Benchmark structural system designs

Parameters and schematic design.—

To develop benchmark building models to conduct a comparative whole-building LCA (WBLCA) study, STC and SCC structural systems were designed at 7- and 18-story building heights that conform to the limits of two tall timber building types added to IBC 2021 (type IV-C and type IV-A constructions, respectively). The benchmark buildings had 3.66-m (12-ft) story heights, resulting in overall heights of 25.6 m (84 ft) and 65.8 m (216 ft), respectively. The column spacing in either direction was selected as 9.14 m (30 ft), resulting in 9.14-m² bays. Whereas girders maintained consistent spacing in both structural systems, floor beam spacing varied between the SCC and STC systems. Internal optimization studies determined practically efficient spans of concrete and timber floor beam spacings of 3.05 m (10 ft) and 4.57 m (15 ft), respectively, as unit plan views of each structural system are shown in Figure 1. The floor spacing also aligned with the practical usage of both structural systems on the basis of the feedback provided by a research advisory panel formed by industry professionals. For the SCC system, 3.05-m spans with two floor beams within a unit square floor grid resulted in an optimal thickness for concrete slabs, with 127 mm (5 in.) of concrete on 51-mm (2-in.)-deep metal decks. The concrete thickness was deemed sufficient for unshored construction, adequate vibration mitigation, and avoidance of excessive concrete material use. For the STC system, five-ply CLT provided sufficient flexural and shear capacity and in-service level deflections capable of reaching a longer span of 4.57 m (15 ft) between beams while meeting the secondary design considerations expected from these systems, as discussed in the next section. Therefore, CLT spans with a single floor beam for the STC system resulted in optimal and practical utilization of the materials.

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Structural analysis and design methods.—

All structural frames were composed entirely of wide-flange steel, whereas floor systems varied between structural systems, as illustrated in Figure 2. Structural analysis and design were completed for the superstructure as a gravity frame and ignored lateral load design considerations. Design live load and superimposed dead load were taken as 3.12 kN/m² (65 psf) and 0.48 kN/m² (10 psf), respectively, to be representative of office design loads. Gypsum and acoustic matting (present in STC systems) were accounted for in the superimposed dead load. Concrete topping and CLT were considered as self weight, independent from superimposed dead load. All floor assemblages were designed in consideration of structural demands, temporary- and permanent-load deflection, airborne and impact sound transmission, vibration, and fire-resistance requirements. The serviceability deflection limits were compared against two load cases provided in ASCE 7, where service load (dead + live load) deflection was limited to span/240 and live load deflection to span/360. Horizontal framing members were designed to have roughly 25 percent composite action, as research has shown that this level of composite action results in roughly 50 percent increase in both strength and stiffness relative to hybrid systems (i.e., ignoring composite action), and that returns diminish at higher levels of composite action (Potuzak et al. 2023).

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Life-cycle assessment

The environmental impacts of each building were assessed using LCA for the entire building structure, which is also referred to as WBLCA. WBLCA is increasingly being used as a critical decision tool for key stakeholders during the building conceptualization and design process. WBLCA quantifies the energy consumption and carbon emissions resultant of raw material extraction, manufacturing, transportation, construction, building operation, and end-of-life demolition/disposal. Comparative WBLCA allows practitioners to make appropriate comparisons, assess the impacts of a design, and implement strategies to reduce total environmental impact (Chiniforush et al. 2018, Gu et al. 2020, Allan and Phillips 2021).

This comparative WBLCA was performed for the benchmark SCC and STC buildings using the commercial software Tally (KT Innovations 2022). There are numerous different software available for performing WBLCA, but Tally was chosen because of its (1) compatible integration within building information modeling (BIM, e.g., Revit) and structural analysis (e.g., SAP 2000) software to seamlessly perform material volume calculations; and (2) widespread use and experience of the academic LCA community to conduct WBLCA (De Wolf et al. 2016, Nwodo and Anumba 2019, Barber et al. 2022). The following sections describe the specifics of the WBLCA, including the system boundaries, choice of functional units, selection of life-cycle inventory (LCI), and major assumptions.

LCA study scope.—

The comparative analysis was based on the LCA modules defined according to ISO 21930 (2017), but the scope of this study was constrained to the modules outlined in Figure 3. Since the focuses of the study were embodied energy and carbon, the scope of the LCA study was chosen to include cradle to gate (A1 to A3), while including optional modules of construction transportation (A4) and end-of-life impacts (C2 to C4, D), as delineated in Figure 3. All benchmark structural building systems were subjected to analyses across these stages where the building life expectancy was 60 years for all structures, which was determined on the basis of the mean lifetime of buildings in the United States (Aktas and Bilec 2012) and was also used in similar LCA studies available in the literature (Lu et al. 2017, Gu et al. 2020). Biogenic carbon in CLT was accounted for in Modules A1 to A3.

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Operational energy and emissions (Modules B1 to B7), which fall outside of the scope of embodied environmental impacts, were not accounted for in the study. Therefore, all the energy required to use the building and associated emissions was not accounted for in the assessment. Similarly, on-site environmental impacts of construction (A5) and deconstruction (C1) were not considered because of the complexities of estimating the environmental impacts associated with construction/deconstruction equipment and processes.

Life-cycle inventory.—

LCI is the compilation of inputs and outputs for the buildings in analysis. Total material quantities were determined through the BIM used in the study with all elements accurately represented in size, location, and material properties, which was paired with LCI materials defined within the LCA software, Tally. Tally draws data for LCI from the commercial LCI database GaBi 8.5 (2018) and environmental product declarations (EPDs) available for certain construction materials.

The LCA results were reported according to the five impact characterization schemes defined by the tool for reduction and assessment of chemicals and other environmental impacts (TRACI) 2.1, where the primary impact method categories reported in this study were the global warming potential (GWP) along with the fossil fuel depletion potential. Despite a robust internal database of Tally, preliminary analyses determined generic CLT environmental impacts to be unrepresentative of current CLT manufacturing practices in the United States. Similarly, the database lacked a rubber material analogous to an acoustic mat. As a result, further refinement of the models was required to obtain outputs reflective of appropriate EPDs. This was achieved by applying adjustment factors to material volumes during LCI to result in appropriate unit production environmental impacts consistent with published EPDs.

The utilized EPDs were incorporated in the production stage data only (stages A1 to A3). Accordingly, environmental impacts of the remaining life stages were products of the assumptions made for transportation, end-of-life processing, and postlife reuse or recycling. Therefore, environmental data for end-of-life and reuse and recycling stages (Modules C and D in Fig. 3) were not modified. The LCI for various elements used in this study are presented in Table 1. Beyond these assumptions, Tally assumes that hot-rolled steel and steel decks are manufactured with 100 and 28 percent scrap material, respectively. This is accounted for by applying a “credit” in Module D, as opposed to the product stage.

Table 1.Life-cycle inventory (LCI) of the analyses.

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The details on the EPD sources, assumptions, and life-cycle scenarios used in the WBLCA study, as well as the material adjustment process for correcting inaccurate default parameters used in the software for the CLT and acoustic mat materials, are presented in Rohde (2023).

LCI data.—

The unit environmental impacts associated with structural components used in this study are reported in Table 2. These values are reflective of the LCAs reported in similar prior studies and publicly available EPDs as presented in detail by Rohde (2023). The environmental impacts and mass of structural assemblages are provided as a single element. As a result, the LCA outputs of hot-rolled steel framing and its associated cementitious fireproofing are presented under the Framing category and reported as one component as fireproofed steel. Similarly, concrete data varied slightly between structural systems as the concrete topping in STC floor assemblages was assumed to be 2,500 psi plain concrete, whereas the structural concrete in SCC floor assemblages was assumed to be 4,000 psi concrete reinforced with minimum steel rebars to minimize temperature and shrinkage cracking.

Table 2.Embodied carbon emissions and fossil energy of structural components normalized per unit mass.

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To validate EPDs and Tally database environmental impacts provided in Table 1 against external data, structural component impacts used in this study were compared with a literature review that synthesized data provided in >70 relevant studies (Cabeza et al. 2021). The entire comparisons are presented in Rohde (2023) for brevity and indicated that the environmental data used in this study were within the ranges provided in their extensive study. Potential variations are expected to result from regional influence on environmental impacts, as the study by Cabeza et al. (2021) encompassed worldwide data, although most of the reported data were resultant of research conducted outside the United States.

Structural design of benchmark buildings

An evaluation of the structural design of the functionally equivalent STC and SCC buildings revealed some significant differences. STC floor member sizes were generally controlled by vibration requirements, leaving low strength and deflection utilizations (demand-to-capacity ratio). However, strength and deflection utilization ratios were near the full utilization for SCC members, indicating a more efficient design for primary considerations. Final member sizes for each benchmark structural frame are summarized in Table 3. Lightweight STC floor systems are advantageous for gravitational force and deflection demands in composite systems where the stiffness and strength advantage of composite behavior has shown to be significant (Potuzak et al. 2023). However, satisfactory vibration design of the lightweight STC floor system required contribution from concrete topping (adding only mass and not contributing to the strength or stiffness) to meet satisfactory performance by remaining below the acceleration limit of 0.005 g for offices, according to the simplified approach provided in AISC DG11 (Murray et al. 2016). Vibration requirements were not considered for roof members, which resulted in flexural strength controlling the design of the composite sections even in the STC structures. Consequently, roof members for the STC floors are generally lighter steel sections than the SCC alternative, despite larger tributary areas of the floor beams.

Table 3.Benchmark building structural frame member design summaries.

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Despite the underutilized composite design of the STC system being controlled by vibration requirements, further analyses indicated that noncomposite (hybrid) STC design of the members would have required oversizing steel members because of not meeting the primary design (strength and deflection) consideration requirements according to the utilization factors presented in Table 4. Member sizing of the STC system for the regular floor system was entirely controlled by vibration limits while assuming 100 percent composite action between the steel and CLT panels, which controlled flexural strength and deflection limits by resulting in relatively low demand/capacity (utilization) ratios that assumed 25 percent composite action in the calculations. The vibration performance of STC systems was assessed using modified guidelines originally developed for SCC systems. Although these methods provide a foundational basis, additional studies are recommended to refine the approach, accounting for unique characteristics such as timber's orthotropic properties and composite action under dynamic loading.

Table 4.Comparison of composite and noncomposite steel–timber design sections for benchmark building scenarios.

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The comparison between the composite and hybrid STC designs indicates that significant structural member optimizations were achieved relative to noncomposite design, particularly when vibration was not the controlling design criterion. Utilizing the composite strength of the roof members resulted in noncomposite utilizations substantially above the capacity of the section.

Superstructure mass comparisons

The 7- and 18-story STC benchmark buildings had 9.7 and 4.0 percent lower superstructure mass, respectively, than the SCC alternatives by STC buildings having normalized masses of 207 and 228 kg/m², compared with the SCC building having 236 kg/m². Table 5 displays total values as well as material contributions to the total mass. The presence of gypsum boards for not exposing any timber elements for the 18-story STC had a considerable impact on total mass, as these elements comprised 8.0 percent of the total mass. Disregarding gypsum would have resulted in the 18-story STC structure having 12 percent lower mass than the SCC alternative.

Table 5.Seven- and eighteen-story (7S, 18S) material quantities.

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Steel framing mass had only negligible variances between STC and SCC, differing by −1.8 and +1.8 percent in the 7- and 18-story structures, respectively. Despite having fewer framing members, vibration demands increased STC floor member sizes, resulting in comparable steel mass per bay. Table 5 indicates that all significant variances in mass were attributed to the floor systems, which averaged 86 percent of superstructure mass in all structures. When studying floor systems alone, timber assemblages reduced floor system mass by 11 and 5 percent in 7- in 18-story structures, respectively. Despite this, the presence of concrete topping for mitigating secondary design considerations in the STC floor assemblage had the most significant impact. The 38-mm (1½-in.) normal-weight concrete comprised an average of 16 percent of STC floor assembly depth, but an average of 41 percent of the floor assemblage mass.

Life-cycle impact assessment results

Life-cycle impact assessment (LCIA) outputs accounted for all life stages within the boundary scope. The final reported values were net impacts, which accounted for biogenic carbon (carbon sequestration of wood) as well as avoided burdens (otherwise referred to as credits) due to recovery and reuse. Tables 6 and 7 detail LCIA outputs obtained in the study for 7- and 18-stories per total building area, respectively. Both STC systems had significantly lower embodied carbon than the SCC alternatives by prompting 50 and 42 percent reductions, respectively, in total embodied carbon. Similarly, both STC systems had lower embodied energy than the SCC alternatives, with 32 and 21 percent reductions, respectively, in total embodied energy.

Table 6.Life-cycle impact assessment (LCIA) results of seven-story (7S) buildings (STC = steel–timber composite; SCC = steel–concrete composite) encompassing cradle-to-gate (A1–A3; including modules A4, C2–C4, and D) embodied carbon (EC) and embodied energy (EE) per total building area (28,800 m²).

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Table 7.Life-cycle impact assessment (LCIA) results of eighteen-story (18S) buildings (STC = steel–timber composite; SCC = steel–concrete composite) encompassing cradle-to-gate (A1–A3; including modules A4, C2–C4, and D) embodied carbon (EC) and embodied energy (EE) per total building area (74,000 m²).

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Tables 6 and 7 present component contributions to total environmental impacts and indicate that the most significant contributor to both embodied carbon and embodied energy in STC systems was steel framing. However, the most significant contributors in SCC systems varied, with concrete being the largest contributor to embodied carbon and steel framing being the largest contributor to embodied energy. Furthermore, as reported in the tables, environmental impacts of STC floor assemblages had a lower impact than the steel framing alone. The opposite was found in SCC systems, where floor system impacts were greater than those of framing.

As discussed previously, steel framing mass varied <2 percent between structural types SCC and STC. Likewise, the portion of both total embodied carbon and embodied energy associated with steel framing varied by roughly 2 percent between structural types. Therefore, environmental benefits due to fewer framing members in the STC system were found to be negligible. Consequently, the environmental benefits of the steel–timber system stemmed primarily from less concrete usage and the negative embodied carbon and low environmental energy impacts of CLT floors.

Isolating floor assemblage in Tables 5 and 6 provided insightful data by displaying disproportionality of environmental impacts and component depth. The floor assembly for the STC system, which was comprised of the CLT, acoustic mat, 2,500 psi plain concrete, and type X gypsum (only for the 18-story building), had significantly lower embodied environmental impacts compared with the SCC system floor assembly, which was comprised of metal deck and 4,000-psi reinforced concrete per unit building area.

The unit embodied carbon and energy impacts of the 7- and 18-story STC floor assemblies are presented in Table 8. The embodied impacts differed primarily because of the gypsum boards as summarized by the normalized embodied carbon and embodied energies. The normalized results indicate that 7- and 18-story STC floor assemblies resulted in approximately 1/6th and 1/4th of the embodied carbon and had 57 and 40 percent less energy of SCC floor assemblies, respectively. Despite comprising only 17 and 15 percent of STC floor system depths, concrete accounted for 150 and 95 percent of STC floor system embodied carbon as well as 51 and 36 percent of embodied energy, respectively. Conversely, CLT made up 77 and 68 percent of floor system depths but accounted for −135 and −86 percent of embodied carbon as well as 11 and 7.5 percent of embodied energy, respectively.

Table 8.Summary of mass and environmental impacts of floor assemblies. Life stages A1–A3.

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Environmental impacts by life stage

The embodied carbon and embodied energy of the structures are presented by grouped life cycle stages in Figures 4 and 5. As previously discussed, carbon sequestration of lumber products causes the largest portions of the STC structures’ carbon footprints to appear at the end-of-life stages. This is because timber avoids net carbon emissions in the product stage (Modules A1 to A3) because of the large amount of carbon sequestered in lumber. Conversely, the SCC structures’ carbon footprints are heavily front loaded. This is due to the production stages accounting for the bulk of SCC structures’ embodied carbon, and the end-of-life and postlife stages of concrete are associated with the material’s environmental credits stemming from recycling and reuse. Similar to the observations made on the carbon impacts, the energy required to produce CLT is relatively low compared with the energy required to produce concrete and metal decking. However, the STC structures have no avoided energy burdens throughout their life cycles, whereas the SCC structures have a net negative embodied energy value in Module D due to the postlife recycling and reuse.

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Tables 9 and 10 present the detailed life-stage impact values of each structure and their relative percentage of the total impact. It is noted that avoided embodied carbon burdens associated with SCC structures in Module D are only a fraction of the avoided burdens associated with STC structures in Module A, as the avoided burdens attributed to 7- and 18-story SCC structures (Module D) are 12 and 14 percent of the avoided carbon burdens attributed to the STC structures (Module A), respectively. A similar comparison between net embodied energy in the structural types is not valid. CLT was assumed to be incinerated for energy recovery in the postlife stage. However, the energy produced by incineration was not significant enough to overcome the remaining STC materials’ energy demands in Module D. As a result, the net embodied energy associated with Module D of STC structures is positive and contributes to overall energy demand. Therefore, avoided energy burdens in this life stage are unique to the SCC structures.

Table 9.Embodied carbon per life stage.

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Table 10.Embodied energy per life stage.

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More points of interest in the life-stage breakdowns are the large embodied carbon values associated with CLT in the product (Module A) and end-of-life stages (Module C). As discussed, lumber products sequester relatively large amounts of carbon before production. As such, the sequestered carbon is accounted for in Module A. This created a very large negative embodied carbon attributed to the production of CLT. However, later in the structure’s life, much of the carbon sequestered within CLT was rereleased into the atmosphere. This is due to the assumptions specified for the material end-of-life scenarios, where 14.5 percent of the CLT was assumed recovered for reuse, 22 percent incinerated for energy production, and 63.5 percent landfilled As a result of these assumptions, 81 percent of the carbon sequestered by CLT was released because of material breakdown in a landfill or emitted because of material recovery and energy production. This resulted in relatively high embodied carbon impacts during end-of-life stages for the STC structures. The comparability of the embodied carbon of STC in Module C and SCC in Module A in Table 9 is also observed.

Sensitivity analysis on inclusion of biogenic carbon

The influence of including biogenic carbon in LCA studies was identified as a major parameter, as it is often disregarded or the inclusions are unspecified within LCA studies conducted by other researchers (Andersen et al. 2021). To investigate its influence, a sensitivity analysis was conducted that compared the inclusion and exclusion of biogenic carbon for both 7-story STC and SCC WBLCAs. Table 11 summarizes the total life assessment embodied carbon outputs of both systems while excluding and including biogenic carbon. The exclusion of biogenic carbon was carried out by turning off the consideration within the analysis software Tally, as well as modifying input data that were extracted from EPDs to exclude carbon sequestration. All material quantities, wood density, transportation distances, end-of-life scenarios, as well as all other structural material inputs, remained constant.

Table 11.Total life embodied carbon when excluding or including biogenic carbon. Life stage modules A1–A4, C2–C4, and D.

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Results indicate that the consideration of biogenic carbon is vital to the embodied carbon performance of the STC system, whereas the SCC system had a minimal difference in this study since the consideration of biogenic carbon only negligibly affects the environmental performance of all the other materials included in the study (acoustic mat, framing, metal deck, concrete). Therefore, the resulting variances of total life embodied carbon due to biogenic carbon consideration were typically <2 percent, except in CLT, which decreased total life embodied carbon by 130 percent. This was expected because of lumber products having net negative embodied carbon, as carbon sequestered within trees causes lumber products to act as carbon sinks.

STC design

Structural design provided insight into nuances associated with the novel STC system since very limited guidance to designers currently exists on STC design:

1. Design methodologies used for limiting floor system vibration of the STC system followed simplified methods through modifying the existing methods for SCC floor systems. This approach yielded member designs primarily governed by vibration considerations, which were mitigated by increased steel framing size and additional concrete topping. The applicability of simplified design methodologies to STC systems, particularly in accounting for timber floor panels and their long-term compatibility with concrete topping layers, is recommended to be verified through more detailed studies with usage of advanced vibration assessment design approaches.

2. For the benchmark building designs developed in this study, CLT panel thickness was determined to be 5 ply to meet the fire-resistance requirements. The selected panel thickness resulted in span lengths that were 15 ft for efficient floor panel use, which was different from the concrete composite floor deck systems (10 ft).

3. Significant structural performance advantages were found to be associated with STC design relative to noncomposite (hybrid) design even at low composite action of 25 percent. This was despite the benchmark buildings’ floor systems design being heavily controlled by vibration limit states where the member utilization factors for primary design considerations such as strength and deflection were largely underutilized.

Life-cycle assessment

Substantial environmental advantages were found to be associated with the STC structures relative to the SCC structures. The LCA analyses provided consistent whole-building comparisons for functionally equivalent buildings and the following conclusions were achieved:

1. Embodied carbon, or GWP, was significantly lower in the STC systems than in the SCC systems since the total embodied carbon was reduced by 50 and 42 percent in the 7- and 18-story structures, respectively.

2. Embodied fossil energy, or primary energy demand, was considerably lower in the STC systems than in the SCC system since the total embodied energy was reduced by 32 and 21 percent in the 7- and 18-story structures, respectively.

3. Steel framing mass and their associated environmental impacts were comparable between systems of the same height. As a result, environmental benefits attributed to STC structures stem predominately from floor assemblages, which comprise an average of 86 percent of total superstructure mass.

4. STC systems had relatively lower embodied carbon, embodied energy, and mass associated with floor assemblages by an average of 380, 98, and 10 percent, respectively.

5. In whole-building comparisons, CLT floor panels averaged 130 percent lower embodied carbon, 93 percent lower embodied energy, and 52 percent lower mass than concrete on metal deck slabs used in the STC systems.

6. Although the volumes of steel frame were nearly equivalent between structural types, the environmental impacts of the material far exceed its proportion to structural mass. In STC structures, hot-rolled steel on average accounted for 14 percent of total mass, 77 percent of total embodied carbon, and 62 percent of total embodied energy. In SCC structures, hot-rolled steel on average accounted for 13 percent of total mass, 42 percent of total embodied carbon, and 45 percent of total embodied energy.

7. Inclusion of biogenic carbon results in substantially different conclusions on the embodied carbon outcomes of the STC system with up to 85% variation between the results. Excluding biogenic carbon results in vanishing all the carbon emission, or global warming potential, benefits over the SCC system.

8. Additional studies are recommended to account for all the energy required for building use and the associated emissions, with a particular focus on operational energy, emissions, and the on-site environmental impacts of construction and deconstruction operations.