What are the biggest problems for designing a structure in high grade steel? - تخصصات بيت.كوم
IDEALISED STRESS-STRAIN CURVE FOR CONCRETE The IS Code permits while the stress strain curve for Fe and Fe steel are shown in Fig. For Fe steel, sv = °x + = y 2xl05 Strains in steel (3 stresses in steel (3 rows): Refer to the design stress-strain curve for Fe [Fig. Figure shows a typical stress-strain curve of concrete in compression, . worked bars used as steel reinforcement are Fe and Fe with the values.
Here using Fe is not economical as it costs more. Practically the civil designs are made by considering the factor of safety aspect. Now days Engineers make use of modern limit state method for detailing. Normal loads taken into consideration are dead load, live load, earthquake load, wind load etc.
Limit State Method – Free Civilengineering subject Tutorial
If the engineer underestimate the load, it is unsafe. Design Engineer take partial safety factor. When combinations of loads are taken into consideration, they consider safety factor1.
It means that the maximum working load is considered1.
If designs are altered by rendering quantity of steel, Fe grade is ok. Otherwise it costs more and is a waste. Balancing of Tensile 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 give ductility property.
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 grade ductility is standardized as minimum Now consider the case of Fe grade. Steel Bars used in civil constructions must have sufficient yield strength.
More over it should be ductile. Then only steel can elongate or deform on heavy loads and safeguard the buildings without breaking up. So the point is that Fe or high grade must be used only when design usage requires it.
Stress Strain relationship for concrete and Stress Strain relationship for steel
Otherwise use of Fe will be safer. Bending problems associated with Fe grade A higher strength and lower ductility means that Fe bars do not bend easily. Fe grade shown itself sensitive to high strains induced in the bending process. It is not tolerant of bending to diameter higher than the minimum bend diameters specified. Reports indicate that if bend diameters are frequently less than the minimum specified that lead to problems failure.
But compressed side of bend shows ribs spitted up. 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 grade bar should be carried out very slowly, not with a jerk. 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 fails at construction sites. The builders of most high rises that use Fe grade rebars procure them in factory cut sizes, avoiding the need to work on them further.
Their designers provide them with the steel detail that helps them procure items of the shelf. Small builders may not have such luxuries. In the first place, their design might not be for Fe steel, negating the savings in quantity of steel. Even if their design is for Fe 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 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 grade Weldability too is an issue with Fe grade steels. The amount of Carbon content in steel has been a major deciding factor for engineers as 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.
Stress Strain relationship for concrete and Stress Strain relationship for steel
Even though the carbon content in Fe is advised to keep max0. It is observed that in the case of welding a Fe or Fe 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 lose its strength of its external case and revert back to steel with lower yield strength.
In short designers should not rely on welding Fe or grade steel bars. ReBending problems associated with Fe grade Even though Re bending or reverse bending is not advisable for TMT grades, occasions do arise on construction sites where they are unavoidable.
This results in reduction of steel strength. The notching strain develops in rebend of Fe grade is considerably high which leads the bar to get snapped off. But steel got surface cracks and strain leading to excessive corrosion.
It is recommended to preheat Fe grade to a temperature Degree Centigrade. But practically it is difficult in sites. Reinforced concrete buildings are exposed to the elevated temperatures during a fire event. Thus, the structure is safe for certain efforts since these efforts do not produce stresses exceeding the material strength. However, the fact that structure is safe for design efforts does not prevent a possible brittle failure.
So, to be considered safe, the structure must also provide ductility, which can be achieved using tougher materials such as steel fiber-reinforced concrete to build structural elements. To verify if the material presents toughness compatible with the level of ductility that must be provided to the structure, the stress-strain curves can be used.
So, toughness is related to the area under this curve and, the greater the area, the greater the material toughness. With the increasing development of engineering and the increasing on complexity of geometry and loads used in the projects, design and analysis of concrete structures have also become more complex. Thus, the use of numerical methods, such as finite element method, has become common practice in analysis of these structures. From this point of view, the stress-strain curves, representing constitutive models for material, become even more important since the accuracy of analysis depends on the ability of constitutive models to represent properly the material behavior.
In technical literature there are reported many analytical models developed to represent the stress-strain curves for plain concrete under compression. Among most important and known models, must be cited models Popovics  and Carreira and Chu . Since the models of the compressive behavior of fiber-reinforced concrete, in general, were developed from models developed for plain concrete, it is necessary the inclusion of some parameters in these models to consider the influence of fibers on the properties of stress-strain curve.
The models of Ezeldin and Balaguru , Mansur et al. The model proposed by Ezeldin and Balaguru  is valid for concretes with compressive strength varying from 35 MPa to 85 MPa.
Steel fibers with hooked ends and with aspect ratio of 60, 75 and were added in volumetric fractions of up to 0. The model proposed by Mansur et al. Beyond these studies, Barros and Figueiras  presented a model developed for concretes with compressive strength varying from 30 MPa to 60 MPa, to which were added up to 0.
In this paper, results of many compression tests with displacement control performed by the authors were used. Concretes presented average compressive strength of 40 MPa and 60 MPa and were produced with steel fibers 35 mm long. These fibers had aspect ratio of 64 and were added in fiber volumetric fractions of 1.
The influence of fibers was evaluated on the peak stress and strain and on toughness in compression. An analytical model to get the complete stress-strain curve was developed based on the model of Carreira and Chu .
Stress-strain curves for steel fiber-reinforced concrete in compression
To these concretes steel fibers 35 mm long were added in fiber volumetric fractions of 1. Concretes with 60 MPa were produced with the same materials, except fly-ash. Table 1 presents compositions of the produced concretes. It was observed that amount of superplasticizer used in each mixture increased as fiber volumetric fractions increased, since the difficulty in adding high amounts of fibers to concrete increases as the amount of fibers increases.
Also, the superplasticizer dosage and the water-cement ratio were calculated in relation to equivalent cement, that is, in relation to a mixture of cementitious materials cement, silica fume and fly-ash with the same density of cement. Three cylindrical specimens mm diameter and mm high were cast for each mixture. In this test, displacements were limited to measurement capacity of the transducers that was 10 mm. However, this limitation applies only to the maximum displacement, although it is possible that some specimens present smaller displacements at the end of the test.
Once stress-strain curves are determined, the average curves were obtained for each produced mixture. Following, these curves were normalized dividing the stresses axis by the peak stress. This procedure removes the influence of compressive strength and allows direct comparison of curves.