Developing a strategy for long span truss installation

– By Kirk Grundahl, P.E. –

Kirk Grundahl, P.E.

Kirk Grundahl, P.E.

A truss collapse can be devastating, not only to the immediate health of those on the job site, but to the installers’ confidence going forward. Training a crew in proper bracing techniques in a way that gets them past the negative experience requires more than a little thinking outside the box.

BBL Buildings & Components Ltd., in Perryville, Missouri
It is said a picture is worth 1,000 words. The following image of long span trusses communicate volumes about the importance of diagonal bracing in the context of lateral restraint (i.e., top chord purlins). The 80 foot trusses for this 80 feet x 130 feet x 18 feet agricultural storage building were manufactured by BBL Buildings & Components, Ltd. (BBL) in Perryville, Missouri, and installed by a local contractor. As the photo shows, the contractor didn’t follow the Building Component Safety Information (BCSI) installation documents for long span trusses provided by BBL with the delivery.

BEFORE Caption 1: The trusses for this agricultural building, which weren’t installed, braced and restrained per BCSI recommendations, bowed out of plumb by three to four feet.

BEFORE
Caption 1: The trusses for this agricultural building, which weren’t installed, braced and restrained per BCSI recommendations, bowed out of plumb by three to four feet.

Soon after installation, the trusses started deflecting and BBL was called to the job site to inspect them. “We went out to investigate and found the trusses weren’t braced properly and were bowing out of plumb by three to four feet. The entire roof system was at the point of collapsing,” said Dale Schemel, General Manager at BBL. “The owner asked what could be done to straighten the trusses, but our recommendation was to take them down immediately.”

Fortunately, no one was hurt in this particular case. After the trusses were taken down, a thorough inspection of the chords, webs and plates on each truss was performed and it was determined they could be used as 2-ply trusses attached with screw fasteners. BBL redesigned the components to alternate every 2-ply truss followed by a new single-ply truss spaced 4 feet-0 inches o.c.

AFTER Caption 2: The original trusses were converted into 2-ply trusses; each 2-ply truss was followed by a new single-ply truss spaced 4’-0” o.c., with proper bracing and restraint.

AFTER
Caption 2: The original trusses were converted into 2-ply trusses; each 2-ply truss was followed by a new single-ply truss spaced 4’-0” o.c., with proper bracing and restraint.

2004 Seneca, Wisconsin Experience
Installing 80-foot clearspan roof trusses for a 108 foot long salt storage building can be a challenging task under normal circumstances. It is even more challenging when the walls are 30 feet high and the installers have already had the trusses fall over twice due to lack of proper bracing. With their first attempt, the installers got 17 trusses up before they fell. In their second attempt, they succeeded in installing 34 trusses before they fell, which was better, but still far from the end goal. Part of the problem was they only used metal spacers at 20 foot on-center as the top chord bracing.

During their second attempt, the installer used lumber stiffeners to keep the trusses straight during the hoisting process, which also stiffened the trusses enough to get 34 up prior to the top chords buckling. The severe lack of adequate top chord bracing over the 34 trusses illustrates how truss stability is not really well understood. Most experts would have assumed these trusses should have buckled long before they actually did.

On the third attempt, I was asked to oversee the truss installation at the job site and help develop a strategy to ensure the trusses were effectively and safely installed. It was a perfect opportunity to implement the BCSI installation concepts developed by the Structural Building Component Association’s (SBCA) component manufacturer members.

In three days, 55 trusses were successfully erected and the fear of installing long span trusses was transformed into a positive sense of accomplishment for the installers.

Ultimately, the performance of structural components succeed or fail because of the work of the individuals who install them. SBCA manufacturer members believe the most effective way to be the hero more often than the goat (we all hate being the goat) is to give an SBCA Jobsite Package to the general contractor (GC) and framing crew on every job.

Generally when working with a GC or inexperienced framing crew for the first time, SBCA members visit with them ahead of delivery, follow the Information for Framers enclosed with the SBCA Jobsite Package and walk them through these SBCA publications:

  • B1 – Guide for Handling, Installing, Restraint & Bracing of Trusses
  • B2 – Truss Installation & Temporary Restraint/Bracing
  • B3 – Web Member Permanent Bracing/Web Reinforcement
  • B4 – Construction Loading
  • B11 – Fall Protection & Trusses

SBCA members typically emphasize key best practices contained in those Summary Sheets that might be particularly important, given the components that will be used on that particular job, such as bracing details for long span truss installation as described above.

In the end, the Seneca truss installation project became a success story both for the installer and for the value of BCSI. In addition, the component manufacturer avoided being the goat!

Kirk Grundahl, P.E., is Founder/President of Qualtim Inc. in Madison, Wisconsin. With almost 40 years of experience in the structural building components industry, he has served the Structural Building Components Association (SBCA) as Executive Director since 1992. Grundahl is also called on regularly to provide forensic engineering and expert witnessing services.

Learn more about proper truss installation from Kirk Grundahl at the Frame Building Expo in Indianapolis March 8-11, 2016. Register at www.nfba.org.

 

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