Ayesha

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Ayesha last won the day on April 27

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About Ayesha

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    Islamabad
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    U.E.T. Taxila
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    Civil Engg
  1. The first thing you need to check is if you have done a modelling mistake or not. ETABS and Manual results shouldn't be off by more than 2%. Here are a few items that you should check and then get back to us. 1. Check Total Gravity Load The easiest way to do it is that to check vertical reaction (gravity only) at a column. If the manual and ETABS column reactions for gravity load are same at different locations in your model, then you can assume that your loading input is correct. As a rule of thumb, you centre columns with biggest tributary areas should have higher reaction than corner and edge column assuming constant grid framing. 2. Check Load Combinations The next step is to check the load combination and that if the loads have been correctly assigned to each load category. The best way to do that is to print your load combinations and check them on paper. Similarly, load cases assignments can be checked as well. Pay attention to signs of +ve and -ve. 3. Check Units When checking assigned load cases, you need to make your that you have applied the loads in the correct units. It is very common to juggle the units while multi-tasking. Verify all units. 4. Check Material Properties I have checked a lot of models where everything was correct except for material properties. Sometimes, the design engineer enters wrong unit weight and it messes all the reactions and design. Be sure to verify material properties. 5. Check Stiffness Modifiers If you are assigning stiffness modifiers, please check them to ensure they are correct. 6. Check Slab Thickness Make sure your slab thickness is correct and not in wrong units. 7. Check member sizes Make sure that the members are correctly modelled. 8. Check Model for Errors You need to make sure that your model is stable. If your model is unstable your results would mean nothing. I don't know what the latest version of ETABS does in-terms of instability warnings but SAP2000 would never give you instability warning. You will have to go in the analysis log and check if the model is stable or unstable. A lot of junior engineer just get their results from unstable models which is dangerous. Check the analysis log and check model for all error including meshing. After you have completed all these steps, you will have more confidence in stating the discrepancy between ETABS and Manual Calcs.
  2. Please share the screen shot so it is useful for other members too.
  3. Here is a solved example as well. Buried Concrete Basement Wall Design.pdf
  4. You can read more on https://en.wikipedia.org/wiki/Lotus_Temple . Here are a few pics. By Wiki-uk - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=18545739
  5. To further elaborate on the my above reply paragraph, the extent to which a wall will contribute to the resistance of overturning moments, story shear forces, and story torsion depends on its geometric configuration, orientation, and location within the building. While it is relatively easy to accommodate any kind of wall arrangements to resist wind forces, it is much more difficult to ensure satisfactory overall building response to large earthquakes when wall locations deviate considerably. This is because, in the case of wind, a fully elastic response is expected, while during large earthquake demands, inelastic deformations will arise. The major structural considerations for individual structural walls will be aspects of symmetry in stiffness torsional stability, and available overturning capacity of the foundations. The key in the strategy of planning for structural walls is the desire that the inelastic deformations be distributed reasonably uniformly over the whole plan of the building rather than being allowed to concentrate in only a few walls. The latter case leads to the underutilization of some walls, while others might be subjected to excessive ductility demands. As far as the general practice goes, elevator shafts and stairwells lend themselves to the formation of reinforced concrete core. Traditionally, these have been used to provide the major component of lateral force resistance in multistory office and residential buildings. A centrally positioned large core may also provide sufficient torsional resistance without requiring additional perimeter framing.
  6. This is a very good question. An optimal location of shear walls in a high rise building depends on its architectural features that dictate the mass distribution of the structure as well as the lateral force resisting system of the structure. As a structural engineer, you will have to try different locations considering that optimal location will be one that shall result in centre of rigidity of structure being closet to centre of mass. This is the general principle.
  7. Vote the one you think is the best not the one where you graduated from.
  8. To me its acceptable as you cover more slab area under an inclined beam (so more load on beam). Just check the FFB-17 manually to make sure it is not being undersigned (due to smaller length that actual in the model).
  9. Center to center modelling assumption helps engineers create simple models to represent real life situations. For low-rise typical buildings, the margin of error due to centre to centre modelling is negligible (for most cases). Here is the explanation for why should you do centre to centre modelling for your case posted above. The columns are pretty wide and the irony is that you have only 1 node to join the column to. That node is at the centre of the column. If you don't draw centre to centre, you are actually resting your beams on top of other beams instead of column. This will result in a conservative beam design.
  10. Questions to consider are: Where does your manual calculation stand.. closer to ETABS or Safe? Does reduced moment in beam result in higher slab moments in SAFE? Are the softwares you using cracked or original?
  11. Excellent reply Uzair.
  12. Remarkable observations. Kudos.
  13. Great question. Current ACI design (2002 onwards) is based on strain limit and based on that you end up with tension controlled or compression controlled sections. Tension controlled being the sections where strain is steel is equal or greater than the limit provided by code (0.005 I believe). Previously (1963 to 2002), the reinforcement design was based on reinforcement ratios. This is where the terminology "under-reinforced", "over reinforced" and "balanced reinforcement" comes from. The simplest word to put this is that any steel that has strain greater than 0.005 (or whatever the code says) at time of concrete failure is termed as tension controlled. Both under-reinforced and balanced reinforcement sections will meet this criteria.
  14. Legit request. Find attachment. 2011-12-lateral-stability-of-purlins-and-girts.pdf
  15. You can model sag rod as bracing (Sag rod properties can be defined in a frame section and modelled as a brace) provided your design condition allows the sag rod to behave like a brace as I mentioned in the previous post. Once you model them, see the forces and design accordingly.