We are going through (on-going) verification of grid location and plumb of columns (steel). One of my bosses asked whether he could weld on targets near the top and bottom to facilitate the process (meaning speed up or make it cheaper). On the face of it, it sounds like a fine idea, but it raises a number of issues such as aim point, angle of prism, locating prism precisely, not having the target or prism damaged or knocked off during transport and erection, etc etc etc.
We are NOT using a scanner, and we will NOT be using a laser scanner. (That is a different discussion).
Curious if anyone has made targets or prisms on columns work, and if so, how did you do so?
I suggest target stickers and reflectorless measurements. If precision beyond that is required the work should be turned over to a metrologist.
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My issue is that at fairly shallow angles, the return from a target sticker seems to be suspect (at least from the published specs).
But, I would still give it a try for comparison, I just need a way to keep them in place during construction.?ÿ
I find that the angle has to be very sharp (well past 45°) before it becomes a problem
I could see the targets helping for plumbness verification, if you are able to place them precisely before installation - if placed top and bottom at the centerline of the beam, you could set up anywhere with good visibility, even without orienting to project control, and check the relative horizontal position of the top target vs. the bottom one. Do that on two different faces of the steel and that's a great check.
We use stick on targets for monitoring soldier pile walls, and like Norman, we have found that they will give a reliable return even past 45 degrees. It can help to (carefully) scribe a mark in the steel before placing the target over it, so that you can replace the target if it comes off for whatever reason. Of course, it also helps to set up in roughly the same spot each session, which you may not have the luxury of doing.
The main advantage to those targets is that they are cheap(er)...we will use the 45-degree ones for control points (again, usually for resecting for monitoring projects) but rarely for the actual monitoring points.
I would be curious if you are able to make this work - at the very least a test on a beam or two.
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Lots of good stuff here, we usually fab our own weldable bases for the M8 adapter, or drill and tap directly to iron. The magnetic bases are really handy, if the application is right. I do not like the epoxy approach, a positive mechanical connection is much better.?ÿ
For pointing with the instrument I almost always use ATR unless really close or special conditions like fog.?ÿ
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Welding on steel structural members could lead to his liability for structural failure. Welding structural steel requires advanced skill and follow up testing, I suggest he not do it. I began my career in civil engineering doing construction inspection and testing. I then worked in building structural maintenance doing inspection and repair design work. Poorly done welding can cause subsequent cracking in structural steel. Cracking is the worst form of structural steel failure and fatal catastrophes. Xray testing of steel welds is much more expensive than medical xrays.
Paul in PA, PE, PLS
Reflector less is accurate enough to plumb a column. A 20 ft column would be allowed to be out of plumb by 0.04' or 1/2 inch. That is from bottom of the column to the top.
We have been checking walls for plumb in earthquake damaged houses.
The easiest solution we have found is to place a crossbeam laser at the base of the wall. Use the horizontal line on the floor to align with the wall,?ÿ and measure from the vertical beam to the wall at various heights with a steel rule.
The absolute numbers don't matter, its the differences as you go up.
It is not hypercritical, but you do want to be careful to make each measurement reasonably vertical above the others.
Our results check well against measuring with a total station (S6 +- 3mm), and we believe are good to around a millimetre.
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I've done it with stick-on targets placed centered on one face of the column , top and bottom and using the reflectorlees (narrow) beam to measure to them. Always measure in 2 faces to compensate for any beam missalignment with the crosshair, especially at steep angles. You can also go about it by just scribing 2 center marks top and bottom on the same face of the column and using the reflectorless measurement. It is very accurate. The iron workers I've seen,?ÿ use just an old , beat up?ÿ 4' level to plumb columns up to 40' high and the same goes for the inspector.?ÿ
As Tim says a 20' column can be out of plumb 1/2 inch. Why not buy a Bazooka Plumb Bob, measure two sides of the column and be done with it?
I personally do not weld. It would be done correctly.
We have multiple tiers of columns, with splices and specs for deviation from plumb for the section as well as overall and specs regarding location of the center of column in absolute terms from the base center and from grid.
Plumb for one section is one thing, but that is not the whole story.
Not familiar with this type of work; will a 20 ft column flex 0.04' or 1/2"?
A lot of my work has been structural. I agree with Norman 100%. You have to go a fair way past 45° (or be a long way from it) before you start running into noticeable errors.
Furthermore, if you want to delve into the world of fixing prisms to columns, then you need to consider whether any of your potential prism centering / axial errors will be less than the tape target / reflectorless method. My experience says no. Fixing something as an offset to the surface you which to measure seems counter-intuitive if you have reflectorless access to the surface itself.
Also don't forget that steel columns move. A lot. They are made from low grade steel, are susceptible to expansion and almost always have external stress introduced during the installation. If someone wants you to chase every millimetre then you might want to have a stern word with them.
