Understanding P-M Interaction Curves in ETABS Section Designer | Part-3 | ilustraca | Sandip Deb

Added: 2026-05-22

[00:00:00]Hello everyone, welcome to Illustrissa.

[00:00:03]So in your previous lecture, we have >> [snorts] >> seen how to uh you can say draw this PM interaction curve for your provided rectangular column.

[00:00:14]Okay?

[00:00:15]So now, what I have done, I have created the same column in ETABS section designer. So if I go to file section properties, frame sections you can see I have created this column of 450 by 450 with concrete grade M40.

[00:00:31]Okay? In section designer ETABS. Okay?

[00:00:34]And I have provided this reinforcement.

[00:00:36]Of 25 dia. If you just right click on the reinforcement, see, it is of 25 dia and HYSD 500 grade. Okay? So if I now see the interaction surface for this particular column, I'll click on here interaction surface.

[00:00:52]Okay?

[00:00:53]And you will see it is giving me these points.

[00:00:57]Right? It is giving me total 11 points.

[00:01:00]And it also gives the capacity for the tension. Okay? So what I will do let us let us see whether the result that we have got, it is uh I mean coming it is whether it is close to what the software is calculating.

[00:01:16]I'll click on any axis because this is a symmetrical section. So whether PM3 or PM I mean PM3 means P with M3 axis, moment about M3 axis major axis and or uh com P with moment about minor axis. Okay? So let us select any PM3 suppose.

[00:01:36]And if you see the first case that we have, there is no moment.

[00:01:40]And the actual capacity that we're getting here that means this point.

[00:01:46]Take my pen.

[00:02:01]>> here.

[00:02:03]So, if you see in this point this point, so it is giving given by the software it is 3 8 6 6. 3 8 3 8 kN.

[00:02:20]How much we got in our calculations?

[00:02:23]Remember remember if I open our calculations, it is around 3 8 6 6.5. So, very near to what we got in the software also.

[00:02:33]3 8 6 6.38 and we got 3 8 6 6.5. So, it is almost similar.

[00:02:37]Right?

[00:02:38]Okay. And then then if you see there are the value that we got for the balance point which is 1 1 2.2 or 3 4 2 uh Let us let us try to find this 3 4 2 2 what we are getting here. So, in between 3 6 and 3 1 there is no further I mean intermediate point given. And in between 11 12 32 8 93 there no further point given. And in between 4 38 to minus 284 minus in that is it is going below below the axis that means tension. So, we actually require the intermediate value may be zero or some value which is to compare our results. So, here why we are getting 11 points because in that if I just click on okay and okay cancel it.

[00:03:32]If you see in the design uh concrete frame design and if I go to view revise preferences, here we have these two options. One is number of interaction curves. So, uh I have explained in the previous uh lecture that uh you have the column like this and if you we we are generally finding the uh you can say capacity curve for the major two axis, right? Major and minor axis. But, you can orient the axis like this, like this, like this, like this, and you can find out the capacity.

[00:04:07]I mean, the capacity curve. So, here uh you can define how many capacity curves are uh you can say how many axes it should consider, how many 2D interaction curve it should calculate and then it it it should combine to uh get the final capacity of the column.

[00:04:25]So, here by default it's 24.

[00:04:28]And then, number of interaction points.

[00:04:30]So, if you have uh suppose like this, um we we found out how many how many points? 1 2 and 4 in our manual calculation.

[00:04:40]So, now here you can define how many number of interaction points it should calculate. Okay? So, the more the interaction point is, the more precise or more you can say smooth this curve will be. If it is if it is four, it will be straight line. It will be ignoring this this portions, right?

[00:05:01]Okay? So, that's why you can increase the number of uh you can say points here. And suppose, let us for now we are putting it a value of 301.

[00:05:13]301. You need to give a value as a odd number. So, 301 I kept. I'll click on okay.

[00:05:19]I'll go to define section properties again, frame sections.

[00:05:23]And this is I'll modify section designer.

[00:05:27]I'll go to direction curve again.

[00:05:29]And you can see now now we have total 301 points calculated. And now it is more precise.

[00:05:37]Okay? More precise. You see, this is okay. 3866.84.

[00:05:42]We will try to find a uh P value which is close to our balanced point that we have you can say estimated which is 112.2.

[00:05:52]Okay?

[00:05:53]And if we here around 112.2.

[00:06:02]So, there is no 112 is not there. It is 111.

[00:06:08]1115.

[00:06:10]Okay?

[00:06:12]How much we got? 1112, right?

[00:06:14]1112.2 and the corresponding moment was around 307. Okay, let us see. For 1115.98, the corresponding moment we are getting it is which is around around 309. So, it is okay if you see. It is very close value. It is 1115.

[00:06:33]Corresponding value we are getting 307.

[00:06:36]Pretty pretty I mean fine that the result that we are getting. Close to our manual calculation, right? And now, let us see if we have P equal to zero value.

[00:06:47]Here, positive P means compression. So, we don't have a P equal to zero, but let us see any value which is close to zero.

[00:06:56]Suppose 6.4, and we are getting a moment of 155. Okay?

[00:07:01]And for zero, what we got? For P equal to zero, the moment we got 154. So, pretty fine that the result that we are getting.

[00:07:10]The result that we are getting in our manual calculation, see, it is very close to what we are getting from our uh software result.

[00:07:18]So, you have to decide how many interaction points want in a particular curve and how many interaction curves you want uh for for one particular set.

[00:07:29]The default values are 24 and 11, but obviously you can change it just to you can say check your manual calculation.

[00:07:36]So, this way this is how you will be I mean uh Let me see. You can assess the interaction curve in any software. So, for this zero point for this zero point, we got a value of 154.

[00:07:52]This is uh 3866.

[00:07:56]And somewhere here we have the balance point.

[00:07:59]Somewhere here we have the balance point for which this is around 111 2.

[00:08:04]Okay?

[00:08:06]Maybe maybe below this, somewhere here.

[00:08:08]1. Somewhere here.

[00:08:13]This 1112.

[00:08:15]And this I think 155 or 154, something like that. Right?

[00:08:20]For the balance condition, how much you got?

[00:08:25]Okay, for balance condition, it is 307.

[00:08:28]So, if I just put my column uh uh you can say cursor here, it is 323 and 1305. If I see somewhere I have to go to some close value.

[00:08:42]11 Okay, 1113, it is around 323. So, somewhere here uh we'll be having the balance point.

[00:08:52]Okay? So, you can see that in software also, whatever we are getting, it is it is very close to what we are getting in our loop.

[00:09:03]This is what we have estimated in our manual calculation also. This is my balance point.

[00:09:11]balanced point This is my pure bending.

[00:09:18]Sorry, pure compression.

[00:09:22]Right?

[00:09:24]bending And then, this point is pure tension.

[00:09:31]Now, how to find for pure tension? Pure tension is nothing but 0.87. We know the steel will be totally yield. I mean yielded. So, maximum stress.87 FY into AST. How much is the AST? 982. So, we can see 435 multiplied by 982. How much we should get?

[00:09:53]In our case, in our condition, let us just calculate.

[00:09:58]Put 435 into 982. 982.

[00:10:05]Getting a value of uh We're getting a value of 427.

[00:10:16]No, in our case, it is it will be not 982 because the whole section is under tension, so it will be 19 uh how much?

[00:10:24]Let me like this.

[00:10:28]435 into 4 into 491.

[00:10:31]It will be 854.3.

[00:10:40]4 into 491 be 8 84 kN. It is in tension.

[00:10:49]Okay, so this point, pure So, it is for pure tension, which I have not drawn in my capacity curve. So, if I if I just show you here in this also, maximum, so it is around 4 853.

[00:11:02]Uh 73. So, I'm getting 850 is very close, right?

[00:11:07]So, like this, you can understand the capacity PMM can say interaction curve by the provided by the software also. You can see this is pretty fine. But, again, I'm saying, I mean how precise you need the value, that you need to decide and you need to put those inputs in the design preference.

[00:11:28]Okay? So, this is how uh you can say draw the interaction curve or the software draws the Okay, so I hope this whole lecture series on the PM interaction curve is helpful will be helpful to you. So if you have any doubt please do write in the comment. I will try to reply as the at the earliest possible as as per my capacity.

[00:11:52]Thank you and we'll see you in the next lecture.