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Badar (BAZ)

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Everything posted by Badar (BAZ)

  1. Badar (BAZ)
    There are major flaws in your structural system. There is no load path for transferring lateral loads from top to foundation. They way you have modeled it, you are relying on moment connections, which, I think is not practical, and also not applicable for open web joists. Why have you selected open web joist for 15 ft span. You can use hot rolled section You have not modelled bracing required to prevent local buckling of compression flanges of your flexural members. You can ensure buckling restraint by running beams in longer directions of the shed as well; Span between the beams will depend upon required unbraced length. You will also need to put cross bracing in the plane of roof deck, as well in the plane of column grid-lines (in both direction) , at-least in one bay, to improve global stability of the system. You will need to check, among others, if the model is using correct values of unbraced length, moment gradient factor (Cb) and effective length factor. If you want to use open-web joist, then you need to ensure that Etabs knows that it is built up section. I suggest you to select compact section for you flexural members, and ensure that slenderness ratio of elements of column-section are low enough to ensure inelastic behavior. You should be using 2D model.
  2. Badar (BAZ)
    I am not using dynamic analysis, and I hav'nt used special seismic effects.
  3. Badar (BAZ)
    You are welcome. Are you not in possession of ACI-318 specifications? You should read chapter 21 of ACI-318 08, or 318-11 (section 21.5 through 21.7). They have already explained it in simpler form.
  4. Badar (BAZ)
    I get different distribution. The question is why am I getting the distribution in this case. Should anyone proportion members for the design forces given by ETABS in stiffer portion? They are too much. An if I increase the stiffness of lower portion, forces keep on increasing. Is it not a violation of equilibrium, forces are much more than what superstructure can transfer. I have also assigned base level to be at top of basement.
  5. Badar (BAZ)
    I have attached snapshots of the distribution of shear force in walls at two different grid lines. Totale base shear of the building is 2600 kipps, and total shear force carried by walls at the ground level is more than twice the base shear.
  6. Badar (BAZ)
    There isn't any major seismic-requirement for OMRF, these frames should be used in non-seismic areas, or areas of negligible seismic activity. Code (ACI 318) directs us to have at least two continuous top and bottom bars for beams, and a stricter design shear requirement for columns (height/column dimension < 5) susceptible to brittle shear failure. For IMRF, Code requires us to confine column core at joints, confine potential plastic hing location in beams, and to design columns and beams for additional shear due to over-strength of beam members at location of plastic hinges. For SMRF, in addition to requirements of IMRF, code has specific provisions regarding size of column and beams, regarding lap length and developments of reinforcement, joints of beams and columns; it also requires us to provide more flexural strength to columns than beams in order to preclude the possibility of development of plastic hinges in columns.
  7. Badar (BAZ)
    It is not about code provisions. It is about results of analysis .
  8. Badar (BAZ)
    Dear All, Can anybody comment on amlification of forces by ETABS in stiffer region when buidling has flexible upper portion (buildings with basements). Regards.
  9. Badar (BAZ)
    You need to connect members in such a way that their centroid should coincide. If your frame members are not connecting with each other at centroids, than use the option "insertion points".
  10. Badar (BAZ)
    You can find solved examples in the books such as Dynamics of Structures by Anil k Chopra, and Seismic and wind design of concrete buildings by S.K. Gosh and David Fanella.
  11. Badar (BAZ)
    I invite your input on a question. In case of seismic design, what is the aim of setting these drift limits. What does code want to achieve with them? Are they intended to comply with serviceability limit state, damage control limit state or collapse limit state?
  12. Badar (BAZ)
    Abdul Qadeer: you will, most probably, provide the reinforcement ( flexural reinforcement in both directions) which will be more than what is required for the temp-shrink. So, I don't think its a valid question. Will temperature differentials have an effect on serviceability in spite of that increased reinforcement? I am not sure.
  13. Badar (BAZ)
    I do not have any published-source in mind that can be referred. As far as I know, expansion joint are needed for two reasons: to arrest temperature/shrinkage cracks, and to fend-off unwanted/complicated behavior against seismic/wind actions. As you have mentioned in your post, different authors have recommended different ways to deal with temperature/shrinkage stresses according to their experience. In this problem you have to decide why are you thinking of providing expansion joints. Is it for temperature/shrinkage , or seismic/wind, or for both. I know two factors that needs to be considered while providing expansion joints for seismic actions:to avoid torsional behavior due to building shape/structural system, or to avoid the complex behavior that the structure may have to deal with in the case of building experiencing earthquake of different magnitudes at its different areas, which can happen if the building/structure is spread over large area (bridges, stadiums etc).
  14. Badar (BAZ)
    You can use those elastic-2D stress-strain relationships to solve this problem. You will need to assume the values of crushing strain, cross-sectional area and elastic modulii of both columns. You will assume same values so that you can compare the effect of confining stress. following relation will be use: strain y = 1/E[stress (y) - Poisson's ratio . stress (x)] For the case without confining stress, stress(x) is zero, and for the other case, it is hydro-static pressure at the base of column, which will also be expressed in terms of height of column. As it is mentioned in earlier comment, you will express stress(y) as function of cross-sectional area and height of column. crushing strain will need to be substituted for strain(y).
  15. Badar (BAZ)
    Aslam Kassimali's book can also be consulted. You can find it on this link: https://skydrive.live.com/redir?resid=35DD3B5349D6D025!3299&authkey=!ACpYP6NJabY6gHw&ithint=file%2c.pdf
  16. Badar (BAZ)
    Dear Fazalarju, even though you have requested an answer from someone else, I feel, I should reply again. The answer to the first part of your question is 'yes'. It was yes in earlier comment; May be, my earlier reply was not lucid enough. Regarding second question, I would say that the assumption, inherent in your query, is so rigid that you are not able to comprehend the answer to the contrary. I understand your query, and I am aware of the distribution of earth pressure.
  17. Badar (BAZ)
    Etabs is not used to analyze roof, or floor slabs. However, you can model it in etabs . You don't need to assign earth pressure in Etabs model. In my opinion, it would be unnecessary to do that. You can deal it with it separately.
  18. Badar (BAZ)
    You will need two values to compute maximum principle stress, minimum principle stress and maximum shearing stress at a given section and at a particular point: these values are bending stress(sigma x) and shearing stress for a particular point at that section. Sigma x is My/I and shear stress can be computed by using equation 65 of that book, i.e the equation for computing variation of shear stress over a rectangular cross-section. You will need to pick a point to compute these values, if you have taken a mid of cross-section, then shear stress will be maximum and bending stress will be zero. Put these two values in
  19. Badar (BAZ)
    You can compute principle stresses, given in the book under the article: "principle stresses in bending". These are the same relations that are used for plane stress problem.
  20. Badar (BAZ)
    Curvature of beam is double derivative of deflection. There are other ways to express curvature; it depends upon what you want achieve. I think you are talking about the relation of curvature that Timoshenko used for deriving equations for deflection. I am not able to understand your next query. Where are these stresses linear, and where are they non-linear? I am not sure what you are referring to? You don't need to derive stiffness matrix and other matrices to compare results with FEM. You need to use equation d, or h, on page 172 and 173 of his book (strength of materials, second edition, the book is in two parts). These two equations can be used to compute deflection of beam ( one for udl and other for point load), and equations, take into account, shear, as well as, bending behavior.
  21. Badar (BAZ)
    In your last comment, I think , you are talking about distribution of stresses along cross-section of member; linear for bending and nonlinear for shear stresses(rectangular section). This distribution corresponds to a state when material is behaving elastically. Non linear distribution of shear stress, along cross section, does not mean that material is in plastic state.
  22. Badar (BAZ)
    Yes, you will easily accommodate those restrictions. I think that is, mentioned in previous comment, the simplest, and widely accepted way of validating your results of elastic FEM. You can also compare distribtution of principle compressive stresses, distribution principle tensile stresses, deflection due to shear deformation, due to bending deformation, plot (ratio of deflection due shear/deflection due to bending) with length/height of beam, and consequently comment on that L/H <4 criteria. Shear contribution of deflection will be more for L/H less than 4, i.e deep-beam behavior. I have attended the course, but that was in context of Geotechnical engineering with emphasis on tunneling; consequently, we were assigned a tunneling problem.
  23. Badar (BAZ)
    The way you have written about your assignment, it can be done in different ways. One way has been mentioned by Umar, which involves calculation of strength of beam. There could be another way, the easier one. Perform linear elastic F.E analysis on your deep beam, and then compare the results- which could be deflection, bending strains and shear strains- with Themoshenko equations that take into account bending , as well as, shear deformations. You can find these equations in "Timoshenko S.P. and Gere J.M., (1971), Mechanics of materials, Van Nostrand Reinhold Company", or "Timoshenko S.P., (1957), Strength of Materials – Part I, D. Van Nostrand Co. Inc., London, England".
  24. Badar (BAZ)
    Centre of rigidity is a measure of 'torsional susceptibility' of buildings, and is computed by calculating stiffness of vertical members, at a particular floor level, to lateral loads. Hence, in elastic range, it is independent of loading. But loading can change the state of member: can push a member into inelastic region. In in-elastic region, center of rigidity will change continuously because stiffness of some members will diminish, depending on the state of members and their mechanical properties, which will depend on loading condition. In design offices, and as far as building codes is concerned, you, primarily, deal with elastic response, and hence it is independent of loading.
  25. Badar (BAZ)
    Vertical resistance of piles will depend on soil conditions; insure enough skin friction, or bearing capacity at bottom of pile according to external vertical forces. In case of machine loading, you will also need to provide for ability to shear and bending stresses. Check for punching shear in pile cap. Check out this book for guidance. https://www.dropbox.com/s/v5ffxjcq4jyqcpt/Pile%20Foundation%20Analysis%20and%20Design_H%5B1%5D.%20G.%20Poulos%20%26%20E.%20H.%20Davis_1980.pdf
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