Loading

Module 1: Overview of Design of Steel Structures

Study Reminders
Support
Text Version

Set your study reminders

We will email you at these times to remind you to study.
  • Monday

    -

    7am

    +

    Tuesday

    -

    7am

    +

    Wednesday

    -

    7am

    +

    Thursday

    -

    7am

    +

    Friday

    -

    7am

    +

    Saturday

    -

    7am

    +

    Sunday

    -

    7am

    +

Video 1
I am going to discuss about the design philosophy of the steel membersand in this course as I have told that we will be designing the members using limit statemethod. So the design philosophy of the limit state method will be discussed in today’slecture. Now different type of Design Philosophy has been followed in last few decades. Earlier oneis called Working Stress Method, this Working Stress Method was used till 2007 in ourcountry.Another Design Philosophy we come across the globe is Ultimate Strength Method. Anotherdesign Philosophy which we will be considering in our course is Limit State Design Method.Limit State method means Limit State of Strength and Limit State of Serviceability. So whywe are not going for Working Stress Method or Ultimate Strength Method and why we aregoing for this will also be discussed in today’s lecture little bit and what are the philosophy oflimit state method that also I will discuss in next few slides. First let us discuss about the Working Stress Method as I told that Working Stress Methodwas used till 2007 in our country and the guidelines are given in IS 800-1984. Basically inworking stress method we consider permissible stress that we divide with some factor ofsafety to get the allowable stress. So, if we see stress strain diagram in case of steel we consider the structure to withstand loadup to yield strength (fy), that means the characteristic strength of the member. So up to Yieldstrength we consider and then we divide it with some factor of safety and then we get thePermissible stress. If you see here I have written that Permissible stress is should be less than yield stress bysome factor of safety. So here, we assume the material to behave in linear elastic manner andstress-strain diagram stress-strain behavior is also considered linear. That means we are notconsidering beyond the yield stress though the member can take certain load after reachingthe yield stress.The factor of safety in different case has been reported in IS 800-1984 the earlier code. So, this is how the working stress method was used earlier but in this case there are certaindisadvantages or certain drawbacks were there. We do not consider load factor that means wedesign based on service load but from the probabilistic point of view, we have to considerthat sometimes it may exceed that load we are consideringTherefore, in that case the structure may fail, so we cannot rely on this Working StressMethod always. Another thing is that sometimes this Working Stress Method become veryconservative because we are taking upto the linear behavior of the stress-strain diagram, thatmeans we are considering up to the yield stress, but after yield stress the member can takecertain amount of load with certain deformation. Therefore, the nonlinear part, the inelasticpart we are not going to consider which is not correct. Therefore, if we consider that in ourdesign, then the construction cost or the size design member will become less and it will beeconomic. And also we have to understand that the member we will design in such a way thatit should not be conservative, it should be economic and of course 100 percent safety has tobe considered. We will not compromise with any safety but at the same time we would try tomake it economic. So that is possible if we go Limit State Method what I am coming later. Than another method which we considered earlier was Ultimate Strength Method. It isbasically a plastic design method, in this case the Limit State is attained when the membersreach plastic moment. That means in this case we go up to say fu, so up to this we considerand then we design and of course, we also multiply some load factor factor with the workingload to get the Ultimate Load.In this method, we do not consider the serviceability condition that means whether theoccupant feel discomfort or not, whether excessive deflection is coming or not that we do notbother. So from the users point of view it is not advisable, so this method also becamenowadays obsolete.Nowadays we prefer Limit State Method the structure is designed in such a way that it cansafely withstand all kind of loads that may act under consideration in its entire design life. Sothat we have to consider the science of reliability based design with the objective ofproviding a rational solution to the problem of adequate safety, that means we are notcompromising with the safety and uncertainty is reflected in loading and material strength. Sowhat we do here, we consider up to ultimate strength and we make use of some factor ofsafety to get the permissible strength or the member.So here we use some sort of factor of safety to ensure the uncertainty factor also we use theload factor as we are not sure that what will be the actual load in the site. We try to find outthe maximum means worst possible combination and we multiply with some factor whichwas obtained from reliability based method and then we try to design with that factored loadthis is Limit State Method but this is Limit State of strength another is Limit State ofServiceability that also we have to consider. So in case of Limit State of Strength we have to consider the stability with Stability againstOverturning and Sway Stability that we have to keep in mind also we have to keep in mindthe Fatigue and Plastic Collapse. Therefore, Limit State of Strength depends on this fewaspects. So in IS 800: 2007 the Limit State of Strength includes this few things which we have to keepin mind like Loss of equilibrium of the structure as a whole or in part, loss of stability of thestructure, then failure due to excess deformation or rupture, fracture due to fatigue and brittlefracture. So, these are associated with the failure which we have to keep in mind and we haveto design under the worst possible combination. So as I told that one is Limit State of Strength, another is Limit State Serviceability. So LimitState of Serviceability when we check Deflection limit, then Vibration limit, Durabilityconsideration and also Fire resistance. So these are few aspects from Limit StateServiceability point of view, so we have to take care we have to keep in mind this limit andwe have to design the structural member keeping all these limits in our mind. So Limit State of Serviceability will be associated with the discomfort faced by the userwhile using the structure that is one is excess deflection or deformation of the structure.Because suppose in structure we are residing in a tall building towards the top floor then dueto vibration means due to cyclone or due to earthquake the building may vibrate considerably.but we know from Limit State of Strength we know that design has been done in such a wayit will not collapse but if you do not consider the Limit State of Serviceability then we areallowing large deflection, so if deflection is more than the occupant will be afraid of stayingthere because of this large vibration.So in such case we have to consider the occupants discomfort ability and we have to takecertain measure so that vibration can be reduced, excessive deflection or deformation of thestructure can be reduced. So this has to be taken care.Then excessive vibration of the structure causing discomfort to the commuters, repairabledamage or crack generated due to fatigue that also we have to keep in mind that we shouldtake care of damage or crack and of course corrosion and durability that also we have to keepin mind. So these are the some parameters which are associated with the Limit StateServiceability. Now coming to Partial Safety Factor, so in case of Limit State of Strength we know thatcertain safety factor have to be considered, one is Safety Factor for Load, this is given inclause 5.3.3, table 4 of IS 800: 2007. Where, Qck is the characteristic load or load effect and Qd the design load or load effect andgamma is partial safety factor for k-th load or load effect. So this f is going to vary from timeto time depending on the type of loading, right. So in table 4 of IS 800: 2007 this partial safety factors are given. Next is what will be the partial safety factor, suppose if we have dead load, live load andcrane load combination what we can do, for dead load we can multiply 1.5 and for live loadwe can multiply 1.5. So this will become as this one load combination will be 1.5 into deadload plus live load. Another load combination we can make that is that dead load plus liveload plus wind load or earthquake load there we are making multiplication of 1.2, so there wecan make like this 1.2 into dead load plus live load plus wind load or earthquake load, eitherwind load or earthquake load we are providing, so this is one sort of load combination.Another load combination is say 1.5 times dead load plus wind load or earthquake load. Sothese are different types of load combinations are reported in the code which we have toconsider and we have to find out the worst combination that means which one will be worstand we have to design the member from that worst combination. In addition, we can see thatfor Limit State of Serviceability under dead load and live load we can multiply just unity 1but when we are going to consider Limit State of Service under dead load live load and windload or earthquake load, we will multiply 0.8. That means for that serviceability loadcondition we can consider that dead load plus 0.8 live load plus 0.8 earthquake load or windload. here it is 1.0, so this is one combination against which we have to check the deflection.So several load combinations will come into picture one is due to strength point of view,another is due to serviceability point of view we have to consider. So for each case we willconsider and we will see whether it is exceeding the limiting value or not, limiting value maybe strength, stress, limiting value may be deflection. And when we are going to check thedeflection criteria, we will multiply either 1 or 0.8 as per the codal provisions given and incase of strength calculation we will multiply either 1.5 or 1.2 as per the type of loading, right. So if you see the table 5, we can see the different Partial Safety Factor has been consideredfor different type of material condition, like in case of yielding resistance governed byyielding so gamma m0 is one safety factor which is considered as 1.10, whereas resistance ofmember to buckling that also as 1.10 that also gamma m0 and resistance governed byultimate stress that we are making 1.25 partial safety factor that means we are dividing thepartial safety factor with the ultimate strength to get the design strength.And for connection for bolt, friction type bolt gamma mf we use 1.25 for shop fabricationalso 1.25 for field fabrication, whereas for bearing type also this is 1.25, 1.25, in case of rivetalso we provide 1.25 and in case of weld we provide in shop fabrication 1.25 and for fieldfabrication we increase that the factor of safety upto 1.5, so this is how the factor safety hasbeen decided and reported in the code which we have to consider and we have to divide withthese factor of safety with the ultimate strength of the material to get the design strength ofthe material.
Video 2
Another is the serviceability criteria, for serviceability criteria deflection limits has beendefined in, table 6 of IS 800: 2007. Different limits have been provided, say for in case ofindustrial building, I am just showing few of them one is vertical deflection another is lateraldeflection, and again design load will be due to live load/wind load, due to live load only. Sodifferent type of design load will be considered and =different type of members have differentlimiting condition. Therefore, for different type of supporting condition and different type ofmembers the limiting deflection has been given in the table 6. So this is the continuation of the table 6 for other buildings, one is industrial building andanother one is other buildings we have given. Now another thing is the Cross Sectional Classification, this is given in clause 3.7 of table 2.one is class 1 which is plastic, class 2 classification is compact and class 3 semi-compact, weknow in IS code different type of steel rolled sections are given. So say for I section we have ISMB, we have ISJB, we have ISLB, ISHB, ISWB like this wehave different type of I sections. Now for different type of I sections, this d the depth of theweb and tw thickness of web, d by tw, which is different. Similarly, this flange width andflange thickness this is different, so its ratio is also different B by tf, d by tw. So we have tosee what is the ratio and different type of structures has been classified according to the crosssection as plastic, compact or semi-compact. So for a particular type of member, we have to decide means particular type of cross sectionwe have to decide means we have to see whether this cross section is under plastic, semicompact or compact and accordingly design criteria will be followed. So these things wehave to keep in mind. Then coming to Load and Load Combinations, load is important because under the particularload we have to design the member and that load may be dead load means due to self-weight,may be live load, may be wind load or systemic load or may be other type of load likeaccidental load or snow load, hydrostatic load, different type of loads are there. So then wehave to know what are the codal provisions, how to calculate the load on a particular member.Then we have to go for the load combination with certain factor of safety that we have seen.So here if we see, the different type of loads are given in IS:875 in part 1 to part 5, IS:875part 1 to part 5, various load and load combinations have been given and now in part 1 thedead loads of the structures have been given. Mass density of different kind of material likebrick, plaster, concrete are given in detail in IS:875 part 1.So the dead load calculation or the self-weight of the structure if we want to calculate, thenwe have to go through the IS:875 part 1 and then we have to see what is the total dead load orself-weight coming into this structure.Next is the live load or imposed load. Live load or imposed load is given in IS:875 part 2, inpart 2 different type of live loads are given like in case of residential building what will be thelive load, in case of industrial building what will be the live load, in case of office buildingwhat will be the live load that has been specified. Again in case of residential building inbalcony, in kitchen, in bedroom what will be the live load, the different live loads arespecified, so that has to be taken care from that code.Another is the crane load crane load also can be found from this part 2, then coming to windload wind load is given in IS:875 (Part-3), so wind load I will be coming details after thisslide. Then snow load in the area, where snow is a factor there we have to consider the snowload and that has been given in part-4, and in part-5 the temperature load, hydrostatic load,soil pressure, fatigue, accidental impact, explosions etc and different type of loadcombinations are given in part-5. So part-5 consist of temperature load, hydrostatic load, soilpressure, fatigue, accidental load, impact, explosions etc and different type of loadcombinations means dead load plus live load, dead load plus wind load, dead load plus liveload plus wind load, like this different load combinations are recommended in part-5.And earthquake load you can find out in IS:1893-2002, in case of earthquake load we knowin our country we had 5 zones, now we have 4 number of zones, zone 1 and zone 2 is clubbedto zone 2, so zone 2, zone 3, zone 4 and zone 5 and zone 5 is the most systematically activezone. So for different zone what is the systemic coefficient for calculating the load that hasbeen given in the code in IS:1893-2002, so detail calculation of load due to earthquake can befound in this code and according to that we have to calculate the load coming to the particularstructure and then we have to apply that load to the structure to design the structural member.Then erection load is given in IS: 800-2007 in Clause 3.3, the details are there and also othersecondary effects such as temperature change, differential settlement, eccentric connectionsthose things also has to be taken care in the load and load combinations because due todifferential settlement and temperature, extra load will come into picture, so that has to bealso taken care in the design calculation. Now in clause 5.3.1 if we see the structural system has been classified in three groups, one isthe permanent action, permanent action means the load which are permanent in nature theseare basically self-weight of the structure which we call generally dead load, so these arepermanent action. Another is variable action, variable action is basically imposed load andwind or earthquake load are not permanent these are temporary and variable so these areunder variable actions. Another is accidental actions, action due to accidental load likeexplosion or due to sudden impact such type of accidental actions happen, so that has to alsobe taken care.And we have told that while designing the steel structure the load combination have to beconsidered with partial safety factor. Partial safety factor I have already discussed that is deadload plus imposed load, here we will multiply with 1.5 and dead load plus imposed load pluswind or seismic load that is 1.2, we will multiply dead load plus wind load here also wemultiply 1.5, like this dead load plus erection load, so these are some load combinationswhich we have to take into consideration for the design of the member. Now very briefly I will go through the wind load calculation because in case of steel structurewind load is a factor for designing the steel members because the steel structures are light inweight so it is vulnerable to cyclone and wind. Therefore we need to calculate what are thewind load coming in to the steel structures and accordingly we have to find out the designcriteria means we have to find out the load coming on a particular member and thenaccordingly we have to design, not only we will design we will check the limiting deflectionbecause serviceability criteria has to be also maintained.So what is the deflection coming due to wind because steel are ductile in nature so lot ofdeflection will come in comparison to concrete structure therefore we have to check theserviceability criteria as well, right. So thinking that I am going to give a brief review on thewind calculation and I told that wind calculation is given in the code IS 875 (Part 3). You willget the detail of wind calculation and here the design wind speed Vz (m/s) is calculated . where Vb is a basic wind speed and these basic wind speed are divided in our country in sixzone, ok in six different zone it has been given like in zone one the basic wind is 55 meter persecond this is the highest speed, then in zone two it is 50, zone three it is 47, zone four 44,zone five 39 and zone six is 33 m/sIn IS 875, figure 1, the basic wind speed for different zone has been given also at the end ofthe IS code in a tabular form it is given for different city what will be the basic wind speed.Next is the probability factor k1, k2, k3, k1 is the probability factor or risk coefficient this isgiven in table 1, I am not going into details if you look through the code you will be able tounderstand all the details have been given. Then, k2 depends on the terrain, height andstructure size, so that factor is given as k2 and in table 2 that is given, you will see thataccording to height the k2 factor is going to increase like this it is going to increase thatmeans the wind speed will go on increasing with the increase of height. Another is k3, k3 isthe topography factor means in what type of topography structure is going to be constructedwhether it is valley or anything else or plain land depending on that what is the slopedepending on that the k3 factor will be calculated these details you can find out in Clause5.3.3. Now Wind Pressure, Wind Pressure we can find out from this formula. The wind pressure at any height of a structure depends on following. Velocity and density of the air Height above ground level Shape and aspect ratio of the building Topography of the surrounding ground surface Angle of wind attack Solidity ratio or openings in the structure
Once we get the design wind pressure we can find out the wind force
Another thing is we have to consider that wind force on roof and walls as an individualmeans if there is a roof suppose we have a building like this and it has a roof so there we canfind out what is the wind pressure is coming from externally and what is the internal windpressure is coming depending on that we have to find out the force and this force can becalculated from this formula. So if we can find out this value the coefficient external pressure coefficient and internalpressure coefficient and the surface area then we can find out the wind force on roof or wallas an individual, right so this is how we can calculate the wind force.why Limit StateMethod is important and why it is more accurate, more practical compared to other twomethods that is Ultimate Strength Method and Working Stress Method. Why we have movedto Limit State Method that is understandable now and tomorrow onwards when we will begoing for design of members or connections, individual members or connections will followthis criteria that means Limit State Method design criteria, where the load factor and thepartial safety factor for the material will be considered and what will be the load combinationfor which we have to design that will be considered and we have seen the what is the ultimatestrength of the member, what is the yield strength of the member for the steel that accordingto the different weight we can find out and we can make use of those parameters for design ofthe elemental means element or member,.So with this I will have to conclude lecture,thank you.