Construction site question: How precisely are support beams placed?

[yllus]

Every morning I arrive at the office and the condo building being built across the street has a little bit more done on it. First they dug an enormous hole, and now they're drilling holes for giant steel beams. A second condo is being built right beside it, and watching them build floor after floor is fascinating. It happens pretty quickly.

So this brought to mind a question: I'm watching this happen from the 8th floor, so I can't exactly peer into the cab of the guy in the machine placing the steel beams; how carefully are those beams placed? GPS is only good to within a few feet; do they use some sort of laser alignment? Or is it completely eyeballed?

I've looked at purchasing a condo before it's been built and one of the disclaimers says something like, "Your condo may not be exactly the dimensions written here." I've heard of people losing a couple of feet due to "issues" during construction. I figure this has to do with those beams not being placed accurately. Anyone in the know?

 

[skyking]

No, any differences in individual units would result from locating things like elevators and chases for plumbing and equipment.
The beams and columns are built to spec, all the holes are pre-drilled.
The ironworkers plumb them for level, and cross check for square as they go up.

 

[yllus]

No, any differences in individual units would result from locating things like elevators and chases for plumbing and equipment.
The beams and columns are built to spec, all the holes are pre-drilled.
The ironworkers plumb them for level, and cross check for square as they go up.

So the guy doing the pre-drilling - what guides his hand? Eyeball?

 

[skyking]

My neighbor works at a fab shop. It is old fashioned tape measure and mark, then mount a magnetic based drill (mag drill) and drill the hole. Us tape measure guys can get 1/32" accuracy all day long.
The mag drill drills normal to the surface for you, so no angled holes, etc.

 

[Triumph]

You can get 1/16" accuracy or maybe even better across almost any distance you could conceivably need to build to using a water level.

http://www.deckmagazine.com/article/64.html

 

[Gibson486]

It is done to spec. If the drawings show a support beam 5 feet 3 inches from the east wall, then it will 5 feet 3 inches from the east wall. It does not have to be exact, but it can't be off by a foot or something.

 

[Modelworks]

You use a transit. Usually two point, sometimes three point if it really has to be precise.
http://en.wikipedia.org/wiki/Theodolite

If you have two people viewing from two different locations you can accurately position just about anything. There is a nat geo special on the construction of a building in China that had to be so precise they used 3 point to align it.

 

[skyking]

You can get 1/16" accuracy or maybe even better across almost any distance you could conceivably need to build to using a water level.

http://www.deckmagazine.com/article/64.html

We used those to set modular buildings and mobile homes. Antifreeze served a second purpose; it was easy to read.

 

[bonkers325]

almost all structural steel you see being installed is pre-measured and pre-drilled in the steel shops. the dimensions are within a specified tolerance, and will always fit together as long as the shop drawings are prepared properly.

as far as beam placement goes, all the guy has to do is line up the holes and bolt them. construction workers aren't paid to think.

 

[MikeyIs4Dcats]

almost all structural steel you see being installed is pre-measured and pre-drilled in the steel shops. the dimensions are within a specified tolerance, and will always fit together as long as the shop drawings are prepared properly.

as far as beam placement goes, all the guy has to do is line up the holes and bolt them. construction workers aren't paid to think.

As a project manager and estimator I can tell you this is 100% wrong.

The rest of your post is typically correct though. There may occasionally be a mistake made or reason something is altered in the field, but typically everything would be within 1/4" or so of the plans.

 

[BoomerD]

almost all structural steel you see being installed is pre-measured and pre-drilled in the steel shops. the dimensions are within a specified tolerance, and will always fit together as long as the shop drawings are prepared properly.

as far as beam placement goes, all the guy has to do is line up the holes and bolt them. construction workers aren't paid to think.

As a project manager and estimator I can tell you this is 100% wrong.

The rest of your post is typically correct though. There may occasionally be a mistake made or reason something is altered in the field, but typically everything would be within 1/4" or so of the plans.

To a point, he's right.

Assembling a steel building is a matter of putting beam (A203) in place with the numbers facing east and to the top of the beam, then connecting (A205) with the numbers facing east and to the top of the beam, then connecting (A204 between (A203 and A205) with the numbers facing north and to the top of the beam...repeat ad nauseum...

I hung steel for buildings for many, many years. It's like a gigantic erector set...each piece has its place...(although there may be dozens or hundreds with the same number for universal fit) and they're USUALLY very clearly marked. It does take a bit of knowledge to read the prints...but it's not rocket science.

However, I'll also disagree with him that construction workers aren't paid to think. (although many are definitely not qualified for it) A GOOD hand thinks all the time, and often catches fabrication mistakes before the piece is put into place...where correcting that mistake later would cost a lot of time and money.

I was on one job where we had most of the columns and at least half of the main beams in place before one of the iron workers caught a simple fab shop mistake...and because it was a school project, we couldn't fix it in the field...or at our fab shop. All the beams and columns had to be taken down and shipped back to the fab shop in Oregon for refabrication.

 

[mizzou]

sometimes when i'm doing woodworking, if the wood isn't exactly straight or level, I force it

i'm sure this doesn't translate well to building making because we lack giants with huge gigantic pneumatic tools

 

[Gibson486]

As a project manager and estimator I can tell you this is 100% wrong.

The rest of your post is typically correct though. There may occasionally be a mistake made or reason something is altered in the field, but typically everything would be within 1/4" or so of the plans.

Yup, they need to think....think of ways to create change orders to charge my clients more...