Structural Design Considerations

CLT panels are typically used as load-carrying plate elements in structural systems such as walls, floors and roofs. Lumber in the outer layers of CLT panels used as walls are normally oriented up and down, parallel to gravity loads, to maximize the wall's vertical load capacity. Likewise, for floor and roof systems, the outer layers run parallel to the longer span direction.

For floor and roof CLT elements, key characteristics that must be taken into account are the following:

• Out-of-plane bending strength, shear strength, and stiffness
• Short-term and long-term behavior:

- Instantaneous deflection
- Long-term deflection (creep deformation)
- Long-term strength for permanent loading


• Vibration performance of floors
• Compression perpendicular to grain issues (bearing)
• Fire performance
• Sound insulation
• Exposure to elements

For wall elements, key characteristics that must be taken into account at the design stage include:

• Load-bearing capacity (critical criterion)
• Out-of-plane shear and bending strength
• Fire performance
• Sound insulation
• Exposure to elements

The U.S. CLT Handbook discusses these and many other issues such as proposed analytical design methods, research and conclusions about lateral design of CLT buildings, enclosure design, environmental performance, etc. This section on structural considerations will focus on two key areas: connections and floor vibration. The following section will summarize important information about fire resistance.

CLT and the International Building Code

In the next version of the IBC, recently approved changes will streamline the acceptance of CLT buildings. The 2015 IBC will recognize CLT products when they are manufactured according to the product standard. In addition, CLT walls and floors will be permitted in all types of combustible construction, including Type IV buildings. Type IV wall provisions require the exterior side (only) of exterior CLT walls to be protected by fire retardant treated wood sheathing, gypsum sheathing, or a noncombustible material; however, there are other requirements for the exterior wall, floors, roof, etc. Floors are required to be 4-inch (10.16-centimeter) nominal minimum thickness and roofs a minimum of 3-inch (7.16-centimeter) nominal. While this code change will not go into effect officially until the 2015 IBC is adopted by a jurisdiction, the information could be used to simplify an alternative methods argument under current codes. (For more information, see the American Wood Council website at www.awc.org.)

Connections and Construction of CLT Structures

Connections in timber construction, including those built with CLT, play an important role in maintaining the integrity of the timber structure and in providing strength, stiffness, stability and ductility. Consequently, they require thorough attention of the designers.

Traditional and innovative connection systems have been used in CLT assemblies in Europe and North America. Common types of connections in CLT assemblies include: in-plane panel-to-panel (floors, walls and roofs), out-of-plane panel intersections, and panel-to-foundation. Basic panel-to-panel connection can be established through single or double exterior splines made with engineered wood products, single or double interior splines, or half-lapped joints. Metal brackets, hold-downs and plates are used to transfer forces at the wall-to-floor/roof interfaces and in wall-to-wall intersections. Innovative types of connection systems can also be used which lead to enhanced performance or quicker assembly.