When you write a letter or series of paragraphs, sometimes it’s difficult to proof the very thing you just wrote. Why is that? Because you don’t approach it with a clean-slate perspective but with an expectation of what you think it says. Likewise in our industry, it’s often said that a better way to check a truss takeoff is to have someone else check the design because the on-looker will have a different perspective. Inspecting trusses for TPI compliance poses a similar issue.
Often we read the truss drawing and then look at the plates used. On occasion, we see what we just read rather than what we see. Just like proofreading or reviewing a truss takeoff, the information sticks in our minds. It’s difficult to flush this data and give the review a clear reflection. It’s human nature. Because of this human fallacy, a great way to measure and record as-built information when inspecting trusses is to record data before comparing data. This can be done with all of the collected information.
For example, instead of looking at the drawing, look at the truss first; e.g.,
- Identify the plate size, gauge, particulars, and then record that information at the joint without concentrating on what the shop drawing actually shows.
- Make a measurement of the overall length, then record the dimension prior to reading it off the drawing.
- When verifying the wood grade and size, look at the truss and record the lumber grade mark in the general location of the mark on the shop drawing. Note which side the grade mark was located prior to looking at the lumber requirements.
If you do the inspection in this order, the chances of overlooking an error will be greatly reduced.
After the data is recorded, the inspector can compare the as-built data with the actual data. Above is an example where some of these techniques have been employed. The data is a clear record of what was built and a reviewer can look at the inspection process and determine with the data shown that the inspection was accurate.
By the way, did you look at the sample truss? Did it fail? Knowing why something fails is the first step in correcting the process issue that allowed the error to happen.