WoodWorks has developed a series of checklists to assist in the design and cost optimization of mass timber projects. The design optimization checklists are intended for building designers (architects and engineers), but many of the topics should also be discussed with the fabricators and builders. The cost optimization checklists will help guide coordination between designers and builders (general contractors, construction managers, estimators, fabricators, installers, etc.) as they are estimating and making cost-related decisions on a mass timber project.
The paper also includes links to a variety of mass timber design resources, most of which can be found on the WoodWorks website (www.woodworks.org).
The checklists cover:
• Pre-design – including design and builder team coordination, cost estimating considerations, contractual considerations, and design goals
• Schematic design (SD) – including material optimization/panel sizes/grids, system coordination, structural systems, fire resistance, acoustics, finish quality, schedule savings, and cost optimization
• Design development (DD) – including additional considerations on material optimization/panel sizes/grids, hybrid considerations, system coordination, fire resistance, MEP systems, finish quality, specifications, and cost optimization
During pre-design, for example, the paper includes:
• Establish design goals through discussions among the owner, designer and builder, and use them as a guide in decision making throughout design and cost analysis. Consider the relative value of the following benefits based on the project’s objectives and constraints. Decide if these benefits are project goals or value-adds.
Following is an excerpt from schematic design:
SD Design Optimization Checklist
Material Optimization/Panel Sizes/Grids
• Identify options for mass timber fabricators and suppliers. o Maximize panel sizes to minimize machine cutting, material waste and costs. Unique geometries and varying slab edge conditions can increase costs; balance design with budget.
• Determine the thickness and grades of roof/floor panels based on preliminary loading. Minimize the thickness of floor/roof panels to control costs. Vibration often controls floor panel thickness rather than structural span capability.
• Approach mass timber design as a modular system. Start to think of your design on a module of 8 or 10 feet. For comparing material systems, the 8-foot module also works well for steel and concrete. The module can be refined early in the design development phase. For projects that include structured parking, consider modules that work well with the parking layout to minimize structural transfers.
• For panels sourced from overseas, shipping container size limits will govern maximum panel sizes. Actual panel size will be limited to a little less than 8 feet by 40 feet.
Ask an Expert: How can I create an efficient structural grid for a mass timber building (www.woodworks.org/experttip/2019-efficient-structural-grid/)
• Optimize structure, vibration, fire resistance and acoustics while minimizing the thickness of the floor/roof panels. These areas usually have the biggest influence on cost. o Coordinate the structure, vibration, fire resistance and acoustic systems. Meet with the project’s consultants/experts to discuss system options and how each impacts the others. o Correlate acoustic and fire-resistance solutions. (They are interdependent.)
• Determine where to use mass timber systems on the project. Consider: floor and roof systems, load-bearing walls, lateral force-resisting walls.
• Select the lateral system and consider how quickly it will be erected. Is the lateral system compatible with the fast speed of mass timber erection? Construction schedule savings can be significant with mass timber (and are a main driver for its use) and the choice of lateral system can greatly impact the construction schedule.
• Consider a mass timber core as an alternative to systems that have long cure times. o Review the sequencing of erection. Do the lateral systems need to be installed prior to the mass timber?
An Approach to CLT Diaphragm Modeling for Seismic Design with Application to a U.S. High-Rise Project (www.woodworks.org/wp-content/uploads/Approach-to-CLT-Diaphragm-Modeling-forSeismic-WoodWorks-Jan-2017.pdf)
• Optimize the construction type to minimize fire rating impacts.
• Consider fire protection approaches and what should be discussed with the Authority Having Jurisdiction (AHJ) during design. AHJs may be unfamiliar with mass timber systems. Early pre-application meetings can help to align expectations and mitigate concerns far in advance of permit application.
Fire Design of Mass Timber Members – Code Applications, Construction Types and Fire Ratings (www.woodworks.org/wp-content/uploads/Wood_Solution_Paper-Fire-Design-of-MassTimber-Members-WoodWorks-Apr-2019.pdf)
Ask an Expert: When designing a mass timber building, what are the key design considerations related to fire ratings, panel thickness/member size and occupancy? (www.woodworks.org/experttip/ designing-mass-timber-building-key-design-considerations-related-fire-ratings-panel-thickness-membersize-occupancy)
- Consider floor topping options and finishes.
- Determine how the wet vs. dry weight of toppings impacts schedule and mass of structure.
- Decide whether the mass timber floor/roof panels will be exposed on the ceiling side or concealed.
- Consider the minimum level of acoustical performance required by code and expected by the owner/occupant.
Acoustics and Mass Timber: Room-to-Room Noise Control5 } Inventory of Acoustically-Tested Mass Timber Assemblies (www.woodworks.org/wpcontent/uploads/Acoustically-Tested-Mass-Timber-Assemblies-WoodWorks.pdf)
- Start to consider the appearance of exposed mass timber elements and the species available from different fabricators. Selecting species provided by local fabricators will typically yield shipping/transportation cost savings.
- Specify desired finish grades (architectural, industrial, etc.) for budgeting purposes. o Work with potential manufacturers to determine their appearance grade options and associated cost premiums in order to arrive at a solution that meets both aesthetic goals and budgetary constraints.
Ask an Expert: How should desired appearance grades and characteristics be specified for CLT, NLT and glulam? (www.woodworks.org/experttip/desired-appearance-grades-characteristicsspecified-clt-nlt-glulam)
SD Cost Optimization Checklist
Mass timber buildings are relatively new to the U.S. and, as such, not enough data is available to predict costs with the unit cost method. Cost estimating mass timber systems requires a holistic approach to compare costs and yield savings. This approach takes into account the considerations/ questions in these checklists, such as the financial benefits of a shorter construction schedule. A direct comparison of the price of a mass timber structural frame to the price of a steel/concrete frame will not provide an accurate overall project cost comparison.
Schedule Savings = Cost Savings
- Mass timber buildings are erected more quickly than buildings made from other materials. Consider the financial benefits of shorter construction schedule and earlier lease/occupancy.
- No cure times means follow-up trades can start working as soon as floor/roof panels are placed. According to Lendlease, Candlewood Suites at Redstone Arsenal was built 37% faster overall and with 40% fewer construction workers than a similar cold-formed steel military hotel. DLR Group projects that the Seattle Mass Timber Tower would be constructed 25% (5 months) faster than the baseline posttensioned concrete building in its case study.
Candlewood Suites Hotel at Redstone Arsenal (www.woodworks.org/wp-content/uploads/4-Story-CLT-HotelWoodWorks-Case-Study-Redstone-Arsenal-01-05-16.pdf)
Optimized Design, Efficient Construction: How One Company Successfully Constructed a Massive Timber Project (www.woodworks.org/wp-content/uploads/webinar_slides-EVANS-SILVA-OptimizedDesign-Efficient-Construction-Webinar-180613.pdf)
Tall with Timber: A Seattle Mass Timber Tower Case Study (www.fastepp.com/wp-content/uploads/181109-Seattle-Mass-Timber-Tower-Book.pdf)
- The unique aesthetic of an exposed wood structure can offer market distinction with potential profits from leasing velocity, less tenant turnover and/or higher leasing rates. 4,12,13
Less Weight = Cost Savings
- Mass timber buildings weigh less than concrete buildings and some steel buildings. The lighter mass requires smaller foundations and can result in lower seismic forces.
- For sites with problematic soils, significant savings are possible because of the lighter foundations, which can result in less soil remediation costs, and an earlier start to the above-grade structure.
- For a better estimate of panel fabrication costs:
o Include allowance for CNC fabrication of panels.
o Include allowance for finish of mass timber, if known. If unknown, either provide a range or include the assumed appearance grade that is factored in the quote (architectural, industrial, etc.).
- The cost of shipping can vary widely. Will material be shipped on standard trucks or will panels require oversized trucks? Can oversized trailers access the site?
- For overseas manufacturers, consider the cost of:
o Shipping containers and rail transit in addition to the typical manufacturing and trucking costs
o Storage in a warehouse or the extra room and logistics to store on site (climate-controlled storage may be needed depending on project location)
Installation & Labor
- Include allowance for installation/labor. Mass timber frames are usually installed with one team lead and 6 to 10 team members.
o Labor shortages are common and predicted to increase. Relatively small crews are needed to install prefabricated mass timber systems.
♣ Lendlease trained unemployed veterans for the Redstone Arsenal project.
♣ Swinerton was able to use entry-level electricians for the First Tech Federal Credit Union.
- Start thinking about who will install the mass timber, GC or subcontractor.
- Consider whether on-site training of crew by the manufacturer or an install specialist will be necessary.
- Capture savings from pre-fabrication. Installation costs of MEP elements can often be reduced.
To download the checklists in their entirety, visit the WoodWorks website at https://www.woodworks.org/publications-media/solution-papers/.
Contact WoodWorks for Free Project Support
WoodWorks – Wood Products Council provides education and free technical support related to the design, engineering and construction of commercial and multi-family wood buildings in the U.S. A non-profit organization staffed with architects, structural engineers and construction experts, WoodWorks has the expertise to assist with all aspects of wood building design, including (but not limited to): allowable heights and areas/construction types, structural detailing of wood and hybrid systems, fire resistance and acoustical-rated assemblies, efficient and code-compliant lateral system design, alternate means of code compliance, and energy-efficient detailing. For assistance with a project, visit www.woodworks.org/project-assistance or email email@example.com. Visit our website at www.woodworks.org.