Roaring Brook Consultants, Inc.

Engineering a Better Future

CURRENT NEWS AND TIMELY TIPS

Engineered Wood Products

Read Part 1 "An Introduction to Engineered Wood"

More about engineered wood products -
Prefabricated Metal Plate Connected Wooden Trusses

In woodtip1, we presented some information about engineered wood products used for simple structural members such as beams and joists. We also mentioned the fact that one of the most common types of pre-engineered wood product is the prefabricated metal plate connected wooden truss. This product is used typically for the construction of floors and roofs in buildings. It is distinguished by the use of graded, dimensioned lumber members that are connected by metal plates with integral fasteners. These plates contain many small "teeth" that are pressed into the wood at the factory using special equipment. The trusses are then delivered to the construction site as complete units, ready for installation.

Prefabricated metal plate connected wooden trusses have been used with great success for many years. When they are utilized correctly, the structures produced are efficient, reliable, and economical. However to achieve reliable results when using wood trusses, you must pay careful attention to fulfilling the assumptions upon which the design of these trusses is based. In particular, the trusses must be correctly positioned and braced and must be handled carefully before and during installation to avoid damage. Design loads must be specified carefully so that the manufacturer can prepare an appropriate design prior to fabrication.

Recognized manufacturers of this type of truss follow rigorous practices and belong to industry organizations that develop guidelines and standards for this type of product. If you are uncertain about what you are getting or the process of purchasing prefabricated trusses, ask the manufacturer to demonstrate its experience and to tell you about the industry organizations to which it belongs and the standards that it uses.

Let's start with a brief discussion of the loadings used in design. It isn't sufficient to tell the manufacturer that you want trusses of a certain span and to assume that the manufacturer will be able to prepare a satisfactory design. A complete description of the use of the finished truss will also be necessary, including but not necessarily limited to a description of the final configuration of the building, definition of the expected service loadings or applicable codes, a description of any equipment that is to be mounted on the trusses, and a description of any finishes that are expected to be supported by the trusses. The manufacturer's engineer can then prepare a design or determine if any additional specific information unique to your installation is necessary for preparing a design. As a matter of course, you should require that the manufacturer provide you with a complete set of drawings for the trusses fabricated for your project. You should also require that these drawings be stamped and certified by a registered professional engineer who is licensed in the jurisdiction where the project is located.

Once you are satisfied that an appropriate design has been prepared and your trusses have been fabricated and delivered, it is important for you to follow the manufacturer's instructions for correctly placing and securing the trusses as part of the finished installation. Correct installation of prefabricated wood trusses is not difficult, but it does require some skill and care. It also requires attention to critical details such as proper lifting procedures and proper installation of required bracing members.

Every load of trusses should be delivered with lifting and handling instructions. These instructions contain both written and pictorial descriptions and are easy to follow. Be sure to follow all of the manufacturer's instructions regarding storage and handling of the trusses. Keep them covered, dry, and out of the weather. Store them on a sufficient amount of blocking to keep them off the ground and well supported, so that the individual members and connector plates do not become damaged. These trusses are long, slender members that are easily damaged by sideways bending. They must be carefully stored and carefully lifted to avoid damage. Always use correct slings and lifting equipment as recommended by the manufacturer.

Prefabricated trusses must be braced in position if they are to perform correctly. During installation the trusses must be temporarily braced in position as they are being erected, and all permanent bracing indicated by the manufacturer or your project engineer must be installed and securely attached to the trusses. Prefabricated wood trusses must be joined into a unified structure to give satisfactory performance. Do not leave this to chance. Follow all of the bracing instructions provided with the trusses and all of the instructions shown on any project drawings.