Mass Plywood’s Future in Warehouse Construction

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Mass timber has become increasingly popular over the last decade for office and multi-story residential buildings. Looking forward, one of the largest potential markets for mass timber is for warehouses and similar big box type facilities.

Construction of warehouses and other big box type structures has evolved over the decades. Currently, the primary methods are site cast or precast tilt-up concrete construction and Metal Building System (MBS) construction. Alternatively, the advantages of mass timber, which are substantial, can be incorporated into these structures. It is anticipated that this new system, or derivatives thereof, will become a common construction system selected for these facilities.
Freres Engineered Wood, a producer of mass plywood, enlisted Crow Engineering, a multi-discipline engineering firm in Beaverton, Oregon, to design a warehouse for its Mill City, Oregon, plant. Crow focuses on industrial, commercial, and warehouse projects and has been advancing the use of mass timber, and specifically mass plywood, for industrial and warehouse applications. The general contractor for the project was CD Redding of Salem, Oregon. The mass timber was erected by Prime Contracting LLC, a timber structure erection company from Cornelius, Oregon.

Since this project was for Freres, it was logical to maximize the use of mass plywood. As such, the building was constructed using mass plywood panels (MPP) for walls and roof deck and mass ply lam (MPL) beams and columns for framing. This proved to be a very effective and desirable way to construct a warehouse—combining speed of construction, carbon sequestering, renewable resources, the use of small cranes, and a small construction crew—resulting in a cost effective and aesthetically pleasing facility. According to a study done by Freres, replacing concrete and steel construction with mass plywood eliminated approximately 429 metric tons of greenhouse gas emissions, representing a total potential carbon benefit of 1,539 metric tons in the construction of their plywood storage warehouse.

The foundation system for the Freres project is comprised a simple 8-inch concrete slab floor with thickened edges to support the walls and thickened sections at the interior columns. Site cast tilt-up requires sufficient cure time for the slab before beginning the process of laying out and forming wall panels, setting rebar, pouring the wall panels, and then waiting for the panels to cure before bringing in a large crane to set the wall panels. Then, once the concrete wall panels are set into place, they need to be backfilled, and the slab extended to the wall panel. Using thickened slab edges eliminated the extra step of pouring stem walls or having to do a pour back strip after setting concrete wall panels.

The wall system selected for the Freres warehouse utilizes MPP in a similar fashion to tilt-up concrete construction. Since MPP panels are plant-manufactured and precision CNC fabricated, the wall panels can be erected as soon as they arrive on site. In fact, on the Freres project, erection of wall panels started before the entire foundation slab was completed, only a few days after placing the slab, shaving months off an equivalent site cast tilt-up concrete building schedule. Solid load bearing tilt-up style MPP walls have no sub-framing and therefore have no dust shelves or exposed insulation to create housekeeping problems for the facility.
The column and purlin spacing and the depth of the girders and purlins were optimized to fully utilize 48-inch-wide MPL billets, with essentially no waste. Purlins are at 12-foot on center, matching the wall panel width, creating a very uniform system, resulting in most of the wall panels being identical. Pockets were routed in the panels by the CNC machine for the purlins to sit in, eliminating all beam hangers at the perimeter wall. The 4-inch-thick wall panels, with an h/t ratio of 80, were designed using a P-Delta procedure developed in-house.

Several options were evaluated for supporting the purlins from the girders. Rather than steel hangers, the idea of screwing a ledger onto the sides of the girders was considered. This eliminated the steel hangers but required many screws to support the loads from the purlins. A further iteration was ultimately implemented. Rather than a continuous face mounted ledger, shallow pockets were routed in the sides of the girders and short ledger blocks were set to bear on the edges of the routed pocket. This was a tradeoff of CNC routing cost against savings on the length of the ledger and greatly reducing the number of screws required. It also automatically indexed the position of the ledgers, eliminating the need for field locating hangers or ledgers. This solution is possible with MPL girders due to the cross-laminations, whereas it would not be acceptable with glulam, due to potential cross grain tension failure.

Two-inch thick mass plywood panels, 12 feet wide by 48 feet long, placed on MPL purlins at 12 feet on center, were used for the roof deck. This results in the deck being a four-span continuous member. The multiple span condition reduces deflections considerably as compared to simple span conditions, thus allowing a 2-inch panel to be used rather than what would otherwise require a 3-inch panel.

This facility, located on the western flank of the Cascade mountains, is in a moderately high seismic zone. Anchorage of concrete wall panels to a roof diaphragm in high seismic zones has long been problematic. Building codes require very large anchorage force levels for this attachment. This force is a function of the weight of the wall, so heavy concrete walls create large anchorage forces. A mass timber wall on the other hand will weigh approximately 20% of what a concrete wall will weigh, resulting in much smaller anchorage forces.

The lateral force-resisting system utilized for this project uses spline connected MPP shear walls and roof diaphragm. The ASCE 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures has some provisions for designing CLT shear walls, however it is very limited in scope. The MPP tilt-up style of construction doesn’t really fall within those limited parameters, so the lateral force system was designed as an “alternate method” per the building code. The State of Oregon Building Division has approved and published the Statewide Alternate Method No. 15-01 “Cross-laminated timber Seismic force-resisting systems” for this purpose. The method requires a very conservative response modification factor to be used in the seismic analysis. Although it is an approved method specific to the State of Oregon, other jurisdictions have accepted the method on a case-by-case basis. The method has ductility requirements that give options regarding which elements of the system must be designed to be ductile. It requires shear fasteners for those ductile elements to be controlled by type IIIs or IV yield modes per the National Design Specification for Wood Construction.

Since this is an unheated building there were no requirements for thermal insulation. Even though no insulation was added to the wall panels, the natural mass and insulation value of the MPP results in the building staying comfortable throughout the year.

The building is classified as Type V-B construction. It should be noted, however, that other than the perimeter bearing walls, the framing complies with Type IV Heavy Timber construction with the inherent fire resistive characteristics thereof.

Any rainscreen/siding material can be used with this system. For this project, 1-inch by 4-foot wide MPP lamella were used as siding, creating an attractive board and batten style appearance.

The project proved to be cost effective. Total cost of construction, including site preparation, was only $118 per square foot, based on the commercial value of MPP. According to CD Redding Construction, this lands between the cost of a basic MBS and a tilt-up concrete building of similar dimensions.

Mass plywood is a cross-laminated veneer panel and is classified as cross laminated timber (CLT). MPP is a code approved product in both the United States and in Canada complying with the APA PRG-320 Standard for Performance Rated Cross-Laminated Timber. Product design values and requirements for multiple grades of MPP and MPL can be found in Freres’ APA Product Reports PR-L325 and PR-L325(C).

MPP panels, manufactured in 8 feet, 10 feet and 12 feet nominal widths, are available in 1-inch increments from 2 to 12 inches thick. The 1-inch increments provide design flexibility when choosing a panel thickness to meet the requirements of the project. The cross laminations of mass plywood and other CLT products provide dimensional stability and cross grain tension strength which is not found in traditional timber products. MPP and MPL are produced at a moisture content of 8%. This is a distinct advantage for dimensional stability in that 8 percent is the typical equilibrium moisture content of wood in a seasoned building.

When designing tall walls of any material, it is generally not practical to meet the code limitation on the height to thickness ratio (h/t) of compression elements. Historically, concrete and masonry walls were designed with pilasters to stiffen the walls to conform with these code-imposed thickness ratios. In the 1970s, a P-Delta analysis method was devised to allow masonry walls to be designed as “slender walls” exceeding the h/t limitations of the codes. After a significant testing program, the method was adopted by the engineering community and codified. It was then extended to concrete tilt-up construction. Standard construction practice now routinely uses slender concrete and masonry walls without stiffening pilasters.

Mass plywood can be utilized in a very similar way. MPP is manufactured under controlled conditions which results in very consistent and predictable properties. A P-Delta analysis is highly dependent on the material modulus of elasticity. This can vary significantly in concrete and masonry materials; however, the manufacturers quality control testing has shown that the most consistent property of MPP is the modulus of elasticity. Deflection is therefore very predictable, so a P-Delta analysis can be considered very reliable.

The deflection considered in the P-Delta analysis should include all potential sources of deflection, as well as the orthotropic nature of the panels. In addition to applied loading, initial straightness, as well as moisture “bowing” should be considered. The author uses Emin rather than E for the P-Delta analysis, which is very conservative. Even such, the panel thickness is normally controlled by service load deflections, not strength.

Historically, the walls of unheated or semi-conditioned tilt-up concrete buildings were left uninsulated, relying on the thermal mass effect of the concrete alone to moderate temperature swings in the building. However, current energy codes require some level of wall insulation in addition to the mass for both semi-conditioned and fully conditioned facilities. This is typically accomplished by pinning fiberglass batt insulation on the inside face of the walls.

MBS buildings are typically insulated with fiberglass batt insulation systems, placed on the outside face of the girts and purlins, but exposed to the interior with an exposed vinyl face. The steel building provides no thermal mass or insulation of its own, so higher insulation levels are required over tilt-up concrete buildings.

Calculations, provided by Freres Wood, have shown that mass plywood can qualify as a “mass wall” per the ANSI/ASHRAE 90.1 energy code if the panel is 4 inches thick or greater. In addition to the mass provided, the panels have a thermal resistance “R value” of 1.25 per inch. Depending on the climate zone, this can result in no added insulation needed to comply with ANSI/ASHRAE 90.1 for semi-conditioned spaces and significantly reduced insulation requirements for fully conditioned spaces. What insulation is used is typically applied to the exterior of the panels, leaving the MPP exposed to the interior. Using solid “tilt-up” style MPP leaves a flat interior surface with no protruding girts and columns.

Research for spline connected CLT/Mass Plywood shear walls would be helpful to establish code prescribed seismic coefficients for nailed spline connected panels and other types of shear wall systems. It is the author’s expectation that research will show that less conservative R-values than required by the Oregon Statewide Alternate method will result.

The Freres warehouse demonstrates that mass timber, and specifically mass plywood, represents a serious alternative for warehouse and other big box structures. The use of a renewable resource such as mass plywood has proven to be very cost effective, thermally effective and environmentally friendly. ■

About the Author

John Bradford, PE, SE, senior structural engineer with Crow Engineering, has 50 years of experience designing heavy (mass) timber structures. Bradford is the SER for the project and developed the methodology used for the MPP slender wall analysis.