floor and the deflection of the floor under a concentrated load at the center of the floor [
As stated above, the goal of this study is to investigate the impacts on design selection with respect to the main TCC floor components, namely, concrete and timber, for specific span requirements on the embodied carbon of the system. The allowable span for a TCC floor system can be developed by considering all ultimate limit states related to timber, concrete, and shear connectors, and serviceability limit states related to deflection and vibration. The ultimate and serviceability limit state requirements for TCC floors with different combinations of materials, dimensions, and connection characteristics were evaluated based on [
In ultimate limit states, a floor system might fail due to concrete crushing, timber crushing, or screw yielding, when the stress demand exceeds the capacity at concrete layer, timber layer, and shear connectors, respectively. The lowest capacity associated with these failure modes would govern the allowable floor span. In the developed analytical model [
Here,
By applying the displacement compatibility condition at the interface, the redundant shear force in the connectors with
Here,
Then, the superposition method can be implemented to determine the vertical deflection [
Here,
The linear-elastic effective bending stiffness [
After yielding of each connector, the stresses in concrete and timber are checked to determine if either of them fails (e.g., concrete compression, timber tension, and/or shear) before yielding of the next connector.
According to [
And the bending stress in the members of each subsystem can be written as
Here,
The stress distributions of the subsystems and the actual system are shown in Figure
Stress distributions in the TCC system.
The extreme fibre stress of timber must not be greater than its factored bending strength as follows:
Here,
The shear stress in timber member can be calculated as follows:
Here,
The top extreme fibre stress of concrete in compression must not be greater than its factored compressive strength and the bottom extreme fibre stress of concrete in tension must not be greater than its factored modulus of rupture as follows:
Here,
Hu et al. [
The deflection can be calculated based on the load combination associated with serviceability limit state in CSA O86 [
As required by the NBCC [
Based on the described methodology, the allowable spans were developed for a variety of TCC floor systems built with MTP (as in Figure
Type of TCC system with MTP and inclined self-tapping screws [
Design properties of the concrete and MTP.
Thickness (mm) | Modulus of elasticity (MPa) | Bending strength (MPa) | Shear strength (MPa) | Compressive strength (MPa) | |
---|---|---|---|---|---|
Concrete | 50, 75, 100 | 26600 | — | — | 35 |
GLT | 89, 140, 186 | 9500 | 11.8 | 1.3 | — |
CLT | 105, 175, 245, 315 | 11281 | 28.2 | 0.5 | — |
Properties of the self-tapping screw.
MTP | Acoustic layer (mm) | Stiffness (kN/mm/screw) | Yield strength (kN/screw) |
---|---|---|---|
GLT | 0 | 15.24 | 15.34 |
CLT | 0 | 14.00 | 16.19 |
GLT | 5 | 7.34 | 14.66 |
CLT | 5 | 7.03 | 15.51 |
The MATLAB software was used to perform repetitive calculations, based on the analytical equations presented in [
TCC allowable floor spans with associated embodied carbon for 250 mm spaced connectors.
MTP | MTP thickness (mm) | Concrete thickness (mm) | Effective stiffness ((106) Nm2) | Span (m) | STC (dB) | Embodied carbon (kgCO2/m2 eq) | ||
---|---|---|---|---|---|---|---|---|
Total | Timber share | Reinforced concrete share | ||||||
GLT | 89 | 0 | 0.08 | 2.62 | 39 | 15.4 | 15.4 | 0.0 |
50 | 1.87 | 4.98 | 49 | 39.0 | 23.6 | |||
75 | 3.03 | 5.39 | 52 | 51.3 | 35.9 | |||
100 | 4.9 | 5.92 | 55 | 62.6 | 47.2 | |||
140 | 0 | 0.61 | 4.26 | 42 | 23.0 | 23.0 | 0.0 | |
50 | 4.61 | 6.26 | 49 | 46.6 | 23.6 | |||
75 | 6.07 | 6.44 | 52 | 58.9 | 35.9 | |||
100 | 8.22 | 6.75 | 55 | 70.2 | 47.2 | |||
186 | 0 | 2.09 | 5.73 | 45 | 29.7 | 29.7 | 0.0 | |
50 | 8.91 | 7.39 | 49 | 53.3 | 23.6 | |||
75 | 10.65 | 7.43 | 52 | 65.6 | 35.9 | |||
100 | 13.06 | 7.60 | 55 | 76.9 | 47.2 | |||
CLT | 105 (3 ply) | 0 | 0.34 | 3.80 | 40 | 11.9 | 11.9 | 0.0 |
50 | 2.7 | 5.47 | 49 | 35.5 | 23.6 | |||
75 | 3.94 | 5.78 | 52 | 47.8 | 35.9 | |||
100 | 5.88 | 6.21 | 55 | 59.1 | 47.2 | |||
175 (5 ply) | 0 | 1.73 | 5.50 | 44 | 18.2 | 18.2 | 0.0 | |
50 | 8.41 | 7.29 | 49 | 41.8 | 23.6 | |||
75 | 10.04 | 7.33 | 52 | 54.1 | 35.9 | |||
100 | 12.35 | 7.50 | 55 | 65.5 | 47.2 | |||
245 (7 ply) | 0 | 4.75 | 6.88 | 47 | 24.6 | 24.6 | 0.0 | |
50 | 19.65 | 9.01 | 49 | 48.2 | 23.6 | |||
75 | 21.73 | 8.92 | 52 | 60.5 | 35.9 | |||
100 | 24.43 | 8.93 | 55 | 71.8 | 47.2 | |||
315 (9 ply) | 0 | 10.2 | 8.16 | 49 | 31.0 | 31.0 | 0.0 | |
50 | 38.35 | 10.63 | 49 | 54.6 | 23.6 | |||
75 | 40.94 | 10.46 | 52 | 66.9 | 35.9 | |||
100 | 44.07 | 10.37 | 55 | 78.2 | 47.2 |
Table
In the TCC system, generally, effective bending stiffness and mass per unit area dictate the performance of the composite floor system. From Table
Effective bending stiffness increase compared to bare MTP versus thickness of MTP in TCC system.
It is also noted that mass per unit area is influenced largely by concrete thickness because of the higher density of concrete (2300 kg/m3) compared to timber (420 kg/m3) and the system deflection decreases with the increase of concrete thickness. Therefore, the effective bending stiffness increases with a thicker MTP, but the total mass per unit area only increases marginally which yields a larger floor span. From Figure
Allowable span versus thickness of concrete in TCC system.
The design of all the TCC floors was governed by the vibration criteria, which generally agrees with the common practice. In addition to changing the properties of the floor, the addition of concrete to the TCC floor increases the weight significantly because of the higher density of concrete compared to timber, which can subsequently create a demand for larger framing members and foundations due to an increase in dead and consequently seismic loads. The influence of the weight of TCC is similar to the influence of concrete thickness on the allowable span. Therefore, an increase in concrete thickness tends to reduce the allowable floor span due to a reduction in natural frequency which is detrimental to vibration performance.
In the developed span Table
Influence of connection properties on TCC floor allowable spans.
Timber | Timber thickness (mm) | Insulation thickness (mm) | Concrete thickness (mm) | |||||
---|---|---|---|---|---|---|---|---|
50 | 50 | 75 | 75 | 100 | 100 | |||
Connector spacing (mm) | ||||||||
250 | 500 | 250 | 500 | 250 | 500 | |||
GLT | 89 | 0 | 4.98 (+) | 4.75 (+) | 5.39 (++) | 5.16 (++) | 5.92 (+++) | 5.71 (+++) |
89 | 5 | 4.76 (+++) | 4.54 (+++) | 5.14 (+++) | 4.93 (+++) | 5.67 (+++) | 5.48 (+++) | |
140 | 0 | 6.26 (+) | 6.00 (+) | 6.44 (++) | 6.15 (++) | 6.75 (+++) | 6.46 (+++) | |
140 | 5 | 5.97 (+++) | 5.75 (+++) | 6.10 (+++) | 5.87 (+++) | 6.40 (+++) | 6.17 (+++) | |
186 | 0 | 7.39 (+) | 7.13 (+) | 7.43 (++) | 7.13 (++) | 7.60 (+++) | 7.28 (+++) | |
186 | 5 | 7.07 (+++) | 6.86 (+++) | 7.06 (+++) | 6.83 (+++) | 7.20 (+++) | 6.96 (+++) | |
CLT | 105 | 0 | 5.47 (+) | 5.23 (+) | 5.78 (++) | 5.52 (++) | 6.21 (+++) | 5.95 (+++) |
105 | 5 | 5.23 (+++) | 5.02 (+++) | 5.50 (+++) | 5.28 (+++) | 5.93 (+++) | 5.72 (+++) | |
175 | 0 | 7.29 (+) | 7.05 (+) | 7.33 (++) | 7.05 (++) | 7.50 (+++) | 7.20 (+++) | |
175 | 5 | 7.02 (+++) | 6.82 (+++) | 7.01 (+++) | 6.79 (+++) | 7.15 (+++) | 6.92 (+++) | |
245 | 0 | 9.01 (+) | 8.78 (+) | 8.92 (++) | 8.64 (++) | 8.93 (+++) | 8.62 (+++) | |
245 | 5 | 8.74 (+++) | 8.56 (+++) | 8.59 (+++) | 8.39 (+++) | 8.56 (+++) | 8.35 (+++) | |
315 | 0 | 10.63 (+) | 10.41 (+) | 10.46 (++) | 10.20 (++) | 10.37 (+++) | 10.08 (+++) | |
315 | 5 | 10.36 (+++) | 10.20 (+++) | 10.14 (+++) | 9.96 (+++) | 10.02 (+++) | 9.82 (+++) |
Remarks: “+” represents the STC is the first level of the acoustic performance or at least 45 dB. “++” represents the STC is the second level of the acoustic performance or at least 50 dB. “+++” represents the STC is the third level of the acoustic performance or at least 55 dB.
The sound transmission class value for each span is also presented in Table
The embodied carbon will obviously increase once concrete topping is added to MTP to form TCC. Figure
Embodied carbon versus thickness of concrete and MTP in TCC system.
Allowable span versus embodied carbon of TCC system (the three points on each curve represent from left to right 50 mm, 75 mm, and 100 mm thick concrete).
By using a new analysis approach [ The increase in concrete thickness to achieve a longer span is only effective for TCC floor systems with spans shorter than about 7 m, due to vibration performance being the controlling design parameter. This is because the addition of concrete beyond this span has the counteracting effect of increasing bending stiffness but reducing the natural frequency. Following from 1 above, it is generally true that it is beneficial to increase MTP thickness instead of concrete thickness to achieve a longer span, especially for a floor span shorter than 7 m, from a sustainability perspective. This is due to the substantially larger carbon footprint of concrete than timber on a per-volume basis. If self-tapping screws are used, there is evidence to suggest that the connection parameters, such as spacing, have a minor influence on the allowable floor span when these parameters are above certain threshold levels. Other connector systems may provide more sensitivity, but further studies are required.
This research focused on embodied carbon as a single metric and did not account for cost or other environmental impacts from the material selection, for example, eutrophication or acidification potential and on-site construction. Further analysis including these effects and accounting for the relationship between embodied carbon and life cycle cost is recommended.
All the data, models, and code generated or used during the study appear in the submitted article.
The authors declare that they have no conflicts of interest.
The authors would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for the financial support through the Industrial Research Chair (IRC) in Engineered Wood and Building System Program.