Margin of error? No more than an inch.
By Chuck Newcombe
A friend of mine recently bought a large metal barn kit. After reading the instructions for preparing the concrete pads for installation, he became concerned and asked if I knew of laser instruments that might assist in precise location of the mounting studs in the concrete foundation.
As it happened, I did! Check out my article about my tests using a Fluke 424D Laser Distance Meter. After writing the article, I just had to add the 424D to my measurement tool arsenal, so I had one readily available for my friend to use.
My friend Mark's barn would measure 50 feet by 60 feet (15.24 meters by 18.288 meters) when done, but the pieces that would form the support arches required mounting studs in the concrete, about 50 feet apart, with an error of no more than an inch, and they had to be at the same level.
The reason for precision
I asked Mark why the placements had to be so precise. It seemed to me that the structure could just have elongated mounting holes to allow adjustment during installation.
The manufacturer of the building stressed the need for precision to ensure that all of the precut pieces would fit and the skin wouldn't be wrinkled.
To achieve that precision in assembly, the center support arch joints had flange faces that were joined with bolts, leaving no "wiggle room" for adjustment.
How to place the mounting studs precisely
Being somewhat of an experimenter like me, Mark conducted some tests, comparing the measurement repeatability of the 424D Distance Meter to what was achieved by his contractors using a steel tape measure.
Mark first made three measurements of 47 feet 8 inches (14.529 meters) to establish the center post to center post width of the building. He determined his gold-standard reference lines using the 424D.
To make his measurements, he placed the laser meter on a tripod 3 feet (0.9144 meter) above the reference line. He used a plumb bob to locate the front of the meter over the line. A 6-foot (1.8288-meter) level, supported by a tripod, was used vertically on the other side. He referenced the edge of the level to the level bubble and a plumb bob.
|47' 8 "||---|
|47' 7 15/16"||2/32"|
He challenged his contractors to tell him how far apart the lines were. They used a steel tape, a two-man method, and measured 3 feet off the ground with a 6-foot vertical level on each side.
They did pretty well for repeatability, showing only ½ inch (1.27 centimeter) difference between their extreme measurements, but the difference between their result and Mark's was about 1 1/2 inches (3.81 centimeters).
The contractor's solution, when holes in a mounting pad didn't line up exactly with the stud, was to have a welder present to burn the mounting holes where needed while the arch was suspended on the construction crane hook.
Note in this picture that no such adjustments were needed when using the 424D to achieve precise stud location.
Diagonal measurement to assure "square-ness" of the building
There was another issue—keeping the building square. Again, the contractor used a two-man technique with a steel tape and two 6-foot levels.
The building dimension is 50 feet by 60 feet (15.24 meters by 18.288 meters). The calculated diagonal should be 78 feet 1/8 inch (23.82 meters). He measured both diagonals after first determining the reference corners using a professional-grade transit.
|78' 1 3/16"||1 1/16"|
|47' 7 15/16"||1 1/2"|
|47' 7,7/8"||1 5/8"|
|47' 7 15/16"||3/4"|
|47' 7,7/8"||1 1/16"|
Contractor errors on the first diagonal were similar to those noted during the stud placement tests, but note that for the second diagonal, their best measurement was equal to the worst reading on the 424, and they still had one error over one inch.
Q: Can you assemble a steel barn without the use of a precision laser measuring tool?
A: Yes, of course. It's been done for years.
Q: Can you make the process more efficient and accurate using a Fluke 424D Laser Distance Meter?
A: The answer is "yes."
First, one person, using an array of tripods and targets, can perform all of the measurements necessary.
Second, you can dispense with the need for a welder to be on-site when your crane operator is placing the support arches, saving both time and manpower.
As you have probably decided by now, my friend Mark is somewhat of a detail-oriented guy. I'm sure he will sleep better knowing that his barn has been correctly assembled.
He was so impressed with the Fluke 424D that he purchased one for his own use. I can hardly wait for his next report on its capabilities.