Beam/column
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Author: chigozie
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Beam/column
#23
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iceman84 wrote:-
1.How did you assume that the crack penetrates up to the link level? Cracks can't occur between links?
Comment: - You had already answered the question by yourself “Cracks can't occur between links” or better said “Structural cracks can't occur between steel reinforcements”. Will cracks develop if we have complete steel section (steel beam)? The answer is no. Why do we have cracks in the first place? Cracks develop in reinforced concrete structural members due to the fact that at certain conditions, they have to be subjected to bending which induces tensile stresses on some parts of the member. If that situation develops, the ability of the structural member to resist tensile stresses is called into action. We know that concrete is a pseudo plastic material as such performs poorly in withstanding tensile stresses. Its strength in tension is about 1/10 its strength in compression. So if we have grade 25 concrete, the design strength in compression = 0.45x25 = 11.25n/mm^2. Then its strength in tension = 0.1x 11.25 = 1.1. If we have 400mmx1200mm concrete beam with section modulus Z = bd^2/6 = 400x1200^2/6 = 9.6x10^7mm^3, then this section can resist in tension only a moment = stress x section modulus Z = 1.1x9.6x10^7 = 1.05x10^8Nmm = 105KNm. The weight of reinforced concrete =25KN/m^2 which implies that for the 400mmx1200mm concrete beam, weight/unit length (w) = 25x0.4x1.2 = 12KN/M or the design weight W = 1.4x12 = 16.8. For simply supported beam, moment M =WL^2/8. This implies that L = (8M/W)^0.5 = (105x8/16.8)^0.5 = 7m ; so we will find out that at a length greater than 7m, the section had already cracked under its own weight (without imposed load). But for steel section, this is not the case as steel is very good in resisting either compressive or tensile stresses equally. So if you impose steel on the way of the cracks, the steel stops the crack from developing or progressing. That was why it said that the crack only penetrates up to the link level and stops since at that level, you have the longitudinal reinforcement (like your good self also confirmed that “Cracks can't occur between links”). This is because the steel absorbs the tensile stresses so that the crack stops at that level.
2. Have you ever calculated a moment-curvature relation (by hand or with a software) for a RC element? If you have I suggest to look at the stiffness in behavior stage II (cracked section) and compare it to the uncracked section stiffness? You will probably obtain a result of 0.3-0.5. That is way it is taken 0.5 (it is an average value) for beams.
Comment:- moment-curvature relation has nothing to do with the present discuss. It may comes in when you are discussing deflection and accessing the capacity of such structures like water tanks that strength does not govern design but are designed based on the serviceability limit condition. This is not same thing as when you are designing under the ultimate limit condition.
3. The writer you are talking about is Thomas Paulay. I'm afraid I can never agree with your opinion on this issue. The writer's personal opinions are sustained by TESTS. Your opinion is sustained by a calculation, which in my opinion is not correct!
Comment:- I am neither challenging the writer’s integrity nor his opinion. What I am saying is that you must have quoted him out of context. Situations could arise in which you have to discount part of the concrete strength in flexion or whatever (like while assessing the strength of leaking concrete tank-serviceability condition). I had already hinted on that in my last post. But that situation does not arise in the present discuss. Again you said …Your opinion is sustained by a calculation, which in my opinion is not correct!...”.
Comment:- Please proof me wrong. Demonstrate (we are engineers as such we have to support our discuss with facts and figures).
4. I have quoted 2 books, as requested. Unless I will be given quotes from different books I (humbly) refuse to answer in this thread.
Comment:- I do not need to quote any books. The codes have already confirmed that that is the right thing to do. Is what I said in line with the codes of practice? Check out this example (on analysis of reinforced concrete frame) in Mosley and Bungey “Reinforced concrete design” pages 37-42. You will see that your proposed modification of moment of inertia from “I” to “0.5I-0.3I for the beam” was not implemented. This is because it lacked bases. Again check out page 264 of “Reinforced concrete designer’s handbook edited by Charles Reynolds et allies”. These books that I quoted are “authorities” as long as civil engineering is concerned. The Reinforced concrete designer’s handbook, on that page calculated the moment of inertia of un-cracked concrete beam section and that for column and found their ratio, It went on to calculated the moment of inertia of transformed cracked concrete beam section and that for column and found their ratio section and compared the differences between the two approaches and arrived at same ratio as such concluded that there is no gain in using the more fatigues approach as it will at the end give same result as for the untransformed section as such will amount to a waste of precious time that will achieve nothing.
One of the gains of belonging to or having access to an association like ours(CIVLEA) is the ability to share and to learn. We all learn every day of our lives. We would be most fortunate if we could have avenues to learn particularly the positive things like we get from this community. It is said that two heads are better than one. One of the reasons why is because, I may be having an idea that I thought of to absolutely correct but which actually was not very correct. If you are existing (solely) on your own, you will have nobody to point it out for you. But if you have the opportunity to share with others, you might get the benefit of the others helping you to correct that idea. When that becomes the fact, like happens to all of us in life, you do not have to be sad over it, but be grateful and thankful to your stars that you got that opportunity. So I see no reason why you should be annoyed. No one is here to humiliate the other. We are all learners and like I said before now, we learn all the days of our lives.
Regards
Teddy
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Messages In This Thread
Beam/column - by chigozie - 06-09-2010, 11:59 AM
RE: Beam/column - by iceman84 - 06-09-2010, 12:20 PM
RE: Beam/column - by tamboon - 06-09-2010, 02:18 PM
RE: Beam/column - by Grunf - 06-09-2010, 02:31 PM
RE: Beam/column - by chigozie - 06-09-2010, 05:09 PM
RE: Beam/column - by iceman84 - 06-09-2010, 06:10 PM
RE: Beam/column - by iceman84 - 06-10-2010, 12:12 PM
RE: Beam/column - by chigozie - 06-11-2010, 12:09 PM
RE: Beam/column - by iceman84 - 06-12-2010, 09:02 AM
RE: Beam/column - by DamirDz - 06-30-2010, 06:39 PM
RE: Beam/column - by RedHorse - 07-01-2010, 01:39 PM
RE: Beam/column - by DamirDz - 07-01-2010, 05:28 PM
RE: Beam/column - by iceman84 - 07-01-2010, 06:56 PM
RE: Beam/column - by DamirDz - 07-02-2010, 05:25 PM
RE: Beam/column - by iceman84 - 07-04-2010, 10:38 AM
RE: Beam/column - by chigozie - 07-06-2010, 10:40 AM
RE: Beam/column - by iceman84 - 07-06-2010, 11:43 AM
RE: Beam/column - by chigozie - 07-07-2010, 09:49 AM
RE: Beam/column - by chigozie - 07-12-2010, 12:13 PM
RE: Beam/column - by iceman84 - 07-12-2010, 12:42 PM
RE: Beam/column - by chigozie - 07-16-2010, 11:51 AM

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