Readers will be interested in the findings of a recent study completed by Cary Kopczynski & Company, that compares the cost of the structural frame of a hypothetical 10-story residential building located in the Pacific Northwest using cross laminated timber (CLT) and cast-in-place reinforced concrete.
Based on a survey of contractors knowledgeable in CLT construction, the cost of the structural frame in the CLT option was estimated to be $48 to $56 per gross square foot excluding costs for acoustical dampening and fire protection. Acoustical and fire protection costs are expected to range from $2 to $6 per square foot.
The cost of the structural frame in the cast-in-place reinforced concrete option is estimated to range from $42 to $46 per gross square foot. Additional cost for acoustical dampening may be required, which is estimated to be in the range of $1 to $2 per square foot.
The results of the study suggest that the cost of the structural frame for the CLT option is about 16 to 29% greater than that of the cast-in-place reinforced concrete option.
The results from this study show that CLT structures cost significantly more than cast-in-place reinforced concrete structures. Square foot costs of some actual CLT projects are also greater than comparable reinforced concrete structures based on national average structure costs. It is expected that the costs would be even greater for CLT projects outside the Pacific Northwest where the majority of CLT production facilities are located.
The study noted also that the choice of structural material has limited impact on the total energy consumed over a building’s life, since the structure affects only Embodied Energy – the energy required to produce the building’s physical materials. Operational Energy – the energy required to heat, cool, light, and operate a building – is dominant, being upwards of 95% of the total energy consumed.
It was observed that for the ten-story building evaluated, the concrete option is less susceptible to floor vibration under dynamic loads, and to excessive lateral drift under wind and seismic loads unless concrete shear walls are substituted for CLT walls in the CLT Option. Concrete shear walls in the CLT Option would require measures to control differential axial shortening between the concrete and gravity load carrying vertical wood elements.
The authors of the original study caution readers to use judgement when drawing conclusions from the data presented in the report. This is especially true for cost and constructability, since the available CLT information is limited and costs vary widely from region to region.
The full study, “Cary Kopczynski & Company, 2018, Cross Laminated Timber Feasibility Study – A comparison between cross laminated timber and cast-in-place concrete framing for mid-rise urban buildings. Seattle, WA.” Is available here. A shorter summary of the report appears as a CRSI Technical Note, which is available here. Graphics sourced from the above noted study.
