Wood+Truss+Failures

=Metal Plated Wood Truss Failure Causes= toc //Tim Emmick, M. Eng. Architectural Engineering, Penn State 2012//

Introduction
Metal-plate-connected wood roof trusses are common components in the construction of light-framed building structures. They are most commonly used in short span (<50') residential structures but are also used in long span (up to 80') commercial applications. The precise manufacturing and pre-engineered aspects of wood trusses allow for quick and easy erection upon their arrival on site. With this ease of assembly comes a variety of important guidelines and instructions for the installer to follow. The failure to follow or observe these guidelines has led to the collapse of many wood roof truss systems. Truss failures are often attributed to improper or lack of temporary and permanent bracing, incorrect loading or overloading during construction, high winds during erection, and weak members or bad joint connections. Many of these problems grow out of the difference between how trusses actually work and how the builders using them understand them (Morse-Fortier, 2006, pg. 569). This paper will discuss a variety of these failures, pinpoint the trigger mechanism, and discuss possible ways to avoid roof truss collapses or failures in the future.

**Keywords**
Wood Roof Truss, Metal Plate Connected Trusses, Collapse, Failure, Bracing, Truss Erection

The erection and installation of wood roof trusses should be done by experienced carpenters and safety should be the first priority. When a truss is lifted and placed, toenails should be used to secure the truss to the top of the wall. Spacing blocks are usually cut to 25 1/2" (for 24" on center) and placed on the top chord either directly above or below the block securing the adjacent truss. Proprietary metal bracing products are also available (BCSI, 2003) The truss needs to be checked to verify it is plumb as slightly racked trusses are highly prone to bending, buckling, and collapse. Once all the trusses are in set in place and have sufficient temporary bracing, the permanent bracing needs to be installed. If there is a gable end truss, it needs to be braced at an angle securing it to the top of an interior wall or partition. This provides lateral stability at the ends of the assembly. The interior trusses need to be braced to the end gable through the open webs as required in the stamped engineer truss drawings. It is important to note that truss engineers design only the roof trusses while the structural engineer of record is responsible for the building structure and is a seperate entity. Typically the truss engineer needs to send the truss drawings to the engineer of record for approval. Once the permanent bracing is installed, the fascia boards and roof clips (hurricane clips) are installed followed by the roof sheathing. The roof sheathing braces the top chord and the roof clips and fascia act as bracing at the ends of the truss. Drywall is used to brace the bottom chord in structures that requires an interior finish but in unfinished buildings such as a garage or warehouse, 2"x4"s are laid across the top of the bottom chord to provide bracing.
 * Typical Installation**

Storage, Handling, and Erection Errors
The improper handling of wood roof trusses prior to erection can create opportunities for failure before the truss is even placed. Upon delivery, trusses should be stored in a dry, flat location, close to the structure to limit movement from storage to erection. When trusses are not stored on flat surface as seen in Figure 1, it creates bending stresses within the lumber and in the joints and can cause the metal plates to "pop" or pull out at the joint connections. Truss packages should stay bundled together to avoid any possible sliding, tipping, or damage prior to erection. Case 3 in "Common Causes of Collapse of Metal-Plate-Connected Wood Roof Trusses" by Dr. Harvey Kagan, a worker was injured in a roof truss collapse. The worker claims the site consisted of rocky and irregular terrain and that several trusses, including the collapsed truss were damaged during the unloading. There was no evidence to support this allegation however pictures of another truss bundle showed one truss with a broken lower chord. When erecting the truss by hand, special care should be taken when rotating the truss into position using fork-like lifting poles supporting the truss at 1/4 points to avoid lateral strain (TPI HET-80, pg. 2). When erecting trusses using machinery such as a forklift or crane, the truss should never be lifted at a single point. A spreader bar allows the truss to be lifted into place while supporting the truss from 2 points as seen in Figure 2 and reducing stress at the panel points.

Temporary Bracing
Temporary bracing is required to hold wood trusses true to line and dimensions, plumb, and in a stable condition until permanent truss bracing and other permanent components necessary for the overall stability of the structure are completed (Silvester and Russo, 2009). Figure 3 shows a common temporary bracing diagram with lateral bracing and diagonal bracing to help stabilize the structure during construction. As shown in Figure 4, ground bracing is typically alligned with top chord lateral braces or at truss panel points and can be attached to the earth, floor, foundation, or slab and should be of no less than 2"x4", #2 grade marked lumber or specified by the truss engineer (TPI HIB-91, 1991). ======

Permanent Bracing
Permanent bracing is required after the erection of the trusses is complete. Bracing such as the roof sheathing, fascia board, connection to the top plate, and ceiling drywall all function as a form of permanent bracing along with their original intended use. While this fully braces the entire perimeter or diaphragm of the truss, the internal webs and members may require additional lateral bracing. This is typically accomplished by installing long 2"x4" members to the internal webs as required by the truss engineer and is shown on the truss profile shop drawings and represented by the cross section of a 2”x4”on a member. Lateral bracing must lap at least two trusses at a bracing break (at the ends of the 2"x4"s) to maintain continuity within the bracing. The most common failure type for trusses missing permanent bracing is compression members buckling due to lack of lateral stability (Morse-Fortier, 2006, pg. 573).

Overloading During Construction
This can occur when placing heavy concentrated loads on the trusses during construction. Some possible sources of these loads are stacks of plywood/OSB for the roof sheathing, gypsum wall board, roof gravel or ballast, HVAC equipment, and roofing shingles. No construction load should ever be placed on trusses without proper bracing and when placed, they should be broken down into smaller units and placed over panel points or main supporting members (TPI HIB-91, 1991).

Material Failures[[image:failures/TJE_SHATIS_Anthony&Drerup.jpg width="369" height="268" align="right" caption="Figure 5: Disengaged Metal Plate- Photo Credit: Anthony and Drerup"]]
Wood is a natural material which has inherent flaws and weaknesses. Typical wood truss construction uses #2 grade lumber or better for the top and bottom chords as well as the internal webs. Wood's strength is reduced by high moisture content and increased temperature, often seen in the attic space of a structure. Truss engineers account for this in their design but there are circumstances beyond this which are not typically considered. Exposure to prolonged wetting from rain during construction can reduce the load carrying capacity at the connection by 40%. After drying, the joint connections still have a strength loss of about 10% and it's stiffness loss can range from 12%-37% (Mtenga et al, 2011 pg. 1). Storing trusses off the ground and covering during inclement weather can reduce these losses in strength and stiffness. Various chemical treatments for fire ratings or pressure treatments for insects and decay will also alter the properties of wood. Mike Drerup of Walter P. Moore, presented a case study at Penn State where fire rated wood, based off an older chemical formulation, was used in an attic truss. Per the calculations, the original design (scissor truss) was sufficient however the fire-retardant treatment (FRT) used to obtain the fire rating, slowly caused the wood to degrade and wood members disengaged with the metal plates as scene in Figure 5. At the time of the failure, it was found the FRT had reduced the strength of the wood by about 50% and was determined the strength loss is progressive over time, rather than a simple reduction factor at the time of manufacturing (Anthony and Drerup, 2011, pg. 1).

Manufacturing Errors
While this is one of the less common occurrences, it does happen for a variety of reasons. During the manufacturing process, pre-cut members are laid out in a jig and the metal plates are placed at the connections by the workers and then pressed into the wood by either a roller or press. Failure opportunities arise if the plate is not fully embedded into the wood, an incorrect or undersized plate is used, a plate is misplaced, or if the joint is missing a plate altogether. It is also important to verify the species and grade of lumber match the truss design drawings. In all of these cases, the strength and stiffness of the joint is not constructed as designed and under certain loading conditions will fail. Thorough field inspections can help identify these issues and the truss engineer can design a field repair to remedy the issue.

Weather
While all the sections above create a failure opportunity, it's usually weather which compounds the the deficiency and causes the failure. Weather acts as the trigger mechanism to cause the overall failure. High winds create strong lateral forces which test the temporary bracing during construction and permanent bracing of the completed structure. In most of the case studies of metal plate roof truss failures, it was improper or lack of temporary bracing combined with high winds which caused the failure. As seen in Figure 6 and described in Silvester and Russo's "Investigation of Collapse of Wood-Framed Warehouse During Construction", it was determined the erected truss system lacked adequate bracing to provide resistance to wind speeds well within typical wind conditions (25-35mph) while the building was under construction. The day of the collapse, there were recorded winds within this range and the roof structure collapsed. Heavy snow accummulations and drifting snow are also a common trigger mechanism to wood truss failures. In the case study performed by Harvey Kagan in "Common Causes of Collapse of Metal-Plate-Connected Wood Roof Trusses", the roof of a 6 year old shopping center collapsed during a (ground measured) snowfall of 25". Upon Dr. Kagan's field inspection, it was observed the section of the roof which collapsed, as well as the sections which were still standing, had no lateral bracing on the internal web members. It was determined that these unbraced compression members exceeded the allowable length to depth ratio for in plane compression without the required lateral bracing, thus causing the members to buckle and fail. Figure 7 shows these bent web members without the bracing on the roof section still standing 6 days after the collapse. The roof was still covered with about 12" of wet snow when this picture was taken.



Conclusions
Metal plated roof trusses, when handled, erected, and installed properly have proven their value within the building industry. However, their value is sometime questioned when failures or collapses occur. In most of the case studies read for and/or included in this document, the installation was not per the Truss Plate Institute guidelines or as designed by the roof truss engineer. Many roof structures failed during the construction process while others took years for an incident occur. Some suggestions to help mitigate further issues include contract language to include the truss engineer's involvement in truss installation (Morse-Fortier, 2006, pg. 569), proper training or certification for carpenters or roof truss installers to comply with the guidelines described in BCSI-03, or annual roof inspections of existing structures to check for issues.

Additional References
Gupta, Rakesh, Miller, Thomas H., Dung, Da-Ren (2004). //“Practical Solution to Wood Truss Assembly Design Problems.”//Practical Periodical on Structural Design and Construction, 9(1), 54-60. <[]>
 * This article analyzes the behavioral issues of joints and members in complex roof assemblies.

Hussein, R. (1999). //“Parametric Investigation of the Buckling Performance of Metal-Plate-Connected Joints.”//Advances in Engineering Software, 45-56. <[]>
 * This article investigates the instability problems of metal plate connected joints and the governing factors into buckling failures.

Truss Plate Institute (TPI) (1976). //BWT-76: Bracing Wood Trusses: Commentary and Recommendations//. Truss Plate Institute (TPI), Madison, WI. <[]>
 * This manual provides industry guidelines and details for bracing of wood trusses.

Truss Plate Institute (TPI) (1991). HIB-91//: Handling, Installing, & Bracing Wood Trusses: Commentary and Recommendations//. Truss Plate Institute (TPI), Madison, WI. <[]>
 * This manual provides industry guidelines for handling, installing, and bracing metal plate connected wood trusses.

Underwood, Catherine R., Woeste, Frank E., Dolan, J. Daniel, Holzer, Siegfried M. (2001). //“Permanent Bracing Design for MPC Wood Roof Truss Webs and Chords.”// Forest Products Journal, 51(7/8), 73-81.
 * This article discusses the proper placement of permanent bracing required for stabilization of specific members of each truss.