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Building Drift in ETABS 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.

I will try to answer some of your questions: Make sure that when you model your shear/ core wall in ETABS, you have meshed it well, so that the wall has enough well distributed support along the length. Further reading.. 1. Can I model a raft like this with beams (12" x 72") under all columns (42" x 42") & (36x72") under core walls (24" thick); not inverted beams but beams with top flushed with raft top Yes, you can; raft is an inverted slab! 2. Do the slab elements between these beams will have a soil support also assigned to them? Yes they would. Assign them. Further reading... 3. when we define slab properties do I have to tick the " thick plate" option or not. What is the difference? Generically, thick Plate should account for shear deformations in your raft/ mat; Increase your moment and shear. I would suggest you to look into SAFE manual to see the exact difference. If in doubt, keep it checked. 4. What is difference between slab types options? slab? footing? mat? Is your question from a software point of view or general? 4. How to I assign wall supports in safe v8. By placing beams of above property on my raft slab & assigning a wall/line support to them (Kv x width of beam) or Assigning null property to beams and use safe v8 option in which it calculate support stiffness from its size. Kv is the vertical sub-grade modulus. I wouldn't bother assigning any supports to beams. Would just assign area springs to slabs and run it. 5. How do we detail thick rafts. Do we provide mid layer of steel. if so can somebody point a reference book or source on how to calculate it. There is no hard and fast rule and things depend on how thick your mat is. Mid layer can be provided for very thick mats. I have also provided say #5 @ 12" c/c if the mat is very deep for shrinkage control. Make sure you provide skin reinforcement on the sides of the mat and horizontal perimeter bars around it. You will always get areas of high stress under high loads. You can neglect some of the high concentrations by judgement. Also, your mesh size will effect your results and avg stress values. Good Luck!