That would make for some very large gutters. Is this project in Seattle?(lots of rain) Have you thought of what might happen if snow and ice builds up in these large gutters(lots of weight)? Just a thought :)
Anyway, it is possible to do as you are asking. Keep plenty of tips on hand for the plasma torch, as that is a lot of cutting(4500 lin ft of cutting = 2250 feet times two cuts). Layout will take a minute or two to make sure each half is equal to the other. A framing square and a bubble protractor will help in finding the centerline of each pipe to divide equally. Someone else may suggest another method, but this comes to mind at the moment. Plasma over Oxy/Fuel I think is a wise choice since you need to limit your heat input to keep the pipe straight and by making cuts on two sides exactly opposite each other will further enhance your chances of keeping it straight. "Buggo" makes a magnetic track that the torch can be mounted on and long straight cuts are easily achieved with very little grinding or cleaning up.
As far as a fabricator being able to handle it... I would think as long as the fabricator has time allowed for the layout and cutting, with some possibilty of some straightening, I would think most could handle it. I work for a structural fabricator and we will cater to a customer provided the cash is there to cover it. :)
Good Luck with your pipe-gutter project, and please post your results when you have completed it and let us here on the forum know how it turned out.
John Wright

Well, providing a no waste factor. You would need about 113 pieces of pipe. Its a lot of pipe but it seems to be a fairly simple task.

If you do decide to do this...you have some expensive and very heavy duty gutters. Dont mind me, im a big fan of overkilling stuff. :)

I dont think cutting the pipes is going to be the problem, its only 1/4 inch thick and any medium/heavy duty plasma should cut it like butter.

If one can devise a way to continually manuver the pipes around and get them to the cutting rack rather quickly..the rest should be simple.

I personally would find some saw horses and set them up probably about 4 would do. Mark out the lines on each end to get a accurate "in half cut". Take a full lenth of angle iron, clamp and tack it to the pipe so you can drag the plasma torch down the whole lenth of the pipe.

thats the way i would do it...there is a lot of pipe to cut, and you may want some heavy duty hangers to hold this thing up..its going to be pretty heavy.

would it be cheaper to have someone install regular commercial gutters?

I'm not trying to drum up some business here, being on the other side of the country from you (Maine). I just thought I'd give my 2 cents FWIW.

Plasma cutting would be the way to go for consistency and minimum distortion. Hand work would not be the best. I would find a place with a 25' long burn table with CNC (burn tables can't cut the full of the table usually, they need a few feet for the bridge mechanism). I would program the cutting to interupt every 4 or 5 feet and restart and inch beyond the stop. The "tabs" that will be left should be hand cut after the pipe has cooled although plasma at high travel speeds won't get the pipe very hot to begin with. But that will reduce distortion even more and prevent the pipe from "laying open" and disturbing the cut line.
The plasma should easily be able to cut at 100"-120" per minute not counting the stops. That will give the least heat input to the pipe and keep distortion down.

A water table would be excellent. Submerging the pipe while cutting will reduce distortion the best. It just might be hard to find a table with 10" of depth unless the operator takes out the supporting cross bars.

You made mention of an architectural end use. Keep in mind that thermal cutting will give you good cut but not a machine-like finish.

However....there will still be some distortion that will require straightening, you can't completely prevent it. If the distortion isn't too bad, I would have the pipe through a set of plate rolls set at light pressure so it doesn't flatten the half pipe, and run it through a few times. Make sure to grind off the plasmas dross (slag) because it is very hard and WILL damage the rolls.
If rolls are not practical for you, I would heat straighten them, using a motorized unit like a Bug-o to run a torch down the length of the half pipe to .

One final thought, I would check around to see if anyone has the forming dies for a bending brake. You could have the half pipes made out of plate and they will probably come out straighter than by thermal cutting. You most likely would be dealing with 8' or 10' lenghts if you go that route.

All in all, what you want done is practical IF you have the equipment available.

CHGuilford

Ask your steel supplier if this could be roll formed (probably polygonal but if it has enough sides who could tell). Probably big setup charge but 4500 feet might be enough to hide it in.
Bill