Sorry about the background color.
I am estimating a project for a new 20" steel water main to be constructed over a bridge in New York City. The design features this main to be installed within a 28" steel casing pipe. I know how to build a water main over a bridge without a casing; I need help with the procedure that includes the casing.
The bridge design features a single span steel girder structure. In one of the bays, the Owner want this main installed. They have detailed sliding Utility Supports made of steel angles and plates etc. Their cross section depicts this support, as well as the carrier pipe, 2-inch thick insulation, spider supports, and the casing pipe.
My current scheme is:
1. erect the bridge girders
2. install the utility supports, attached to the girders using connection plates
3. set those supports at a temporary/interim elevation (slotted holes)
4. install the water main (welded joints, 20' lengths)
5. construct the deck, surfacing, parapet, the fencing & guardrail etc
6. allow the superstructure to deflect under its dead load and superimposed loads
7. get access under the bridge and adjust the main and its supports to exact correct elevation, filed drill holes, then bolt up
I am looking for thoughts on step four. If there was no casing required, I would A. install timber planking, supported by the new girder bottom flanges, to provide a surface for the workers; B. Install the supports; C. set the 20' sticks into bay; D. weld the joints; E. test joints etc.
But with the casing, do I install it at step four in its entirety, then insert/push/jack the new pipe into that? Or do I install the casing and carrier simultaneously, as in - section of casing/section of carrier/section of casing/repeat?
What do contractors typically do?
I thank you for your help and experience.
From previous experience in NYC, it's done simultaneously.