When GPS systems first became available for personal cars, we started hearing stories about folks turning left onto a railroad track or following a cow trail through the valley to a dead end. In 2012, three Japanese tourists in Australia drove into the Pacific Ocean after their GPS told them to. The students were trying to reach North Stradbroke Island, but the GPS neglected to mention the 9 miles of water between it and the mainland.
So, the device manufacturers analyzed the situation and made corrections, created disclaimers, and improved accuracy and performance. Folks learned to translate the device. Although we still hear a story or two on occasion, the flurry of humorous tales has diminished.
Like GPS technology has done for navigation, lasers have revolutionized the truss industry when it comes to “exact” placements. Lasers are one of the best investments a component manufacturer (CM) can make. But like our humorous stories about GPS, the same could be said about lasers used in truss manufacturing. “Operator error” will continue to be a problem, even when the laser has determined what is “correct.”
Therefore, when using lasers, there are a few items that you might not have considered but should.
One issue involves lumber dimensions. In the following illustration, lumber is designed to be located on the black lines. Due to the thickness of the material or the accuracy of the cut lumber, however, the webs shift, as shown in red. [For image, See PDF or View in Full Issue.]
The illustration is somewhat exaggerated, but in essence this is what is happening as the truss builder attempts to remove gaps in the joints. When this happens, specific locations assumed by the laser are no longer accurate.
Issues that can occur
- Chord widths are often greater than or less than the exact dimensions used in calculating the laser projection, resulting in a data error (see preceding illustration).
- Web members are very often cut short or long. Inaccuracy of 1/16 inch will add up, especially in floors.
- Long scarf cuts on component saws vs. linear saws are particularly susceptible to error.
- Parallax error can occur if the height of the projected item is not calibrated or per the assumption of the laser system. (Parallax is when the light of the laser looks like it’s at a different location compared to a dead vertical projection. This is caused by the angle of the laser and its distance from the center of the laser.) The farther away, the more the error.
- Sometimes CMs spread their laser heads too far apart to be effective.
- Locations projected onto a connector that has not be installed, sitting on the top of the lumber, will be different after the connector has been pushed into place.
- The unevenness of the gantry table will cause inaccuracies.
Results that can cause non-compliance
- In the situation where chord splices are designed on panel, this means that the original plate intended for the joint and splice no longer syncs up correctly.
- Connector plates are mis-located.
- Splices are not covered properly resulting in non-conforming tooth count issues.
- Builders “adjust” or introduce gaps between members. This should be avoided and can create deflection issues as the building is placed into service. [Note: I see this sometimes as component manufacturers build floor trusses attempting to hold the duct chase to a certain dimension; for reference, see my August article, Why is the Gaps Limitation So Important to Truss Fabrication?]
Action to be taken
- Truss drawings should be followed and reviewed with the laser prompts.
- Laser systems should be regularly calibrated.
- Tables should be leveled regularly. Even a steel top table needs to be leveled every 6 months.
- Design settings should be adjusted to increase fabrication tolerance to accommodate plate placement.
- Plate placement should be reviewed using the plate placement method (PPM), tooth count method (TCM), or digital QC (DM).
The Bottom Line
Laser information should be interpreted rather than blindly followed. As simple as lasers make plate placement, training must be conducted with truss builders, and results must be measured with a good quality assurance program.
An ANSI/TPI 1 3rd Party Quality Assurance Authorized Agent covering the Southeastern United States, Glenn Traylor is an independent consultant with almost four decades of experience in the structural building components industry. Glenn serves as a trainer-evaluator-auditor covering sales, design, PM, QA, customer service, and production elements of the truss industry. He also provides project management specifically pertaining to structural building components, including on-site inspections and ANSI/TPI 1 compliance assessments. Glenn provides new plant and retrofit designs, equipment evaluations, ROI, capacity analysis, and CPM analysis.