Mass Timber Primer
TEXT AND DIAGRAMS J. David Bowick
The most novel architectural material in Canada right now is an old one—wood. But when made into mass timber beams and panels, the age-old material gains structural and fire-resistance qualities that open up new possibilities for construction. We asked structural engineer David Bowick to walk us through the basics of working with mass timber structures. Here’s what he told us.
Mass timber construction—which makes use of glue laminated (glulam) beams and columns with products like cross-laminated timber (CLT), nail-laminated timber (NLT), and dowel-laminated timber (DLT)—is in its early days. With concrete and steel, there are clear templates for commercial construction—the 9×9 bay concrete flat slab, with 4 metres from floor to floor, and for big box retail, the 9×12 bay with steel beams and open web steel joists. With mass timber, it seems every project is constructed using a unique structural system. There’s an opportunity to go beyond purely pragmatic concerns with mass timber. The uniqueness of the system can become a design goal.
What are the baseline rules of working with this family of materials? Mass timber systems follow inherently different rules than concrete and steel. Square bays are effective when you have a system that has similar strength and stiffness in both directions, but are inefficient with wood, which is necessarily “stick built” with “one-way” elements (elements that are much greater in one dimension than the others, or decks which are much stronger in one direction than the other) stacked on perpendicular one-way elements. Because mass timber systems rely on beams, they are necessarily deeper than slab systems, requiring greater floor-to-floor heights. The beauty of wood, and the cost premium in constructing with mass timber, means that architects and owners often want to expose and express the structure. Fire-protecting exposed wood structures and protecting them from the elements during construction creates design challenges and can be costly.
One also needs to consider construction economics. With mass timber construction, the cost of fibre represents roughly two-thirds of the cost of the structure. Mass timber elements are manufactured off-site in large pieces, often taking advantage of CNC milling, which minimizes both factory and site labour. The impact is that systems that minimize material use may prove economical, even if they result in increased complexity in the system.
As the use of mass timber becomes more widespread, it may become more affordable. For many years in North America, the cost of construction has been dominated by the cost of labour. Current estimates place labour at more than 50 percent of the overall construction costs. When other contributing costs are considered, such as equipment and general conditions, the cost of material in construction represents a relatively small part of the overall budget for a building.
Moreover, the inevitable shift to a carbon-based economy means that those technologies that have the lowest contribution to greenhouse gas emissions will ultimately prove to be the most economical. Wood is a renewable resource that encapsulates carbon. It is part of the solution, rather than part of the problem. The only question is how long this shift will take, and how bad the climate crisis will get first.
In the meantime, opportunities and challenges await the architects and engineers working with mass timber. At the conception of this article, I planned to present the relative merits of a half dozen ways one might assemble a mass timber floor. I quickly realized that there are many, many more than that. Here are a few.
If there was such a thing as a “normal” system in mass timber, this would be it. In fact, Toronto’s “first commercial timber building in 100 years” at 80 Atlantic uses this configuration, so it’s worth discussing first.
Gary Williams, president of fabricator Timber Systems, once told me “the system that has the least expensive deck will be the least expensive system.” This may seem counter-intuitive since, in general, beams are the big expensive components of floors. But with mass timber, beams account for the minority of the material—so it is the deck that drives the cost of the system. Consider, too, the fact that to reduce the cost of the deck, one must reduce the deck span, which means adding beams. But as beams are added, the demand on those beams drops proportionally. So while the volume of timber used in the deck goes down, the amount of material in the beams stays about the same.
If you are using a beam and deck system, it makes sense to orient the beams in the long direction and the deck in the short direction, since the beams, being deeper, resist bending more efficiently, resulting in a structural system with less fibre overall. However, this also results in a deep system, so it might not be right if floor-to-floor heights are critical.
In cases where the floor-to-floor height is critical, it may make sense to orient the beams in the short direction, since this will result in a shallower overall floor system.
Because mass timber components are made of wood laminations, the sizes jump in increments. For CLT, these are 105 mm, to 175 mm, to 245 mm, to 315 mm, and so on. GLT and NLT follow the dimensions of sawn lumber: 89 mm, 140 mm, 184 mm, 235 mm and 286 mm. In the case of dissimilar spans that are close to each other, it is reasonably likely that the deck is the same either way, so it may make sense to minimize the beam size by spanning the deck in the long direction.
Beam and deck systems can minimize “beam shadowing” in largely glazed buildings, if beams are oriented perpendicular to the perimeter walls.
A system of beams and girders is a strategy for ensuring minimum cost, by keeping the deck as thin as possible. Beams are spaced at the maximum span of the most economical deck. Girders must then be provided to support the beams and transfer load to the columns.
Any system with beams has an impact on the distribution of services and potentially, as a result, on floor-to-floor heights. Providing beams and perpendicular girders further reduces flexibility, although it may generate the most economical structural scheme.
Beam and girder systems result in a greater amount of “beam shadowing” relative to beam and deck systems.
Two-way beam systems are particularly interesting to consider when bay sizes are equal in both directions. By alternating the deck orientation in
a basket weave pattern, the beams in both directions are loaded equally. Each beam receives half the load that it would see in a unidirectional beam system, and as a result, the beams can be smaller and lighter.
Any system of sticks that is exposed to view on a ceiling will tend to organize the perception of the space, creating a primary and a secondary orientation. An advantage of a two-way beam system is that there is no dominant orientation, which may have a beneficial impact on space planning.
A reciprocal frame is an apparently whimsical system that, nevertheless, offers some distinct structural advantages. The whimsy arises from the fact that beams do not necessarily span from support to support, but form a network that researcher Olga Popovic Larsen describes as “mutually supporting beams in a closed circuit.” The colours red and cyan in the diagram above represent the warp and weft directions of the frame, and highlight the reciprocal nature of their support.
As with a beam and girder system, the density of the reciprocal frame can be set to optimize the deck, offering economic advantages. Unlike a beam and girder system, though, it is a democratic framing arrangement without a dominant orientation.
Another benefit is that in a reciprocal framing system, all members simultaneously contribute to the support of the load, which helps mitigate floor vibration. While the total static deflection under load may be the same, optimized to the design criteria, the single point load deflection—which is an indicator of vibration performance—is much less.
A reciprocal frame system that spaces beam elements equal to the width of a CLT panel allows the deck to span simultaneously in two directions. This is of limited advantage in normal loading conditions, since the CLT will manage short spans comfortably without the two-way benefit, but it can be a significant benefit in the event of fire, since all laminations are effective in resisting load.
A distinct disadvantage of a reciprocal framing arrangement is that the system is not self-supporting until it is complete, and will require some amount of falsework to complete erection.
A two-way, point-supported CLT framing system takes advantage of the two-way bending strength of CLT to eliminate beams altogether, creating an extremely thin floor system. In particular applications, this can be a tremendous benefit.
The system is limited, though. CLT is much weaker and less stiff in one direction because of the lay-up of the laminations. It also only comes in widths ranging from 2.4 to 3.0 metres, so this establishes your column spacing in the direction perpendicular to the grain.
Fire is also particularly challenging. After about 1 ½ hours, you have burned through two laminations of the CLT in a five-ply panel. This leaves only two laminations in the strong direction and one lamination in the weak direction, which may not sustain the fire load case. Thus, CLT used in this manner will often have to be encapsulated in drywall for fire protection, concealing it from view.
Michael Green has used point-supported CLT ingeniously to create beam-free zones for mechanical distribution, in buildings which otherwise use beam-and-deck systems.
When an objective is to achieve long spans coupled with thin structural depth, wide flat beams made from glulam or CLT can be used. Bending capacity is proportional to width, but varies with the square of the depth, so wide flat beams are less efficient than narrow deep ones. As a result, the structure will be less economical.
While there is an apparent reduction in span, the system does not achieve the savings in deck that one might expect. If the edge of a wide flat beam is loaded, it will topple over, so the bending stiffness of the deck is needed to resist that. Effectively, the deck has to span to the middle of the beam, regardless of its width.
With a wide flat beam system, there is an opportunity to hang the deck from the beams, as opposed to supporting it from below. It makes
no difference to the deck, which has to span to the middle of the beam regardless, although it adds the complication of designing hangers
(or possibly long screws) and fire-protecting those hangers.
On top of the deck, the floor can be made flush with a raised floor system, or the cavity between beams can be filled with EPS foam billets, and the whole made flush with a concrete topping.
This system can it be helpful in space planning and possibly has aesthetic benefits by eliminating beam shadowing. Alvar Aalto’s Villa Mairea gives you a sense of the extraordinary impact of natural light penetration on a wood ceiling uninterrupted by beams.
It is possible to use wide flat beams in a system where they are flush with the deck. This will provide a very thin system overall, and potentially an economical one, because in this case, the deck span is only the distance between beams, not centre to centre. It has the toppling problem though. If conventional columns are used, the system is unstable and the beams will topple under unbalanced loads. To resist this, the columns must be made to be wide—almost as wide as the wide flat beam—so that unbalanced moments are resisted by the column. It also presents other challenges for the designer, who must develop a flush hanging system for the deck and prove the torsional strength and stiffness of the beam; these properties are not well understood or documented.
This system was advanced by Fast and Epp for the Arbour, with Moriyama & Teshima Architects and Acton Ostry Architects. The team writes: “The long-span timber-concrete-composite ‘slab band’ system creates a near flat ceiling for easy service routing and space flexibility. The system was inspired by underground concrete parking structures, where very shallow, very wide bands are commonly used.”
All of the wide flat beam systems present a minimal amount of surface area relative to volume, so their fire performance is near to optimal.
The staggered deck system has been used by Michael Green Architects and advanced by Equilibrium Consulting. It consists of two parallel layers of deck—a top deck and a bottom deck—gapped and offset, with a relatively small overlap between the two. The overlap is fastened with diagonal screws in a way that develops longitudinal shear. The impact is that the deck may be thin, as the structural performance reflects the overall thickness, similar to corrugated metal deck or cardboard. It is a means of achieving long spans with minimal material.
The term cassette—literally a box—is used in mass timber construction to describe manufactured assemblies of beams and deck. These are optimized to crane capacity, minimizing the number of pieces to be erected and accelerating construction. One form of the cassette is the timber box beam, which consists of CLT top and bottom flanges with mass timber (possibly glulam) webs. This combination can be very efficient, since the least effective material near the neutral axis has been removed. Such a system would be capable of very long spans.
The void may be left open for distribution of services, or may need to be filled to prevent a cavity which can spread fire.
“Stressed skin” is an expression used in wood construction to describe a system where the deck contributes to the flexural strength of the system by carrying compressive and tensile forces, and doesn’t merely transfer load to the beams. A box beam, as illustrated earlier, is one example of a stressed skin, as is a structural insulated panel (SIP).
A stressed skin lattice is a system which attempts to gain the benefits of a stressed skin system in a two-way system. Since wood is inherently a one-way material, strong only parallel to its fibres, the “skins” (CLT panels) are used in opposite orientations: the top skin in one direction and the bottom skin in the other. The web must work in both directions as well. One layer transfers shear across its width, and the other carries the axial chord force, balancing the skin.
The above diagram shows, in cyan, the upper lattice which acts composite with the bottom skin, forming a shallow vierendeel truss oriented into the page. The lower lattice, which acts composite with the upper skin spanning parallel to the page, is shown in tan.
What would Italian architect and engineer Pier Luigi Nervi do? With material cost high relative to labour costs, Nervi developed structural forms in concrete that optimized the use of material and minimized weight.
The mass timber industry is in a similar situation today. The cost of the raw material is high (at roughly two-thirds of the overall cost) and CNC fabrication allows complex forms and assemblies to be constructed relatively economically.
The structural strategy used by Nervi for the Palace of Labour in Turin could be adapted to a mass timber solution. In this proposal, 16 radial beams span directly to each column support, eliminating the need for a girder—another example of using wood in a two-way system. Eight of the 16 beams are curved and therefore subject to torsional forces. In order to resist the torsion and prevent these beams from rotating under load, an annular ring of blocking is provided.
Voided concrete timber composite (VCTC) is a timber concrete composite (TCC) system with a thick topping, roughly equal to the thickness of the mass timber. The “voided” part is about removing the concrete where it is least effective, closest to the neutral axis, saving weight and reducing the carbon footprint of the system. The voids can be formed in any number of ways. Commercially available voiding systems such as Bubbledeck, Cobiax or Sonovoid can be used, as can custom voids such as EPS blocks or even plywood boxes.
The significant advantage of a VCTC system is the elimination of beams and the ability to achieve long spans. The direction perpendicular to the wood fibre is carried by the concrete alone, which acts as a wide flat beam within the thickness of the topping. The elimination of beams results in a thinner structural system overall. In addition, space planning is simplified and beam shadowing is eliminated.
A secondary benefit is that the system is very versatile. The same system can be adapted to accommodate transfer beams or exceptional spans (such as for an auditorium) without changing the overall thickness or appearance, by varying the balance of concrete to timber.
A VCTC system presents the minimal surface to fire, making it extremely fire-safe. By adding rebar to the bottom of the topping, just above the wood, the system can be made to sustain fire load in the complete absence of the wood.
Because the system has a strong direction (parallel to the wood fibre) and a weak direction (perpendicular to the wood fibre), it is ideally suited to situations with unequal bays in an aspect ratio of around 1:1.5.
The VCTC system was developed by Blackwell and proposed in a wonderful (but unsuccessful) submission for the Arbour project at Goerge Brown University, with MJMA and Patkau Architects.
The Peikko DELTABEAM system is an extremely effective solution to the challenge of achieving both long spans and a shallow depth. The triangular-section delta beam was originally developed for use with hollow core precast and has been adapted for mass timber.
The DELTABEAM is a flush steel beam, consisting of a tapered box with a wide bottom flange that serves as a seat. The webs of the box are perforated with large holes that allow the beam to fill with concrete from the topping. The concrete topping is fully composite, with both the deck and the steel beam contributing to the bending strength and stiffness of the overall system. Reinforcing steel may be placed inside the box, and made capable of sustaining load in the case of fire without the contribution of the bottom flange—so the bottom flange may not need to be fire protected.
This system is being used on 77 Wade Avenue, an eight-storey commercial office building designed by BNKC Architects with Blackwell Structural Engineers.
As mass timber matures, many proprietary and non-proprietary systems have shown up in the marketplace—and between writing and publishing this article, there will no doubt be a few more. Some will have legs and endure, while others will not. Many are composites, combining wood with steel and concrete to maximum benefit.
Cree by Rhomberg is one proprietary, modular system that has gained some traction in Europe. It consists of mass timber beams with a composite formed concrete deck, pre-manufactured in panels of 2.5 to 3 metres in width by one full bay long. At the exterior edges, the concrete is turned down to form an edge beam. The exterior may be supported on columns or modular wall panels. In the interior, the panels are supported on a steel box beam, with a wide bottom flange which acts as a seat.
When German architect and engineer Friederich Zollinger invented the lamella roof in the 1920s, it was to respond to a severe shortage of housing and building materials following the First World War. The system uses simple, standard segments of timber in a rhomboid pattern. The Zollinger Lamella is a reciprocal framing system, where the beams are oriented in the fashion of a diagrid. One of the great benefits of the Zollinger Lamella comes from the arched form of the roof. The system has benefits when on the flat as well. The high density of the lamella means that individual pieces can be made small and the deck panels may be made as thin as possible. Orienting the deck orthogonally on a diagrid of beams creates a very strong diaphragm, which is a benefit for resisting wind and seismic loads.
The triple beam system is one of a number of all-wood, shallow-beam systems. With a triple beam system, the middle beam is discontinuous, allowing the column to pass through to support the column above. This configuration prevents the problem of crushing perpendicular to the wood grain, and of large shrinkage perpendicular to the grain. The two side beams may be continuous past the column, adding significant strength and stiffness benefits.
By assembling the beam out of multiple pieces, a designer is able to push past the limiting width of 365 mm usually linked to glulam beams. While wide shallow beams are less efficient, there is a benefit from a fire perspective, as they present less surface relative to volume; in addition, the side beams provide fire protection to the connection of the middle beam. The savings in depth with this system can be critical as timber buildings get taller.
J. David Bowick is a principal with structural engineering firm Blackwell.