im a student studying Land Surveying in London, i was wondering if any land surveyors could give me the best and most accurate way they would set out the structure of a motorway gantry which is 62m long. The two pillars on each side have to be constructed within a +/- 2mm specification. The task is set out in a way that one side of the motorway will not be available to the surveyor. Which is the best way of actually setting out the correct position for the structures to be built in and be able to line up within 2mm from both ends. The motorway gantry has been pre constructed and will be put onto the motorway in one night. How would a surveyor set the positions up for the bolts? the only thing i got is setting out from a known coordinate on a local grid. What is a professionals view on this?
thank you for your time.
I'm assuming that this is a theoretical exercise, but a few more facts are needed to give a complete answer. I've had a number of similar jobs, including dropping bridges onto piled trestles in strong tidal waters (tide range 40 foot).
If the surveyor can't get to the far side base to set the bolts then that begs the question of how the base will be constructed. Obviously somebody has to get there to put the foundations in, even if daytime access for survey is not possible.
If that's the case then two suitable targets need to be fixed by the night gang, at opposite ends of the base, so that they can be observed by the surveyor from the accessible point. you can then provide a drawings based on these observed positions to enable the bolts to be placed correctly, BUT....
... on the construction side:
Bolts will presumably have baseplate anchors and will be sleeved so that there will be some freedom of movement once they are cast in. However, the thickness of the gantry baseplate and the size of the bolt hole will limit the amount of movement available, so it is essential that the bolts are vertical when cast in. They should be jigged at the top into a replica of the gantry baseplate and the base anchors should be tack welded to rebar so that they remain correctly spaced at the bottom.
Things tend to move when concrete gets poured and with only a couple of millimetres tolerance the bolts really need to be surveyed as soon as the concrete is in, while there is still a chance to move them slightly.
... on the surveying side:
Before you do anything you need to ensure that any EDM you are using is checked ON SITE to determine the instrument-prism constant. Do this by setting three tripods in line and measuring AB, BC and AC in each direction, USING THE SAME PRISM, which will then be used on site for all future measurements with that instrument.
To find the prism constant (e) AB+e+BC+e=AC+e, hence AC-AB-BC=e. Either set this in the instrument or apply it to all readings. Then you know the distances measured are as good as the instrument can provide. Now compare your "true" target with any stick-on targets you might be using, so that you know if there is any correction to apply to those.
Then check all your tribrachs to make certain the plumb is true. The check all the tripod legs to make sure everything is tight and no slack anywhere.
With no normal access you need to adopt a non-contact method for any pre-checks. Don't rely on EDM non-contact for accuracy, do it by intersection.
You probably need to set up a baseline on the accessible side, at least 50 metres long, so you can intersect the bolt group (note that if the bolts are fully jigged you just need two reference points on that jig). Ideally you need two instruments so that they can be set at each end of the line - that means you don't have to keep taking the instrument down, moving it to the other end and resetting.
Coordinate the bolt positions for the nearside leg in relation to this baseline. Initially, as mentioned earlier, you can coordinate the first two targets to provide the dimensions needed to position the bolts prior to concreting.
Decide which two bolts you will use for the checking when concrete is poured and calculate the bearings to these from the baseline. When the bolts are set but before being concreted, with two instruments on the baseline you can then turn off the required angles to the first bolt - check it from both instruments and then repeat for the second bolt. You can then make any minor adjustments.
Repeat the exercise when the concrete is poured, to make sure the bolts don't move.
Finally, don't panic : in the middle of the night it's amazing what a big hammer and burning gear can do!
From Arkansas to England
Gantry? is that a pre made bridge?
1.) Find the control that was used to DESIGN the thing.
2.) Re run that control, especially in the vicinity of the immediate construction.
3.) IF that control is good and tight, then work off of it, to set up the final bolt positions.
4.) If the control is not so good, check that portion immediatly near the project. Determine if it will hurt the project. This determination is made by good old fashioned red neck good sense.
5.) Lay it out according to plan.
6.) Remember what you are doing. What you are there for.
7.) Take before and after pics.
8.) Post them here.
9.) Take a coffee break.
10.) Get a bonus for saving the project!
(This one calls for a complete calibration of all the tribrachs, and instruments that will be used, BEFORE doing anything else!) Maybe Calibrate afterwords too, because you may have the legal need to PROVE your work, and actions.
N
From Arkansas to England
Nate,
Gantry in UK usage would be a "lightweight" bridge type structure, not usually used for vehicles.
It might be for holding up signage (as in this case), signals, it could be used to describe a crane trolley support, a pipebridge type support or the description could be applied to a pedestrian viewing platform (temporary or otherwise - such as a viewing gantry over the operations area in a nuclear facility).
In all cases it would tend to be long and narrow.
The size given on this one is similar to a standard UK 6 lane motorway signals and signage gantry, which would have two end piers (either concrete or braced steel columns) and two cross braced lattice girders forming the gantry itself.
Bases are normally concrete, standard design - just the piling varied to suit the ground conditions. erection is normally in an overnight road closure, drop the upright piers onto the founds, drop the gantry onto those, bolt everything up and clear site.
Nate
The problem is that these nice fellows speak English, and we don't.
Nate
Um... yeah, what he said!!
