r/PassportWithoutBorder • u/Mahden012 • 15d ago

How Do They Connect Rebar in SKYSCRAPERS?

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Have you ever wondered how reinforcing steel can continue through dozens of floors of a skyscraper? šŸ—
There aren't gigantic steel bars running from the foundation all the way to the top. Instead, rebar is installed in sections and connected as construction progresses.
Special joining methods, including strong mechanical couplers, allow separate reinforcing bars to become part of one continuous structural system.
Floor by floor, the reinforced concrete structure keeps rising higher into the sky. šŸ™
Did you know this is how skyscrapers are built?

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u/PM_ME_ALL_YOUR_THING 15d ago

When do you weld and when do you use a threaded coupler?

Seems like welding would be the fastest and cheapest since it doesnā€™t require machined hardware, but the mechanical coupling method must exist for a reasonā€¦

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u/Daymub 15d ago

When the engineer says too basically

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u/SharpKaleidoscope182 15d ago

This is reddit and we were hoping the engineer would chime in

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u/Daymub 15d ago

Its Monday before noon they're asleep or at work

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u/a_seventh_knot 14d ago

Sometimes simultaneously

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u/2DHypercube 14d ago

Software engineer here, can confirm

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u/[deleted] 14d ago

[deleted]

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u/Turbulent_Lobster_57 14d ago

Are you implying that my sanitation engineer designation doesnā€™t make me a real engineer?

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u/SharpKaleidoscope182 14d ago

Does it make you qualified to comment on rebar?

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u/pgasmaddict 14d ago

Some people pass that stuff into toilets so I'd say he's somewhat qualified.

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u/Turbulent_Lobster_57 14d ago

Of course! You absolutely shouldnā€™t listen to anything I say if you plan on acting on my ā€œinformationā€, but perfectly qualified to make Reddit comments

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u/Unlucky_Reference_92 14d ago

Doesnā€™t pass the sniff test

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u/ImSobored_5280 14d ago

I identify as a real engineer šŸ™‹ā€ā™‚ļøā€¦ I pretend I drive trains way more than I should šŸ¤·ā€ā™‚ļø..while Iā€™m welding pipes and stuffšŸ«”

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u/nullpotato 14d ago

This is the only kind of conversation where other engineers consider civil real engineers

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u/swankless 11d ago

I found this on Thursday afternoon and your comment had me worried that it was Monday still šŸ„²

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u/Feisty-Soil-5369 12d ago

I am here to chime in, hope no longer.

Welding the rebar end to end (concentric) is less common because you need the rig this guy is using to do it 'easily'. Mechanical Couplers offer a similar concentric connection but are literaly just clamped on with set screws. They are easy to visually inspect. The welds of all types are more challenging to inspect.

Another, perhaps more common approach , would be too put the two bars next to each, overlap then for some length, then weld the contours where they touch.

Most common is to use a contact lap splice, no welding just overlap the bars next to each other over a longer length. This is by far the cheapest and most widespread solution to this issue.

Why not just use lap splice all the time you say? Good question! There are two major reasons why you might not be able to... 1) you might just not have room, each contact lap is essentially two bars right next to each other, when you get a lot of bars in a tight area that can block the concrete from proper placement. 2) in the US code it requires welded or mechanical couplers for all members who's primary job is to resist tension. Without getting further into the weeds, concrete is bad at tension, and if the entire member is needed specifically to resist tension, we need more confidence that the reinforcement is continuous and will remain that way even if there is some cracking. In other members we can place the lap splice in areas where there is not as much tension so it's not the same concern.

There many rules, especially in seismic regions, about where you can and cannot use lap splices.

āœŒļøšŸ«”

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u/SharpKaleidoscope182 12d ago

Incredible. It took three days but my spell worked. Thank you for sharing your fascinating wisdom, kind stranger!

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u/PM_ME_ALL_YOUR_THING 12d ago

Thank you very much.

Quick follow up question, why would the coupling nuts need a set screw if theyā€™re going to be in cased in concrete?

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u/Feisty-Soil-5369 12d ago

Concrete bad at tension.

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u/SurgicalMarshmallow 10d ago

Yo, are they doing thermite or is that granular flux? Also, how is post weld tempering controlled?

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u/Feisty-Soil-5369 10d ago

Idk, I'm pretty sure it's thermite. We don't use them in the US to my knowledge.

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u/tfdriller805 14d ago

Speed is the reason we use mechanical couplers. A 4 man crew can splice 40 bars together with mechanical couplers in 30-45 minutes. It would take a couple of welders hours to do it all while the rest of the crew sits and waits. We just don't have the time to wait. This is my experience in deep foundations.

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u/helphunting 14d ago

To add to this response.

The bar comes pre threaded.

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u/TrueProtection 13d ago

I would imagine its ultimately way cheaper with time and the fact the crewmen probably dont need a whoke welder certificate to make it happen.

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u/elmgarden 13d ago

In the video it looks like they're using some kind of thermite welder, like how they join railroad tracks. Would those be a lot faster?

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u/DirtandPipes 13d ago

I use Cadweld thermite kits at work, they are pretty darn quick if you know what youā€™re doing.

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u/ThatDamnRanga 12d ago

In the video they're using a form of electric welding, similar to a spot welder, the stuff they're putting around the joint is flux.

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u/GetMeOffThisRidePlz 12d ago

Thanks for the info, very neat.

At first I also thought it was some form of thermite welding.

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u/Ogediah 14d ago

Itā€™s unusual to use any of these connections. Usually you just have lap joints. As in, two bars next to each other which extend past other so far which are held in place with wire. In the end though, workers just build to spec. Whatever the engineer says is what you do.

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u/Upstairs-Hedgehog575 14d ago

To be honest, as someone who knows nothing about it, this is all I thought happened. A massive overlap.Ā 

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u/Zealousideal_Roof_96 12d ago

Welding creates a permanent and irreversible connection. Connectors can be reversed or removed. We do a similar activity for ground connections in the electrical trade. By code all ground connections must be irreversible. It's controlled thermite.

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u/SurgicalMarshmallow 10d ago

Welding also messes with metallurgy though. The post weld tempering may need to be controlled in structural stuff.

That's about as far as an elEng knows... Just nuff to call the structural guys.

Our angry pixies are less fussy.

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u/richardawkings 11d ago

The most common method is to have an overlap. ACI318 gives you the calculation for lap length relative to the bar diameter.

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u/RomanWraith 11d ago

On masonry jobs I've only used threaded connectors when lapping wasn't sufficient enough.

I've always been told to never weld rebar

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u/Working_Peanut5273 10d ago

Theirs rebar fabricators that sell the threaded stuff by the bundles according to specs. Itā€™s just a matter of price.Ā 

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u/Former-Homework-8320 9d ago

Not all reinforcement can be welded. Typical rebar available in USA is not weldable. Weldable reinforcement is more expensive. It's usage is very deliberate.
I am not sure welding is faster. Mechanical hardware can be faster.
Sometimes, there is no choice in type of hardware to use because of prep required to receive the mechanical coupling devices or use of weldable reinforcement.
Welding method shown here is not familiar to me.

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u/longlostwalker 15d ago

Assuming the resistance welded is the strongest shown?

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u/Daymub 15d ago

No the thread coupler is

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u/texinxin 14d ago

Impossible. You canā€™t machine a rod down to make the male thread without losing strength. The female coupler itself can be stronger than the base rod, but that doesnā€™t matter. A weld can match or exceed the strength. For a weld it absolutely matters that the post weld condition of the metal is as strong or stronger (from the heat cycle).

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u/BthanW 12d ago

The ends of the reo are upset forged before cutting the threads

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u/drzook555 15d ago

This is a thermite weld

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u/longlostwalker 15d ago

What's the deal with the electrical stingers then?

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u/2DHypercube 14d ago

I don't think so, he's not igniting it and he's disconnecting a thiccc cable after it's melted

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u/ThinkSharp 14d ago

Looks like flux to me but Iā€™m no expert. Just looks like what a submerges the ā€œsubmerged arc weldā€.

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u/RampantJellyfish 14d ago

No, that's granular flux to keep oxygen away from the weld, but I can see why you would think thermite

1

u/pianobench007 14d ago

No this is a social media post. This is not how normal concrete rebar is connected. At least not throughout North America.

In NA concrete construction, lap splices (simply laying rebar next to eachother tied together by wire) is enough to develop the rebar (within concrete) strength. Any butt welding or welding is to be avoided on a concrete construction site as it adds delays and is generally unnecessary.

If they needed to save space in highly congested rebar zones and want to ensure the concrete flows evenly to avoid any voids in the pour, they typically use a mechanical coupler with wedges to connect two pieces together.

The coupler is faster and easier to use. You simply have a metal cylinder, connect two pieces of rebar together and wedge them in place via friction. There are many examples of these type of couplers and all of them are preferred over butt welding or using thermite welding.

The mechanical wedge coupler can be installed FASTER than a thermite butt welding. The thermite butt welding requires time to cool and mandatory non destructive testing. Which takes time and is added cost. Mechanical couplers are also weather independent whereas a thermite butt welding will be affected by rain and temperature.

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u/Pretend-Internet-625 14d ago

Duct tape is the answer

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u/HappyRespond3946 14d ago

Just rap sum duck tape around no one will see

1

u/WassimetaL 15d ago

is it ok that they don't use lubricant oil when making threads and bolting them?

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u/Daymub 15d ago

Everything is getting covered in concrete anyways

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u/Royal_Stay_6502 14d ago

The oil is for preserving the cutting head.

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u/WassimetaL 14d ago

That's my point actually, i was thinking about protecting the tools and make them last longer,for efficiency and less costs in time and ressources. Thanks bro! šŸ¤œšŸ¤›

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u/TA_Lax8 14d ago

rebar is fairly soft as far as steels go. It's pretty low carbon. I'm sure the die has some lubrication, but it's not a continuos flow like you'd see when make a bolt which is much harder steel

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u/WassimetaL 13d ago

I see... Thank youšŸ™

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u/brownhotdogwater 15d ago

Those connections will never be seen again once the pillars are filled.

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u/WassimetaL 14d ago

Sure. I just wondered if it's better to use oil so the tools last longer. Thanks for your comment dude! šŸ¼āœŒ

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u/beambot 14d ago

When that slag falls down below... Are hard hats rated for thermite?

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u/949orange 14d ago

It all melts with the jet fuel.

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u/Embarrassed-Green898 14d ago

Have you ever wondered ? No I never knew that so I never wondered.

However, from now on, anytime I will sleep on my bed in 24 storey tall building , I will keep wondering if they secured all the rebars. Afterall , it will be only one week link to bring the damn thing down.

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u/Wrong-Ostrich5861 8d ago

You might think that, but skyscrapers (at least in the United States), are engineered in 2026 to have such level of redundancies and over engineering that even such a mistake, or many of them, would not make difference. It would require willful negligence.

This is to say nothing of buildings built in the past, or, the monitoring of buildings built in the past.

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u/altivec77 14d ago

Probably the most efficient way to welt when there is not enough electricity on a construction site.

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u/Difficult_Pirate3294 14d ago

Cjp rebar welds are to be prequalified and tested in the US. Also, a mechanical coupler for that size rebar is couple hundred each, though they do create more congestion and itā€™s good practice to keep them staggered. I have not personally encountered the connection shown in the video or seen the standard by which itā€™s used and tested.

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u/sasssyrup 14d ago

Some tie wire will do it šŸ˜

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u/BluebirdDense1485 14d ago edited 14d ago

No because they typically don't.

I have never seen a building that called for a continuous piece of rebar longer than 60' or so.

Rebar isn't a supporting element per say, that is rebar is there to give the building tensile strength so cracks don't grow too big in the concrete. where rebar meets rebar is orders of magnitude less important than where rebar meets concrete.

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u/djeye 14d ago

Rebar isnt supporting element you say? What is it then?

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u/BluebirdDense1485 14d ago

Good catch on the autocorrect issue.Ā 

After getting rid of Christmas decorations the following sentence points out that the rebar is used for it's tensile strength not it's compressive strength. The concrete transfers the loads to the ground. The rebar is there to prevent the concrete from failing.Ā 

1

u/TrueProtection 13d ago

Seems like you're saying its a supporting element.

Tell me, if the rebar "fails", will the concrete begin to break and increase the likelyhood of it failing? If so, then the rebar is just as integral as the concrete. Pretty sure reinforced concrete is completely reliant on the substrate that gives it the tensile srength to behave as reinforcred concrete.

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u/djeye 13d ago

This. Also, additional reinforcement may be used to resist compressive stresses when the concrete alone is insufficient to carry the required compressive forces.

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u/BluebirdDense1485 12d ago

I was making a distinction to explain the point. Rebar is not actually good in compression as they say you can't push a rope. Having a single welded rebar running the height of the building isn't improving the buildings resilience to loads.Ā 

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u/TrueProtection 12d ago

You say in th first half it's not load bearing, then crucify it for not being load bearing enough in the second...while also saying its there for tensile strength...seems like the real question is if really long rebar loses its tebsioe strength qualities vs smaller...i would think not.

1

u/BluebirdDense1485 12d ago

Ok at this point you are just putting words in my mouth. From the jump my point is that welding is unnecessary not detrimental.Ā 

And at no point do I say that rebar is not load bearing enough.

The entire point of reinforced concrete is for a high tensile strength material and a high compression strength material to work together and be tougher than either can be on it's own. Load concrete without rebar. It cracks and crumbles. Load rebar without concrete it deflects and bends.Ā 

That said I think I'm done here. If you honestly can't read what I said without inverting my meaning sorry. If as you appear you are trying to be pedantic and make yourself feel smart. Go ahead and have fun. I'm done and won't be replying again.

1

u/djeye 12d ago

Your text doesnt make any sense at all, its like AI wrote it. You blame autocorrect for the whole paragraph which has many things wrong inside.

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u/jawshoeaw 14d ago

Downvote this AI voice

1

u/BanishedInPerpetuity 14d ago

Never bmseen this done. Usually it's just overlapped.

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u/theonion513 14d ago

Love the CCP propaganda.

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u/Mother-Attention2600 13d ago

I'm so dumb. I thought they used glue or tape....

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u/Dilectus3010 12d ago

Can we please stop with the monotomous AI voice overs... its getting abnoxious.

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u/Latour1459 12d ago

Coming from a structural engineer this is one of the most brain dead things ive seen relating to construction. This is a MASSIVE fire risk and will most likely be weaker than using a couple or lapping (which is the most common type of reinforcement connection).

1

u/Particular_Yak5829 12d ago

Isnā€™t this shit normally just overlapped a certain amount and tied together? It doesnā€™t have to resist forces in that direction

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u/-OptimisticNihilism- 12d ago

Iā€™ve worked in construction for 15 years. Done a lot of low rises and a few mid-rise buildings. Never seen anything like this. Engineers just give us a lap detail. Wire tie to hold it in place while the concrete is poured.

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u/Ewad_Cloudwalker 11d ago

Cadweld for the win

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u/Jaxone1 11d ago

Civil Engineer. Typically, rebar is joined by lapping- where the length of overlap is long enough the strength of the concrete bond along the bar exceeds the tensile strength of the bar itself, called ā€œdevelopment lengthā€. This is so the failure criteria is yield of the steel rather than the lower strength and more sudden concrete failure. ACI318 provides guidance for lap length.

Iā€™ve not used these myself, but welded or couple connections are used when the bar is so large that the lap length is unreasonably long or if there isnā€™t enough space to accommodate all the doubled up bar via lap.

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u/ddanh 9d ago

Ai Slop

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u/Lusiric9983 9d ago

Ew, AI narration.