How much Rebar in a 6.3Meter ring beam ?

[SurinBeach]

Gents,

I'm building a large sala, about 6.3Meters x 6.3Meters, my Farang builder and his team just wanted to put in 5 lengths of 12 mm rebar in a beam that's 20cm wide 45cms high.

My farang architect mate is suggesting 3 pieces of Rebar at the top and bottom each, and 2 half way up. Rebars not so expensive so I'm happy with this too (due to length of span)

Any comments greatly appreciated as we'll be pouring concrete sometime Monday afternoon I reckon !

Regards

SB

[cooked]

Concrete fails under tension, not compression. This means that steel at the bottom of the beam is important, but at the supports there should be more steel also, at the top. If an engineer designs it, he will have some of the rebar bending up to the top of the beam over a pillar. I spent a lot of time designing beams and stuff (long time ago) and eventually realised that the money to be spent by hiring an engineer could be saved by using the very simple formula, area of steel in a cross section = 4% of the beam area, which always worked out in excess of what an engineer would recommend. I just put in some foundations and they had 4 12 mm rods built in.

So you have 900 sq. cm section, you should have 900 x 4% = 36 sq cm of steel. I reckon 12 mm is very weak as one rod is 2.25, sq. cm making sq cm. 12 sq. cm section with 5 rods. Six metres is a big span and to be taken seriously. The concrete alone is very heavy. I would be thinking along the lines of at least 16mm ribbed steel. with two- three rows at the bottom and one- two at the top. Sixteen 16 mm rods. I may be wrong, I hope somebody comes up with a better response.. You may have to cancel the pouring. Sleep well!

[SurinBeach]

Thanks Cooked ! Don't think I'll sleep at at all as that's twice the number of rods as we were gonna upgrade to tomorrow !

Many thanks for your reply

Regards

SB

Concrete fails under tension, not compression. This means that steel at the bottom of the beam is important, but at the supports there should be more steel also, at the top. If an engineer designs it, he will have some of the rebar bending up to the top of the beam over a pillar. I spent a lot of time designing beams and stuff (long time ago) and eventually realised that the money to be spent by hiring an engineer could be saved by using the very simple formula, area of steel in a cross section = 4% of the beam area, which always worked out in excess of what an engineer would recommend. I just put in some foundations and they had 4 12 mm rods built in.

So you have 900 sq. cm section, you should have 900 x 4% = 36 sq cm of steel. I reckon 12 mm is very weak as one rod is 2.25, sq. cm making sq cm. 12 sq. cm section with 5 rods. Six metres is a big span and to be taken seriously. The concrete alone is very heavy. I would be thinking along the lines of at least 16mm ribbed steel. with two- three rows at the bottom and one- two at the top. Sixteen 16 mm rods. I may be wrong, I hope somebody comes up with a better response.. You may have to cancel the pouring. Sleep well!

[SurinBeach]

Would this be your same 4% Mr Cooked ?

7. The maximum area of either the tension or compression reinforcement in a horizontal element is 4% of the gross cross-sectional area of the concrete.

Concrete fails under tension, not compression. This means that steel at the bottom of the beam is important, but at the supports there should be more steel also, at the top. If an engineer designs it, he will have some of the rebar bending up to the top of the beam over a pillar. I spent a lot of time designing beams and stuff (long time ago) and eventually realised that the money to be spent by hiring an engineer could be saved by using the very simple formula, area of steel in a cross section = 4% of the beam area, which always worked out in excess of what an engineer would recommend. I just put in some foundations and they had 4 12 mm rods built in.

So you have 900 sq. cm section, you should have 900 x 4% = 36 sq cm of steel. I reckon 12 mm is very weak as one rod is 2.25, sq. cm making sq cm. 12 sq. cm section with 5 rods. Six metres is a big span and to be taken seriously. The concrete alone is very heavy. I would be thinking along the lines of at least 16mm ribbed steel. with two- three rows at the bottom and one- two at the top. Sixteen 16 mm rods. I may be wrong, I hope somebody comes up with a better response.. You may have to cancel the pouring. Sleep well!

[cooked]

Would this be your same 4% Mr Cooked ?

7. The maximum area of either the tension or compression reinforcement in a horizontal element is 4% of the gross cross-sectional area of the concrete.

Concrete fails under tension, not compression. This means that steel at the bottom of the beam is important, but at the supports there should be more steel also, at the top. If an engineer designs it, he will have some of the rebar bending up to the top of the beam over a pillar. I spent a lot of time designing beams and stuff (long time ago) and eventually realised that the money to be spent by hiring an engineer could be saved by using the very simple formula, area of steel in a cross section = 4% of the beam area, which always worked out in excess of what an engineer would recommend. I just put in some foundations and they had 4 12 mm rods built in.

So you have 900 sq. cm section, you should have 900 x 4% = 36 sq cm of steel. I reckon 12 mm is very weak as one rod is 2.25, sq. cm making sq cm. 12 sq. cm section with 5 rods. Six metres is a big span and to be taken seriously. The concrete alone is very heavy. I would be thinking along the lines of at least 16mm ribbed steel. with two- three rows at the bottom and one- two at the top. Sixteen 16 mm rods. I may be wrong, I hope somebody comes up with a better response.. You may have to cancel the pouring. Sleep well!

Not quite. We were taught that this was a quick fix if you couldn't be bothered to calculate the steel required. Actually I had the figure of 4.3% in my head but that was nearly 50 years ago. I don't remember the 'maximum' bit though. So it probably does amount to the same thing, yes. When I compared results using the scientific method with this one, there was a difference of about 20 % in quantity if I remember aright.

Edited February 1, 2015 by cooked

[Anthony5]

Would this be your same 4% Mr Cooked ?

7. The maximum area of either the tension or compression reinforcement in a horizontal element is 4% of the gross cross-sectional area of the concrete.

Concrete fails under tension, not compression. This means that steel at the bottom of the beam is important, but at the supports there should be more steel also, at the top. If an engineer designs it, he will have some of the rebar bending up to the top of the beam over a pillar. I spent a lot of time designing beams and stuff (long time ago) and eventually realised that the money to be spent by hiring an engineer could be saved by using the very simple formula, area of steel in a cross section = 4% of the beam area, which always worked out in excess of what an engineer would recommend. I just put in some foundations and they had 4 12 mm rods built in.

So you have 900 sq. cm section, you should have 900 x 4% = 36 sq cm of steel. I reckon 12 mm is very weak as one rod is 2.25, sq. cm making sq cm. 12 sq. cm section with 5 rods. Six metres is a big span and to be taken seriously. The concrete alone is very heavy. I would be thinking along the lines of at least 16mm ribbed steel. with two- three rows at the bottom and one- two at the top. Sixteen 16 mm rods. I may be wrong, I hope somebody comes up with a better response.. You may have to cancel the pouring. Sleep well!

Yes. Actually I had the figure of 4.3% in my head but that was nearly 50 years ago. I don't remember the 'maximum' bit though.

Would be interesting to find out what the 'minimum' bit is.

[cooked]

Sorry, I corrected my post. To calculate the 'minimum' you need an engineer. I learnt my stuff in pounds per square inch and so on, so I'm no use to you. I did quite a bit of building and eventually stopped calculating, you get a feeling for what is needed once the form work is set up. The 4% formula is a good guide though. Just bung it in!

[cooked]

Go to bed.

[SurinBeach]

Agreed re the 4% and bung it in , its trying to persuade my builder(s) its required eventhough its me who's paying for it !

Would you say the 2% I will/might have is definitely not enough, ...remember I have no slab, just supporting a roof for the sala ie like a carport ....

You in bed yet ? 5555....

[Anthony5]

Agreed re the 4% and bung it in , its trying to persuade my builder(s) its required eventhough its me who's paying for it !

Would you say the 2% I will/might have is definitely not enough, ...remember I have no slab, just supporting a roof for the sala ie like a carport ....

You in bed yet ? 5555....

I wouldn't risk with 5 x 12mm on a beam that's almost twice the length without support as recommended.

I hope you're also aware that there are 4 grades of rebar in Thailand, and that you only can know which one you have in hand if you have a trustable supplier or put them on the scale.

Did I really use the word trustable in connection with a Thai supplier ?

[eyecatcher]

Did i miss something but there id nothing to say the 6m ring beam is unsupported.

It can be 10m or 30m long and it makes not much difference IF the ring beam is sset in a ground trench as opposed to 3m high.

If its in a ground trench 3 at the bottom for assurance would be fine.

A sala did you say? Thats a concrete deck with a fancy pergola atop isnt it?.

The question to resolve is where is the ring beam ground or elevated.

[Anthony5]

Did i miss something but there id nothing to say the 6m ring beam is unsupported.

It can be 10m or 30m long and it makes not much difference IF the ring beam is sset in a ground trench as opposed to 3m high.

If its in a ground trench 3 at the bottom for assurance would be fine.

A sala did you say? Thats a concrete deck with a fancy pergola atop isnt it?.

The question to resolve is where is the ring beam ground or elevated.

Doubt he gone put the roof at ground level.

.remember I have no slab, just supporting a roof for the sala ie like a carport ....

[Crossy]

Our ring beams are 0.2 x 0.4m. One span is 6.8m it contains 6 x 15mm bar (4 at the bottom, 2 at the top) supported by 6mm rectangles every 175mm.

About 1.5% steel area.

This design was checked by the local building engineer when he approved our house plans.

EDIT Just checked the drawings and corrected span to 6.8m.

[cooked]

Well then, use 16mm and you'll feel better.

[Crossy]

This page http://www.rcsolver.com/en/eurocode-concrete-design-articles-ACI_318_11_concrete_beam_design_parameters has some details on both maximum and minimum steel areas.

[cooked]

Yes, plenty of those, but unless you know the tensile strength of the steel and the strength of the concrete as well as other parameters not much use. One thought: your contractors WILL be using a concrete vibrator of course, will they not?

[IMHO]

Seriously, consult an engineer.

[SurinBeach]

Morning all and thanks for the feedback, this is the ring beam that will be about 2.5 meters up in the air supporting the roof ....

The ground slab is poured already

School runs beckon, will reply after the littluns have been delivered !

[SurinBeach]

Here's some pics ....

I know (will I imagine) that my issue is the span being 6 Meters, just to highlight this is the ring beam that will just support the roof, there will no slab on this ring beam in question ....

Can you get lightweight concrete in Thailand ?

Regards,

SB.

[SurinBeach]

Hi Cooked,

I'm (badly) trying to do some calculations of the percentageof Rebar to be used in my Beams ...

I'm assuming you are right and I am wrong so can you assist my my failed O levels maths ....

You say the area of 12mm rods = 2.25cm.

Assuming area = Pi x Radius Squared

When I calculate 12mm diameter, I'm saying radius squared = 36

Multiplied by Pi = 36 x 3.14 = 113mm

Which is exactly half yours ? What am I doing wrong ?

Best Regards

SB

Concrete fails under tension, not compression. This means that steel at the bottom of the beam is important, but at the supports there should be more steel also, at the top. If an engineer designs it, he will have some of the rebar bending up to the top of the beam over a pillar. I spent a lot of time designing beams and stuff (long time ago) and eventually realised that the money to be spent by hiring an engineer could be saved by using the very simple formula, area of steel in a cross section = 4% of the beam area, which always worked out in excess of what an engineer would recommend. I just put in some foundations and they had 4 12 mm rods built in.

So you have 900 sq. cm section, you should have 900 x 4% = 36 sq cm of steel. I reckon 12 mm is very weak as one rod is 2.25, sq. cm making sq cm. 12 sq. cm section with 5 rods. Six metres is a big span and to be taken seriously. The concrete alone is very heavy. I would be thinking along the lines of at least 16mm ribbed steel. with two- three rows at the bottom and one- two at the top. Sixteen 16 mm rods. I may be wrong, I hope somebody comes up with a better response.. You may have to cancel the pouring. Sleep well!

[SurinBeach]

Anyways so the plan now is, since the rebar is "up" and consists of 5 x 12mm rods

To add 3 x 16mm rods so there are 3 bottom 2 middle 3 top ....

Will work on my percentage area when I know I'm doing the correct calculations !

Many thanks

SB....

[SurinBeach]

One more item, the new Carport is next, and the size is 4meters wide x 9 meters long supported by a column 4meters from one end (5 meters from the other)...

Suggestions for beams and columns/pillars please ?

[SurinBeach]

Aghhhhhhhhhh !

They're putting in 16mm Rebar but have just cut the 10 meter sections into 2 x 5 meters and I guess will splice to save THB rather than use one continuous piece to cover the width which is 6.3 Meters ....

Is this OK ? .......

I'm not pouring concrete until I get the Rebar work checked by an Engineer now !....

[Anthony5]

Aghhhhhhhhhh !

They're putting in 16mm Rebar but have just cut the 10 meter sections into 2 x 5 meters and I guess will splice to save THB rather than use one continuous piece to cover the width which is 6.3 Meters ....

Is this OK ? .......

I'm not pouring concrete until I get the Rebar work checked by an Engineer now !....

Haha, the TV experts got another victim.

Ever realised how relaxed life can be when you don't visit this forum?

[SurinBeach]

Actually here's the areas ....

http://en.wikipedia.org/wiki/Rebar

So

12mm diameter = 113 Area (mm²)

16mm diameter = 201 Area (mm²)

Edited February 2, 2015 by SurinBeach

[SurinBeach]

Worrying is OK otherwise I would have had a cracked concrete beam some time down the road (if I let them pour the concrete on the rebar they insisted was FINE !...

I'll worry now, get it done right, then relax under my sala drinking cold Chang "knowing" its not coming down on my head any time soon...

(Hopefully)

Aghhhhhhhhhh !

They're putting in 16mm Rebar but have just cut the 10 meter sections into 2 x 5 meters and I guess will splice to save THB rather than use one continuous piece to cover the width which is 6.3 Meters ....

Is this OK ? .......

I'm not pouring concrete until I get the Rebar work checked by an Engineer now !....

Haha, the TV experts got another victim.

Ever realised how relaxed life can be when you don't visit this forum?

[cooked]

Hi Cooked,

I'm (badly) trying to do some calculations of the percentageof Rebar to be used in my Beams ...

I'm assuming you are right and I am wrong so can you assist my my failed O levels maths ....

You say the area of 12mm rods = 2.25cm.

Assuming area = Pi x Radius Squared

When I calculate 12mm diameter, I'm saying radius squared = 36

Multiplied by Pi = 36 x 3.14 = 113mm

Which is exactly half yours ? What am I doing wrong ?

Best Regards

SB

Concrete fails under tension, not compression. This means that steel at the bottom of the beam is important, but at the supports there should be more steel also, at the top. If an engineer designs it, he will have some of the rebar bending up to the top of the beam over a pillar. I spent a lot of time designing beams and stuff (long time ago) and eventually realised that the money to be spent by hiring an engineer could be saved by using the very simple formula, area of steel in a cross section = 4% of the beam area, which always worked out in excess of what an engineer would recommend. I just put in some foundations and they had 4 12 mm rods built in.

So you have 900 sq. cm section, you should have 900 x 4% = 36 sq cm of steel. I reckon 12 mm is very weak as one rod is 2.25, sq. cm making sq cm. 12 sq. cm section with 5 rods. Six metres is a big span and to be taken seriously. The concrete alone is very heavy. I would be thinking along the lines of at least 16mm ribbed steel. with two- three rows at the bottom and one- two at the top. Sixteen 16 mm rods. I may be wrong, I hope somebody comes up with a better response.. You may have to cancel the pouring. Sleep well!

Hi hi I was drunk! Seriously for some reason I screwed up there. Sorry