Good joint detailing at the design stage can help truss builders accurately position truss plates, including the back plate which is essentially positioned “blindly.” Laser projection is excellent for positioning the top face plate, however, it is subject to the same limitations for the back plate and then some. (For more on that subject, see Glenn Traylor’s article, “Should We Follow Lasers Blindly?”)
Visual Plate Placement (VPP) is a technique of joint detailing specifically for the accurate placement of the back plate. With VPP, the designer reviews the computer-generated plate detail and re-positions and/or upsizes the plate so that at least one or two plate edges, and/or corners if needed, provide placement reference points for the back plate. [For all images, See PDF or View in Full Issue.]
Shown in the first image is a 3x4 two-web joint that is fully symmetrical and the plate center matches the standardized joint center as depicted with the black dot. However, the back plate will have to be placed “blindly” by feel and will have a risk of angulation. The designer may want to upsize the plate to a 4x4 and position the plate to the edge of the green web’s vertical scarf, which can be spotted visually for accurate back plate placement. (For more on this issue, see Glenn Traylor’s article, “How Important is Angulation? Why is It important?”)
Shown next is a good example of VPP. The back plate has an excellent chance of being placed accurately because three edges of the back plate can be visually positioned.
The next pair of images shows a 5x7 plate on a two-web joint that is fully symmetrical and the back plate can be positioned accurately. However, as shown in the image on the right, the software should be overridden and the plate re-positioned so the placement has better accuracy.
Good joint detailing enables Visual Plate Placement at the time of fabrication and quick QC visual verification during inspection by standardizing joint details with “rules of thumb” for VPP in the design office coupled with continuous education and continuous training of the truss assemblers. Many joints can be standardized with charts and rules of thumb. Training is further simplified if TVs are placed above the truss build tables for training. During fabrication, the truss builders only have to glance up at the screen to see how the plates should be positioned.
What’s more, proper use of a claw hammer will free up two hands to accurately place the back plate at the table:
Step One: Pry up the truss chord
Step Two: Rotate the hammer grip down to the table surface
Step Three: Release the hammer
Step Four: Use two hands to accurately position the back plate to match the position of the top plate
Unfortunately, I see the majority of truss assemblers stop at Step One and place the back plate with just the one hand. But, as shown in the photos, it’s easy enough to let go of the hammer and then proceed with the positioning! (For additional reference on this and other avoidable common mistakes, see Glenn Traylor’s “How Do Your Manufacturing Errors Rank on the List?”)
By implementing good joint detailing and visual plate placement, truss plants can improve production accuracy, reduce errors, and streamline the quality control process.
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