Design Guidance for Effective Connections

Connections obviously need to provide the structural strength necessary to transfer loads. As discussed above, well-designed connections also mitigate potential adverse effects of moisture ingress. Finally, well-designed connections minimize the potential for excessive tension perpendicular to grain stresses—under design conditions as well as those related to unusual loading conditions. Chapter 10 of the ASD/LRFD Manual provides details for a variety of design configurations (e.g., beam-to-concrete or masonry wall connections and beam-to-beam connections). Many begin with a brief discussion of the pertinent design challenges. For example:

Beam-to-Concrete or Masonry Wall Connections

Design concept: Concrete is porous and “wicks” moisture. Good detailing never permits wood to be in direct contact with concrete.

A. Beam on shelf in wall. The bearing plate distributes the load and keeps the beam from direct contact with the concrete. Steel angles provide uplift resistance and can also provide some lateral resistance. The end of the beam should not be in direct contact with the concrete wall.

B. Similar to detail A with a steel bearing plate only under the beam.

C. Similar to detail A with slotted holes to accommodate slight lateral movement of the beam under load. This detail is more commonly used when the beam is sloped as opposed to flat.

Five Stories and Higher

With the exception of height, the outward appearance of mid-rise wood-frame buildings has not changed a lot over the years; however, the quality and precision of design and construction has increased considerably. Connection-related design aspects that are more critical in a five- or six-story wood-frame building include accommodating greater dead, live, wind and seismic loads, which increase with the additional height.

Going from four to six floors, for example, increases the gravity loads by 50% but lateral forces due to wind or seismic forces may actually double.

While the size and capacity of connections might change and designers may use more specialized, high-capacity connectors, the connectors themselves are largely the same as they are for any light wood-frame building—nails, bolts and lag screws used with and without proprietary connectors.

Heavy Timber Construction

Heavy timber construction utilizes many of the standard connections described above, including bolted connections, timber rivets, split rings and shear plates. Innovative proprietary connections are also widely used, and today's designers have a tremendous range of options. Innovative examples include, among many others, grid plates, concealed kerf plates with self-drilling dowels and “lagscrewbolts,” which offer the bearing strength of a bolt and a withdrawal strength greater than that of shorter lag screws.

Because heavy timber buildings lend themselves to innovative design, many require custom connections to arrive at the sensitive balance between safety, efficiency, serviceability and aesthetics.

Traditional heavy timber details are available in the publications, Heavy Timber Construction Details by AWC and Glulam Construction Details (Form T300) by APA – The Engineered Wood Association. These details are consistent with the construction requirements outlined in IBC section 2304.10. They are representative of those you would find in existing historic buildings, and are useful when retrofitting or restoring historic structures.

 

The reThink Wood initiative is a coalition of interests representing North America’s wood products industry and related stakeholders. The coalition shares a passion for wood products and the forests they come from. Innovative new technologies and building systems have enabled longer wood spans, taller walls and higher buildings, and continue to expand the possibilities for wood use in construction. www.rethinkwood.com