Introduction
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 between 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.
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. 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.
About
Pres-Lam
About
Pres-Lam
Introduction
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 between 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.
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. 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 by a team lead by Professors Stefano Pampanin, Alessandro Palermo and Andy Buchanan in collaboration with PreStressed Timber Limited (PTL). The system stems from techniques developed during the US PRESSS research programme at the University of California in San Diego during the 1990s under the leadership of New Zealand structural engineer Prof. Nigel Priestley.
Beginning in 2008 a 5 year research campaign was begun in New Zealand under the Structural Timber Innovation Company. During this period the first completed examples of Pres-Lam structures were completed in New Zealand. Following the system’s success, international research efforts have begun at ETH Zurich, the University of Basilicata, Washington State University 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.
History
The concept of Pres-Lam was developed at the University of Canterbury in Christchurch, New Zealand by a team lead by Professors Stefano Pampanin, Alessandro Palermo and Andy Buchanan in collaboration with PreStressed Timber Limited (PTL). The system stems from techniques developed during the US PRESSS research programme at the University of California in San Diego during the 1990s under the leadership of New Zealand structural engineer Prof. Nigel Priestley.
Beginning in 2008 a 5 year research campaign was begun in New Zealand under the Structural Timber Innovation Company. During this period the first completed examples of Pres-Lam structures were completed in New Zealand. Following the system’s success, international research efforts have begun at ETH Zurich, the University of Basilicata, Washington State University 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.
Components
Pres-Lam uses unbonded post-tensioned steel tendons or bars passing through internal ducts in large timber box beams, frames or walls. The post-tensioned steel tendons or bars can also be placed externally. In Pres-Lam moment-resisting frames, the horizontal steel tendons in the beams also pass through the columns, creating a moment connection. 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.
The main structural elements are engineered wood columns, beams and walls, which are the most visible part of the Pres-Lam system. The final configuration of the elements depends on the engineered wood product which is used:
- Glue laminated beams and columns are often made in standard sizes which are block-glued together. In most cases, the internal cavity is made by routing a channel in each half of the beam before the two halves are glued together, providing a full-length duct for the post-tensioning steel. Most columns are made of a solid glue laminated member with no post-tensioning. 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. Most often several sheets of LVL are glued together to form a beam, column or a wall. During gluing of these layers, full length internal ducts can be left for the post-tensioning steel.
- Cross Laminated Timber is made in wide sheets of varying width and length. When internal post-tensioning is required, some boards can be left out in the manufacturing process to create a duct. External post-tensioning can also be used. Cross-Laminated timber is not easily used in Pres-Lam frames.
It is 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. 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. High strength steel is desirable in order to reduce the size of post-tensioning elements and also to ensure they remain elastic even under extreme loading such as an earthquake. When flexible 7-wire strands are used, the tendons can draped to reduce section heights in gravity beams. In some cases additional steel devices, viscous dampers (dashpot) or friction dampers may be added increasing connection strength and improving seismic performance.
Image 1 (top):The two-storey STIC test building was subjected to normally devastating lateral load with only minor damage.
Image 2 (right):A three storey Pres-Lam frame building on a shaking table at the University of Basilicata ready to be tested.
Components
Pres-Lam uses unbonded post-tensioned steel tendons or bars passing through internal ducts in large timber box beams, frames or walls. The post-tensioned steel tendons or bars can also be placed externally. In Pres-Lam moment-resisting frames, the horizontal steel tendons in the beams also pass through the columns, creating a moment connection. 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.
The main structural elements are engineered wood columns, beams and walls, which are the most visible part of the Pres-Lam system. The final configuration of the elements depends on the engineered wood product which is used:
- Glue laminated beams and columns are often made in standard sizes which are block-glued together. In most cases, the internal cavity is made by routing a channel in each half of the beam before the two halves are glued together, providing a full-length duct for the post-tensioning steel. Most columns are made of a solid glue laminated member with no post-tensioning. 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. Most often several sheets of LVL are glued together to form a beam, column or a wall. During gluing of these layers, full length internal ducts can be left for the post-tensioning steel.
- Cross Laminated Timber is made in wide sheets of varying width and length. When internal post-tensioning is required, some boards can be left out in the manufacturing process to create a duct. External post-tensioning can also be used. Cross-Laminated timber is not easily used in Pres-Lam frames.
It is 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. 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. High strength steel is desirable in order to reduce the size of post-tensioning elements and also to ensure they remain elastic even under extreme loading such as an earthquake. When flexible 7-wire strands are used, the tendons can draped to reduce section heights in gravity beams. In some cases additional steel devices, viscous dampers (dashpot) or friction dampers may be added increasing connection strength and improving seismic performance.
Image 1 (top):The two-storey STIC test building was subjected to normally devastating lateral load with only minor damage.
Image 2 (right):A three storey Pres-Lam frame building on a shaking table at the University of Basilicata ready to be tested.
Image 3: Pres-Lam wall systems-
The most common frame arrangement consists of horizontal straight tendons passing mid-height through hollow beams and through the supporting columns as shown in fig 1.a. 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 as shown in Image 1.b.
Any Pres-Lam frame system can also be braced with diagonal bracing elements to provide additional lateral strength and stiffness.
Applications
Pres-Lam wall systems
Pres-Lam 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 shown in Figure 1(a), however pairs of coupled walls in the same plane shown in Figure 1(b) have the added advantage that vertical shear forces in the coupling devices between the walls will induce additional axial loads into the walls and provide additional strength.
Core walls around a lift shaft or stairwell are stiffer and potentially stronger than stand alone or coupled shear walls because they create a vertical tube. 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 Figure 1. In all cases the vertical dotted lines show the location of the internal post-tensioning element, which provides the tensile connection to the foundation, in addition to any supplementary reinforcing which is not shown.
Image 3: Pres-Lam wall systems-
The most common frame arrangement consists of horizontal straight tendons passing mid-height through hollow beams and through the supporting columns as shown in fig 1.a. 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 as shown in Image 1.b.
Any Pres-Lam frame system can also be braced with diagonal bracing elements to provide additional lateral strength and stiffness.
Applications
Pres-Lam wall systems
Pres-Lam 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 shown in Figure 1(a), however pairs of coupled walls in the same plane shown in Figure 1(b) have the added advantage that vertical shear forces in the coupling devices between the walls will induce additional axial loads into the walls and provide additional strength.
Core walls around a lift shaft or stairwell are stiffer and potentially stronger than stand alone or coupled shear walls because they create a vertical tube. 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 Figure 1. In all cases the vertical dotted lines show the location of the internal post-tensioning element, which provides the tensile connection to the foundation, in addition to any supplementary reinforcing which is not shown.
Characteristics of Pres-Lam
Design of Pres-Lam systems normally requires a performance based design approach meaning that the structural system is 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.
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 when the floors span between the beam elements.
Under wind loading the lateral movement of the building during a frequent event (1 in 25 years) often provides the governing design criterion. In this case Pres-Lam walls and frames are designed with a large clamping force from the post-tensioning so that gap opening will not occur and the connection will remain as stiff as possible.
All timber buildings perform well under earthquake loading because they are general light weight and do not generate large forces under acceleration. Although the post-tensioning alone creates a strong connection between members, in some cases additional reinforcing elements are added to provide additional moment capacity or to reduce the required post-tensioning force. Under ultimate seismic load these additional reinforcing elements can be designed to dissipate energy which lowers the seismic demand on the building. A Pres-Lam building can be designed with a damage-avoiding structural system by making these additional elements replaceable in the case of a serious earthquake.
Characteristics of Pres-Lam
Design of Pres-Lam systems normally requires a performance based design approach meaning that the structural system is 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.
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 when the floors span between the beam elements.
Under wind loading the lateral movement of the building during a frequent event (1 in 25 years) often provides the governing design criterion. In this case Pres-Lam walls and frames are designed with a large clamping force from the post-tensioning so that gap opening will not occur and the connection will remain as stiff as possible.
All timber buildings perform well under earthquake loading because they are general light weight and do not generate large forces under acceleration. Although the post-tensioning alone creates a strong connection between members, in some cases additional reinforcing elements are added to provide additional moment capacity or to reduce the required post-tensioning force. Under ultimate seismic load these additional reinforcing elements can be designed to dissipate energy which lowers the seismic demand on the building. A Pres-Lam building can be designed with a damage-avoiding structural system by making these additional elements replaceable in the case of a serious earthquake.