Setting Grid Control on Fresh Concrete

Preferably, I’m out there immediately after the concrete finishing crew laying down my grid lines because the longer you wait the more stuff will be placed in the way and you’re impeding the fourth dimension of construction. It does not matter whether it is slab on grade, steel deck, post tension or any other type. You put down what you have for your ground control and then come back later and put in your “final” grid lines once the construction has progressed to the next stage… after the shoring has been removed or the post tension cables stressed.

Grid lines go on ply wood and as soon as the fresh concrete is hard enough to stand on, the layout begins.

Plywood

Lay out crew waiting on Concrete

Concrete still being finished, Gun UP--Lines Down

Ralph

Ralph, you might want to remove that first photo showing no perimeter protection and guys walking around without the proper gear… big brother may be watching!

I would but that's not the case. The reason for no perimeter protection is because that's still the early erection stage.
If you erect from the core the perimeter is the last thing to go up. In order to put railing up you need plywood and ribs. So that stays unprotected for a couple of hours. The floor below is protected and there is a Net surrounding the building that gets jacked up to 2 stories below the erection deck.

Ralph

So perimeter netting is a legal substitute for cables and safety harnesses?

Thanks for the replies.
So, if you set your grid immediately after the pour and then come back later to set final grid, how do you mark each so that the trades using the grid don't get confused?

I've set grid intersections on fresh concrete in colored pencil and then scribes for final. Also, I spray clear paint over the final grid chalk lines to make it last longer, which I wouldn't do for grid set on fresh concrete.

However, my primary question isn't about when you set grid. I'm just curious how you educate whoever is paying for the survey costs that they have to pay you to set grid on a concrete slab twice. I'm sure this would be easy to explain when dealing with prestressed concrete, since everyone expects it to move much more.
I've met contractors who don't think a regular concrete slab will shrink an appreciable amount. I'm not a concrete expert, but to me I feel that due to a number of variables (bldg dimensions, conc mix, weather, etc.)a slab will shrink and warp to it's final form, which can't be precisely predicted. Some grid points may come in 1/16", some 1/4", some more. In any case, the shrinkage would compromise our grid and our tolerances that we shoot for would probably be exceeded.

I use white chalk lines for the initial "rough" grid lines and then another color for the final grid lines. If they're within an 1/8", I usually don't change the alignment. I’ll just "pop" a new line on top of the old one, but I have worked with perfectionists who'll place a new line 1/16th away from an old one.
Plus there’s a lot more to it than concrete shrinkage. Depending on the type of structure (concrete, steel or composite) the entire structure moves as you’re building it.

There is no such thing as cable and Safety Harnesses in the plywood erection phase in NYC High Rise. I think you need to understand the Process. Where are they going to Tie off on?? There's nothing there. I other words, "What do you connect a cable to in Mid Air"?

Now here's what it looks like when the perimeter has been erected note the railing and the pour stop

Now when the rebar is in, you embed angle iron at specified locations and you also embed hooks in the deck for the floor below. That angle iron will support the cable and the mesh you see in the photo.

Here are the harness straps. They get embedded in the concrete along the perimeter and are exposed when the floor is stripped.

Ralph

I'm just the messenger, but if OSHA shows up someone's going to pay, especially if there's a fatality involved. The way we do it is to use the column rebar (if it is big enough) if not, then we set a 6' piece of angle iron in each column (2' embedded & 4' projecting) then you string your cables to it and tie off to the cables. Roadhand does a similar process with bridge columns & beams.

I've written extensively on the NYC High Rise Process on the other forum, I don't think I'm going to re-visit it at this time. However if you can figure out how to search the archives of the other forum , type my name and NYC High Rise and you'll get the results.
I will say however that it's been done that way for the last 100 years and there is no City with as many skyscrapers as NYC.

Ralph

Be interesting to measure that shrinkage. I looked at some formula for calculation, but there is too many variables to state a general rule. I would think it would be negligible for all but the most stringent layout work, probably less than a chalkline width. But, as noted, there are many, many variables.

One can eventually expect shrinkage on any early erection.

Sorry...somebody had to say it.