Rana

Super Moderator
  • Content count

    782
  • Joined

  • Last visited

  • Days Won

    210

Everything posted by Rana

  1. Farooq bro, I meant the sketch for the the loads and bending moment diagram from ETABS. I am afraid we cant open and check your ETABS model.
  2. But keep in mind, this would change the building system and R factor.
  3. Are you running cracking analysis? If yes, thats the reason. If no, then are you using the same modifiers as in ETABS?
  4. Please provide a sketch.
  5. Just be extra cautious about the uplift due to geometry of the structure under gravity loads. Make sure you provide enough thickness to counter it.
  6. Just to add what Ayesha said, I remember way back, that to convert the triangle and trapezoidal loads to equivalent UDL you multiply it by 1.33 or 1.30. As I mentioned refer to Reynold's book, it explains various cases in detail. Following is an attachment from Reynolds's Reinforced Concrete Designer's Handbook - 10th Edition
  7. Great announcement; 1) I do not use facebook, so cant jump in here. 2) Need more background about this. Not clear what actually is it.
  8. I think its time now to categorize questions based on the difficulty level. For example; A toddler asking very basic question like what is mass source or time period, should be classified under lets say Easy or 0 or Beginner or Simple etc category. And as such specific moderators should be assigned to take care of that forum. Then we can make other sections like moderate or difficult. Where one really need to relearn/review the question and think about the answer carefully. I think all the internet is full of the same basic questions like time period, base shear etc. Isnt it the time to take SEFP a step ahead and not just another forum full of the same basic questions? How can we re-categorize the SEFP sections to give it a new feel, not just like concrete, steel, software issues etc. What do you think?
  9. Also see Modern structural analysis book by Iain A Macleod section 5.6.5 Eccentricity of members at a joint, if you want to go deeper.
  10. I could not find a clear reference for this. This brings us to another topic of uncertainty (As structural engineers, we deal it every day). Of course, this problem in the post could be more rationally solved by sound engineering judgement, but I want to highlight the approximate methods. When there is uncertainty, try to be consistent all the time. Errors and approximations are obvious in structural analysis. Just keep consistency. For example, personally I'd like to keep the Y/X in these cases, less than 10% (strictly speaking 8% based on 500 Y and 6000mm X). Set a criteria for yourself and follow the same approach in modelling assumptions. What are these X and Y, well Y is the center to center distance of columns, and X is the distance between these columns (or the span of beam). What I could discern from the provided image is that; Y=1.8' X=5.3+7.5=12.8' so the beam connecting center to center has an in-plane slope of 14%. a. Of course the first approach in the pictures provided is not accurate as other members have pointed out. b. Second approach is also not suitable as the slope is more than 8%. However, This could still be done if column sizes are more than adequate as compared to spans and loading. In your cases the bigger column size is almost 28 inches, so in my opinion should not have much problem due to this eccentricity. But.....visually the situation looks awkward, although ETABS is not Sketchup, but things like that will draw attention of the reviewer/checker of your models and probably will focus more in this area. In order to avoid this use option c below. 3. Model the bigger column as a shell element with just one support point (important) at the base.
  11. Dummy frame is just another section defined as 50mmx50mm. Try this and then update us if still T is higher.
  12. First, do not apply loads to null lines. Always develop a habit to define a dummy frame sections of lets say 50x50mm and then model these lines for such loads. Second, you are having huge values.......huge values of what? displacement?
  13. Dear Bhuvan, Instead of overtly throwing your models at us, you could have explained the problem in more detail may be with some snapshots. We all are working in different places, and you could imagine it is not always possible to open some third party models/files/work during working hours. Please try searching the forum for the similar questions and try to limit the problem with your own research and searching skills utilizing the past posts in this forum. If still you do not get the answer, please try uploading a snapshot or two of a specific beam in your model explaining your concern.
  14. Another thought; 1. Vertical effects are not required in ASD analysis of UBC-97 but are required in ASCE 7-05. 2. Inclusion of vertical effects make no sense according to Gary R.. Searer; SEAOC adopted the concept of vertical effects for the sole purpose of adjusting the seismic design in line with 1.4D factor of dead loads in earlier codes. This factor was changed to 1.2D due to better approximation of dead loads in the new codes. So in order to rule out the discrepancies vertical effects were introduced. "Certain unintended consequences of this action were only discovered after the code was published"... For example one design that was safe in ASD suddenly became unsafe in strength design combinations. In near-fault areas of Ca=0.6, resistance to overturning decreases by a huge margin to 60% of dead loads. The author explains that; With the exception of a single story structure, ignoring live load totally in the resisting load combination is very very conservative. In order to accelerate portions of the building rapidly upward, the upward forces must exceed gravity by a large amount, thus resisting weight is significantly greater than just 1.0 dead load. The problem became more complicated with the inclusion of vertical effects in ASD in ASCE 7-05 in a try to align the ASD design with strength design as far as vertical effects were concerned. He recommends using 1.2D +- E + f1L + f2S or 0.9D +- Eh. Refer to: 2006 Annual Meeting of the Los Angeles Tall Buildings Structural Design Council Alternative Procedures for Design of Tall Buildings POORLY WORDED, ILL-CONCEIVED, AND UNNECESSARY CODE PROVISIONS Gary R. Searer, S.E. Consultant Wiss, Janney, Elstner Associates, Inc https://www.scribd.com/doc/285836953/Poorly-Worded-Ill-Conceived-and-Unnecessary-Code-Provisions
  15. AQ, there is nothing wrong with the conversion. I stated "87.5mmx10=87.5cm" which is 87.5mm for each storey x 10 storeys = 875mm = 87.5cm. This is the total drift of top floor relative to base 0,0.
  16. I am such a big fan of Dr. Naveed, wish he delivers a lectures in Dubai.
  17. I am not sure how the slab is supported? Is it a flat slab? How many stories? Beam flexure also plays part in seismic resistance. You have to see what is your system and how much beams are resisting the lateral forces.
  18. Share the diagrams you calculated? What do you mean the shear force goes only upward? You mean like a cantilever in isolated footing? If yes, then strip footing is not a cantilever but like a continuous frame. And one way shear, yes one-way shear could also be a problem (bigger problem than the 2-way punching shear) even in isolated footings. Dont under estimate it. For example if the bearing reaction too huge.
  19. Building Drift in ETABS waseemrana.com Drift is a very complex topic in structural engineering. It involves too many factors to arrive at a suitable decision. It involves engineering judgment, the phenomenon fresh engineers might not feel. In this article, I have tried to explain what is building drift, allowable limits, ways and means to check in ETABS models and to control the excessive drift. Please keep in mind, this article is not about the building drift as far as structural science is concerned, rather this topic of drift is related to ETABS software. First of all you must be familiar with the term story drift. For convenience, I am quoting here the definitions from UBC-97 code:- STORY DRIFT is the lateral displacement of one level relative to the level above or below. STORY DRIFT RATIO is the story drift divided by the story height. 1) Maximum Limits Now what for story drift limits? What is the maximum permissible value? Well it depends upon the type of drift. Is it seismic or wind? For seismic, I will refer to UBC-97 code which in section 1630.10.2 talks about drift limits for earthquake. Now in simple words, the maximum limit for seismic drift is:- delta M shall not exceed 0.025 x story ht (if building seismic period is less than 0.7) delta M shall not exceed 0.020 x story ht (if building seismic period is equal or greater than 0.7) Important to note here is that it talks about SEISMIC drift so SEISMIC building period not the WIND period. Now delta M = Max inelastic response displacement = 0.7R delta S where R = from Table 16-N delta S = displacement from static, elastic analysis this value is read from ETABS. you multiply this value by 0.7R to get delta M This was all about seismic drift, but for wind drift code is mute. I will refer you to ASCE 2005 commentary CC.1.2 So we can understand that the limit for wind drift is "on the order of l/600 to l/400" for "common usage". This is common thing, however, in reality this figure can be up or down depending upon the ductility of cladding material and finishes. However for common usage value of l/400 is thought to be well satisfactory. Here l means story ht. The concept of drift limits is same throughout all the governing codes, and the typical limits of story height by some number is same, but obviously you have to take care of the process of calculating the wind force or seismic forces. You should not calculate wind force from one code and apply limits of another code. 2) Load Combinations Once the drift limit has been determined separately for seismic and wind forces, now is the need to check the actual drift vs the limit. Determination of actual drift depends on the load combination and the period of recurrence. If not properly calculated, this may dramatically increase or decrease the accepted drift values in model. Seismic force E is always already factored so that's the reason its factor is always 1.0 in load combinations of ACI/ASCE code. The recurrence period for seismic force is 50 years. In seismic drift we do not convert it into service seismic force. Seismic drift is checked against the direct load case of EQx, EQy etc in ETABS. For wind drift, we need to convert 50 year wind to service wind force. It has been recommended by ASCE commentary CC.1.2 To convert 50 year service wind force to 10 year service wind force it is multiplied by 0.7, as the equation says, and other gravity loads; D and 0.5L are also added. So in a nutshell we create following load combinations in ETABS to check our drift:- DRIFTWx1 = D+0.5L+0.7Wx DRIFTWx2 = D+0.5L-0.7Wx DRIFTWy1 = D+0.5L+0.7Wy DRIFTWy2 = D+0.5L-0.7Wy For seismic drift, as discussed earlier, we do not need any combination, drift will be checked just on EQx and EQy load cases only. 3) How to check in ETABS Now we have obtained both the actual drift and the drift limit, but how can we do this in ETABS easily? Well, after creating the drift combinations as discussed in step 2, we need to do as below:- For seismic drift goto File>Print Tables>Summary Report Select the file name Scroll down to the end of the page, you will find out a section about drifts, similar to this one:- It displays the max drift for each lateral load case for each story. As we want the drift for wind to be on drift load combinations and not on wind load cases, so we will not compare this wind drift without limits. In this table we are going to check just the drift values of our ETABS model for individual seismic load cases; EQx and EQy. As you noticed, this table shows us values in fraction format. For example 1/105 that becomes 0.009523809524. This 1/105 value is story drift divided by story ht. It means delta S / story ht. Now this value is delta S. First we need to convert it to delta M by multiplying it with 0.7R. Assume R here is 3.5 so delta M = 0.7 x 3.5 x 1/105 = 7/300 = 0.023333 which is less than 0.025 so safe ( if T<0.7). So instead of calculating every time by 0.7R we can check these limits in other way. If our limit is 0.025 then the limit we get is 0.025/R/0.7. Assume R=3.5. Now the values in ETABS are inverse so our limit is 0.7x3.5/0.025 = 98. In ETABS the drift is reported as 1/x where x is some number. Now as long as x (some number) is greater than 98 our limit of 0.025 x story ht is being satisfied. This way you can quickly check and compare seismic drifts. Now for the wind drifts, goto Display>Show tables, select Point displacements>Story drifts and then select only drift combinations for results. Click on and then copy the table to EXCEL. To save time you can right click on EXCEL taskbar and select maximum and minimum. Then just select the column H or I and see the maximum value that should be less than H/400 to H600 limit (0.0025 t0 0.00167). Again the values reported in ETABS are divided by story ht. http://4.bp.blogspot.com/-9qv8XKHgL8Q/UALNKflmVsI/AAAAAAAAAEQ/AwKBYWt2iys/s320/image022-773193.jpg 4) Controlling Excessive Drift Values sometimes you may face problem of excessively large values in drift tables in ETABS. Well we are not going to talk about different measures and modeling techniques to control the drift values. We are going to talk about large numbers in drift tables. Sometimes it happens that a point or node is free in the model or is connected to a NULL line or very flexible section. Drift tables for example the story drift table in wind captures the maximum displaced points. Obviously the displacement of several meters in tables is not what we are looking for. Drift values (relative) may be still okay for these points, but it requires you to check the displacement values too before checking directly the drift. Unlock the model and remove all free points, check for any discontinuity and modify your models to remove all the errors. ranawaseem.com
  20. Could you please explain that logic? Furthermore; seismic inter-story drift is limited to 2.5% of story height. If story height is 3.5m, the allowable inter-story drift is 87.5mm. If all floors were to be within the drift (so code requirements are satisfied) then a 10 story free standing building would have total roof drift of 87.5mmx10=87.5cm. Thats a lot, yes. But I do not see any limits in code for the seismic total drift as long as you have satisfied the inter-story drifts. Ofcourse, more the building overall drift, more will be p-delta effects, (that could be huge for a massive tall building). In wind design, overall drift is limited due to problem of acceleration at top floors. But in a seismic event, obviously human criteria is not something you should aim for.
  21. Because there is none except that you have to provide sufficient seismic gap between two buildings. If its one free standing building then no limits except inter story drifts.
  22. I invite other members to also shed some light. For example it could be something like this; If a post is related to let's say wood design (and not to a specific software) we could ask the user to include tags such as "wood" in the question. I am opposed to making so many categories. That would create mess. We could control the sub categories by appropriate tags (strict policy to be imposed on users to input appropriate tags based on the materials /pre-defined sub-categories). All users to be requested politely to contribute in the proper section. For example some one asking how to calculate the time period should directly go to BASIC FAQs section and search the thread, if not found then post the question. Or we could enforce new users to post only in the #4 section, unless we see that the user is not a university student but a more experienced one. So we, as moderator team, could grant him permissions to post in all the forum anywhere he/she likes. This could be a bit more work for moderators but how many 'ACTIVE' users we have? Also this would keep the forum 'clean' and decluttered. Don't you think each and every group on Linkedin and other places are full of one famous question about ETABS i.e. stiffness modifiers? Every new comer post the same question. And also don't you think following two questions should be in two different categories; 1. What is the purpose of stiffness modifiers in ETABS. 2. Although not required by ACI codes, how do you guys model the shear stiffness of shear walls? To reduce f12 or not? etc. Most of the times a design issue is always related to a specific software issue, such as no.1 . So the categories I have recommended are not the final words. Just a direction to start the discussion on the topic.
  23. By consistent, it means that in order to draw the ‘design’ response spectrum you got to know the PGA, PGV and PGD. Usually when these are unknown, PGA is taken as 1g. PGD and PGV are estimated by the relationships given by Newmark and Hall (see Chopra book). However, in your case, you already got PGV and PGD for PGA=1.0g, right? Now you have to make the pseudo-accelration/velocity/deformation spectrums based on this. After that, you have to scale the calculated response spectrums for 0.25g simply by multiplying them by 0.25. For more on this refer to Chopra book (4th edition) section 6.9.
  24. That might have caused some inconsistencies in the calculations (very small/large values) and could have caused this error. There is not much diff in behaviour if you change from 0.01 to 0.0001 so keep the previous value. That would not solve the problem i reckon.