Robot Structural Analysis Forum
Welcome to Autodesk’s Robot Structural Analysis Forums. Share your knowledge, ask questions, and explore popular Robot Structural Analysis topics.
cancel
Showing results for 
Show  only  | Search instead for 
Did you mean: 
Reply
Message 1 of 27
hussamshihabi
7816 Views, 26 Replies

seismic loading

hello to all

i want to ask if there is document on how does the robot analyse seismic loads is there a clear vision on how it works i mean i have been using ETABS to resolve seismic cases...so i need the philosophy in which ROBOT works to resolve the cases ... any way another question .... is there a video tutorial on modeling and analysing and designing a building with all wind loads and seismic loads application with the design of all the structural elements (shear walls , foundations , frames , bracing systems .... etc. ) thanx for all the help

26 REPLIES 26
Message 2 of 27

Hi,

 

I am attaching 2 step by step examples how to define sesmic cases.

Then you have to create either manual or automatic combinations to be used in design.



Rafal Gaweda
Message 3 of 27

thanx for the help ... is robot better for seismic analysis than etabs ?

and if u have a link of a video tutorial for the whole modeling process i would appreciate very much 

thanx for ur time 

Message 4 of 27



and if u have a link of a video tutorial for the whole modeling process i would appreciate very much 

 



http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Tutorial-videos-for-basic-and-advanced-use-o...



Rafal Gaweda
Message 5 of 27
Message 6 of 27
vmorkunas
in reply to: Rafal.Gaweda

Dear support team  

I found this usefull worked example posted by you, but it is done in Robot Millenium, is there anything mroe recent? I see that Robot 2014 adn 2015 has more advanced seismic analysis options? Thanks.

Message 7 of 27
Ilona.Michalska
in reply to: vmorkunas

Hi vmorkunas,

You can find the example how to run and set up a seismic analysis in this tutorial:
http://help.autodesk.com/view/RSAPRO/2015/ENU/?guid=GUID-88B74D07-4C31-46DF-B415-76658FFF549B
I hope this helps.

Regards,
Ilona

Message 8 of 27
vmorkunas
in reply to: Ilona.Michalska

Thanks a lot, Michali, this is more or less what I was looking for!
Message 9 of 27
Rafal.Gaweda
in reply to: vmorkunas
Message 10 of 27
hherrera
in reply to: Rafal.Gaweda

I noticed that when working with stories, walls between stories will default 100% to the upper story.  You can see that graphically when turning on the"Stories - legend by colors."  When reviewing the seismic weight calculation that robot does, it will place the entire seismic wall weight on the story that the wall belongs to.  We would like to see the seismic weight split by tributary height of the wall, between the upper and lower story.  Is there a way to have robot split the seismic self weight of the walls between stories?  

Message 11 of 27

Hi, you can easily assign linear load over the beam or the slab that create splitted weight of the walls, for example half on the first floor and half on the second one. Easy

Hope I've help you

Best regards

PasProStudio

www.pasquiniprogetti.eu

Structural + Detailing engineers
Message 12 of 27
Tuctas
in reply to: hherrera

That's a good question... Try to seperate the wall that is placed between two levels (stories) in to two parts, so the one "half" to be included in to the upper storey and the other one to try to assign it manually to the lower storey. I am not sure if you will be able to do what you want, see also the post below because there are some limitations on the use and the definition of stories:

http://forums.autodesk.com/t5/Robot-Structural-Analysis/Assigning-Stories/td-p/3748125

Message 13 of 27

If you choose a modal analysis with consistent matrix, and walls correctly meshed, I don't see why the mass would be assigned only to the upper level, it will "stay" at location of the FE nodes. 

With lumped matrix, it true that you will "loose" the weight at the base level (modal mass assigned to the support will not move and with lumped matrix it can be half of the OW of vertical element.

 

But correct me if I'm wrong, to me stories and modal/seismic analysis doen'st influence each other, stories is just a way to classify your structure but doesn't interfere with calcs. Lika when defininfga concrete bar is a colum or a a beam in the inspector.

 

To be followed, i'm curious ..

Message 14 of 27
Tuctas
in reply to: t.sautierr

t.sautier, have a look at the link that i attached in my previous post to see what's going on with the stories+modal analysis...

 

About the consistent mass matrix and walls i agree with you. However, sometimes it is prefered to model walls with linear members (plus using proper rigid offsets) instead of F.E, so the seperation that StefanoPasquini is talking about should be done (this case of using linear members instead of F.E helps to illustrate better the results of modal analysis because this way there are less d.o.f's so the eigenvalues don't get "confused", as it happens sometimes with F.E. This way of model whould be also convenient for ductile walls BUT Robot hasn't develop them yet...).  

Message 15 of 27
t.sautierr
in reply to: Tuctas

Interresting post tuctas, as far as i've understood, stories can influences modal analysis in case of defining excentricities in the modal analysis. Precisely, it seems to happen from the addition of nodal masses if I follow correctly Arthur.

 

Did you do the test by defining the excentricitiy by offsetting the matrix mass? as the matrices are the same whatever are the stories, maybe when having stories definition problem, this method which ins not using stories will work.

 

Thanks for reporting this "detail", I never noticed that before.

 

Actually I barely use stories, I calculate displacement, drifts etc ... using excel and nodal results. 

Message 16 of 27
Tuctas
in reply to: t.sautierr

In the model of that older post, i didn't define any eccentricity in the mass matrix from my oun, but because of the different definition of stories the C.M of them were shifted by the programe. This shifting caused the differentiation on the modal results, so the definition of stories aren't so "harmless"... 

 

Message 17 of 27
t.sautierr
in reply to: Tuctas

So I did create changes even when using matrix offsetting ... indeed it is not so harmless. I guess the majority of the building it is ok but for buildings with semi-levels .. one as to be carefull ... ok. fine.

Message 18 of 27

Instead of trying to tweak the model in order to "maybe" behave the way that we would want it to, I would probably just take StegnoPasquini's recommendation of manually placing seismic linear dead loads based on tributary heights of the walls.  Wanted to avoid that, because every time with a new model we would need to make sure to remove wall selfweight from the structure selfweight, but it looks like it will be the only way to get accurate results.

Message 19 of 27
Pawel.Pulak
in reply to: hherrera

Some clarification (or complication? :)) to remarks of hherrera, Tuctas and t.sautier

 

When discussing the distribution of mass (or weight) between stories it is important to remember what type of results or data is discussed.

 

As concerns the seismic mass considered in modal and seismic response spectra analysis it is not quite true that 100% mass of the meshed wall goes to the upper story.

 

I will try to illustrate it on the extract from some structure shown in the screen capture below:

adjacent_stories.png

 

It shows 2 walls from adjacent stories 2 and 3 and 3 finite elements from the slab between them. Node numbers and finite element numbers are displayed.

Walls are meshed with 2 rows of finite element along the height.

When composing mass matrix the mass is distributed to nodes and in such case something like tributary distribution is used. For instance the mass from finite element 214 is distributed to nodes 261 262 287 288. It means that it is distributed to both story 2 and story 3.

It is done independently from the mass matrix type: consistent, lumped with or without rotation.

As you see in case of such density of mesh the distribution of mass from the wall is not uniform - top story "takes more".

But for instance in case of only one finite element along the height of the wall it would be distributed between walls fifty/fifty

 

The above way of distribution is independent from the existence of stories and that is why the results of modal analysis are the same with or without existing stories. But it is true ONLY if mass eccentricities are NOT defined in modal analysis.

 

If mass eccentricities are defined in modal analysis and stories exist then eccentricities are calculated separately for nodes of each story (opposite to global calculation of them when stories do not exist) and in case of different geometry of various stories it may result in different results of modal analysis when stories exist and do not exist.

 

The above description shows also another effect - the mass considered in for instance node 262 is "taken " from finite elements 201 203 523 526 (belonging to Story 2) and finite elements 214  216 (belonging to Story 3). All these mass components are added without remembering what part of it originated from which story. So when activating mass eccentricity this mass will be modified considering parameters of Story 2 and ignoring the fact that part of it originated from Story 3.

 

Analogous approach is used when calculating FX, FY and FZ forces in stories on Reduced forces tab of the table of stories. These forces are calculated considering forces distributed to nodes of appropriate stories.

 

It is different as concerns masses shown in the table of stories - they are calculated directly (without the distribution to nodes) from masses of objects (walls, slabs, bars) assigned to given story and/or from loads applied to these objects and converted to masses.

 

---------------------------------------------
If this post answers your question please click the "Accept as Solution" button. It will help everyone to find answer more quickly!

 

Regards,


Pawel Pulak
Technical Account Specialist
Message 20 of 27
Tuctas
in reply to: Pawel.Pulak

Thank you for the clarification Pawel.
So if we enforce some elements and nodes to belong to a story out of the "normal" procedure of the program, we should wait the results to be "as expected" (I.e the shifting of mass due to eccentricity actually follows the assignment of stories, manually assigned or not) shouldn't we? I am actually referring to that example of the older post of mine...(attached above).

Can't find what you're looking for? Ask the community or share your knowledge.

Post to forums  

Autodesk Design & Make Report