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Post-Tensioned Timber A Sustainable System for Clients Seeking to Innovate


Entuitive’s drive to deliver Uncompromising Sustainable Performance in the built environment has us continually pushing the bounds of engineering, researching better, more innovative ways to design structures. Recently, we wrote about timber-concrete composite systems, which offer many advantages when it comes to building design.


Post-tensioned timber is another innovative structural system that offers specific advantages over concrete and steel construction. Below, we’ll discuss what post-tensioned timber is, how it works, and the opportunities we see with this innovative system.


What Is Post-Tensioned Timber?


Post-tensioned (PT) timber is a technical solution combining engineered timber structural elements, such as beams, columns, and walls, with high-strength steel bars or tendons. These tendons are anchored to the timber elements and tensioned using hydraulic jacks to place the timber elements in a state of precompression to counteract the anticipated applied loads.


Post-tensioned timber was originally investigated at the US Forest Products Laboratory in the early 1960s. Its first practical application was for stress-laminated bridge decks developed by the Ontario Ministry of Transportation in the late 1970s. It has been significantly developed for building use over the past couple of decades by researchers at the University of Canterbury with the Structural Timber Innovation Company (STIC)1,2 in Christchurch, New Zealand, ETH Zurich in Switzerland, and more recently other research institutions in the US and Canada.


In collaboration with Carleton University in Ottawa and the University of Canterbury, Hailey Quiquero, a designer in our Toronto office, carried out research on the numerical modelling of post-tensioned beams both at ambient conditions and under fire exposure. This research was undertaken as part of her thesis in 2018, in fulfillment of her MASc in structural and fire safety engineering.


Post-tensioning technology has been commonly used for concrete structures and allows for increased structural efficiency. In timber structures, beams and slabs may have longer spans and smaller cross-sections for the same loading due to increased capacity. Entire building systems become more ductile and can be designed to self-center, resulting in highly effective seismic performance.


Post-tensioned timber beams are typically box-sections built up using fasteners or adhesives with glulam or laminated veneer lumber (LVL) products. In some cases, these engineered products are used only for the top and bottom chords of the hollow beams, with plywood used for the webs. A simple schematic of a PT timber beam is depicted in Figure 1. Columns may also be post-tensioned integrally with beams to create portal frame action. PT timber shear walls are typically fabricated from CLT panels with vertical PT bars or tendons, either internal or external.


Figure 1: Schematic of a PT timber beam showing a built-up wood box-section, high-strength steel tendons, and anchorage.


As mentioned above, the advantages of post-tensioned timber construction include high seismic performance, increased structural efficiency, rapid on-site erection, simple connections, and a lower carbon footprint.


High Seismic Performance


Post-tensioned timber offers increased seismic resilience compared to other structural systems, having been developed in New Zealand for this purpose. In a post-and-beam system, the elements may be post-tensioned across bays to create continuous portal frames. In CLT shear wall systems, the walls may be post-tensioned to create continuity in components vertically along the height of the building. The continuity of these high-strength steel tendons offers self-centering tendencies that serve to pull the building back to its original position after a seismic event.


Additionally, the tendons and relevant connectors may be designed to yield (and are frequently coupled with U-shaped flexural steel plates – UFPs) to increase energy dissipation and create “fuse” elements, rendering this a low-damage option for a lateral force-resisting system. Like electrical fuses, such elements may be replaced after localized damage, reducing the need for more extensive building repair following a significant seismic event.


Increased Structural Efficiency


Post-tensioned timber allows for increased structural efficiency when compared to timber alone. The prestressed steel tendons reinforce the timber elements, counteracting gravity loads and deflections. This also allows the timber elements to withstand heavier loads and enables longer spans, such as those needed for large open-concept spaces like community centres or theatres.


Simple Connections & Rapid Onsite Erection


Wood offers the advantage of speed when it comes to building construction. Panelized and modular timber elements are manufactured off-site and erected faster, lowering site time and on-site labour costs. A mass timber building can be constructed up to 20% to 30% faster than a steel building or traditional cast-in-place concrete building. Mass timber has a further schedule advantage over concrete because there is no need for shores and re-shores, allowing the follow-on trades to begin their work sooner.


Post-tensioned timber systems have the added benefit of simplified connections due to the nature of the PT tendon continuity and anchorage. For example (as depicted in Figure 2), a PT beam could be erected on a simple angle or corbel and then tightened into place once the system is pre-stressed, such that the need for complex bolted or screwed connections is greatly reduced.


Figure 2: Schematic of a PT timber gravity frame connection.


Sustainability


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 helps lower the building’s overall carbon footprint.


One of the most attainable ways of reducing embodied carbon in buildings is to maximize the efficiency of the structural materials. Post-tensioning timber elements to increase the performance under gravity and/or lateral loads allows for the most optimal use of the material properties, thus reducing the overall quantity of materials in the building.


Furthermore, resilience in buildings is important not only for property protection, but also to maximize material reuse and minimize required remediation or waste in the event of an earthquake. The added self-centering and innovative low-damage options available with PT timber systems allow for highly resilient and sustainable structures.


The Future of Building Design – Post-Tensioned Timber Opportunities


We see several opportunities when it comes to PT timber. But before we elaborate, let’s take a quick look at some of the challenges of working with this innovative system.


At the moment, there is a lack of code guidance regarding PT timber. Codes are just now catching up to the possibilities of mass timber in general, so it will still be a few years before PT timber is also reflected in building codes. However, Entuitive has extensive experience with performance-based design and guiding our clients through alternate approvals processes.


A few other challenges include contractor experience, since few PT timber structures have been built in North America, as well as structural analysis software, which generally is not yet set up to include PT timber. That being said, our experience with both post-tensioning and timber have positioned us well to take on these challenges. Our integrated approach and internal collaboration are ideal for this system for clients willing to innovate.


Seismic Design


As mentioned above, PT timber was developed in New Zealand with seismic design in mind. With the new height limits for mass timber buildings in the 2021 International Building Code (IBC) and the 2020 National Building Code of Canada, we have more opportunities than ever to build large-scale and taller buildings in timber. For the time being, most of these buildings will have concrete or steel lateral load-resisting systems. But where an owner or developer would like to explore an all-timber building, including the lateral system, post-tensioned CLT walls are a natural fit for taller buildings in high seismic zones or where a high level of resiliency is desired.


Recreation Centres


Sports & recreation centres typically feature long spans to create the wide, open spaces needed for sporting events and training facilities. Mass timber has already been used on many such projects by Entuitive, such as the Angus Glen Community Centre & Library, Shane Homes YCMA at Rocky Ridge, and the Toronto FC Practice & Training Centre. Where a client is looking for the elegant aesthetic of mass timber, the PT system allows for the increased structural efficiency needed of these facilities.


Bridges


We see PT timber opportunities in the bridge sector as well. Stress-laminated timber decks are an attractive solution for lightweight decking. They consist of nail-laminated timber, post-tensioned perpendicular to grain to increase interlaminar friction, thereby improving load-sharing between laminations. In addition, our team recently co-authored a design example for stress-laminated decks for the Ontario Wood Bridge Reference Guide.


High-Rise Residential


Mass timber is gaining popularity as a material for use in high-rise residential applications. Entuitive has extensive experience in the residential sector, with such projects as Pier 27 in Toronto, Pier 8 Mixed-Use Development in Hamilton, and private timber residences like Ansty Plum in the UK. As we mentioned above, as codes catch up, we are seeing more opportunities to build high-rises with timber, with PT timber offering even more specific advantages.


Commercial & Institutional


Commercial and institutional buildings generally require longer spans. With post-and-beam systems in mass timber, the increased structural depth required results in a larger floor-to-floor height, which increases the cost of the building envelope as well as the volume of air that needs to be heated and cooled. In cases where there is a zoning height limit, this might also mean fewer floors can be built before reaching that limit. PT timber can be used to reduce structural depths, avoiding these challenges.


Our team has extensive experience in tall building design, with such projects as Brookfield Place Calgary, Comcast Innovation & Technology Center in Philadelphia, and 16 York in Toronto.


Entuitive’s commitment to a sustainable future and uncompromising performance means we’re constantly seeking to deliver great projects for great clients. Post-tensioned timber is an innovation that offers specific advantages for the right project.


To learn more about how we can work together on your next timber project, reach out to Eric Gordon


Sources

  • A. Palermo, S. Pampanin, A. Buchanan, and M. Newcombe, “Seismic design of multi-storey buildings using laminated veneer lumber (lvl)”, Proceedings of the 2005 NZSEE Conference, Taupo, New Zealand, 2005.

  • A. Buchanan, B. Deam, M. Fragiacomo, S. Pampanin, and A. Palermo, “Multi-storey prestressed timber buildings in New Zealand”, Structural Engineering International, vol. 18, pp. 166-173, 2008.

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