Steel Strength

Specific to discussions about rebar and metal concerning strength, suitability or other considerations.

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Steel Strength

Postby dozer » Fri Jan 18, 2008 1:24 pm

There was a recent question on how to tell if you are getting good steel, but it was a slightly different discussion, so I thought I would start this thread.

Recently I was checking the structural characteristics of a ground beam with another board member who is also a structural engineer. I had also been using a program called eTabs to check the structural characteristics of a roof truss. For both the user must know the strength characteristics of the steel that is to be used (ie. this will be an input to any computer program which is used to verify the steel used in any building structure).

The steel sold in Thailand primarily comes in light (bow), full (dem) and industrial (Baugh laa saw). However these are not meaningful as per the strength of the steel. As it turns out the specification that identifies the strength of the metal is on the metal itself. The full grade (dem) is SD30. You can see this imprinted on the side of the rebar itself, at least on the 12 mm (4 hun) rough.

This number of SD30 means that a one millimeter thick strand could support 30 kilos before become deformed. (I welcome corrections)...

You can see some comments by a Thai manufacturer about various steel strengths and note that SD30 is the minimum suggested for high-strength reinforced concrete
http://www.namhenggroup.com/steel/en/steel_home_en.php
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Postby steady » Tue Jan 22, 2008 8:26 am

Dozer,
You say "This number of SD30 means that a one millimeter thick strand could support 30 kilos before become deformed. (I welcome corrections)... "

Does this mean (that using my rudimentary maths) that a 16mm diameter rebar has an approximately 6 ton limit before deformation. My thinking is: area of 16mm dia = pie r x r
3.142 x 8 x 8 = 201.

That's 201mm, therefore 30 kg x 201 = 6032 kg.

If this is the case does it follow that if the concrete is good a support made with say 8 pieces of rebar has a strength to support approx 48 tonnes?

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Postby dozer » Wed Jan 23, 2008 8:57 pm

As to your first point, in theory correct. If one one millimeter strand supports 30 kg, 2 would support 60, etc. Once you have you steel encased in concrete different considerations may come into play. You need to test various structures such as ground beams etc. with a stress testing program such as ProKon which will give you as an output how much rebar is needed for a certain load.
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Postby atlas_shrugged » Fri Jan 25, 2008 10:49 am

> That's 201mm, therefore 30 kg x 201 = 6032 kg.
>
> If this is the case does it follow that if the concrete is good a support made
> with say 8 pieces of rebar has a strength to support approx 48 tonnes?

Yes, but "support" in this case doesn't mean what you think it does. Steel of that cross section could be used to LIFT such a load, as in a cable used to lift a heavy truck. In a house, the supporting is done by columns, and the load in a column is supported by the concrete, not the steel (to a first approximation). This is why columns typically use so much less steel than beams.

Concrete is used to resist compressive loads as it's VERY strong in compression, such as the load on a column. Steel is used to resist tensile loads such as you'll find on the bottom at the middle of a beam.

Inside a beam the loads multiply massively so you need structural analysis software to calculate the steel required.

For simple analysis of beams and slabs, get Prokon CalcPad software. Don't be afraid-- it's much easier to use than a guess.

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Re: Steel Strength

Postby AussieBoy » Mon Aug 11, 2008 7:45 pm

Steel has always been tensile strength, the amount of force needed to stretch a piece of steel before it fails to return to it given length,

In Oz reo bar is 450Mpa tensile, simple terms 45,000 tonnes would be needed to pull 1 mt2 of steel, above that the steel would stretch

So if you had a block of steel 1 metre square and lets say 2 meters long, if you could hold one end and pull the other, you would need to apply a force greater than 45,000tonnes to stretch it.

Concrete is good in compression 200KSC or 20Mpa would be 2,000 tonnes pressure over 1 mt2

for a 12mm reo bar in OZ thats about 5 tonnes force before it streatches

I dout that Thailand would have such high strength steel as standard, only 1 standard here for reo, which is 450MPA, high strength can be made to special order

10 odd years ago it was only 300 mpa reo bar BHP up the strength Or Bluescope Steel sold off from BHP

trying to bend a 16mm reo bar here by hand is dam difficult, had no problems with the Thai bar, standard bar brought from Global

general steel is now 400-450 for most steel columns upgrades in last 2 years from 350mpa here in OZ

Would be interesting to know what the MPa rating is for thai reo

just looking on the web found thai steel comparison to OZ standard, steel below 32mm dia meets OZ requirements, doesnt say what grade was used from Thai, so I presume it would be stand available steel.
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Re: Steel Strength

Postby warder » Thu Mar 11, 2010 1:38 am

guys with a beam .2m width x .4m depth what size and how many strands ( I was thinking of using 16 mm ) would I need to span 4m and 5m. plus would I need to increase the concrete dimensions also.
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Re: Steel Strength

Postby jazzman » Thu Mar 11, 2010 10:30 am

0.40 x 0.20 is standard. The metal depends on what it is going to support. if it is going to support a concrete floor and /or brick walls, take RB1 as an example for 3.50 to 4.0 m
Much over 4.00 m you will need to look at RB2 or RB 1'
You should not use the cheapest (low density) steel for ring beams. Use deformed steel (DB) bar rather than smooth round bar (RB).
However, you should preferably be letting your architect decide all this. I accept no responsibility for the accuracy of this information or its accompanying illustration.
The braces are generally every 20 cm and of round 6 mm bar. The cheapest will do - all it does is keeps the main bars apart and in place.
Concrete in beams MUST be vibrated, although most Thai teams that I have seen don't even bother or own the tool. A vibrator costs only Baht 5,000 and it's not worth skimping on. The concrete should be laid in one contiguous pour, meaning that you should order ready-mix, prefereably from a genuine CPAC (blue and white striped trucks) and silos, or Nok Pech Insee (red & white trucks and silos) depot where it is mixed and loaded into the trucks by computer. Cheapo concrete mixed by guesswork and shovelling the ingredients in to the truck tends to have far too much water - not good at all for strength. To be sure, use a slump test if you know how to do it, or fill a bucket and if there is more than a thin film of water on the top, or you can push your finger down into it too easily, it is too wet. Slump test - if you turn a bucketfull upside down, the concrete mix should be stiff enough to hardly collapse round the sides, but fluid enough to fill all the corners of the formwork. Use steng 210 or better (Portland cement).

Local price Khon Kaen as at 10 March 2010: baht 1,770 per cu (m3).
Click the image for full size.
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How to build a $20,000 / £14,000 house and a $???? MOTEL Updated 21 March 09 - with BOQ and costs
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Re: Steel Strength

Postby warder » Fri Mar 12, 2010 2:54 am

Jazzman many many thanks for your reply very informative I will buy you a beer one day, we will be half way between nakhonratchasima and khon kaen.
P.S will not hold you responsible if the lounge lands on my car.
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Re: Steel Strength

Postby jazzman » Fri Mar 12, 2010 4:20 am

That beer might be sooner than you think - I drive through that very area once or twice a week :D
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Re: Steel Strength

Postby warder » Fri Mar 12, 2010 11:11 pm

No problem should be moving there for good I hope in July this year. If the move is delayed for any reason we will be there in May for a month.
:D Mark
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Re: Steel Strength

Postby developer3d » Mon Jun 14, 2010 1:07 am

Bringing together both of these important elements ( steel and concrete ) is the fact that the steel reinforcement must be placed within correct tolerances within the concrete to be of design strength.

You can use the correct steel and the best concrete mix but if the steel is not correctly placed in the concrete you will have a massive reduction in the designed strength of the structure.

The diagram that Jazzman supplied above shows the bars "embedded" inside the beam, there are minimal requirements for this on all structural concrete.

You must check how the steelwork is being held in place, it MUST NOT sit on the bottom on the formwork, its too heavy and rigid to be "hooked" and if does rest of the formwork when the concrete sets where will the steel be in the beam ?

The steel must be in the beam not on the edge of the beam.


And whilst I appreciate that this is Thailand a lot of the "bar chairs" etc are not available but you can do this easily like this.

fig0624.jpg
fig0624.jpg (19.48 KiB) Viewed 2903 times


fig0626.jpg
fig0626.jpg (14.59 KiB) Viewed 2903 times


For those interested in the "technical discussion" here is a link to a "steel placement handbook" that shows the strength loses associated with incorrect placement of reinforcing steel.

http://www.iowadot.gov/construction/.../steelplacementhandbook.pdf
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Re: Steel Strength

Postby geordie » Mon Jun 14, 2010 3:27 am

tried the link does not want to play
my comments may be wrong but never deliberately
If it aint broke, dont fix it
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Re: Steel Strength

Postby developer3d » Mon Jun 14, 2010 8:12 am

Yes it is a deep link and I think that is affecting it. File is too large to attached ( forum is understandably limited )

Summary

Too little concrete cover is bad - Too much concrete cover is bad

Another study entitled "DURABILITY PROBLEMS OF CONCRETE STRUCTURES IN THAILAND AND THEIR SOLUTIONS" found

"Most of the damages were visible in form of steel corrosion" and further that "Most of the steel corrosion problems were due to poor construction such as insufficient concrete cover or honeycombing" - a problem of mix proportioning

So here is the 'meat"of the document - apologies for those not interested in this much detail.

Credit for information - Iwoa DOT http://www.iowadot.gov/

CONCRETE COVER

When the reinforcing steel is placed with less concrete cover than required by design, the life of the reinforcing steel can be
shortened due to corrosion from increased exposure to deicing materials and/or the elements. Corrosion of the reinforcing steel will cause an increase in the diameter of the steel, which will cause the concrete outside the steel to sometimes spall off and the concrete around the reinforcing steel to become debonded from the reinforcing steel. This debonding of the concrete from the reinforcing steel can reduce the strength of the structure by preventing the necessary interaction between the concrete and the reinforcing steel. Proper bonding between the reinforcing steel and the concrete can also be prevented by debris on the bar such as dirt, mud, oil, or corrosion when the concrete is poured around it.

Too much concrete cover, on the other hand, will reduce the strength of the structure. A common misconception is that if the minimum concrete cover is good then more concrete cover is even better. However, if the reinforcing steel is placed with more cover than designed for, the strength of the structure will be significantly reduced, as explained later in the FLEXURAL DESIGN THEORY section.

STRUCTURAL BACKGROUND

Concrete is strong in compression and weak in tension. Steel, however, has strength in both tension and compression. In a
reinforced concrete structure, the two materials are used together in a manner that will make the best use of the strengths of each.

Consider the situation shown in figure 1. This is a common situation for a reinforced concrete section between two supports.
This situation could represent a bridge slab supported between two beams or supported between an abutment and a pier, or it could represent a culvert slab, wall, or floor.

The reinforcing closest to the surface of the concrete is usually the main reinforcing and runs from one support to the other. The main reinforcing can be either at the top of the section or the bottom of the section depending on how the section is designed to be loaded. The function of the reinforcing that is placed perpendicular to the main reinforcing is to distribute stresses between the main reinforcing and help control cracking of the concrete.

The location of the main reinforcing steel depends on the direction of flexure of the structure. The main reinforcing in a
reinforced concrete box culvert wall is the vertical reinforcing steel on both sides of the wall. The main reinforcing in a single
barrel reinforced concrete box culvert slab is the bottom transverse reinforcing steel. The main reinforcing in a bridge slab
supported by beams is both the top and the bottom transverse reinforcing steel.

FLEXURAL DESIGN THEORY

Somewhere between the main reinforcing and the opposite edge of the concrete is an imaginary line called the neutral axis, (see figure 2). Material on one side of the neutral axis is in compression and material on the other side of the neutral axis (the side containing the main reinforcing) is in tension. The neutral axis has neither compression nor tension since it is the point at which the stress changes from tension to compression.

Consider the situation shown in figure 1. The main reinforcing in this situation would be the lowest layer of reinforcing bars.
Notice that these reinforcing bars run from one support to the other. When the section is loaded as shown in figure 2, the section flexes downward. When the section flexes downward the concrete and reinforcing steel above the neutral axis are in compression and at the same time, the concrete and reinforcing steel below the neutral axis are in tension. As the section below the neutral axis is stretched by the tensile force, the steel and concrete stretch together at the same rate due to the bond between the reinforcing steel and concrete. During this stretching, the reinforcing steel elongates and retains its tensile strength. The concrete in the tension zone however, cracks and contributes no tensile strength to the structure (see figure 3). Therefore, the concrete below the neutral axis is disregarded when determining the flexural strength of the structure.

The theoretical design section shown in figure 4 is a cross section of the structure looking along the main reinforcing showing only the materials considered to be contributing to the strength of the structure. The dashed lines in figure 4 represent the area of cracked concrete that does not contribute to the strength of the structure. The steel in the compression zone does not significantly increase the compressive strength of the concrete and therefore the area above the neutral axis is assumed to be solid concrete for design purposes.

Placing the reinforcing steel with more than the design cover causes the neutral axis to be shifted higher in the section, which reduces the area of concrete that is in compression and increases the cracked area of concrete in tension. This decrease in useful concrete and increase in useless concrete greatly reduces the strength of the structure.

LAP LENGTH

The design lap length is usually a minimum length required to transfer stress from one bar to another. Consider for example, the longitudinal steel in the slab of a bridge or in a culvert, or the vertical steel in the back face of a culvert wall. It would be ideal to make these bars one continuous bar, this however would be impractical due to difficulties in transporting and handling the steel.

In order to achieve the same effect as having one continuous bar, the design will call for shorter bars and minimum lap lengths. If the actual lap length is less than the required lap length, the stress may not be transferred to the other bar, which could cause a failure in the structure at that lap location.

DEVELOPMENT LENGTH

The development length is often shown on the plans as a minimum embedment length. The purpose of the development length is to anchor the reinforcing bars beyond the area where the strength of the bars is needed. Without the required development length, the reinforcing bar would pull out of the concrete surrounding it and the structure could fail. A typical example of this situation would be the top transverse reinforcing bars in the cantilevered section of a bridge slab outside the exterior beam of a bridge. The critical section of the slab is just outside the outside edge of the beam supporting the slab. The reinforcing bars must extend into the slab beyond the critical section for a required length. If the reinforcing bars do not extend beyond the critical section sufficiently, they will be pulled out and the structure will fail.

BAR SPACING AND BAR SIZE

The amount of reinforcing steel in the tension area of the structure also has a large impact on the strength of the structure. Both the spacing and the size of the reinforcing bars control the amount of steel in the tension area. The impacts to flexural strength from deviations in bar spacing and bar size are shown in figure 5.

steelplacementhandbook_Page_09.jpg
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Re: Steel Strength

Postby ningnong » Tue Jun 15, 2010 2:06 am

Thanks developer3d, that digest was very useful and has addressed quite a few missing areas in my understanding of the method.

If you know the answers would you mind answering a couple of more questions?

1. Why do they not treat the steel to prevent rusting before embedding it in concrete?

2. Some re-bar is smooth, but some has corners and has been twisted, and another sort appears to have some ridges or other surface features, presumably to provide additional friction - are they used for different purposes or are they interchangable?

Thanks
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Re: Steel Strength

Postby developer3d » Tue Jun 15, 2010 9:31 am

The bond strength that concrete makes with the steel is proportional to the contact area surface of the steel to the concrete. In other words, the greater the surface of steel exposed to the adherence of the concrete, the stronger the bond.

So you are correct in assuming the deformed reinforcing bar are better in this regards than plain round or the square one ( I haven’t seen square in Thailand )

In fact, when plain bars of a given diameter are used instead of deformed bars, approximately 40 percent more plain bars must be used.

Each type is useful for a different purpose, and engineers design structures with these purposes in mind. Plain bars are round in cross section. They are used in concrete for special purposes, such as dowels at expansion joints, where bars must slide in a metal or paper sleeve, for contraction joints in roads and runways. They are the least used of the rod type of reinforcement because they offer only smooth, even surfaces for the adherence of concrete.

Or where someone is trying to save a few baht - yes I know that round bar is everywhere in everything in Thailand but it is NOT as good as deformed bar in the way it bonds to the concrete.

Re the “rust treatment” question the adherence of the concrete depends on the roughness of the steel surface—the rougher the steel the better the adherence. Thus steel with a light, firm layer of rust is superior to clean steel, but steel with loose or scaly rust is inferior.

Loose or scaly rust may be removed from the steel by rubbing the steel with a sack. The requirements for reinforcing steel are strong in tension and, at the same time, ductile enough to be shaped or bent cold.

The problem with corrosion is long term exposure of the steel to the elements – ie ground, water, air,

If all else fails you could just do this …. Haha

bamboo-reinforced-concrete.jpg


bamboo-reinforced-concrete-7.jpg


Actually the use of bamboo as a reinforcing material in concrete is very well documented but that is a brand new set of shop houses not some dirt poor farmers house and I think they could afford some steel for the driveway and floors..
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