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Pres-Lam is a method of mass engineered timber construction that uses high strength unbonded steel cables or bars to create connections between timber beams and columns or columns and walls and their foundations. As a prestressed structure the steel cables clamp members together creating connections which are stronger and more compact than traditional timber fastening systems (Below and Sarti 2016). In earthquake zones, the steel cables can be coupled with internal or external steel reinforcing which provide additional strength and energy dissipation creating a damage avoiding structural system (Buchanan et al. 2008).

Pres-Lam can be used in conjunction with any mass engineered timber product such as Glue Laminated Timber, Laminated Veneer Lumber or Cross Laminated Timber.

History
The concept of Pres-Lam was developed at the University of Canterbury in Christchurch, New Zealand (Palermo et al. 2005) in collaboration with PreStressed Timber Limited (PTL) (Prestressed Timber Limited 2007) stemming from techniques developed during the US PRESSS at the University of California in San Diego during the 1990s under the leadership of New Zealand structural engineer Nigel Priestley (Priestley 1991).

Beginning in 2008 a 5 year research campaign was begun under the Structural Timber Innovation Company (STIC 2013b). During this period the first completed examples of Pres-Lam structures were completed in New Zealand (Devereux et al. 2011). Following the systems success international research efforts have begun at ETH Zurich (Wanninger and Frangi 2014), the University of Basilicata (Smith et al. 2014), Washington State University (Ganey 2015) and several other research institutions. In 2017 the NHERI Tallwood project was started with funding from the U.S. National Science Foundation focused on further validation of Pres-Lam in North America (Pei et al. 2017).

Components

Pres-Lam uses unbonded post-tensioned steel tendons or bars passing through internal ducts or placed externally to large timber box beams, frames or walls (Beerschoten et al. 2015; Sarti et al. 2015). In moment-resisting frames, the horizontal steel tendons in the beams also pass through the columns, creating a moment connection (Newcombe et al. 2008a). Pres-Lam structural walls have vertical post-tensioned tendons in a centrally located duct to anchor the walls to the foundation. In some cases, additional elements are added to supplement the strength of the post-tensioning (Newcombe 2005).

Engineered wood columns, beams and walls create the majority of the structure and are the most visible part of the system. The final configuration of the element depends on the engineered wood product used: It would also be possible to use other engineered timber products such as Parallel Strand Lumber or Laminated Strand Lumber although no current examples or testing exist in these materials.
 * Glue Laminated beams and columns are often made of standard sizes beams block glued together (STIC 2013a). In most cases, a cavity is routed in the centre of the two pieces of the beam before to provide the cavity for the post-tensioned steel. Where possible columns may be made of a single glue laminated beam. Glue laminated timber is not easily used in Pres-Lam walls
 * Laminated Veneer Lumber is produced in long lengths of a standard width and thickness. Often several sheets of LVL are required to be glued together to form a beam, column or wall. During gluing, cavities can be left for the post-tensioned steel.
 * Cross Laminated Timber is made in wide sheets of varying width and length. When post-tensioning is placed internal to the sheet boards may be left out to create a duct (Dunbar et al. 2014). Cross-Laminated timber is not easily used in Pres-Lam frames.

Pres-Lam post-tensioning steel normally consists of either 7-wire strands or high-strength steel bars (Macalloy, Dywidag, or similar). These systems are common and readily available from the concrete prestressing industry (Macalloy Ltd 2007). High strength steel is desirable in order to reduce the size of post-tensioning elements but also to ensure they remain elastic even under extreme loading such as an earthquake. When flexible 7-wire strands are used tendons can draped reducing section heights in gravity beams (Rug and Pötke 1988; Pohlmann 2004).

In some cases additional steel devices (Palermo et al. 2012), viscous dampers (dashpot) or friction dampers may be added increasing connection strength and improving seismic performance (direct to Earthquake Engineering).

Applications
Pres-Lam wall systems

Walls may be used as, Isolated cantilever walls, pairs of coupled walls in the same plane or core walls around a lift shaft or stairwell.

The simplest design case is the full-height isolated cantilever wall however pairs of coupled walls in the same plane shown in Figure 4.1(b) have the added advantage that vertical shear forces in the coupling devices between the walls will induce additional axial loads (direct to Normal Force) into the walls and provide additional strength (Kelly et al. 1972).

Core walls around a lift shaft or stairwell are stiffer and potentially stronger than stand alone or coupled shear walls. Axial forces in the transverse side walls (perpendicular to the plane of loading) contribute to the overturning resistance.

Free-body diagrams of walls under lateral loads are shown in the figure below. In all cases the vertical dotted lines show the location of the internal post-tensioned element, which provide the only tensile connection to the foundation, in addition to any supplementary reinforcing which is not shown.

Pres-Lam frame systems

The most common frame arrangement consists of mid-height straight tendons passing through hollow beams and through the supporting columns (Newcombe et al. 2008b). However, for higher gravity loads it becomes more appropriate to use draped tendons where the tendon is used to provide additional gravity support as well and beam end moment resistance (Mayo 2015).

Any Pres-Lam frame system can also be (direct to Braced Frame) braced in order to provide additional lateral strength and stiffness.
 * 1) Straight horizontal tendon, mid-height of beams


 * 1) Draped horizontal tendon, mid-height of beams

Characteristics of Pres-Lam
Pres-Lam systems normally adopt a performance based design approach meaning that they are tuned to respond in a pre-determined manner under gravity loading, wind loading (direct to Wind Engineering), seismic loading and any other relevant load case (Sarti et al. 2011).

Pres-Lam walls can be used as part of the vertical load resisting system through the use of corbels or collector beams. Pres-Lam frames may also be used as part of the vertical load resisting system by spanning floor between the beam elements (Smith et al. 2016; Moroder et al. 2017).

Under wind loading the lateral movement of the building during a frequent event (1 in 25 years) often provides the governing design criteria. In this case Pres-Lam walls and frames are design with such a clamping force from the post-tensioning that gap opening will not occur and the connection will remain as stiff as possible (Buchanan 2016).

Timber buildings perform well in earthquake loading as they are general light weight and do not generate large forces under acceleration (Blass et al. 1994). Although the post-tensioning alone creates a connection between members in some cases additional reinforcing elements are added in order to provide additional moment capacity or reduce the required post-tensioning force. Under ultimate seismic load (Eurocode 8 2004; Standards New Zealand 2004; ASCE 2010) these elements can be designed to dissipate energy which lowers seismic demand. Should the project team desire, a Pres-Lam building can target damage limiting principals by making these element replaceable in the case of damage (Brown et al. 2012).

Notable structures

 * The Nelson Marlborough Institute of Technology Arts and Media Building – The world’s first Pres-Lam Building
 * The College of Creative Arts, Massey University – Uses Pres-Lam frames to augment vertical load carrying capacity and well as high seismic loading
 * The Kaikoura District Council building – Subjected to the 2016 Kaikoura earthquake
 * The ETH Zurich house of Natural Resources – the first Pres-Lam building to be constructed outside of New Zealand
 * Peavy Hall – a three storey mixed use education building under construction Oregon State University campus in Corvallis, Oregon, United States.
 * The Framework building – a 12 Storey Pres-Lam building to be built in Portland, Oregon