Critical setting-out of steel posts to support large decking sub-frame - advice needed

[B52s]

Looking for some guidance as I'm not sure how best to set-out steel posts (to be set in concrete) to ensure that the 150x150 top plate of each of the 16 No. box section posts will match perfectly with the L-shaped designed decking sub-frame to be supported and bolted through onto the top plates below. A friend has given me a loan of his Dewalt Crossline Laser DW088 to assist with the setting-out procedure, but I'm not confident that this will be sufficient, or the correct tool to use, to achieve the critical setting-out of the posts required.

Any advice would be greatly appreciated. TIA

Edited 3 hours ago by B52s

[Nickfromwales]

A laser with 2 beams will give you a perfect x & y, so you’ll need to set out from one straight line (with that as your x) and then you can reliably cast the 90° perps (y) off that. 
 

Then it’s just down to measuring everything twice.

 

Do you have a grid plan you can post up? 

[Russell griffiths]

Don’t set them in concrete, bolt them onto a concrete pad, this way you can build all the structure up and bolt them down when you are happy. 
too many things get drawn up with little thought as to how to build it. 
 

or you need two wooden posts and a string line. 
 

or the alternative method is to dig your holes and build the entire structure, but don’t concrete the posts, just hang them in thin air by putting temporary blocks under the structure. 
when you are happy it’s all correct get a load of concrete at once and concrete in all the posts at the same time. 

Edited 2 hours ago by Russell griffiths

[saveasteading]

56 minutes ago, Russell griffiths said:

 hang them in thin air 

This works and is some concreters preference,  but can also go badly wrong. Maybe you can find a way so they don't get displaced.

I started doing columns this way but moved away from it.

 

@B52s what stage are you at now? Is the concrete in place. 

My preference is always to have the concrete in first, to an absolutely precise level or with 25mm space for metal shims or an extra nut beneath. NB this can affect your subsequent detailing so keep the concrete low to do this.

 

Then to draw the bolt cluster positions as 4 points but with the square projected, as you will lose the marks. Then drill for threaded rod and epoxy.

 

Some people prefer to have a ply or metal template to hang the bolt clusters but they can be displaced. 

 

As @Nickfromwales and @Russell griffithssay...  check and check, and have a permanent reference line.

 

Checking diagonal distances with a tape is important, even if you've used a right angle laser... they can be faulty or be used incorrectly... I never found out which.

This process goes wrong a lot, even with experienced operatives, so you're right to ask and keep asking.

 

[Nickfromwales]

As above, oversized provisional concrete pads up to 95% of the height is a good idea, then just use plates and threaded bar to connect to floating plates. Adjust the bars for height, and when on point drill and fix the plates.