Transcript for:
Fundamentals of Reinforced Concrete Design

[Music] welcome to the course design of reinforced concrete structures so what do we mean by design design means to plan or to make drawings for appearance or to show the structure which is not yet constructed so the structure which is not built for that we have to plan we have to make drawings and that is called design and now the next question that what is reinforced concrete reinforced means actually as per the oxford dictionary it calls that make it stronger reinforce means make it stronger and reinforced concrete we make it with that steel bar steel bar so we make concrete stronger with the steel bars that is called actually reinforced concrete so coming to the point the design of reinforced concrete structures so we have to design reinforced concrete structures so that particular one we shall do in this particular course and one sec it may be habituated so what is concrete concrete is made by mixing cement fine aggregates coarse aggregates and i have kept that and water because water i have kept its cement fine aggregates coarse aggregate these are the three materials which we shall consider but water also is another governing element if we add water less then also that it will vary the strength of the concrete if you add water more then also the strength of the concrete will vary so water plays an important role in this particular that concrete making so that's why we kept it water and how do you describe that water we describe this one as water cement ratio so what are cement ratio will tell us that whether that particular concrete we shall achieve that target strength you can find out the steel steel it is made still it is made in the workshop and it is in a controlled environment whereas we are talking concrete concrete is made in the field itself and you can find out that particular one there are so many factors play while making concrete we just simply cannot put a particular proportion and we cannot say that concrete is made there are lot of other things also even the concrete is made there is one more important part that is called curing that means even if we make concrete then you have to cure it generally we do it that water if the curing is done properly then also we can say that significantly that changes will be there in the strength so if we come to this point just to give this particular one generally this course aggregates made of the stone chips and this one obviously in that bulk amount that is the major amount you can say this one that fine aggregates and that one we can say this is generally the sands and which we can get get it from the river beds and the binding materials here that is cement and here we are keeping that one that we have to measure that water also and this one that water cement ratio for a particular case if we have say one kg of say for this case cement then we require say 400 milliliter say we are considering water cement ratio 0.4 generally it comes in that range 0.4.5 in that range depending on the target strength so coarse aggregates fine aggregates cement and water please note this particular one because whenever we are talking is a reinforced concrete we should remember this particular what are the three different parts generally it comes say one is to two is to four if we consider one is to two is to four that is ratio of cement sorry concrete then we mean to say that one means cement two means sand and four means coarse aggregates we generally do not specify that water we generally specify that one is to two is to four one is two one and half is to three so these are called different uh generally that guiding one we call it nominal mix so here we designate m fifteen if we consider m fifteen we mean to say that we have to achieve the strength of the concrete after 28 days that is 15 newton per square millimeter similarly we specify say m 20 m 20 means the design makes m for mix and 20 means 20 newton per square millimeter so that means we shall achieve that one after 28 days so if you test that one on 28th day we should achieve that particular target strength just let me give an example of that say your slab this is actually slab and whenever we specify this concrete reinforced concrete then we have these bars these bars because this one we are considering slab that where you are sitting what the what the roof the top that is also that is called slab generally we have four components one is called that slab another one is called beam other one is called column and the column footing so these are the four components we have to design and that we shall learn in this particular class so slab horizontal one just like a plate beam horizontal one so like this we can say beam and then we have that one column and column footing which will transfer the load on the ground that soil and that also depends that soil that bearing capacity that is that how much load it can take if we take say here just only say single one say very very pinned one then sharp one then what will happen it will just simply pierce the ground so that we have to make a square type of thing or circular type of things so that the load could be transferred to the ground and that depends on the bearing capacity of the soil that is another aspect that means that one will be told by geotechnical engineers who are looking after that soil so that is another aspect so we shall get the features or properties of soil where we are going to construct that particular building and then on the basis of that we can find out the footing size so i repeat the number one that we have to know how to design the slab number two we have to know how to design the beam then column which will transfer the load on the ground and since the column dimension that is not sufficient that is less which could pierce the ground thats why we are making that column size on the bottom little bigger which we are considering that footing considering this particular reinforced concrete element this is nothing but the slab element here we can have that concrete then we are having bars in two directions we have bar and you can see at the top also and at the bottom also the bars at the top also at the bottom and these bars you have to specify the dimension what should be the dimension of the bars what is the spacing of these bars what should be the depth of this concrete we cannot simply provide the bar at the bottom of the slab we should give little bit of cover that means we cannot give that one so if you see the slab from the bottom you cannot see the bars you have because it will be little bit up may be say 25 millimeter 30 millimeter depending on the situation if the environment say for normal case generally we keep say 15 millimeter for slab if it is a beam then we have to keep 25 millimeter if it is a column then we give 40 millimeter that means from the outside dimension of the column we have to give it inside 40 millimeter so that we have to give that particular dimension just i can say if we have say a beam so this is your dimension and let us say we are having the slab it will start from here ok so we have to provide the reinforcement because concrete is very weak in tension concrete can take compression that means if you press it it can take enormous load but if you just pull it or bend it such a way so that the tensile one it is very very less almo we ignored it we neglect it that means that concrete does not take tension and that but we just supplement with this reinforcing bars so we are having the reinforced concrete though we are talking say one single element but it has two different parts one part is taking compression and another part is taking tension tensile one will be taken care of by steel which is manufactured in the workshop and compression one will be taken care of by the concrete which we do cast in the field itself so here also you will find out little bit difference that means one is the control environment another one is the you know that environment is not so controlled because there could be so many furious thing can enter to that particular one but anyway we shall not take care that particular one we shall assume that we shall achieve that particular target strength now when we are talking say cover regarding this particular one so this is called actually this whenever you are talking this particular one the bar that means the bar the beam is like this this is your beam let us say the longitudinal direction let us specify something say your axis that will be then it will be more clear so if i say this is your say x ok and then we can move this particular one say your y and the other one say your z so whenever we are talking this particular one x means along the longitudinal direction so this particular cross section it means it means we are considering this one as say y and this one we are talking series so is it clear that means we mean to say this portion that is one cross section and there if we see the bars actually along this direction and if we take a section if we look this particular like this so we have to specify say a a that means i am taking a section along this particular line and if you look that one then i shall see and this is nothing but say section a a let us say there are two bars we are specifying two bars so we shall get just to round the circular thing maybe it could be say what is the bar diameter generally the bird diameters we are having say six millimeter it starts from six millimeter then eight millimeter ten millimeter twelve millimeter so we can have bird diameter six mm eight ten twelve fourteen also available but generally we do not use it because generally that it does not it is not manufactured in a regular 16 whatever we specify in our say drawing in construction that only i am telling 16 then 20 25 so we go in this particular order there are so many others also but generally for our residential building school buildings or the common type of structures we are happy with this type of bars why we we need it that if you do the construction say the planning design all those things done in the say your design office so now what you have to do when you will give the drawing immediately they cannot start so what you have to do the site the stores they will ask for that what type of steel you are using in your design that you have to specify because if i give the drawing now they cannot start immediately so this would have to have some tender for say procurement of different materials concrete for concrete stone chips and then your cement then as well as steel also so first thing the steel because still that you have to give order then it will come it it may take some time so thats why they ask for that what type of bars you are going to use generally we use for slabs say eight millimeter eight and ten generally we use it for slabs then we start say your beams we generally start from say twelve sixteen twenty that we generally use and for columns we again use 12 16 20 25 it all depends on the designer also because every analysis is something you are doing you are following a method and it is straight forward and it is you can say that particular one analysis that one you can say that almost everybody of us will use the same formula everything here also in design also we shall use the same formula but everybody has its own design philosophy so you can find out that there are different school of thought that ok he prefers this way the other one people but we are using the same formula but whenever we are providing say for example for slab one can say ok i shall use only eight millimeter a designer may have some kind of allergy of using say ten millimeter diaper also he is always is interested fond of using eight millimeter this is one kind of say philosophy but there is nothing wrong on it so the thing is that eight millimeter ten meter like this but somebody is interested ok whenever i feel ok let us use 10 millimeter similarly for say beams says somebody is interested using say 12 millimeter and 16 millimeter only similarly say your for columns we shall start to we can start with 12 millimeter then we can start 16 20 whatever it is that we can start we can use it also so now coming to the point i started with that cover when we told the cover we mean to say the center line central line of the bar from there to the end of this one we shall say that is called effective cover so effective cover it means this one let us say that is d dash we can have other one that is called clear cover so we have effective cover and clear cover clear cover means from the end of the bar the end of the that section that is called actually clear cover so we have one clear cover and another one is called effective cover now clear cover let us say let let us make it little bigger then this is the bird diameter bar this is the end of the section clear cover we mean an effective cover it means this is called effective cover so what we can do actually here if we say clear cover say c c then effective cover will be c c plus diameter of the bar half of the diameter of the bar so five by two so whenever we calculate that one so effective cover and clear cover we shall make it like we shall come in detail that at the time of design now coming to this particular point why do we need clear cover one important aspect that is your say corrosion of steel so that the steel is not corroded so that is one aspect and there are so many others also we can find from the environmental aspect that is the major one we can consider that we have to give proper we just simply cannot give immediately at the bottom so that part we have to consider here there is one aspect that is your say this is your junction that what we consider here that this part is called that slab and sorry and the other one is called this this one is called your say your beams that beam detailing and this is your column and this portion you are having that slab so actually i am okay just a minute i'm just coming okay so this portion ah i am not still habituated right this portion is called that cos slab this one is called your slab and the other one slab and there is a junction here say if we consider that particular junction what we can do here that if we consider let us say this is a plan and we are considering that four different parts we can make let us give certain dimension this one is a 10 meter and let us take this one also say 10 meter ok what we can do here if we use 10 meter you can find out that 10 meter of one plate if we consider then what will happen you will get deflection in the middle too much because 10 meter by 10 meter that particular one if you have to make say maybe you can consider the paper maybe even if you consistently say cloth itself a big one if you just hold it with say four corners then you will find out the deflection at the metal too much so what we can do we can give two horizontal bars like this in the middle so that your middle point deflection is less and but you will get deflection here also in these four points you will get the deflection but we can reduce our deflection so that means if we can make say here beams the horizontal one which we call beams so if we put two beams here in addition to these four beams in the periphery so we can increase the stiffness and we can reduce the deflection because deflection is also is an important criteria now coming to this particular point let us say that we shall provide four columns say here say like this and also we can provide here also columns so one such example here when we consider that one such example here this particular one if we consider that this column and there are two beams this is one beam this is another beam and the one column that one also intersecting so now this one whatever that we have to provide the reinforcement for these beams we have to provide enforcement for these beams and also you have to provide reinforcement for slabs and also you have to provide information for the columns so you can find out that it is really a chaotic situation because there are so many bars you have to provide and each of them has their own bird diameter so when you have to consider that part and that is called detailing detailing is very very important in reinforced concrete design even if you design that one that also that how to provide the reinforcement that is very very important aspect so this is one such example that you can see that how the bars are provided and through these through these all the bars and there are so many also there are some kind of say lapping also you can find out that means it is not possible that we shall get one single bar coming from the bottom to top maybe we shall get say five meter or say eight meter or 10 meter that length whatever we can get from the workshop now when we are considering that particular bar that bar you have to provide so you have there should be a lapping so the thing is that obviously we can understand that if we if we have say two things like this this is one but this is one bar and let us consider just with the quad we would like to tie it out if we make this one small obviously one can simply pull it and take it out but so that means we should have some kind of overlapping and obviously it depends on the diameter of the bar if we consider say two wires the wires are very very small diameter then we can have whatever overlapping required if the wire having the larger diameter then we need overlapping mode that means with the quad itself we have to tie it out and if we pull it so that it will not go out so this is called lapping or and we have to provide the lap length also so that it will not tear it out so these are the different aspect we have to consider in the reinforced concrete design there is another aspect this is your column another aspect that bars whatever we are having we have to provide for the bars we call it tie the tie that means it is tied those these are these bars are called longitudinal these birds are called longitudinal and these birds that call tie the tie you provide we provide ties so this is your longitudinal bars which are going up and we have to provide time say like this so we have to provide the diameter of the tie bar and the spacing this is called spacing that means so we have to provide the tie bar and spacing so this one we have to provide so that if if we do not provide what will happen if we do not provide then it may happen that it will bulge like this so thats why you have to tie it out so that it will not go out it will not separate out so this is also we have to provide and it depends on the your say column dimension lot of other things when we shall design that column that time we shall do this particular aspect so this is another one we are not going to detail of that particular one this is your that at the construction site we generally are habituated we generally see the finished product but if you see the construction there are so many things actually there if you see this portion this is once a block this is another block these blocks all those things which are given actually to provide the proper clear cover these small small blocks you will find out at different places just to provide that one for the proper clear cover otherwise you at all it will be only at the bottom of the that particular say your surface so you have to lift it up and you are lifting up with that is a small blocks that is made of say motor motor is nothing but sand and cement so with that we make that particular one this is your say circular column and here you can see that this one having say spiral the tie we are making say spiral one we are making so this is another one we generally make that for and this is actually one they are going to construct circular column this is your base at the bottom that footing after this they will start the circular that your section and this one obviously we have to use this type of say arrangement otherwise it is so heavy you can understand otherwise that all everything will fall down your just concrete because fresh concrete it does not have any strength green concrete does not have any strength so it will simply fall down so you have to use this supports to withstand that particular load and only after say 28 days or maybe after 14 days only you will get the required strength and then if you take it out then it will so generally we have to take it out after 28 days and then you can see that particular structure that is built it can take the load for which it is designed there is another aspect here that is this is called beam this one is called column and this is your say beam this particular beam has a special name we call it tie beam we call it tie beam so whenever we are having say this one we are talking say one elevation and what we do this is a ground what we do let us say another column is there what we do say maybe we can provide here or maybe we can provide we shall provide say here one beam and this is called at the bottom this one we can provide one beams at the first floor level another beam at the second floor level like that we can go this beam has a special name we call it tie beam and it is rectangular that means if you seek the cross section you will see like this you can see here also you can see this is the one we can see that particular one having say your this particular one here that one section if you consider that is a rectangular you can see that particular one say your rectangular that you can see so we can come to this particular one so tie beam that is a special name that we have to consider and i have already told that you are securing curing is generally done like this that jute bag all those thing we just cover it and we just put water and that is called actually curing and that one you you have to do that curing also that is very very important even if you cast it properly all the cares you have taken care of even then you will not achieve the target strength for which it is designed if it is not done that curing properly so that is very very important this is another aspect of that from the site just i have taken this particular one and here also you can see there the construction is going on in our that campus itself so this particular construction and you can see the tie beams there are so many tie beams here so that one at the bottom why we need that one because from there we shall that wall that greek wall will be started that particular one we have to use it now coming to the point here one aspect that is we are doing the test we are doing the casting at the field itself and we know the proportion let us say m 20 grade of cronkite that means we would like to achieve the target strength that not target strength we shall say characteristic strength we call it that characteristic strength that we have to achieve if we say m twenty why i am telling every time m twenty because that is the one we generally use m 20 mix 20 that is originally used for common design residential in school building like that college building so any public building generally we are happy with that m 20 grade of coin that is a common mix we use it when we use that m 20 grade of concrete mix 20 20 newton per square millimeter we have to achieve the strength that is called the characteristic strength we have to achieve and that we shall have to find out how do i know that we have achieved that strength for that what we do actually we take we at the same time we cast 150 millimeter cubes that 150 millimeter of cubes that we cast it this particular one the three cubes that we cast it at least three cubes we have to cast which having dimension one fifty millimeter dimension and that we have to test and that we test it with this type of machine this is your say we call it say universal testing machine that is commonly available that this testing machine that you can use it for bending you can use it for say your compression there are so many others also for tensile strength also that also you have to use it what happens actually if you test it this particular one you just keep on loading and this one will come wherever it will have the crack and failure only it will go up to that and then it will go back so that we can find out what is the whether we can achieve that what is the failure load the crushing load that we have to find out so we just do the test like this that we put the keep the cube in between this say one and we do the test then we find out the failure the different cracks it will come and i have just given we do not go up to that but it will crush like this so the thing is that that that one hand which is the you know say just coming to the previous one let me show you so this one what will happen it will if you add the load it will go move on move on like this and it will go only up to failure so from there you can definitely you can find out and it will not go beyond that then it will go back so from that you can find out the strength that your say crushing load you can find out and from that you can say that what is the failure load and from there you can find out that what is the strength how much you have achieved that particular load you can achieve this one say we are having in our laboratory say 60 ton this particular machine can take say maximum sixty two one then one hundred twenty two one and three hundred ton maxima it can go to thinner because these are three different scale so we can depending on the situation that concrete we can go to 60 ton whether we shall adjust it or 120 ton or say 320. so this is your that ah another machine where we do the testing for steel similarly we have to do the testing for steel also so steel bar that we put it like this and we give the tensile the for concrete we do the compression for still we test it for tension so that we put the bar like this that a closer view and where we can do and this one also the load will be such that it will just simply tear it out this particular one whenever we shall do the test it is not the there should be a neck formation and finally it will just simply tear it out and then we can find out the failure load and also we can find out the elongation so that is also equally important because that one will give us the strain one is the stress another one is strain so these are the two different aspects that we have to find out this is another machine this is now computer controlled this is called instron this is this machine is called instron so this one also another universal testing machine which can do the work for say testing of different parts a tensile compression bending these are the major three that we can do it with this machine and that's why we call it universal testing machine i am talking this one whatever available in our departments the structural engineering laboratory so this is another closer view of that particular one ok now this is another important aspect that is called that how what should be the guidelines because concrete and steel these are the two different materials what should be the guidelines we should have certain kind of fixed rules and regulation which we shall all of us we shall follow and so that we shall come to always in a definite result every time so for that we have that indian standard course every country has their own standard codes so in uk they are having their own code then also american concrete institute they are also having their own code there is one famous that is american standard testing of materials astm so that particular one we have astm they are also having different course at how to test materials so these codes so there are so many codes and indian codes having that wealth of information lot of things we can get it from that indian standard codes ah just for [Music] one thing that and each code having their own name each code having their one two three four like that so can you tell me that what is that is one indian standard one indian standard one the that is made for a specific purpose for a specific thing that is for national flag that is for national flag similarly here we are having so many codes and we shall follow is four five six revised in two thousand we shall follow this particular code for our and post concrete design so is four five six revised two thousand fourth division plane and reinforced concrete code of practice we shall follow this particular code and that is our guideline you can consider this is our bible for this particular one because the different people can design different way where we differs we differs with the say diameter of the bar one can argue why shall we not provide that eight millimeter bar in the column but the thing is that but code says that you have to provide that minimum 12 millimeter so the thing is that now if one specifies say less than eight millimeter less than twelve millimeter then obviously there there should be if nothing goes wrong there is no problem when something goes wrong then only lot of debate comes that why we have used that particular bar when the code says so code is the one that which will give us that some consensus ok we shall follow this particular guideline and everybody will follow that particular one so this way that our code will give for different cases you will find out for different cases it has given different things different formulas we have the formulas also sometimes you can find out empirical even we are using empirical based on certain experiments all those things so then also we have to know and we have to use certain say your guideline and which we shall get it in this particular code but there are few other codes so that is another aspect that your say design loads this is another important aspect the whenever we are talking that particular one say load that say for this particular building we would like to design this back building so how much load we shall consider that we have to consider because somebody can say ok it is not the one that from our experience ok this is the classroom it is the capacity of say your may be say 100 students so calculate on the basis of that load maximum say your weight of the nh student on the basis of that we can find out but that is not the usual way we should have certain kind of load say we shall consider maybe say 250 kg per square mil square meter load it will come in one square meter 250 kg may be say 300 kg depending on the situation what type of building what for it is made whether it is a public building whether it is a residential building so depending on that that we can find out the load how much load will be there if we consider so similarly here also the dead load dead load we mean should say that is the self weight of the structure when a structure is designed that time when a structure is designed the it should also take care its own self weight and it is also enormous maybe sometimes it is a 50 percent the total load whatever you can consider that imposed load and itself weight if you consider 100 so self weight will come say 50 percent that means you have to consider that we have to withstand that its own self weight also and that for that we need the unit weights of building materials and stored materials whatever we have this is your say this part 1 that is is 875 part 1 revised in 1987 indian standard 875 part 1 1987 so that code gives code of practice for design loads and please note other than earthquake because earthquake is considered separately for buildings and structures part one that is dead loads unit weights of building materials and stored materials so this one if we know the unit weights of different materials from there we can find out the cell point of the structure the way it is made the different constituents if we know then we can find out the sale weight of the structure and it is revised the second revision in 1987 this particular code we shall follow for the self weight because design load that load actually obviously you can understand that load if you take it different then your design final design will come will different so thats why that load is also another important part that you have to consider the other one that is called imposed loads and we call it live load also so self weight we mean to say dead load and imposed load we consider live load because it can live load we mean to say that it can vary because say orientation of the room of say different tables chairs your furnitures lot of other things we whatever we can fix it one day that we can change the cg and then we can find out that is the way that we can do it so this that's why we call it live load so dead load and your say live load and that we shall get it in this part two the other one is called that wind loads other one is that wind loads that we have to consider because where the structure is being constructed or to be constructed what is the wind condition over the years so on the basis of that we have to find out and obviously we have the map of india where that what is the wind speed at different zone different areas from there we can find out that how much pressure we shall get it how much load we shall get it due to wind that is called so we shall get that part three wind load we do not need it so far ah only we needed in the himalayan today and all those things the snow loads but generally we do not need it we needed say dead load live load and wind load these are the three we get it from this eight seventy five eight seventy five we shall get it and the other one since it is a separate one so we need indian standard one eight nine three right now we are having only part one we had one 1984 revision that is only one code is 1893 now after the bhuj earthquake then we have that particular because it has given a really an impact and we are having so many programs on say earthquake engineering in our country and funded by ministry of home affairs funded by ministry of human resource development so that is national program and that one it is really that some kind of awareness is coming but the code is very old it is nineteen eighty four now it is revised at part one that is two thousand two and that is for criteria for earthquake resistant design of structures the general provisions and buildings for buildings this particular code will tell that how much load we have to consider because after all that earthquake load you can consider the strictly speaking that is probabilistic that is not the deterministic load the load is probabilistic so whatever earthquake will occur today that our past earthquake on the basis of that whatever load you can calculate the another earthquake the structure can take which may be more than the load whatever way it is designed so we can say this particular load is a probabilistic but even then we have to consider in a deterministic manner and that we shall find out in this particular code one more important part that is is 875 part 5 this one says the special loads and combinations this is one important aspect that we have to consider because the combination of load that how shall we consider that particular load the different loads generally we consider here the dead load then live load or impose load wind load and earthquake load so d l l l w l and e l so you have to find out the different combination that combination of load we have to consider live load that is the one that for which we are going to design live load is the one for which we are going to design that means we have to take care this load this is the load that for which we have to design the structure but other loads are coming because for different aspects because where you are going to construct that building what type of wind load is there when speed is there what type of earthquake load whether it is that earthquake prone or not so all those things whenever we are considering generally we consider it should be sufficient if we consider say dead load plus live load because we have to design for this live load only if you have to design this particular floor we have to design for this live load only but only live for live load we cannot design we have to consider the self weight so this is your one this is the primary combination without these we cannot design whenever you have to design for this load we have to design for this you have to include the dead load also but we do not know where is the structure going to made we have to know that where we are going to make the structure construct the structure what is the wind speed so for that we have to consider dead load and live load obviously there plus wind load the other aspect we consider here the dead load plus live load plus earthquake load i am giving this one a broad idea generally we do not consider wind load and earthquake load in the same combination we assume that the cyclone and you would say earthquake will not occur simultaneously and that is a reasonable assumption because if we take care both of them ok it is good but we shall not get an economic design so that is also another aspect we have to consider safety is one important part definitely but the other aspect also that you have to consider the economy also so these are the combination also i am just telling in general but there should be some other factor all those things are there that also we have to consider now regarding books the books which one we shall follow what are the books we shall follow this is one book that properties of concrete that neville this one here you will get the properties of concrete because you will get in your structural engineering laboratory also you will find out this particular one that properties of concrete that you will get it and then this particular book because i read this book in my student life also and in my teaching also i am using this book where both of them actually faculty of our civilian department that mulli can gupta but mainly i shall follow this one we shall follow this particular book vague is also possible also in our departments there are so many books available in the market and all are equally good but i am familiar with these books i am familiar with this particular book three books but mainly we shall follow this bc vargas this particular book that can limit design of reinforced concrete that we shall do it so the as a first part we can conclude this particular one up to this and we shall continue that all different aspects in detail that materials we shall consider materials we shall consider and then we shall go to that different aspects of design and finally we shall come up with a problem say design of earthquake resistance structures that we shall come with with few one example at least we shall go from the analysis to design that we shall cover up and so that we can have the full idea or say design of and post concrete structures okay thank you