在住宅建筑中使用500或高级TMT钢筋的一些问题

Some contractors report a tendency among few design Engineers to specify Fe-500 or higher grade steel in residential buildings citing its high strength. Customers are advised that if they use Fe 500 or higher grade steel in building construction, that would help in decreasing the quantity of steel used and reduce the column sizes.

除了玫瑰色的部分，Fe 500级钢铁在施工过程中可能会构成许多特定地点问题，尤其是对于小型建筑商而言。考虑到报告的失败和FE-500级或更高级的问题，建议在住宅和商业建筑中使用Fe 415，并且仅在根据该级别进行整个设计时才能使用FE500。这在下面说明；

Usually, Concrete, a mixture of cement, sand and aggregate, is considered by far as the most stable of building compounds. But it has negatives also: low tensile strength and ductility. This means that concrete’s ability to stretch and to withstand pressure at an angle without breaking is very less.

Here Steel has its role. Steel in the form of bars has great tensile strength and ductility. It can reinforce concrete. Thus the quality of steel has an important role in deciding the quality of concrete. Quest for better material has produced various kinds of steel bars with qualities that make concrete more stable. It started with mild steel Plain bars, evolved through deformed plain bars and then came Cold Twisted Steel Bars (TOR steel) .Now the Market is dominated by various grades of TMT.

Fe 415 TMT bars became the most commonly used steel grade in construction of houses in the state as it is easier to produce the desired strength equivalence of CTD bars through the TMT process than through the conventional CTD bar production process apart from the other advantages of TMT over CTD.

TMT Steel-FE 500及以上的较高等级可能在高层建筑中使用。这是因为使用FE 415钢将需要更多的钢筋来提供所需的强度。那将增加列的大小。在空间处于溢价的时候，这是一个严重的问题，为什么要高涨。

Fe 500是使用用于生产Fe 415的相同TMT工艺生产的。TMT工艺的美丽是，它可以通过在过程中进行略有更改来产生不同等级的钢。如果将钢淬灭一些，则为钢钢筋提供高强度的外部马氏体层变得更厚，以赋予其具有延展性的软内芯为代价。这正是Fe 500 TMT钢中发生的情况。

Fe500 allows us to save cost

Usually steel producers suggest their clients that use of Fe500 grade steel instead of Fe 415 saves cost. But it is true only when the entire design is made according to that grade. Indian standards specify that the MS bars have a tensile strength of 250 N/mm²。

When steel is detailed it is designed to resist a higher load than what it really has to struggle with. Similarly while designing RCC for buildings factor of safety ‘3’ is considered for cement concrete for the cube strength. In the case of steel “1.8” is considered as factor of safety. In short, permissible strength is equal to yield strength divided by 1.8.Thus while calculating load of a structural member and on designing its steel, we provide sufficient factors of safety.

Thus an engineer who makes his design with Fe 415 usually considers the steel to deform at 230 N/ mm²。But as you know even if it is considered to deform at lower loads, Fe-415 resists changes only if it crosses its yield strength. As an example for a residential building or a 3 storied commercial complex, the engineer designs at 230N/mm²and the use of Fe-415 already gives sufficient protection. Here using Fe-500 is not economical as it costs more. Savings materialize only if the design is made for Fe-500. Practically the civil designs are made considering the factor of safety aspect. Engineers make use of limit state method for detailing. Normal loads taken into consideration are dead load, live load, earthquake load, wind load etc., and their combinations. While designing special attention is taken to consider both acting load & material strength. If the engineer underestimates the load, it is unsafe. If load is overestimated, it is safe but turns uneconomical and against the basics of engineering

(Design load = Characteristic load x practical safety factor for load)

Where characteristic load is the maximum working load that the structure could withstand

Design Engineer take partial safety factor. A factor of safety of 1.5 is taken when loads like dead load, live load, wind load are considered separately. But when combinations of loads are taken into consideration, they consider a factor of safety of 1.2. It means that the maximum working load is considered 1.2 times. If all the designs are altered by considering the parameters of Fe- 500 grade, then it’s usage is OK. Otherwise it costs more and is a waste and is against engineering.

Balancing ofTensile strength Vs Ductility

Dual core in Thermo mechanical Steel bars contributes to two distinct characteristics of steel bars. The outer tempered Martensite layer gives required tensile strength to the TMT while the inner ferrite –pearlite core gives it ductility. In any TMT grade, these should be in equilibrium. If one core exceeds the other, TMT will not have sufficient characteristics. Suppose outer core is more than inner core, TMT bars will have more strength; but compromising on its ductility .If the inner core is more, TMT will be called more ductile but with less strength.

It is clearly observed by Bureau of Indian Standards that in Fe-415 grade ductility is standardized as minimum 14.5 % elongation. Yield strength of Fe-415 is standardized to be minimum 415 N/mm²。Now consider the case of Fe-500 grade. Here yield strength should be minimum 500 N/ mm²。这是可观的；但在其延性特性上妥协。FE-500级的延展性标准化为最低12％。对于550级延展性（不突破的变形能力）再次减少到8％。

Steel Bars used in civil constructions must have sufficient yield strength. More over it should be ductile. Then only can steel elongate or deform on heavy loads and safeguard the buildings without breaking up. So the point is that Fe-500 or high grade must be used only when design usage requires it. Otherwise use of Fe-415 will be safer.

与Fe-500等级相关的弯曲问题

较高的强度和较低的延展性意味着Fe 500 bar不容易弯曲。Fe 500级对弯曲过程中诱导的较高菌株敏感。它不耐受弯曲的直径高于指定的最小弯曲直径。报告表明，如果弯曲直径通常小于指定的最小值，则可能导致故障等问题。在某些情况下，钢不会破裂。但是弯曲的压缩侧显示肋骨分裂。

For example, a 12 mm rebar of Fe 500 breaks when bent into a perfect ‘U’, unlike a similar rebar of Fe-415 grade. It might cause it to crack, too. Manual bending takes its toll on the masons. Hence hydraulic bending machines have to be used to bend the bar.

Bending a Fe-500 grade bar should be carried out very slowly, not with a jerk. Bending done on a bar bending table/ block is always very sharp. It weakens the TMT at tension side of bend portion as tempered martensite layer there, gets softened and it breaks. So on practical use, Fe-500 fails at construction sites.

使用Fe 500级钢筋的大多数高层建筑商可以采用工厂切割尺寸，避免进一步加以解决。他们的设计师为他们提供钢细节，以帮助他们购买架子的物品。小型建筑商可能没有这样的奢侈品。首先，他们的设计可能不是Fe 500钢，而是否定了钢数量的节省，同时支付了更高的购买价格。

Even if their design is for Fe 500 steel, they might not be able to take the due advantage due to a variety of reasons. They might not have access to a steel detail that gives the precise number of various types of structural steel elements needed for the building. Even if they have the steel detail and can buy factory-cut steel bars based on it, transporting them to work sites in the interiors through roads that hardly allow truck to pass is a difficult task. That is the main reason why factory cut steel has not picked up in Kerala.

In such a situation they will have to resort to fashioning the required steel elements at the work site itself. And then, an absence of the machinery required to bend the Fe 500 rebars would lead to an increase in labour costs and a decline in quality. This is especially true if the rebar has to be shaped into tight curves. In, short they would have to incur the extra cost without getting the perceived benefits.

Welding problems associated with Fe -500 grade

可焊性也是一个问题,铁500年级的部件l. A minimum level of carbon content is essential in steel to achieve the required strength. At the same time, excess carbon content threatens its property of weldability. Even though the carbon content in Fe -500 is advised to be kept at max 0.30%; practically steel with C ≥ 0.25will be better weldable. It is observed that in the case of welding a Fe -500 or Fe- 550 steel bar, the bar is raised to a temperature above its tempered temperature. Then without controlled quenching and tempering process, it is cooled to the ambient temperature. Through this cycle, steel bar loses the strength of its external case and reverts back to steel with lower yield strength. In short designers should not rely on welding Fe -500 or 550 grade steel bars.

ReBending problems associated with Fe- 500 grade

即使不建议使用TMT等级的抑制或反向弯曲，但在不可避免的建筑工地确实会出现。据报道，在相对较低的温度下，在较高的TMT棒中可以在较高的TMT棒中进行重大软化，同时进行弯曲 /反向弯曲。这导致钢强度的降低。在Fe 500级升级期间产生的缺口应变高得多，这导致酒吧被抢断。在某些情况下，折断不会存在。但是钢将消除表面裂缝和应变，导致过度腐蚀。建议将Fe 500级预热至100度的温度。这可以最大程度地减少工作硬化和延展性损失。但是实际上在网站上很难。

Performance of Fe 500 & Fe 550 at elevated temperatures

Reinforced concrete buildings are exposed to elevated temperatures during a fire event. Most often the elevated temperatures exceed 500 degree C. Unfortunately this level of heating is also above the tempering temperature of Fe-500 TMT bars. Thus prolonged exposure to elevated temperatures would result in retempering of the outer skin resulting in reversion to the strength of the core steel, which is vastlylesser.

因此，对于具有Fe 500或550年级设计的建筑物而言，RCC建筑物框架的加速故障更有可能。

使用FE-500等级的地震性能考虑

The main argument against the use of higher grade TMT is its behavior under cyclic loading. Studies in several seismic prone parts of the world notably New Zealand, Italy etc. have pointed to the difficulties associated with the use of Fe-500 and Fe 550 grades under cyclic loading particularly in Seismic zone 3 and 4. Kerala State is in Seismic Zone 3. A maximum limit for yield strength is desirable to be specified in standards used for earthquake design. The absence of such a maximum limit may lead to brittle shear failure of the structure. Requirements specified in IS: 1786 for Fe- 415 grade TMT bars are in line with the requirements of other countries for ductile design. However this doesn’t hold well for rebars of grade Fe 550 as per IS 1786. Cautious approach should be adopted in using rebar grades higher than Fe 415 especially Fe 550 grade where ductility of rebars is necessary for inelastic deformation of structural members as demanded by design philosophies. Such design cases are, earthquake designing, designing for impact load, designing of beams/ slabs with adjustment of support moments load, against gravity load etc.

In short, engineers must be cautious in the use steel of higher grades where yield strength in maximum is not limited and where ductility is lower, while doing building designs for seismic zone areas.

静态应力应变图的重要性

TMT酒吧presently are used for construction of concrete structures. IS 456 provides design stress strain curves of TMT bars. Usage of design curves of CTD bars, while doing design for TMT grades is not correct. If BIS comes out with design stress strain curve and design value of the yield strength of TMT bars, then only the design turns out to be economical. Using Fe 500 or Fe 550 grades using the design curves of CTD bars doesn’t yieldany economic benefit.

避免可能混合不同等级

Some engineers show a tendency to specify Fe-500 or Fe-550 grade steel where it is not required. It may be used in one part of the building. But the pragmatic decision is frequently taken to make all steel the same grade to avoid possible mix ups. However what happens in practice is that suppliers offer alternatives in order to reduce costs. Sometimes clients also look for other grades. Decision must be so cautious in recognizing this possibility of mix-ups in sites.

在网站和商店/仓库中堆叠和存储更高等级的TMT酒吧的问题。

从质量的角度考虑堆叠高级TMT棒。必须针对不同的直径杆进行优化的堆叠高度。堆叠高度越多，在下层的杆上的负载就越多。过大负载可能会损害TMT的表面特征，从而降低拉伸强度和粘结强度。还应避免使用粗糙的处理，冲击载荷和Fe 500和Fe 550级钢的降低。

t的重要性esting of Fe 500 & 550 grades

Designers/ Engineers should accept TMT Bars only after proper testing & verification of the same irrespective of the name of the brand/ manufacturer. Nowadays the market is flooded with so many inferior products which fail in mechanical testing in labs. They are marketed as Fe 500 or Fe 550 grade for cheating customers having half knowledge. Asking for a test certificate & a computerized plotted stress – strain graph will solve the issue. Also be vigilant if some suppliers give you Fe 500 or 550 grades at the cost of Fe 415. It is impossible as much, care, systems & cost is involved in production of 500 or 550 grade.

One should also be on the lookout for fraudsters who sell other grades of steel under the Fe 500 label. The increasing demand for Fe 500 grade steel in the market and the inability to solve the -problems associated with its production process provides ample room for such fraudsters.

If the Fe 500 steel you bought bends easily and offers no issues with workability, you might have been taken for a ride。

因此，在您打算为梦想中的钢铁购买钢铁之前，请向自己保证自己选择合适的年级。

Fe 415栏将是MAL建筑物的最佳选择，因为其强度和延展性的正确组合。以延展性为代价增加更大的力量可能不是您梦dream以求的家的最佳解决方案。实际上，IS：13920，《地震力结构的延性细节的实践守则》，建议使用Fe 415级或更低的钢筋。只能将其伸长率超过14.5％的Fe 500钢筋固定，可用于正常的12％。

如果您有溢价空间，这是一种考虑使用Fe 500钢并可以使用工厂切割钢的设计，那么这一切都适合您。其余的，FE 415将是选择。如有疑问，请询问您的工程师。

Author

Jismon Issac，机械工程毕业生在钢制制造和质量控制方面拥有近14年的经验。

联系方式: 09447065360

参考：

1)“地震R的关键属性esistant Rebars” – Dr.A.M.Elmaghrabi P.hD (Inorganic and analytical Chemistry)

2）“新西兰标准3101：2006”：混凝土结构

3) “Indian Standard Specification for High Strength deformed bars and wires for concrete reinforcement (third revision),IS 1786:1985”; Bureau of Indian Standards,New Delhi

4) “The impact of 500 Mpa reinforcement on the ductility of concrete structures”-Proceedings of the concrete Institute of Australia 2001.

5) “A clear and present danger in use of High Grade TMT in Seismic Zones”-Emilio M.Morales(Fellow in Civil Engineering, Carnegie Mellon University)