So we got roped into doing way more high tech bridge construction than we bargained for. Mixed with low tech for good measure.
Charlie Russell Choo-Chooo. Damaged in 2011 flooding, two towers on eight piers needed fixin'. They will re-use original girders, and new towers are to be weathering steel to quickly take on the rustic look of the remainder of the 2200' long trestle. Mid span is 180' above the Judith River.
Everything has gone semi-smooth through pouring the piers and setting the anchor bolts.
They are currently at 2/3 height on the tower erection. The final 1/3 will be built by end of next week. We need to go back and provide alignment on top of the new towers to place the re-used girders, and to tweak anything if needed before they crack the bolts.
I can't use radial staking methods to meet their 1/8" tolerance (0.01' to me). I need to use optical line of sight directly down line, but I'm afraid of my slop factors in transferring control up to the existing trestle top. That is less of a concern because I will need to remeasure existing girders in place to derive my line. Still, reflectorless at 300' to a rusty chunk of steel is pushing my error budget already.
So back to old school. I want the contractor to run high tensile wire between existing girders and scribe his own marks on new towers. I just don't feel that with modern robotic equipment that I can do any better.
What I'd like to know is if anyone could recommend a long range horizontal & vertical laser system. This would be advantageous so the crane and man basket don't have to continually dodge the piano wire. I could mount a pair on the ends of one girder, then sight to the same offset value on the other girder. Our distance across is 305' or so, and new tower tops should match the slope of existing towers. However I will need to measure that height difference to shim if necessary.
Any ideas, methodology changes, equipment, procedures, special provisions for differential temperatures, silly comments etc. are very much appreciated.
We will as-built the towers as soon as they crack the bolts. I've got an elaborate scheme involving dueling total stations and a flying man in a basket.
BTW, my partner Lenny is really scared of heights. I only got him about 150' out from the end of the trestle one day and he was white as a ghost and had to turn around. I told Lenny to wear his brown pants next Friday...
Criss Cross Control
Ground points on either side of the South bridge end are used to set your column top girder alignment points on the North end and vice versa. This minimizes the slope in your shots.
Paul in PA
I do not mean any condescension or offense, but is this being over thought? I guarantee that the type of tolerances you suggest weren't in place when this was first built. Probably done with a transit and tape.
And I see gravel ballast on top of the steel so this probably absorbs any slop.
I would not make a guy who is uncomfortable working at heights headchain at height.
Beyond that it does not seem like height is really the issue but transferring existing line 305 feet which does not really sound all that daunting to me.
Assemblies that clamp directly to iron and can hold a tribrach can be really handy, assuming that the structure is stable, if the structure is not stable I think that kind of throws the whole eighth of an inch bit out the window.
I have used the PLS style laser and had great results in tight areas but never over large distances. There are the big honking industrial lasers for mining that put out the big diameter beam but I do not know how tight they are.
I think you should try using two rotary lasers, one for line and the other for grade. Before you commit to purchase take em out to the street and have trials to convince yourself of their tolerance. Build a assembly to mount them, presumably with a slot for the hz controlling laser to wiggle in.
After that you will need to come up with a little speech for the contractor about how it takes a surveyor to properly employ a rotary laser.
I have done greater distances in the horizontal but only like 80 feet overhead by simply wiggling in on line and transferring overhead with a 90 degree eye piece.
Thanks guys.
Paul, agree with backing off to lessen the vertical angle. Two problems, first the ground baselines I've set and worked from ever since are only occupiable on the South Pier. North pier points are set on concrete columns in the river, not big enough for a tripod. Second, they are 4' offsets outside pier center. I may not be able to occupy a point transferred directly upward to the ballast/trough. This is how I'll do it next time though, offset inside for ground points so I can occupy a transferred point up top.
Dave, agree entirely. Non surveyors don't understand error tolerance versus error budget. Everyone wants to split hairs over an elephant house. I've made it very clear that some old boys in 1903 built this sob with much simpler tools than we have now. I don't know if the horizontal alignment in this section was disturbed during the flood, or just built that way, but this section is about a foot "upstream" of overall trestle best fit alignment. So yeah, don't tell me they can't live with some slop.
Chefy, don't worry about poor old Lenny. I won't make him get in the man basket as I too will be wearing the brown pants. I will get a cart to get him out the 1400' on rail, some blinders so he can't see over the edge, and a railing put up at his girder end. With two guns we can simultaneously as-built the bolted in place structures. Contractor will provide me with flying man I hope. Some Least Squares magic to our control net and as-built shots, and a report I can rely upon for positional uncertainty (but only 95% sure...) Looking at the PLS laser 1000' variety. Like what I see, but need to try it out in similar conditions. Clamped somehow to the steel and aligned by surveyors for sure.
D**n, Reed! What have you gotten yourself into? :-O
Lesson to be learned here is STAY AWAY from construction layout. Other then that I have no advice for you.
dig...out!
> D**n, Reed! What have you gotten yourself into? :-O
>
> Lesson to be learned here is STAY AWAY from construction layout. Other then that I have no advice for you.
>
> dig...out!
I can't say that I disagree with you at all Mr dig.
Sometimes it's better to punt than try that 4th & 2 play for what seems to be big buckaroos.
My worst bridge job I ever did was when the engineer (great client) said, "...oh yea - this is in metric.... whatever that is". We just laughed and I said "...Bud you have to be shyting me. I can think in metric, but never have worked in it..." So we laughed some more and had a beer.
Talk about a mind eff.
:good: :good:
My thought would be to set 2 new independent points on top of the bridge on either side of the span. Locate the existing girders and use those points to "stake to line" or create points where they should be.
You can also stake the girder lines on top setup on one and sight the line so you dont have to turn angles. I do this a lot on bridges.
If you can see a few of your control points below resection would be a good way to tie in to your control with little slop.
1/8" seems extreme to me, if I had a dime every time a contractor says a 1/4" was nothing id be retired.
I don't understand the comment about never doing construction layout, I love it!
I don't call myself a construction surveyor, but that sounds like the keep it simple approach and I'm all about that!
Good thing those girders they plan on reusing aren't warped. Probably just the image in my brain that's all twisted up from a ten hour day of drafting and putting out one fire after another. o.O
