Innovations in Timber Concrete Composite Structures

To view the figures, equations, and tables associated with this article, please refer to the flipbook above.

Most engineers were trained to use certain structural systems for a building archetype based on their mentor’s experience. The passage of this institutional knowledge is invaluable, however the prevalence of sustainable design practices in contemporary architecture means we must challenge those preconceptions. Structural engineers’ ability to influence the amount of embodied carbon in buildings gives them unique opportunities to think outside the box and encourage clients to use more environmentally friendly materials in construction. While steel, concrete, and masonry will continue to serve an important role in the built environment, there are certainly applications where replacing them with timber will result in a more sustainable structure.

Historically, timber-concrete composite (TCC) construction was used as a rehabilitation mechanism for bridges and buildings. Some of the earliest cited examples of TCC systems are found in Europe when a lack of available steel stimulated the development of alternative methods of construction after WWI and WWII. Following these developments, the use of TCC designs spread to the refurbishment of historic buildings in Europe. Its use has since evolved, now gaining traction as a method for achieving long spans in new buildings.

Despite its use in Europe, Canada, and other regions, few projects in the U.S. have implemented TCC systems. In fact, the Woodworks Innovations Network (a database that catalogs wood projects across the U.S. and Canada) lists only 11 instances of building designs that used TCC systems in these countries. This may be a result of the lack of guidance and design provisions that pertain to this construction. This contrasts with steel-concrete composite (SCC) systems which have well-developed provisions and design equations that can be implemented by designers in the same way that non-composite elements are designed. However, interest in TCC systems is beginning to gain traction, with the completion of some recent high-profile projects such as Limberlost Place in Toronto and the KF Aerospace stair in Kelowna, British Columbia. These projects have achieved impressive results; the composite CLT girder slab-band reaches a clear-span of about 30 feet using a 7-ply CLT panel at Limberlost Place, eliminating the need for deep glulam beams and keeping the floor structure remarkably thin. The KF Aerospace stair spans 70 feet between floors over the length of its spiral form.

The primary motivation for using a TCC floor system lies in the fact that floors in mass timber buildings often contain a cementitious topping. This topping is most often required for acoustic treatments or fire protection but is not typically taken advantage of for its potential contribution to the structural capacity of the floor. The topping can be gypcrete, but using concrete has certain advantages acting as an exposed topping slab, to route conduit, or as a diaphragm. By connecting timber framing to the topping slab to transmit internal shear forces, designers can create strong and stiff members while utilizing less timber or achieving longer spans than a non-composite system.

While it can be challenging to achieve the same strength and stiffness as a concrete or steel-concrete composite frame, using timber beams produces a structural system that is more sustainable than these other materials. Where sustainability is a high priority outcome, several case studies have shown that replacing carbon-intensive materials with timber provides a considerable decrease in the embodied carbon of the structural system. Aesthetics and biophilic design are also reasons to use exposed timber in a structural system. Implemented as a refurbishment of buildings for adaptive reuse, TCC systems can extend a building’s life-span for future generations.

The typical materials used in timber-concrete composite systems are the concrete slab, a timber member (often a wood beam or plank), and the interface shear connector that ties these two materials together (synonymous with the headed shear studs typically used in SCC systems).
The topping slab needs to be thick enough to carry the internally induced compressive forces and act as a proper base material for the anchorage of the interface shear connectors. A slab thickness of 3-to-4 inches is a good starting point. A thicker slab will induce unnecessary weight to the system, and a thinner slab might not be able to provide adequate anchorage for the shear connectors.

The timber component can be either a beam or a plank. Solid-sawn elements or engineered wood products (EWP) are acceptable, but it is not ideal to use a built-up beam (for example one that is a multi-ply Laminated Veneer Lumber beam) due to the complexity of providing a shear transfer mechanism that equally engages all the individual plies. The wood member should be a species recognized by the NDS or similar governing standard for material design that provides design stress ratings for bending, shear, tension, and compression based on the grade and species of wood.

The shear interface connectors that transfer horizontal stresses between the timber and concrete components are a critical element to define and evaluate as part of the analysis process. Typical mechanical shear connectors are self-tapping wood screws, truss plates, or lag screws. The timber member can also be notched, allowing the concrete to interlock with the wood. The shear connector must have a known strength, typically gleaned through laboratory testing using the same base materials as the composite system (timber and concrete). In addition to establishing the yield and ultimate strength levels of the fastener, it is critical to establish the stiffness of the fastener as this property plays a major role in the stiffness of the overall composite section.

FP Innovation’s Design Guide for TCC Structures in Canada (hereafter referred to as the “Design Guide”) suggests the use of the Gamma Method to determine capacity. This method is recognized in Eurocode 5 as a closed-form solution for calculating the stiffness of a partially composite beam. The procedure may not be intuitive to designers who are accustomed to SCC systems. Some of the unique aspects of TCC evaluation are:

A critical parameter for determining the stiffness of the system is the failure mode of the interface fasteners. The Design Guide notes that when the fasteners fail in a brittle manner, or are not allowed to yield, the moment capacity is determined by the effective bending capacity of either the timber component or concrete component, using the Gamma Method (To view the equations associated with this section, please refer to the flipbook above.):

Brittle fasteners must be strong enough to preclude failure prior to the failure of the timber or concrete component. The strength of the fasteners is evaluated as part of the shear analysis for the beam.

A limitation of the Gamma Method is that it assumes each component exhibits linear-elastic behavior. This may overestimate the capacity of the composite section if the fasteners yield prior to failure of the timber or concrete components. In the case of ductile fasteners, a third limit-state is introduced: the Elasto-Plastic bending moment resistance (M_r,EP).

The shear capacity of the composite section is also influenced by the failure mode of the fasteners. With brittle fasteners, the shear capacity of the entire composite section is likely to be controlled by the fastener strength alone, with the fasteners at each critical section being evaluated for the horizontal shear stress at that location.

Varying the spacing of the fasteners along the span of the beam can provide an economic design.

For ductile fasteners, when the highest-stressed fasteners near the end of the beam yield, the internal forces redistribute to the timber or concrete components until the weaker material fails, similar to the behavior of steel-concrete composite framing. In this scenario, designers can take advantage of the behavior to gain higher shear strength by considering the Elasto-Plastic shear capacity of the composite system.

Note that the connection strength is not directly evaluated. Instead, it is a component of the equations used to determine the shear strength of the timber or concrete components, evaluated using the Gamma Method or Elasto-Plastic model as indicated above.

The shear capacity is also checked against the fastener’s strength under service-level loads to preclude the risk of fatigue failure.

While the specifics of evaluating the bending and shear capacity are complex, the take-away for the designer is to recognize the critical role the interface connector plays in the capacity of the composite section. Those accustomed to the provisions relevant to SCC composite systems should be prepared to account for aspects of the interface shear connector that do not typically drive the design of SCC systems. The type of failure (brittle versus ductile), stiffness of the fastener, and long-term creep effects must all be accounted for in the strength equations.
In conclusion, the design process for TCC beams is a bit more complex than other types of framing. The influence of each component on the strength and stiffness results in more equations to be calculated, and each step of the analysis must be evaluated twice: once for ultimate strength and a second time for long-term effects. Fortunately, because the equations offer a closed-form solution, the process can be automated through the use of spreadsheets or other analysis tools. More information on determining the capacity of TCC framing systems can be found in the Design Guide.

Utilizing the analysis procedure presented in the Design Guide, the author conducted a study of a structural framing bay for a typical office building. The study evaluated a 30 feet x 30 feet column grid framed with three different systems: steel-concrete composite members, non-composite timber members with cross-laminated timber (CLT) decking, and TCC members.

The proposed TCC system consisted of the concrete topping, glulam framing, and CLT planks spanning between purlins, acting as the forming surface for the concrete topping and gravity support for loads between the purlins. Because the concrete must pour down to bear on the glulam beam for a proper connection of the shear fasteners, the CLT plank cannot be continuous (Fig. 1). The author proposes that while this design is unorthodox, it is viable by simply re-orienting the boards of the CLT so that the outer plies (the strong direction) are along the short dimension of the panel rather than the long dimension. This would permit CLT panels to be placed with the long dimension parallel to the supporting joists rather than perpendicular, allowing for similar erection efficiencies experienced using standard mass-timber construction techniques (Fig. 2). At the time of publication, at least one North American CLT manufacturer has published it is capable of providing a transverse layup in their CLT panels. Other layout techniques may be possible but were not considered as part of this study.

The goal of the study was to determine whether it was possible to create a floor framed using a TCC system that has a similar depth to an SCC floor for a given bay size, since the depth of the framing is often a critical design constraint and can cause design teams to steer away from timber systems. The system was designed to be constructed without the need for shoring prior to the concrete curing; an important consideration as shoring can lengthen the construction schedule and add cost. The study concluded the following:

TCC systems currently in use and those evaluated in the design guide example problems are generally limited to sections created using concrete and timber planks rather than beams. This limits the practical applications of the system and does not acknowledge the benefits that can be achieved by creating sections that incorporate a beam element. These systems can be used in place of more carbon-intensive materials such as steel and concrete, while still providing the long spans and minimal depths building owners have come to expect from these other structural systems.

In order to facilitate adoption, the author proposes:

Some design and construction firms have already begun to study the positive benefits of TCC Systems. With better familiarity, resources, and testing of this system, more implementation among designers could occur. ■

About the Author

Kirby Beegles is a structural engineer with Martin/Martin. He is a leader on their Wood and Mass Timber Subject Matter Expertise team and heads the Advocacy and Education components of their SE2050 committee. This article draws from a case study he completed as part of his graduate research at the University of Colorado at Denver.