May 19 2020
Timber-Concrete Composite Systems: Lighter Weight and Lower Carbon
Entuitive’s drive to deliver Uncompromising Performance in the built environment has us continually pushing the bounds of engineering, researching better, more innovative ways to design structures. Our people are the engine of this drive, and lately our own Pascal Urech has been researching timber-concrete composite systems.
A timber-concrete composite system consists of a timber element with a concrete slab above and a shear connection at the interface. This system takes advantage of the compressive strength of concrete and the tensile strength and light weight of timber.
A timber-concrete composite panel.
The Advantages of a Composite System
Timber-concrete composite systems offer many advantages when compared to single-material schemes, including lighter floors, lower embodied carbon, and longer spans.
Using timber as a primary component of the structure results in a lighter structure overall than a traditional steel frame or cast-in-place concrete scheme. For new structures, this reduced mass can help save on foundation costs. For existing structures, a lighter weight helps enable overbuilds (added stories) while minimizing the need to reinforce existing columns and foundations.
Lower Embodied Carbon
Timber is a renewable building material that naturally sequesters carbon dioxide from the atmosphere. In a complete lifecycle analysis, sustainably harvested timber has the lowest carbon footprint of the major building materials, particularly in contrast to concrete. Using timber to replace some of the concrete in a structure therefore helps lower the building’s overall carbon footprint.
Timber-only framing schemes are typically difficult to achieve with long spans (9 metres and above) due to deflection and vibration considerations. Adding concrete and composite action helps to provide the necessary stiffness to maintain occupant comfort.
How Entuitive Is Preparing for Timber-Concrete Composite Projects
Pascal Urech, a Designer based in our New York office, has been researching timber-concrete composite systems and working on design tools to that end.
An RFEM model for timber-concrete composite.
Timber Beam + Concrete Slab
Timber-concrete composites function similarly to the traditional steel beam-concrete slab composite system, but with timber replacing steel as the tension element.
Generally, glued-laminated timber (glulam) beams are used for beam applications. However, solid sawn timber can also be used. In order to provide composite action, some type of shear connection is required between the timber and the concrete.
For beams, this connection often consists of shear plates (typically perforated gauge metal epoxied into the timber and projecting up into the concrete) or fully threaded screws (also installed into the timber, typically at a 45-degree angle and projecting up into the concrete).
Formwork, such as plywood, must be installed on top of the beam and under the concrete topping slab to ensure that concrete can be cast. The concrete slab will typically be reinforced to control crack widths, either with steel reinforcing bars, welded wire mesh, or fiber reinforcement.
Timber Panel + Concrete Slab
Several options are available if timber panels are used. For spanning in one direction, products that orient all the wood fiber in the same direction provide the greatest strength and stiffness, though they are also more susceptible to dimensional changes perpendicular to the grain caused by changes in moisture content.
Examples of such one-way products are panels made from nail-laminated timber (NLT), glued-laminated timber (glulam), dowel-laminated timber (DLT), or laminated veneer lumber (LVL).
Products that orient the wood fiber in two directions have reduced strength and stiffness in the primary direction compared to a one-way product of the same depth, but they are able to span in two directions and also provide greater dimensional stability when subject to changes in moisture.
The most common example is cross-laminated timber (CLT). Mass plywood panels (MPP), which are fabricated in a similar way to CLT but with thin veneers instead of thick laminations, can also be used.
In addition to the connections mentioned above, notches in the timber, with or without screws, can also provide shear transfer for panel applications.
Flat Floor Systems
In a recent study for Forestry Innovation Investment, Entuitive has been researching flat floor systems. While most timber-concrete composite systems use dropped beams, a flat system can be achieved by using hybrid systems that combine timber-concrete composite panels with concrete and steel beams.
Timber-concrete composite floor panels spanning in one direction and supported by precast concrete beams or steel-concrete composite beams spanning in the opposite direction were the focus of the study. An example of such a flat floor structure is shown below, where CLT-concrete composite panels are used in combination with the DELTABEAM® system by Peikko.
Flat floor structure comprising CLT-concrete composite panels with the DELTABEAM® system by Peikko.
Structural designs must meet vibration criteria regardless of material, but timber is particularly prone to vibration problems because of its low mass. Our team has been researching how to model a timber-concrete composite floor using RFEM, one of the structural analysis software packages used at Entuitive.
The Future of Building Design
Entuitive’s commitment to a sustainable built environment means taking the lead in driving our industry towards better design practices that result in structures with as low a carbon footprint as possible. Timber-concrete composite systems are a great step in that direction.