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1 - Introduction Introduction Metallurgy of Steel Mechanical Properties of Steel The Manufacturing of Steel Structures Corrosion Steel Structures subjected to fire Fatigue of Steel Structures Summary References 2 - Limit State Design Introduction Analysis procedures & Design Philosophy Other Design Requirements Summary References 3 - Connections Introduction Bolted Connections Welding & Welded Connections Bolt Group Weld Group Beam and Column Splices Summary References Examples 4 - Tension Members Introduction Behaviour of Tension Members
- Animation for Load Elongation of Tension Member Design of Tension Members Summary References Structural Steel Design Project
- Animation for Calculation of An 5 - Compression Members Introduction Animation - Animation for Compression of Short Column
- Animation for Compression of Long Column Elastic Buckling Strength Curves Strength of Compression Members Concept of Effective Lengths Torsional & Torsional-flexural buckling of columns
- Animation for Folded plate twist under axial load
- Animation for Buckling of plate with unsupported edges Design Strength
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Types of Column Sections Steps in Design of Axizlly Loaded Columns Summary Concluding Remarks 6 - Beams Introduction Concept Of Limit State Design of Beams
- Animation for Excessive Bending Triggering Collapse
- Animation for Lateral Torsional Buckling
- Animation for Failure by Local Buckling of a flange in compression
- Animation for Failure by Local Buckling of a web due to shear
- Animation for Failure by Local Buckling of a web under compression due to concentrated loads
- Animation for Local failure by shear yield of web
- Animation for Local failure by local crushing of web
- Animation for Local failure by buckling of thin flanges Behaviour of Steel Beams Summary References Examples 7 - Beam Columns Introduction Concept Of Limit State design of Beam Columns Design of members subjected to Combined Force Summary References Examples
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Steel is by far the most useful material for building structures with strength of approximately ten times that of concrete, steel is the ideal material for modern construction. Due to its large strength to weight ratio, steel structures tend to be more economical than concrete structures for tall buildings and large span buildings and bridges. Steel structures can be constructed very fast and this enables the structure to be used early thereby leading to overall economy. Steel structures are ductile and robust and can withstand severe loadings such as earthquakes. Steel structures can be easily repaired and retrofitted to carry higher loads. Steel is also a very eco-friendly material and steel structures can be easily dismantled and sold as scrap. Thus the life- cycle cost of steel structures, which includes the cost of construction, maintenance, repair and dismantling, can be less than that for concrete structures. Since steel is produced in the factory under better quality control, steel structures have higher reliability and safety.
To get the most benefit out of steel, steel structures should be designed and protected to resist corrosion and fire. They should be designed and detailed for easy fabrication and erection. Good quality control is essential to ensure proper fitting of the various structural elements. The effects of temperature should be considered in design. To prevent development of cracks under fatigue and earthquake loads the connections and in particular the welds should be designed and detailed properly. Special steels and protective measures for corrosion and fire are available and the designer should be familiar with the options available.
1.2 Metallurgy of steel
When carbon in small quantities is added to iron, ‘Steel’ is obtained. Since the influence of carbon on mechanical properties of iron is much larger than other alloying elements. The atomic diameter of carbon is less than the interstices between iron atoms and the carbon goes into solid solution of iron. As carbon dissolves in the interstices, it distorts the original crystal lattice of iron.
This mechanical distortion of crystal lattice interferes with the external applied strain to the crystal lattice, by mechanically blocking the dislocation of the crystal lattices. In other words, they provide mechanical strength. Obviously adding more and more carbon to iron (upto solubility of iron) results in more and more distortion of the crystal lattices and hence provides increased mechanical strength. However, solubility of more carbon influences negatively with another important property of iron called the ‘ductility’ (ability of iron to undergo large plastic deformation). The a-iron or ferrite is very soft and it flows plastically. Hence we see that when more carbon is added, enhanced mechanical strength is obtained, but ductility is reduced. Increase in carbon content is not the only way, and certainly not the desirable way to get increased strength of steels. More amount of carbon causes problems during the welding process. We will see later, how both mechanical strength and ductility of steel could be improved even with low carbon content. The iron-carbon equilibrium diagram is a plot of transformation of iron with respect to carbon content and temperature. This diagram is also called iron-iron carbon phase diagram (Fig. 1.2). The important metallurgical terms, used in the diagram, are presented below.
0.25%. Upto 2% carbon, we get a structure of ferrite + pearlite or pearlite + cementite depending upon whether carbon content is less than 0.8% or beyond 0.8%. Beyond 2% carbon in iron, brittle cast iron is formed.
1.2.1 The structural steels or ferrite – pearlite steels
The iron-iron carbide portion of the phase diagram that is of interest to structural engineers is shown in Fig.1.2. The phase diagram is divided into two parts called “hypoeutectoid steels” (steels with carbon content to the left of eutectoid point [0.8% carbon]) and “hyper eutectoid steels” which have carbon content to the right of the eutectoid point. It is seen from the figure that iron containing very low percentage of carbon (0.002%) called very low carbon steels will have 100% ferrite microstructure (grains or crystals of ferrite with irregular boundaries) as shown in Fig 1.2. Ferrite is soft and ductile with very low mechanical strength. This microstructure at ambient temperature has a mixture of what is known as ‘pearlite and ferrite’ as can be seen in Fig. 1.2. Hence we see that ordinary structural steels have a pearlite + ferrite microstructure. However, it is important to note that steel of 0.20% carbon ends up in pearlite + ferrite microstructure, only when it is cooled very slowly from higher temperature during manufacture. When the rate of cooling is faster, the normal pearlite
- ferrite microstructure may not form, instead some other microstructure called bainite or martensite may result.
We will consider how the microstructures of structural steel are formed by the slow cooling at 0.2% carbon. At about 900o^ C, this steel has austenite microstructure. This is shown as point ‘i’ in Fig. 1.2. When steel is slowly cooled, the transformation would start on reaching the point ‘j’. At this point, the alloy enters a two-phase field of ferrite and austenite. On reaching the point, ferrite starts nucleating around the grain boundaries of austenite as shown in Fig. 1.3(a). By slowly cooling to point 'k', the ferrite grains grow in size and diffusion of carbon takes place from ferrite regions into the austenite regions as shown in Fig. 1.3(b), since ferrite cannot retain carbon above 0.002% at room temperature.
Fig 1.3. Different stages of formation of pearlite
At this point it is seen that a network of ferrite crystals surrounds each austenite grain. On slow cooling to point ‘l’ the remaining austenite gets transformed into ‘pearlite’ as shown in Fig 1.3(c). Pearlite is a lamellar mixture of ferrite and cementite. The amount of ‘pearlite’ for a given carbon content is usually calculated using the lever rule assuming 0% carbon in ferrite as given below:
Volume fraction of Pearlite % of Carbon =0.8% of Carbon
For example for microstructure of a 0.2% carbon steel would consist of a quarter of pearlite and three- quarters of ferrite. As explained earlier, ferrite is soft and ductile and pearlite is hard and it imparts mechanical strength to steel. The higher the carbon content, the higher would be the pearlite content and hence higher mechanical strength. Conversely, when the pearlite content increases, the ferrite content decreases and hence the ductility is reduced.
1.2.2 Strengthening structural steels
Cooling rate of steel from austenite region to room temperature produces different microstructures, which impart different mechanical properties. In the case of structural steels, the (pearlite + ferrite) microstructure is obtained after austenitising, by cooling it very slowly in a furnace. This process of slow cooling in a furnace is called ‘annealing’. As, mentioned in the earlier section, the formation of pearlite, which is
Structural steel sections are produced by hot rolling process, which involves the temperature range of austenite. During rolling at this high temperature, the heavy mechanical deformation results in finer size grains. In addition to that, rolling at the temperature of austenite, they are allowed to cool in air (normalising) and hence both the procedures aid the formation of smaller size crystals and hence increased mechanical strength.
1.2.3 Rapid cooling of steels In the earlier section we saw that steel is made to under-cool by normalizing (by giving lesser cooling time than required by the equilibrium state of the constitutional diagram), it results in finer microstructure. However, if we cool steel very rapidly, say quenching in cold water, there is insufficient time for the shuffling or diffusion of carbon atoms and hence the formation of ferrite + pearlite is prevented. However, such a fast cooling results in ‘martensite’. Slightly less rapid cooling could result in a product called ‘bainite’ which is dependent on the composition of steel. Bainite is formed above a temperature of about 300°C and between a cooling rate of 8.4°C/sec to 0.0062°C/sec. Martensite is formed by rapid cooling rate less than 8.4°C/sec. Very slow cooling, say full annealing does not form both Martensite and Bainite.
Martensite is very hard and less ductile. Martensitic structure is not desirable in structural steel sections used in construction, because its welding becomes very difficult. However, high strength bolts and some other important accessories have predominantly martensitic structure. The hardness of martensite is a function of carbon content. When martensite is heated to a temperature of 600°C it softens and the toughness is improved. This process of reheating martensite is called tempering. This process of quenching and tempering results in very many varieties of steel depending upon the requirement for hardness, wear resistance, strength and toughness.
1.2.4 Inclusions and alloying elements in steel
Steel contains impurities such as phosphorous and sulphur and they eventually form phosphides and sulphides which are harmful to the toughness of the steel. Hence it is desirable to keep these elements less than 0.05%. Phosphorous could be easily removed compared to sulphur. If manganese (Mn) is added to steel, it forms a less harmful manganese sulphide (MnS) rather than the harmful iron sulphide. Sometimes calcium, cerium, and other rare earth elements are added to the refined molten steel. They combine with sulphur to form less harmful elements. Steel treated this way has good toughness and such steels are used in special applications where toughness is the criteria. The addition of manganese also increases the under cooling before the start of the formation of ferrite+ pearlite. This gives fine-grained ferrite and more evenly divided pearlite. Since the atomic diameter of manganese is larger that the atomic diameter of iron, manganese exists as ‘substitutional solid solution’ in ferrite crystals, by displacing the smaller iron atoms. This improves the strength of ferrite because the distortion of crystal lattice due to the presence of manganese blocks the mechanical movement of the crystal lattices. However, manganese content cannot be increased unduely, as it might become harmful. Increased manganese content increases the formation of martensite and hence hardness and raises its ductile to brittle transition temperature (temperature at which steel which is normally ductile becomes brittle). Because of these reasons, manganese is restricted to 1.5% by weight. Based on the manganese content, steels are classified as carbon-manganese steels (Mn>1%) and carbon steels (Mn<1%). In recent years, micro alloyed steels or high strength low alloy (HSLA) steels have been developed. They are basically carbon manganese steels in which small amounts of aluminium, vanadium, mobium or other elements are used to help control the grain size.
Table 1.1 Chemical composition of some typical structural steels
Type of steel Designation
IS
code: C S Mn P Si Cr Carbon equivalent Fe410A 2062 0.23 0.50 1.5 0.50 - - SK 0. Fe410B 2062 0.22 0.45 1.5 0.45 0.4 - Sk 0.
Standard structural steel (^) Fe410C 2062 0.20 0.40 1.5 0.40 0.4 - K 0. Fe440 8500 0.20 0.50 1.3 0.50 0.45 - - 0. Fe540 8500 0.20 0.45 1.6 0.45 0.45 - - 0.
Micro alloyed high strength steel
Fe590 8500 0.22 0.45 1.8 0.45 0.45 - - 0.
K- killed steel SK- Semi Killed steel (Explained in section 1.4.2)
1.2.5 Stainless steels
In an iron-chromium alloy, when chromium content is increased to about 11%, the resulting material is generally classified as a stainless steel. This is because at this minimum level of chromium, a thin protective passive film forms spontaneously on steel, which acts as a barrier to protect the steel from corrosion. On further increase in chromium content, the passive film is strengthened and achieves the ability to repair itself, if it gets damaged in the corrosive environment. 'Ni' addition in stainless steel improves corrosion resistance in reducing environments such as sulphuric acid. It also changes the crystal structure from bcc to fcc thereby improving its ductility, toughness and weldability. 'Mo' increases pitting and crevice corrosion in chloride environments.
Stainless steel is attractive to the architects despite its high cost, as it provides a combined effect of aesthetics, strength and durability.
Stainless steels are available in variety of finishes and it enhances the aesthetics of the structure. On Life Cycle cost Analysis (LCA), stainless steel works out to be economical in many situations. Increased usage of stainless steel in the construction sector is expected, as awareness on LCA improves among architects and consulting engineers.
Fig 1.5. Stress strain curve for mild steel
Depending on the steel used, εsh generally varies between 5 and 15 εy, with an
average value of 10 εy typically used in many applications. For all structural steels, the modulus of elasticity can be taken as 205,000 MPa and the tangent modus at the onset of strain hardening is roughly 1/30th of that value or approximately 6700 MPa. High strength steels, due to their specific microstructure, do not show a sharp yield point but rather they yield continuously as shown in Fig. 1.6. For such steels the yield stress is always taken as the stress at which a line at 0.2% strain, parallel to the elastic portion, intercepts the stress strain curve. This is shown in Fig. 1.6.
Fig 1.6. Stress strain curve for high strength steel
The nominal stress or the engineering stress is given by the load divided by the original area. Similarly, the engineering strain is taken as the ratio of the change in length to original length.
1.3.2 Hardness
Hardness is regarded as the resistance of a material to indentations and scratching. This is generally determined by forcing an indentor on to the surface. The resultant deformation in steel is both elastic and plastic. There are several methods using which the hardness of a metal could be found out. They basically differ in the form of the indentor, which is used on to the surface. They are presented in Table 1.2.
In all the above cases, hardness number is related to the ratio of the applied load to the surface area of the indentation formed. The testing procedure involves forcing the indentor on to the surface at a particular road. On removal, the size of indentation is measured using a microscope. Based on the size of the indentation,
Fig 1.7. Experimental set up for notch toughness test
Fig 1.8.Test specimen for notch toughness test
1.3.3 Notch-toughness
There is always a possibility of microscopic cracks in a material or the material may develop such cracks as a result of several cycles of loading. Such cracks may grow rapidly without detection and lead to sudden collapse of the structure. To ensure that this does not happen, materials in which the cracks grow slowly are preferred. Such steels are known as notch-tough steels and the amount of energy they absorb is measured by impacting a notched specimen with a heavy pendulum as in Izod or
Charpy tests. A typical test set up for this is shown in Fig. 1.7 and the specimen used is shown in Fig. 1.8.
The important mechanical properties of steel produced in India are summarized in Table 1.3. In Table 1.3, the UTS represent the minimum guaranteed Ultimate Tensile Strength at which the corresponding steel would fail.
Table 1.3 Mechanical properties of some typical structural steels
Yield strength(MPa) Thickness (mm)
Type of steel Designation
UTS
(MPa) <20 20-40 >
Elongation Gauge 5.65 S 0
Charpy V - notch values Joules (min)
Fe410A 410 250 240 230 23 27 Fe410B 410 250 240 230 23 27
Standard structural steel (^) Fe410C 410 250 240 230 23 27 <16 16-40 41- Fe440 440 300 290 280 22 - Fe540 540 410 390 380 20 -
Micro alloyed high strength steel
Fe590 590 450 430 420 20 -
