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Module 1: Overview of Design of Steel Structures

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Video 1
Hello, today I am going to discuss the different material properties of the steel, because we aregoing to use the steel as a structural material. So before going to use that, we must know whatare the behaviour of the steel. We should know the composition of the steel then how thecomposition are going to vary along with the structural properties, what are the advantages ofsteel and disadvantages of steel so that we can wisely use the steel as a structural material. Sothese aspects will be discussed.Steel is a to some extent ductile material. So how it behaves under stress-strain curve, means, thestress-strain curve of steel, how the strain is going to vary with stress, those aspects will bediscussed in today’s lecture. So, before going to use the steel as a design material we will try tounderstand what are the advantages and disadvantages of the steel. So if we look into the different type of advantages, one is the better quality control. The betterquality control means, the steel is basically factory product, so its quality is maintained in abetter way, as compared to concrete material. So therefore, its property will be uniform and itsproperties are well defined. So when we are going to design we know with confidence what arethe properties we are going to get, unlike in case of concrete, we are not very sure, whatproperties we are going to achieve. There will be gap between the target strength and the designstrength, but in this case that gap is quite less.This is the advantageous property of the steel another property is , it is lighter with respect toother building material, it is quiet light, light in the sense the strength to weight ratio is very high.So with less amount of space we can provide the member and that member can withstand largeamount of load with a small free space unlike in case of concrete structure. Because of thiscertain advantages dead load on the structure will be less. So the space in the site will becomparatively high compared to concrete structures.Now another aspect we have that it is faster to erect, means unlike concrete RCC structure, thiscan be erected at site very quickly, because all steel rolled sections are available. So once it istransported to the site just we need the connections properly and then we can erect the structureas we desire. Another thing is the reduction in construction time, so time of construction iscomparatively less which is advantage for construction purpose.Then large column free space and thus amiable for alteration. In case of steel structures, size ofthe column will be comparatively less with respect to the RCC column. So the free space will bemuch more as compared to RCC construction.Then less material handling at site, in case of RCC structure we have to handle, courseaggregate, fine aggregate, cement then water and lot of material handling has to be done and lotof wastage also occur, lot of hazards come into picture at the site. But in case of steel structurewe will be free from those type of hazards and has less material handling , wastage will becomparatively less.Then less percentage of floor area occupied by structural element. the most important thing isthat, steel is ductile and hence, (it is) it can carry the lateral load in a better way and that means itis earthquake resistant and wind resistance . If the structure become more ductile thenearthquake energy can be absorbed and without failure, it can be transferred to the ground.So if the structure is ductile we can have better resistant due to seismic excitation, also it willperform better in case of cyclone. So these are the few of the advantageous property I discussedand similarly, we need to know the disadvantages of steel material, because unless we know thedisadvantages we cannot make use confidently. So one disadvantage is the skilled labor is required. Unlike RCC structure here skilled labor isrequired for connections, because connections has to be made properly and that connections maybe weld connection or bolt connection or may be rivet connection, with a higher degree ofaccuracy. So that the load is transferred from beam to column, column to foundation and alsofrom secondary beam to beam, from floor to beam etc.Another thing is higher cost of construction; actually material cost of steel is quite high ascompared to concrete. So construction cost will be quite high as compared to concrete. So wehave to make use of steel wisely, if it is not required exhaustively then we should not go for steelconstruction, otherwise cost will increase. So this is another disadvantage, because it is high cost.Then another thing is maintenance cost is quite high. After construction due to humidity andother problem, it get corroded, so because of corrosion steel strength get reduced. So time to timefrequently we need to make painting, we need to make maintenance. so unlike concrete structurehere maintenance cost will be disadvantageous .Next is poor fire proofing. At 1000 degree Fahrenheit that means 538 degree centigrade, almost65 percent strength remains. So 35 percent strength vanishes. Similarly, at 1600 degreeFahrenheit 15 percent strength only remains that means 85 percent strength got reduced. Sotherefore it is less fireproofing so we have to be cautious about the fire safety while using thesteel as a structural material.Then another problem is that electricity, which may may be required if we construct in a remotearea. Where electricity is a problem , we may not be able to go for welded connection properly.So sometimes we need electric connection, which may not be available at the site, so in that casewe have to face problem. So, these are the few disadvantages of the steel using as a structuralmaterial. Now coming to chemical composition of the steel, basically steel is an alloy which mainlycontains iron and carbon, apart from the carbon a small percentage of manganese, silicon,phosphorus, nickel and copper are also added to modify the specific properties of steel. Here, inIS 2062-1992 and IS 8500 the chemical composition of structural steel have been given. So someof the chemical composition of different structural grade of steel has been reported in this tablelike Fe410 of grade A,B,C the percentage of carbon has been shown. So different percentage ofcarbon, manganese then sulpher, phosphorus, silicon and carbon equivalent has been given in it.So with the different ratios of this chemical component we can achieve a particular grade of steelthat means a particular strength can be achieved.Here,Carbon Equivalent = (C+Mn)/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15So, this summation is called carbon equivalent, which is given here, right and the terms inbracket denote the maximum limit of the flat products So if we want to produce a particulargrade of steel then composition that can be found from this table. Now coming to types of structural steel we can see that one is carbon steel , basically differentsteel have been produced based on necessity by changing chemical composition andmanufacturing process. So in case of carbon steel carbon and manganese are used as extraelement and another type of steel is high strength carbon steel. By increasing the carbon contentthis type of steel can be manufactured, which basically produces steel with comparatively higherstrength, but less ductility. So for this type of steel you will get high strength, but less ductility.This is high strength carbon steel, another steel is stainless steel, in this type of steel mainlyforeign materials like nickel and chromium are used along with small percentage of carbon. Now being a structural engineer, we will try to see what is the properties of structural steel.Being structural engineer or a steel designer, we will be focusing on ultimate strength, yieldstrength and the ductility. These three things are very important. These are very important forusing the steel and also weldability, toughness; corrosion resistance and machinability are alsosome of the mechanical properties and in this last four properties are important for durability ofmaterial and often associate with fabrication of steel members. So for durability consideration,this last four properties are very important we have to keep in mind and mechanical properties ofthe steel largely depends on this five things, one is the chemical composition.So we have to know what is a percentage of carbon and other different elements. So dependingon that the mechanical properties of the steel will vary. Then how the treatment is going to bemade for producing steel, then stress history, rolling methods and rolling thickness are important.So these are the few things, few parameters which we have to keep in mind for getting thestructural properties of the steel, because these structural properties of steel largely depends onthis. Now the structural steel whatever we use, should conform the IS 2062-2011 which is hot rolledmedium and high tensile structural steel. So it should conform to this code and we use mostly theFe 410 grade of steel. Few physical properties of structural steel are given in IS 800-2007 inclause 2 .2 .41, because these properties also will be required like unit mass of steel (ρ) is 7850kg per meter cube. This is required, because when we are going find out the self-weight of thestructure, self-weight of the steel structure, then we need unit mass of steel.Next is the modulus of elasticity, this is also important to find out the stiffness of the steelmember and this we consider as 2.0 × 105 N/mm2. Poisons ration also is important, which weconsider in general 0.3 and modulus of rigidity G , G we considered as = 0.769 × 105 N/mm2.Co-efficient of thermal expansion for heat related problem, we have to consider this thermalexpansion that is alpha is equal to 12 × 10-6 /°c. because steel can expand or contract and becauseof this stress may develop so that thermal stress whatever is going to be developed that has to becalculated and that additional forces has to be calculated while analyzing their structure. So inthat case thermal expansion coefficient is important.
Video 2
Now coming to mechanical properties as I told, the three things are very important that is one isyield stress, what is the yield stress of the steel? What is the ultimate stress and the minimumpercentage elongation? o these we can find out, in table 1 of IS 800-2007 mechanical propertiesof structural steel like, we use Fe 410 grade of steel. So, in that case, the yield stress is 250 andultimate stress is 410 in MPa and elongation percentage is 23.Another yield stress is given for thickness from 20 mm to 40 mm, we can achieve 250 ifthickness is less than 20 mm, but if t is 20 to 40 then 240 and if thickness is more than 40 mmthen the yield stress is going to be considered as 230 MPa. So for Fe 410 grade of steel what weuse we use yield stress either 250, 240 or 230 and ultimate stress as 410 and elongationpercentage as 23. Similarly, Fe 440 grade of steel we can find out yield stress as 300 and ultimatetensile stress as 440 and elongation is 22.So in this way we can find out the important properties like yield stress, ultimate stress andpercentage elongation from the grade of steel, if a particular grade of steel is given then we canfind out what is the yield stress, ultimate tensile stress and grade of steel, right. Now coming toductility, a very important parameter in case of steel design is ductility. So, I will discuss little more about ductility. Ductility is basically the ability to deform undertensile force and it undergoes large inelastic. Inelastic deformation means permanentdeformation without loss of strength under the application. So if we see the stress-strain diagramof the material if this is strain and this is stress then this portion is basically the ductility portionwhere stress is not developing as such, but the strain is going to be increase. If we release theload, it will be coming to its earlier position, of course not in same path, because it is inelastic,but it will come to its earlier position with deformation. If the material is ductile that means itwill be much more seismic resistance. Therefore, we prefer ductile material so that deformationsare allowed without failure.Then another property we also come across which is called hardness. Hardness is one of themechanical properties of steel by virtue of which, it offers resistance to the indentation andscratching. So hardness can be measured by different test (the) like rock well test, rock wellhardness test. Another test we make which is called Vickers hardness test and then another testthrough which the hardness is measured is called Brinell hardness test. So through this one cantest the hardness of the material and another property also we come across is called toughness. So I am discussing some property, mechanical properties and other properties of steel, which isimportant to know for designing the structure and when we are going to design a member wemust know what is a behavior of the member under load. Say for example, if we make stressstrain diagram of a material, say stress and strain, so brittle material means it will be like this andsuddenly it will fail brittle material and ductile material means it will not fail, it will undergoesstrain. So, this is ductile material.Now toughness is the ability to absorb energy up to fracture. This toughness is measured by thearea under the stress-strain curve. So stress-strain curve of this material and stress-strain curve ofthis material, the area we can find out and we can measure the toughness. It is a one type ofmechanical property of steel. So basically it offers resistance to fracture under the action of theimpact load. So this is one property another is fatigue. Fatigue means the repeated loading. Itmeans damage is caused due to repeated loading, repeated fluctuation of stresses and which leadsto progress of cracking of the structural element and due to cyclic loading damage and failure ofthe material may happen which is called fatigue.In addition, another is resistance against corrosion. In presence of moisture, corrosion of steelhappens. So to avoid that what we can do? We can go for painting or metallic coating. So eitherof these two can be made to take care the corrosion. So this is one property which we have tokeep in mind and then another property is residual stress. Residual stress come into picture, because of uneven heating and cooling, because of unevenheating and cooling the residual stress is in the member develop. So how the material has beenproduced depending on that the residual stress can be calculated, accordingly the design of themember can be done. Then another is stress concentration when certain changes of geometryproperties are there, say stress concentration. It is basically a highly localized state of stresswhere at particular location stress is concentrated and, because abrupt change of the shape. So,sudden change at the vicinity of notch can make the development of stress concentration and alsoduring near the hole the stress also generated several times greater than the actual stress. And forthat we have to take care that means when we are going to design there may be chances of failureat certain localized point, because of concentration of stress. So we have to make the section insuch a way there stress concentration can be avoided. Now we will come to the stress-strain curve. So stress-strain curve of the mild steel we will seefirst, say this is strain which is called epsilon and this is stress, which is called sigma, right. Nowin case of mild steel an ideal curve is looks like this, right. So this is the origin from where stressstrain curve develops. So and this is point A, up to point A that means from O to point A is calledlimit of proportionality. This portion is called limit of proportionality that means up to this, it islinear and it obeys the hooks law.So OA is basically called the limit of proportionality then from A to B, actually after reachingpoint A, change in strain is rapid compared to that of stress, but still the material behaveselastically up to elastic limit of B. So, point B is elastic limit then point C . Point C is theupper yield point means if we go on increasing the force then we will observe that yield pointmeans it reaches upper yield point. So after upper yield point again, it will come down to C. So,C is the lower yield point, right. This observation of C and C point depends on the rate ofloading it means depending on the rate of loading we can observe the point C and C .Then CD part, so beyond yield point, the material start flowing plastically without anysignificant increase in the stress and material goes large deformation. So CD part is basicallyplastic part. So it means it flow like plastic and without any increase of the stress, the strainincreases then up to point E, DE means after reaching point D strain hardening occurs in thematerial, the requirement of higher load to continue the deformation. This phenomenon is callstrain hardening that means it resist deformation and needs more load to deform.So after CD means where load was not increasing, but after that point, it is starts resistingdeformation. So strain hardening occurs, so with the increase of stress, strain also is going toincrease up to certain level, which is highest point E and E this E represents the fu the ultimatestress, right and after that the stress is going to be reduced and at a certain point it breaks. So F isa breaking stress, right F is the breaking stress, so this is how the material behaves.So when we are going to design a steel member we have to know what is a properties of steelunder load that means stress-strain diagram how it varies, in case of mild steel, it varies in a way.In case of tor steel it varies another way, so we have to know and accordingly we have to find outwhat is the fu value and what is fy value and what will be the strain at fu and strain at fy. Thatmeans how much ductile this material is, depending on that we can think of designing themember properly.So when we go to the design a steel member under certain procedure like working stress methodor limit state method or ultimate stress strain design method we have to know the stress-straindiagram, stress-strain behavior of the material so that we can understand that up to what level weare going to allow the deformation and then how we are going to find out the maximumallowable stress and then according to that design criteria will be decided. So this is all about thetodays lecture about the steel as a structural material, thank you.