Bridge Deformation Study
When performing a bridge deformation study with a total station, what should be the minimum precision of the total station to accurately report movement to the nearest 0.001’? This is for a wood bridge with a short span. It’s for a two-lane road, which crosses over a small river.
Can't be done. Ridiculous Spec if that is the accracy requested
There is a major league difference between reporting to the nearest 0.001' and reporting accuracies to the nearest 0.001', and from what I see a majority of clients (and surveyors, which is admittedly pathetic...) don't understand the difference. I've seen a lot of clients say something along the lines of well XYZ Company says they can detect movement within 0.01' or 0.001'! In reality, they're usually just reporting decimal precision to 0.01' or 0.001'... The actual accuracies achieved are usually vastly different.
I agree with John, though. Ridiculous spec.
I agree with John. The bridge will move a lot more than that while my Labrador Retriever walks across it. Or when the sun shines on it. But for any monitoring job a higher spec'd gun is always better. I reiterate my comment in your other thread about tribrachs, tripods, etc.
I've done many monitoring jobs with a 3" gun and, even at that level, have been sufficiently convinced that structures move by a hundreth, or much more, in the normal course of a day. Bridges in particular are often quite flexible and move enough by the passage of vehicles to be visible to the naked eye.
A wooden bridge will bounce around as you stand on it with cars crossing.
I believe you're a public employee, so there's not much liability for your survey department to publish numbers without any meaning. However, a private firm should never agree specs like that. It's better to simply walk away from those jobs if they won't tone down the requirements.
Any one asking for that accuracy or promising that accuracy has no business being involved in the bridge project.
@ john-hamilton
Thanks for the reply. Reading all the replies makes me want to talk with whoever is in charge of the project.
John is spot on. Unfortunately, every monitoring spec I see now days is for 0.001'. Every time I see it, I have to educate the engineers that you might be able to those results in a climate-controlled environment but even that is a big 'might'.
@ Steinhoff
Thanks for pointing out the difference between reporting a number and its actual accuracy or should that be precision? Do precision requirements for this kind of job call for 1 sigma, 2 sigma, 3 sigma, etc. levels?
You can report 50 sigmas but that doesn't mean the instrument can actually measure that. I think that's what the guys are getting at, you gotta tell the client our instruments aren't good enough for what they're asking.
