Watch your crew

[thailazer]

Was at a friend's project the other day and saw a crew taking forms off beams the day after a pour. Yikes, I said. After a bit of discussion we found out the crew was planning on placing concrete planks on the beams in the next few days. Double yikes. Seeing that U Condo collapse in the news, thought it was worth putting out a thread here on CTH to warn builders to ensure beams do not have any loads on them until they have reached sufficient strength. We left our forms on for 14 days to ensure good strength, and bottom bracing even longer.

Crews usually rent the forms so they want to remove and return them soon. Pay for the rental yourself so the crew doesn't take them off too soon. There is bountiful information on the web on the compressive strength of concrete in the first month after a pour, and it certainly needs time for the reactions to progress enough to support those heavy planks and subsequent floor pours. Heard plenty of "no problem" from the crew at my friend's place, but at least we caught them before they took off the bracing and forced them to delay 10 days for planks. Of course, the crew chief thinks us farangs are nuts as they want to proceed as fast as possible. Mai Pen Rai...until it all collapses!

[Roger Ramjet]

Was at a friend's project the other day and saw a crew taking forms off beams the day after a pour. Yikes, I said. After a bit of discussion we found out the crew was planning on placing concrete planks on the beams in the next few days. Double yikes. Seeing that U Condo collapse in the news, thought it was worth putting out a thread here on CTH to warn builders to ensure beams do not have any loads on them until they have reached sufficient strength. We left our forms on for 14 days to ensure good strength, and bottom bracing even longer.

Not to labour on too many points, but it's the columns that carry the vetical weight. I was going to post a number of articles about the high rise collapse from the Bangkok post (small p), except they changed their story so many times and blamed so many people in each story that it would be pointless.
The Thai Engineers Association who did the first inspection said the blame lay in the approval of the design which did not conform to current engineering building standards having only 4 load bearing columns situated in the wrong place. They also stated that the building next to it was also designed the same way, as was the one before it..... will they be torn down? TIT. People are living in one of them.
It also depends on the type of concrete used, the amount and type of rebar. Some highrise buildings have a new floor added every 48 hours or even less in some cases.
But in this case the latest news was that the sub-contractor told the workers to pour the floor against the protests of the on site engineer......well that's the latest guess from the Bangkok post. I think the reporter's orders was: Take some photos and then guess up a story.

[pipoz]

Some highrise buildings have a new floor added every 48 hours or even less in some cases.But in this case the latest news was that the sub-contractor told the workers to pour the floor against the protests of the on site engineer......well that's the latest guess from the Bangkok post. I think the reporter's orders was: Take some photos and then guess up a story.

Its the amount of form work & back propping that you keep in place on high rise projects that counts,(along with making sure the reinforcement is left in place and the concrete is the correct mix) so as to take up the wet concrete on the last pour and to ensure that you don't overstress the earlier floor pours immediately below that, given they are yet to achieve full concrete strength. I have seen workers pull reinforcement out of a deck at night, after the inspections took place earlier that afternoon to save a Dollar (in Indonesia). No doubt it happens elsewhere.

Few high rise building project achieve a 2 day cycle floor to floor as there is no need and no point in doing it. Finishing the structure early doesn't automatically translate into finishing the project earlier.

Most high rise over 30 floor try for 5 to 7 day floor to floor cycle. 5 days is considered very good. With a five day cycle you would keep 4 possibly 5 floors of back propping in place, although I doubt the Thais would think or build this way.

pipoz

[Roger Ramjet]

Its the amount of form work & back propping that you keep in place on high rise projects that counts,(along with making sure the reinforcement is left in place and the concrete is the correct mix) so as to take up the wet concrete on the last pour and to ensure that you don't overstress the earlier floor pours immediately below that, given they are yet to achieve full concrete strength. I have seen workers pull reinforcement out of a deck at night, after the inspections took place earlier that afternoon to save a Dollar (in Indonesia). No doubt it happens elsewhere.

Oh, I totally agree, it's all in the formwork, rebar and correct mix, you also have to have the right design that has been checked and double checked. If you get the columns right (the formwork on them can be removed after 24 hours) and then wait about 7 days, you should have no problems doing the beams, and if the formwork under the beams is left in place for the same period it should hold any concrete flooring you throw at (over/on) it.
The problem in Thailand is getting the columns right in the first place. Most of the columns I've seen are not designed to carry the load that's placed on them being too thin and with the wrong sized rebar in them....and then a whacking great roof of tiles put on top. You cannot cutback on the size and what's in the columns no matter what, unless you use Colorbond or similar, but I've read it over and over again here.... "The plans say 4 hun but my builder says we can/we will use 3 hun....no problems, I can save money there." Which is one of the points I was trying to make....it's not how long they are left to cure, it's what's in them, as well as how long they are left to cure.

[sirineou]

I Happen to work in the High-Rise building industry, in one of the most regulated areas in the world NYC Currently involved in a 54 flr project in Brooklyn
A two day cycle depending on the square footage is not unusual, especially when the building goes typical.
A couple of hours after we pour the deck, as soon as we can walk on it, we are erecting the next one, in two days they are stripping the floor below, and installing reshoring .
In a two day cycle we need three floors worth of forms.
the reshoring consists of legs with jacks. spaced every 8 ft , the reshoring stays for 28 days. In a two day cycle job, it is not unusual to have 12 floors of reahoring
So as long as they shore up the structure, there is nothing wrong with stripping the forms in two days.

[Mike Judd]

AH ! now we know why the Twin Towers came down so fast after the planes ? hit, and building 7 with some office fires.

[canopy]

Whatever you do, don't watch how long someone else leaves their forms on to judge how long you should no matter if it is a skyscraper or house. The minimum time depends on a number of factors specific to your project. If no one really knows the answer for your project, you've got problems. Ripping forms off immediately is indicative of that.

On my last slab after reviewing ACI 308, I made an agreement with the builder in advance his wood forms would stay on for 1 week after the pour. My curing program then continued for another month. To my amazement, not a single crack ever developed anywhere. Not even at any of the control joints. I can only surmise a lot of things were done right, starting with my insistence on using a plate compactor all the way through leaving the forms on. I didn't want a normal thai slab I see everywhere that is brittle with spider web and gaping cracks where termites and ants come through. Later on 20 wheel double trailer trucks drove over it with no effect whatsoever.

Curing impacts every single attribute of concrete. Everyone should be getting this right as it is virtually free and makes such a big difference.

[Klondyke]

AH ! now we know why the Twin Towers came down so fast after the planes ? hit, and building 7 with some office fires.

And in addition the fire supported by the cooked books of the AIG insurance

[thailazer]

I Happen to work in the High-Rise building industry, in one of the most regulated areas in the world NYC Currently involved in a 54 flr project in Brooklyn
A two day cycle depending on the square footage is not unusual, especially when the building goes typical.
A couple of hours after we pour the deck, as soon as we can walk on it, we are erecting the next one, in two days they are stripping the floor below, and installing reshoring .
In a two day cycle we need three floors worth of forms.
the reshoring consists of legs with jacks. spaced every 8 ft , the reshoring stays for 28 days. In a two day cycle job, it is not unusual to have 12 floors of reahoring
So as long as they shore up the structure, there is nothing wrong with stripping the forms in two days.

I always wondered how fast they "moved up". Thanks for posting that.

Here is a link to a basic graph on the strength curve. Although the columns have to hold the load, the beams have a bending moment if loaded and those forces can be in tension which is not concretes strongest asset. Hence my warning.....

[pipoz]

I Happen to work in the High-Rise building industry, in one of the most regulated areas in the world NYC Currently involved in a 54 flr project in Brooklyn
A two day cycle depending on the square footage is not unusual, especially when the building goes typical.
A couple of hours after we pour the deck, as soon as we can walk on it, we are erecting the next one, in two days they are stripping the floor below, and installing reshoring .
In a two day cycle we need three floors worth of forms.
the reshoring consists of legs with jacks. spaced every 8 ft , the reshoring stays for 28 days. In a two day cycle job, it is not unusual to have 12 floors of reahoring
So as long as they shore up the structure, there is nothing wrong with stripping the forms in two days.

Hi Sirineou,

The 2 days cycle must be more unique to America, as we don't wast our time with it in Australia, Asia and rarely try to achieve it in the Middle East. Plus it depended on your floor area.

We are quite happy with the 5 day cycle once we got past the podium and into the typical floors and we are going to 62, given our schedule is driven by the MEP to floors and the Curtain Wall installation. We strip and then back prop to recover the Table Forms (Doka system) and start our Services rough-in 5 floors below the working deck (of 1600 m2)

pipoz

[sirineou]

AH ! now we know why the Twin Towers came down so fast after the planes ? hit, and building 7 with some office fires.

And in addition the fire supported by the cooked books of the AIG insurance

I did not work at the Twin Towers, but I have many friends that did,my deck foreman and brother in-law was there, and we often talk about it, they worked mostly on the foundations and encasing steel beams at the garaged levels.
So aside from the humanitarian disaster, and national pride, we all had a personal investment, we are still very saddened by the event.
Non the less these buildings, were steel. In NYC all residential buildings are concrete, including hotels, and all office buildings are steel.
Whether they came down quick, or not , is arguable.
As we all know steel is forged in a furnace, the ensuing fire from the jet fuel and wind situation at that elevation created a furnace affect contributing to the impact damage. I don't believe there is a steel building in the world that could resist these conditions for long.

[sirineou]

Hi Sirineou,

The 2 days cycle must be more unique to America, as we don't wast our time with it in Australia, Asia and rarely try to achieve it in the Middle East. Plus it depended on your floor area.

We are quite happy with the 5 day cycle once we got past the podium and into the typical floors and we are going to 62, given our schedule is driven by the MEP to floors and the Curtain Wall installation. We strip and then back prop to recover the Table Forms (Doka system) and start our Services rough-in 5 floors below the working deck (of 1600 m2)

pipoz

[/quote]

Hi Pipoz, a pleasure to talk with an other professional.
I would love to work all over the world, like you , if I never see NYC again it would be too soon I hate it here, Unfortunately when ever I mention leaving or retiring, my boss,the owner of the company, has a heart attack and he is family, so I am stuck here.
We also use the Doka system, we used to use their H20 beams but we import our own from China now,but we still use their legs,platforms and cocoon system, for forms we use the semens system. and make our own forms out of plastic coated plywood at our yard.
That's my Job, I run the Yard and logistics.I used to run site safety, but we are too busy now.
Columns, stair racks , complicated beam forms, etc are made in the yard, and are delivered to the job and dropped in by crane. This can save us a lot of time.
We are a Union company and do only Union jobs, have a highly trained crew of professionals most of who have being with us for years, every one knows their part and we work as a well oiled machine.
Being a Union job where the basic hourly wage for a carpenter is $49 plus benefits,I dont want to mention how much the crane and operators cost us, we can't a afford to linger. NYC is a very expensive market and any day we save is a day earned.
If a two day cycle is achievable we have to go for it, especially during the typical floors, where we like to Bank days, so we can use them when we get to the top mechanical,or for days we lost in the first few floors. and for days we loose do to, blowouts, accidents, and D.O.B. shut downs etc

The point being, is that stripping the forms with in a couple of days, if necessary precautions are taken is nothing unusual .
Wrap column in the hot sun, so they don't loose moisture, water decks, use proper rehoring. Of course we don't wrap columns or water decks, by the time they are exposed they are covered by the erection of the next floor, but for a climate such as Thailand it is a important to keep concrete properly hydrated to achieve proper strength.

[pipoz]

Hi Sirineou,

pipoz

Hi Pipoz, a pleasure to talk with an other professional.
I would love to work all over the world, like you , if I never see NYC again it would be too soon I hate it here, Unfortunately when ever I mention leaving or retiring, my boss,the owner of the company, has a heart attack and he is family, so I am stuck here.
We also use the Doka system, we used to use their H20 beams but we import our own from China now,but we still use their legs,platforms and cocoon system, for forms we use the semens system. and make our own forms out of plastic coated plywood at our yard.
[/quote]

Careful calling me a Professional, it could be an insult to Others. Spent the last 18 years working oversees for one reason or another, as it got a bit claustrophobic and expensive in Australia. Ultimately it enticed me to visit Thailand and from there no turning back.

Have done a few Tower in my time and all are different, some of a composite structure (structural steel, metal decks & concrete), others of RC & Post tension slabs and the current one is a straight up Reinforce Concrete structure. Have used some Doka copy system from the Ukraine & Turkey on other projects, plus the RMD system, but Doka always comes out on top. Doka for me is the easiest system to use

One good thing about working overseas, No Unions to contend with, and I will leave it at that.

I have known of projects that tried to achieve the "Holy Grail - Two Day Cycle" and some have came close with three day cycle, only to see stripped out floors remain idle whilst the following Trades struggled to never catch up. If a floor sits idle after stripping with no work taking place then it raises the question of "Why it was so urgent to pour in the first place"?

For me (and this is for a purely all Reinforced Concrete Structure) a 5 day floor to floor cycle, with 4 - 5 floors of back propping and the following trades another 2 floors below the last propped floor, it works smoothly with no major hic ups. Granted it may be different for Composite Structures.

Anyway, welcome to Thailand when you get their fill time. I am still 4 years away

pipoz

[sirineou]

Hi Sirineou,



Careful calling me a Professional, it could be an insult to Others. Spent the last 18 years working oversees for one reason or another, as it got a bit claustrophobic and expensive in Australia. Ultimately it enticed me to visit Thailand and from there no turning back.

Have done a few Tower in my time and all are different, some of a composite structure (structural steel, metal decks & concrete), others of RC & Post tension slabs and the current one is a straight up Reinforce Concrete structure. Have used some Doka copy system from the Ukraine & Turkey on other projects, plus the RMD system, but Doka always comes out on top. Doka for me is the easiest system to use

One good thing about working overseas, No Unions to contend with, and I will leave it at that.

I have known of projects that tried to achieve the "Holy Grail - Two Day Cycle" and some have came close with three day cycle, only to see stripped out floors remain idle whilst the following Trades struggled to never catch up. If a floor sits idle after stripping with no work taking place then it raises the question of "Why it was so urgent to pour in the first place"?

For me (and this is for a purely all Reinforced Concrete Structure) a 5 day floor to floor cycle, with 4 - 5 floors of back propping and the following trades another 2 floors below the last propped floor, it works smoothly with no major hic ups. Granted it may be different for Composite Structures.

Anyway, welcome to Thailand when you get their fill time. I am still 4 years away

pipoz

No disrespect to others intended, meant professional in the High-rise construction industry , many of them have a thing or two to teach me about residential construction and have learned a lot from them in this forum.