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Module 1: Overview of Design of Steel Structures

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Video 1
I welcome to the limit state design of steel structure class. The limit state design of steelstructure has been introduced in our Indian code in 2007. Before that, we used to design thesteel structure by the use of Working Stress Method and from 2007 onwards, the codelprovision has been changed and in 2007, the IS 800-2007 has been published where the entirecode has been discussed based on Limit State Design method.Now coming to this course, today I will discuss about the probable content of the course,which I am going to cover in next 60 lectures. I will inform what are the books you canfollow and what are the codes required for learning this course, and I will show you somesteel rolled section. What are their properties, how the parameters vary, how the parametersare considered? Which parameters are considered? Which are given in the IS handbook SP-6, I will just give you touch on that.Now coming to the Limit State Design of steel structure, one thing we can say that now adays, in every city or every country, wherever you see some skyscrapers, these are mostlybuilt with the steel. Steel has lot of advantages, which I will come later, and because of itsadvantageous properties, we, the designer prefer for designing the structure with the steel.Steel sections are massively used particularly in bridge structure and in transmission tower,refinery well structure. Sometimes some water tanks, some high rise buildings, manyindustrial buildings, sheds are made of steels. Steel is much stronger than the conventionalconstruction material like concrete, its strength to weight ratio is very high i.e. its weight isvery less and strength is very high. It is much ductile compared to concrete and, because ofits advantages designer prefers steel structure, though it is costly.So if we design properly in a cost effective way, the cost can be minimized withoutcompromising the safety. Therefore, we need to know the design procedure properly andmay be in later, by knowing the design procedure we can make a computer code, we cangenerate some computer algorithm, by the use of matlab or any other standard language likeC language, C++, java or we can use excel sheet also. In steel design, we need to iterate thedesign process. We will assume certain member with certain dimensions and we will checkfrom limit state point of view, from ultimate strength point of view, from serviceabilitycriteria point of view. We will check certain steps and if we see these are not satisfied then wehave to increase the size and if we see it is satisfied then we can go for this particular section.In case of limit state design, there are lot of procedures and calculations. Hence, it is verydifficult to perform manual calculation. Therefore, if you can develop a computer algorithmof if you can develop an excel sheet then just by putting the value you can check, whether itis okay or not. In some cases we have seen that when the designer is saying that the assumedsection is okay then they simply go with that section, but that is not economic, because maybe with some lesser section under the same loading, it may be safe.Therefore, we must go for the lesser section, though sometime designers do not go for it,because lot of calculations have to be done. It is a tedious process. But I will suggest you thatif you know the design algorithm, if you understand the design process of a particularmember then you make a customized program at your own requirement and then go for thisprogram for the design of the section and in that way you can make that design optimized,economic and cost-effective. Now coming to the anatomy of the structure we will see that any steel structures are havingthese types of members, as one is beam. Beam is a flexural member where the flexural actioncome into picture. Therefore, we need to know the design of a flexural member of the beam,and then the load will come to the column. Column takes basically the compressive loadalong with certain eccentric load, so sometimes moment also come into picture, thereforecolumn has to be designed accordingly. Floor carries the load and that load is transferred tothe beam and column. Bracing system is an important system in case of steel structure.Bracings has been provided just to increase the load carrying capacity in terms of horizontalload. Now the entire super structural load are going to the foundation, so we need to know thefoundation design also. Therefore, in foundation we will see the slab base, gusset base, baseplate and how the load from the super structure has been transferred to the sub-structure bythe use of base plate and other members. An important part, which we do not generally comeacross in case of RCC structure, is connection.These connections are very important in the sense, that we know the rolled steel sections areavailable or some other type of sections also can be made available in terms of built upsection etc., but it required to be joined properly. Therefore, beam and column need to bejoined properly by the use of bolt, rivet or weld connection.So, when I will be going to teach this subject, I will discuss about the connection at first,because if we know connection then we can go for further in design. In steel design as awhole, connection plays a very important role. We do not give much importance to theconnection, though, I will recommend that when we are going to design a entire system wehave to specially look for the connections and we have to design connections properly, so thatload is transferred from one system to another system, one member to another memberproperly. Sometimes moment also has to be transferred, sometimes only axial force.Therefore, based on the requirement we have to decide what type of connections we will begoing for. Therefore, I am briefly discussing the syllabus, which I am going to cover, but it is tentative,because I do not know how much time I will get to finish. So first module will beintroduction, we will discuss about steel as a structural material and we will discuss itsadvantages and disadvantages, because unless we know the disadvantages of steel we will notmake use of it properly. Then we will go for different types of sections, I. S. rolled sections,which are available in the market, and their properties are given in the code. Then we will gofor some material overview, means how the steel behaves with the increase of load, howstress strain varies in case of steel and how the mechanical properties are dependent onchemical properties of the steel that we will discuss. Then basis for structural design will bediscussed means what type of structural design we are going for in this course.Then we will come to the module 2, which comprises of connections. So different types ofconnections will be discussed and in that connection we will discuss about the boltedconnections, its advantages and disadvantages, then design of bolted connections, itsefficiency and design of joints, then welded connections, advantages and disadvantages ofwelded joints, design of weld connections, fillet weld, butt or group weld, plug and slot weld.So these are the probable topics, I am going to cover. Then in next module, we will discuss about the eccentric connections, because sometimesconnections become eccentric in terms of its loading. So, different types of eccentricconnections are observed, so we will discuss about bolted and welded connection with loadlying in plane of joint and load lying perpendicular to the plane of joint.Then we will discuss about the design of eccentric connection using bolts and weld. Inmodule 4, we will discuss about the design of tension members. In tension member, we willfirst come into different types of failures, we will find out the gross and net sectional area,then rupture of critical section then strength calculation, block shear failure, then slendernessratio of the member and then we will come to design of tension members, considering all thisand then gusset plates, lug angle, tension splices, design of tension members subjected toaxial and bending, those things will be discussed. In module 5, we will discuss about the compression members. In compression member, firstwe will see what are the types of failure occur in case of compression member andaccordingly the strength calculation will be discussed. Then how to calculate slendernessratio of the compression member, we will find out and then design of the compressionmember and design of eccentrically loaded compression member that means when momentalso will come into picture.Sometimes the rolled steel section is not sufficient for withstanding the huge amount of load.So, in that case we will go for built-up compression member, their design procedure will bediscussed. Then in built-up compression member, we need to add the lacing systems. Sodesign of lacing system and design of batten plates will also be discussed in this module.In module 6, we will discuss about the flexural member which is called beam. So we willgive you some introduction to flexural member and then design procedure of beam. Beamwill be basically two types, laterally supported beam and laterally un-supported beam, forboth the cases, design procedure will be discussed. Built-up beams and design of purlins alsowill be introduced in this module.Then we will come to column base, design of the gusseted base and slab base along with baseplate will be discussed here. If we get time then we will discuss about the gantry girder,because in steel structure gantry girder is an important component, which we need to know.So in last module we will discuss about the design aspects of gantry girder, various type ofloads will be there. Therefore, what are the loads coming into gantry girder and how to designthe gantry girder will be discussed. Now I would suggest to follow these two books along with my lectures. These books are verywell written; one book is design of steel structure written by Dr. Subramanian Narayananwho has directly involved with the development of I. S. 800 2007 code. He has written thisbook which is very exhaustively written and well documented. Another book also you canfollow that is written by S. K. Duggal, Tata McGraw Hill publication, limit state design ofsteel structure and the first book is published by Oxford publication.So, other than these two books you can follow some other books also if you want, but I haveseen many books are available in which design calculations are not given properly. However,in these two books the design calculation and procedures and very well documented in asimplified manner. You can go for some reference also like design of steel structure by EliasAbu-Saba, this is CBS publication, you can use this book as a reference book.Then design of steel structure by E. H. Gaylord et al. by McGraw Hill and structural steelwork by S. S. Ray, Blackwell science. However I will suggest you to at least follow one textbook which has been suggested earlier in earlier slide that is either Dr. Subramananyn’s bookor S. K. Duggals’ book.
Video 2
Now these codes are required while learning the design of steel structure, first is the earliercode IS: 800, 1984 which was based on working this method. Now the codel provision hasbeen changed and in IS 800-2007, the code of practice for general construction in steel hasbeen given, in which the limit state design method has been followed. Therefore, when youfollow my lecture I will suggest you to sit with this code, because lot of codal provisions arethere, which has to be followed while listening my lecture. If you follow the codes at thesame time, it will be easier to understand the lecture. There is a handbook that is SP 6(1)-1964 for structural engineers where the Indian steel rolled section properties are given.All the standard rolled section (I section, channel section, T section, angle sections)properties are given in that IS handbook. So we should follow that and also we can follow theIS 808-1989 where certain properties are given. You can use steel table of any standardpublication where the properties are given. Other than that, I will suggest you to follow IS875-part I to part IV for design loads other than earthquake for buildings and structures. I willcome with this code in details later and also IRC code vehicle load in bridge structures maybe useful apart from this for seismic consideration IS 1893-2002 . Now coming to rolled steel section, I have told that different type of rolled sections areavailable in the market and their properties are given in IS handbook in SP-6, so like here Ihave shown that Indian Standard Junior Beam which is termed as JB also. So, IndianStandard Junior Beam (ISJB), Indian Standard Light Beam (ISLB) and Indian StandardMedium Weight Beam (ISMB), Indian Standard Wide Flange Beam (ISWB). So differenttype of I sections are available in the code. Indian Standard Heavy Weight Beam (ISHB),Indian standard column section are also available there. Now, the overall depth of I section is called D, in the SP-6, the properties of I sections aregiven. If you say ISMB 250 that means it will refer to a particular I section of medium beamISMB, that 250 means the overall depth of the section will be 250. Therefore, this D will be250 overall depth.The width of flange is called B and sometimes we call bf also, and web thickness is called t ortw. Thickness of flange is measured at (B - t)/4 distance and we mention the thickness of theflange is T or tf.So if you see in the SP-6, the properties of I section suppose if it is ISMB 250 then its depthis 250, its weight, its cross sectional area and the geometrical properties everything ismentioned there. Not only the geometrical properties, but also Ixx, (moment of inertia about xaxis), Iyy (moment of inertia about y-axis), Rxx (radius of gyration about x-axis), Ryy (radius ofgyration about y-axis), section modulus Ze, Zp, gauge distance can be found. Therefore, thatgauge distance is also standard for particular section. So all the relevant properties can befound out from that code. This channel section are mainly used for column. Indian standard junior channel (ISJC),ISLC Indian standard light channel (ISLC) Indian standard medium weight channel ISMC,different types of channel section are available in code. Here, ISMC 400 means the overalldepth D will be 400. Once depth is known, other properties can be found out from SP-6.The width of the flange is termed as bf and the thickness of the flange T or tf is defined at adistance (B-t)/4, tw is the thickness of web, R1 and R2 are the radius of curvature. Then Cxx, Cyy, flange slop, α are also given in SP-6. In the code, ZZ is written in place of XX, so Ixx isrepresented by Izz. Angle sections are of two types, one is Indian standard equal angle and another is Indianstandard unequal angle. Equal angle means leg length of both of the legs are same, but if leglengths are unequal then it is unequal angle. So standard way of writing is ISA 90 × 90 × 6,that means both of the leg length is 90, thickness of the leg is 6 mm. This is another section, which is called Tee section, Indian standard normal Tee section and incase of Tee section, the total h is the depth. There are rolled steel bars, which is called Indian Standard Round Section (ISRO) and this isIndian Standard Square Section (ISSQ). ISRO100 means it is a round section of diameter 100, again ISSQ50 means it is a squaresection of each side 50 mm. So, this is how it is designated. Then rolled steel sheet and strips are also used, those are termed as Indian Standards steelsheet section and Indian Standard steel strip section. 50 F 8 represent a flat of width 50 mmand thickness of 8 mm are used. Square hollow sections and hollow pipe sections are also used in design of steel members. Soin this first lecture it is shown that different Indian rolled sections are available for designingand their geometrical properties are given in SP-6 which will be frequently used for design ofstructural members.