ASCE7 Equivalent Lateral Force Procedure in Autodesk Robot - Part 4: Software Implementation

[@donaldkhanye9011]

Hello Dr. CEE Thank you for this comprehensive “ASCE7 EQ Loading course - Equivalent Lateral Force Procedure”. Although I am not using the ASCE code, I find your course structure, content and presentation style of the related concepts to be super helpful and relevant to be able to be applied appropriately when using the code in my region or even when using or considering other similar codes in other regions and in general. I particularly appreciate you taking the time to produce parts 1 to 3 (and part 4 of course); some may have rushed to implementing the calculations in a software with very little to no details of the underlying concepts and references, I find such a rushed approach to be dangerous and often resulting in haphazard use of otherwise excellent software programs. Well done to the CEE team, you work is amazing as always. I could not agree with you more regarding the blurry lines of “risk category II or III”. I also get “iffy feelings” each time I am to classify residential buildings, residential flats, or even low-rise office buildings. I mean where exactly does one consciously draw the proverbial lines regarding “……the failure of which could pose a substantial risk to human life….” or “…… for which seismic resistance is of importance in view of the consequences associated with a collapse…….”? Your argument in this regard is fair and it is well presented given what is currently available in the codes of practice. Regarding the “orientation of the columns”. Would it also be acceptable to define one column section and then orientate that section as may be required by using the “Gamma-angle”? or are there nuances in doing it that way instead of defining separately orientated sections? Still on the subject of orientation of the lateral force resisting elements; and in order to avoid or to minimize “inherent torsion” due to Center of Gravity not coinciding with Center of Rigidity. I recall from “Part 10 - why structures twist under gravity loads and Part 11 - reducing twisting” of the “on-going 😊” series on Modelling and Design of RC buildings that “adding stiffness at appropriate locations” did help to reduce the twist. Granted that the structure used in today series example is practically symmetrical (and for good reasons); will it help in other structures which may not be symmetrical and will it make a structure more efficient against EQ actions in general if one “plays around” with the orientation and positioning of lateral force resisting elements or even adds or removes additional elements from a structure with the aim to try and “converge” to a situation where the structure “center of gravity” is reasonably or sufficiently close to coinciding with the structure “center of rigidity”? And if so, how may one quantify “reasonably or sufficiently close centers” by using or by comparing the coordinates values of the three centers provided by RSAP viz: Center of gravity of a floor slab (F), Center of gravity of a storey (G), and Center of rigidity (R)? in other words, how close enough in terms of numbers could be considered reasonably or sufficiently close as a rule-of-thumb? could variations in the location of the center of rigidity from the center of gravity of up to 5% be considered a reasonable rule-of-thumb number seeing that the code would allow for a 5% accidental torsion in any case? Alternatively, does it really matter if the centers are not close since the “rotation caused by the torsion” shall be translated into horizontal forces and if the lateral forces resisting elements within the structure can resist such forces, then the structure would be deemed to be sufficient? Thank you for also highlighting and showing the “common mistakes” or how one can (unintentionally) mess up the input procedure and consequently the resulting calculations values in RSAP. I have enjoyed this series and I am look forward to future videos Kind regards, DK

[@CivilEngineeringEssentials]

Hi there Engr. DK, sorry for the late reply. Have a ton of tasks to finish at my University (end of academic year stuff) so I am a little bit late with my replies. I am happy that you were able to follow along despite the fact that you are not an ASCE guy ^_^ it means a lot to me Indeed, I think I once had a video about it, but thought that this form of explaining theory in details and then applying (with a check) is much better. Risk Category II vs III is kind of vague. I have seen some comments about it, but it kind of fails to convince me. For me, it is either black and white or left to engineer's decision. The way Risk Category is presented seems to look like "black and white" but in reality, it is not. Yes, you could simply use a gamma angle. I think that is a nice idea for a quick video, but here is the explanation for it (you may need to read this multiple times): There exists 3 coordinate systems in a FEM software 1) the global coordinates 2) the local element coordinates 3) the section coordinates I never talked about section coordinates before, but now it is time. Section coordinate axis system comes into play whenever you have a gamma angle involved. For example, we know that the inertia of a rectangle is basically 1/12 bh^3. This works around its own two axis (Switching the variable that has ^3). Also, for the third axis, we have the polar moment of inertia J. Notice that, if the gamma angle is 0, those section coordinates are identical to the local coordinates of the element. If the section rotates, then the inertias have to be "cosined and sined" onto the local coordinates of the element (it is more than just cosining an sining). Those in turn get multiplied by the transformation matrix for a global stiffness matrix. Now if you thought: hmm... this means I need a transformation matrix for the gamma angle to get it to local, and a transformation for local to global, then guess what.. you are right! Yep, it ties into that point that: if you only have vertical loads on a structure, then it twists if the vertical load carrying elements are unsymmetric. of course, in Seismic action / wind action, the inertias will play a role. Yep, I did a full symmetric structure to facilitate the hand-calculations. Here is the trick we do to converge the CG with the CR: we first add our shear walls (or frames) in places we can easily add. Meaning those were our ARCH friends are not going to have a problem. Then, we start tweaking the inertias. This becomes a delicate process of tweaking vs cost. What I mean is, you could sometimes allow some eccentricity to exist. you would think: wait a minute! doesn't this mean more forces and moments and reinforcement? Yes. Does it mean that my structure is more expensive? It depends. Ok explain? Well, how do you make CR and CG close to each other? you play with the inertias. You would assume a minimalistic section then start increasing it for tweaking purposes. Now the realization might start to sink in. But let me elaborate if it did not: increasing the section to minimize torsion increases cost. So now the question is: Is it worth increasing the section to minimize the load? Or should I just take it as it is and increase the reinforcement? Depends on the $$. And I do not mean this as an "ecape hatch" or to "run-away from the reality" On the contrary. BTW, now that I have phrased this "optimization problem", I am inspired of doing a research paper about it. Sorry to drag you down such a deep rabbit hole. I thought that this discussion would be appropriate to try give you an answer for your question, as there are no rules of thumb for that. The code allows the 5% accidental, in my humble opinion, due to the impossibility of having your SDL and LL be uniform on the entire area. Now I think I explained it as the "CG" having an accentricity not the CR. You could look at it as being the CR having eccentricity, and you would be right and you would get the same answers, but in my humble opinion, given the rationale of the code, I am tempted to say that it is the CG that actually has an eccentricity (although when doing hand-calculations, we would move the CR... I did not do those hand-calculations in my videos). You are always most welcome. I am also sorry for the little error I had in those slides. 0.446 vs 0.466 The slides in the description are fixed. All the best, CEE

[@donaldkhanye9011]

@@CivilEngineeringEssentials Hello Doc, I do not mind the wait at all, not even a bit. It is more than enough for me that you still make the time to read and to respond to my questions and those of other CEE community members in the level of detail that you do, you are just simply amazing! Strange as it may sound, I actually find that the deeper the CEE rabbit hole goes, the more and more I get to learn from those holes, so do drag away 😊 I insist. Although I had been aware of the section coordinates system, I had not really thought of it in relation to the other two systems, at least not in a way that you have just explained. And because of your outstanding work on the FEM series, I am now able to meaningfully relate the other two coordinates’ systems; and further to your reply today, I can appreciate the intertwined relationship of all three. I appreciate the helpful pointers in the quest to converge the CG with the CR. I concur, the balance between $$ and the effort that one could afford to spend on optimization is indeed delicate and would be influenced by many things specific to the particular project and setting. The Editor CEE had done an excellent job to ensure that viewers are aware of the small error in the slide. The concept is what really matters and as usual, the CEE has nailed it (again)! Thank you and have a wonderful day Regards, DK

[@CivilEngineeringEssentials]

I'm very happy I was able to add new infos, even with the limitations of written text. Oh yeah, unfortunately, there was an error, so the full slides (including all corrected values) are available in the video description. It is rather an annoyance, but well, nobody is perfect, I guess. Regards, CEE

[@abooodis]

Finally the accedental excentricity now make sence, really appreciate your in depth explinations the Story Siesmic FX force affect CG or CF ? regarding load to mass Converstion, whats the diffrance between Global mass and Dynamic Mass for the Added mass issue, I do compare total DL reaction vs story mass (in stories/Total table) Your work is much appreciated

[@CivilEngineeringEssentials]

I am really sorry for being late in my reply. The seismic story affects the CG. As for the difference, here is a quote from Autodesk: "1- Whats the difference between global mass and dynamic mass ? when we can use global and when we can use dynamic. The prior applies also to forces of inertia (body forces) analysis." Hope it helped, CEE

[@aliyoussef1000]

Excellent job!! Looking forward to more training videos . Thanks

[@CivilEngineeringEssentials]

Happy those videos helped you out. There are of course more and more videos on the way. Stay tuned for those.

[@ralphabella6234]

Thank you so much CEE!!!

[@CivilEngineeringEssentials]

You are most welcome. Sorry for the late reply If possible, please share the video with your colleagues. It helps a lot ^_^

[@mutomboguauburu]

Hi CEE, thanks a lot for sharing your knowledge for all of us to learn. Your work is helping a lot of us we really appreciate. I have a question and it is about max seismic spectral acceleration S1 and Ss. The inputs for the program, are in % of g? I guess so because if not, forces seems to be quite high although working in a low seismic location. For me it is a little bit confusing because it is not clearly indicated if it is in m/s2 or %of g.

[@Mjajsq]

Hello dr. CEE I have a question please. When I design column using provided reinforcements after applying lateral force method. I get a warning that says: seismic depositions, no requirements! Knowing that this column is part of a building. What can I do to solve this. Or can I ignore this warning? Thank you so much!

[@kengineerings9366]

Thank you for the excellent tutorials! I have a question: I'm an expert in ETABS and recently started learning Autodesk Robot as a secondary analysis tool. I modeled a 10-story building in both softwares and found the outputs to be almost identical. However, I've noticed a difference in period calculation. ETABS considers the factor in Table 12.8-1 and multiplies it by the empirical period from equation 12.8-7. In contrast, Autodesk Robot directly uses the empirical period without factoring in Table 12.8-1. How can I make Autodesk Robot account for the values in Table 12.8-1 when calculating the period? Thank you!

[@CivilEngineeringEssentials]

Sorry for the late reply. I'm happy my videos are helping. I had some university work to do (we are approaching the end of the academic year, so tons of paper-work). Ok so to your question: (We are here referencing ASCE7-16): Table 12.8-1 gives you the coeff. Cu, which is used for the upper limit of the period T. This is used in clause 12.8.2 to provide an upper limit for the period T. Note that 12.8.2 talks about the real period determined by the engineers. Not the approximate. This 12.8.2 basically states that: T Cu*Ta which would immediately invoke article 12.8.2 limiting it to CuTa Hope my explanation made sense. Now in RSA, it has a different approach, in which it calculates the period from Ta directly. Unfortunately, you cannot change this setting in RSA> I hope I understood your question correctly. If I missed a point you raised, please feel free to ask again. I am always happy to help. Regards, CEE

[@kengineerings9366]

@@CivilEngineeringEssentials First of thanks for your time to answer my question, yes answered part of it , and you are right, in ETABS after it do the modal and get the analytical period (Tm), it compare it ot Ta CU, and if TaCu is smaller than Tm, it will use TaCu other wise it will use Tm.

[@CivilEngineeringEssentials]

This is actually a plus point for Etabs. As the base shear may get smaller. Still, the ASCE7 allows the usage of Ta so one can still defend what RSA is doing. I will be continuing my series on ETABS during the summer vacation. At the moment (I am an Assistant Professor at a University) our hands are full with final exams and paperwork Anyways, stay tuned for more content. If you have any questions, please feel free to ask them. Also, if possible, please share this channel with your colleagues. It helps a lot. Regards, CEE

[@mwangibenson5199]

Which cases do you consider for load combinations DR.?

[@CivilEngineeringEssentials]

I did not do any load combinations in this video. But if you mean the cases considered for the Seismic Mass, I considered only the DL

[@mwangibenson5199]

@@CivilEngineeringEssentials I meant if you are to do a load combination for instance ALS, are you going to consider all seismic cases generated?

[@CivilEngineeringEssentials]

oh, of course. Those seismic cases (all of them) should be considered. For example: let's say you have a combination called: 1.2D + 1.0L + 1.0 E this (single combination) would be actually more than 5 combinations. Comb1A = 1.2 D + 1.0 L + 1.0 E (x + without eccentricity) Comb1B = 1.2 D + 1.0 L + 1.0 E (y + without ecc.) Comb1C = 1.2 D + 1.0 L + 1.0 E (x with ecc x) and so on... sorry for not understanding your first question. Kind Regards, CEE

[@mwangibenson5199]

Great. Thanks Dr.