ETABS - Sequential Construccion Case - ERROR - Printable Version

22/11/2013 ETABS - Sequential Construccion Case - ERROR - Printable Version

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ETABS - Sequential Construccion Case - ERROR - Printable Version

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ETABS - Sequential Construccion Case - ERROR - pchnext21 - 12-19-2009 09:56 AM

Hi... Im an civil engineer from Peru, (about 5 years)I work with ETABS an theres some time when the"Sequential Construccion Case" doesnt work (I mean when the iterattion doesnt move beyondsome stage, mostly in the 1st or 2nd stage)but most of the time is 'cause :

1.- An element is not properly putan it falls when you look the deformed shape from DEAD.2.- Missing restraints

But this time I really try to fix this but failed...It happend to all of us I guess

I upload the Etabs Model in this linkhttp://www.megaupload.com/?d=RT0IS040

If someone could take a look to the modelI really apreciate it.

Thanks.

Ing. Pedro Cruz(Lima - Peru)

RE: ETABS - Sequential Construccion Case - ERROR - vinnunsam - 12-19-2009 10:58 AM

Dear friend. I appreciate ur interest in sharing your prob. But i first suggest you to read the rules of posting before posting threads. You r bound to use 'code' before posting any links. Justfor ur info.

RE: ETABS - Sequential Construccion Case - ERROR - faros3000 - 12-19-2009 11:31 AM

I briefly reviewed your model and can point out some comments:

1. There are a lot of dummy beams as well as auxiliary line beams with very small cross-sectional areas. Have a look at your vertical deformed shape, and you will find that this small-section beams deform beyond reasonable limits as compared with the deformation of the rest of the beams, slabs and walls.

I personally don´t think they really represent an actual state of the structural system/sub-systems; as Etabs is an object oriented software, you should better use larger slab-objects butincreasing the number of restraining points on both slabs and walls, as well increasing the interal meshing. I have dealt with this problem for years, and found that that is the mostaccurate way of analysing a building like yours. Moreover, if you are going to analyse your structure for seismic exitation, you will miss a lot of compatibility of deformation betwenelements within your model as it is now.

2. Your DL seems to be pretty small. But still in case it is correct, you should have 2 cases for the dead load: a DL that contains only the self-weight of the structure, and an aditional DLthat includes the dead load that will be applied to the building after the self-weight exist.

Be careful with this. As most (but not always) of the time a Sequential Construction (SQC) analysys relaxes the forces/moments in the structure, considering all the dead load (self-weight+ all additional DL) for this condition will underestimate the subsequent deformation for a given story after the story below has been loaded, and consequently will overestimate thereduction of forces/moments in the structural elements on and above that given story.

3. Regarding your SQC analysis, I run your model with both the automatic and manual definition of active groups and in both cases it run fine the nonlinear SQ analysis. The only warningthat I get is as follows:

STAGE NUMBER = 10ENTIRE STRUCTURE IS ACTIVE = YESCHANGES TO ELEMENTS IN STRUCTURE = NONE

STEP 0

* * * W A R N I N G * * *THE APPLIED LOAD IS ZERO IN NONLIN-STATIC CASEDEAD-SQ ,THE ANALYSIS WILL PROCEED BUT THE STRUCTURE WILL NOT BE LOADED

but take into account that this is only for STAGE number 10. Remember that your building is only 9-stories height, so the stage number 10 is the one where all your active groups are fullyloaded but at this point all of the 9 stories have already been loaded individually. So, when the program tries to load the full structure, it finds no new load to apply and then it gives youthe message that the applied load is zero. But this is not an error on the SQ case.

Hope this helps.

http://forum.civilea.com/index.php

http://forum.civilea.com/thread-7450.html

http://forum.civilea.com/user-1955.html





RE: ETABS - Sequential Construccion Case - ERROR - pchnext21 - 12-19-2009 02:09 PM

THANKS man, you really help me a lot.(we never stop learning, I guess...)

About your comments, The reason I didnt use a shell meshing on the floors (internal or external) in the model, is that it takes to long run, so I used menbrane in the slab, but as you know you it only can bedone with 4 edges per menbrane so some elements definitly deform more. Also almost every beam has a realese in M3, because basicly the beams in reality and in a mesh analiys is part ofthe floor, so the forces here are really low and most of the reinforcing is in the slab.

A.-Is there a way to make an analysis with menbranes (for the slabs) better? more accurate I mean.

B.-If the deformation compatibility only happend when using shells(for slabs) and considering that model with menbrane only transfer the loads (Lets say like a sheet, doest not aport rigiyty at all); For the walls in general we have almost the samereinforcemente in both cases, do you agree?

About using 2 Dead loads, I see you point and I absolutly agree...

C.- You're, for some some unknown reason now the model runsevery stage. One question though : In the 1st stage :

Limit 200 50 200 10 1.00000 1.00000 1.00000Cur 2 4 10 Conv 3 0.18750 1.00000 1.00000

But in the rest of the stages :

Limit 200 50 200 10 1.00000 1.00000 1.00000




Limit 200 50 200 10 1.00000 1.00000 1.00000Cur 0 0 1 Conv 1 0.18750 1.00000 1.00000

As you see the first 4 values of the second row are differentdo you know why? (I think its maybe its just the equations converging quickly??)

Then again THANKS for all.

(12-19-2009 11:31 AM)faros3000 Wrote: I briefly reviewed your model and can point out some comments:

1. There are a lot of dummy beams as well as auxiliary line beams with very small cross-sectional areas. Have a look at your vertical deformed shape, and you will find that thissmall-section beams deform beyond reasonable limits as compared with the deformation of the rest of the beams, slabs and walls.

I personally don´t think they really represent an actual state of the structural system/sub-systems; as Etabs is an object oriented software, you should better use larger slab-objects but increasing the number of restraining points on both slabs and walls, as well increasing the interal meshing. I have dealt with this problem for years, and found thatthat is the most accurate way of analysing a building like yours. Moreover, if you are going to analyse your structure for seismic exitation, you will miss a lot of compatibility ofdeformation betwen elements within your model as it is now.

2. Your DL seems to be pretty small. But still in case it is correct, you should have 2 cases for the dead load: a DL that contains only the self-weight of the structure, and anaditional DL that includes the dead load that will be applied to the building after the self-weight exist.

Be careful with this. As most (but not always) of the time a Sequential Construction (SQC) analysys relaxes the forces/moments in the structure, considering all the dead load(self-weight + all additional DL) for this condition will underestimate the subsequent deformation for a given story after the story below has been loaded, and consequently willoverestimate the reduction of forces/moments in the structural elements on and above that given story.

3. Regarding your SQC analysis, I run your model with both the automatic and manual definition of active groups and in both cases it run fine the nonlinear SQ analysis. Theonly warning that I get is as follows:

STAGE NUMBER = 10ENTIRE STRUCTURE IS ACTIVE = YESCHANGES TO ELEMENTS IN STRUCTURE = NONE

STEP 0

* * * W A R N I N G * * *THE APPLIED LOAD IS ZERO IN NONLIN-STATIC CASEDEAD-SQ ,THE ANALYSIS WILL PROCEED BUT THE STRUCTURE WILL NOT BE LOADED

but take into account that this is only for STAGE number 10. Remember that your building is only 9-stories height, so the stage number 10 is the one where all your activegroups are fully loaded but at this point all of the 9 stories have already been loaded individually. So, when the program tries to load the full structure, it finds no new load toapply and then it gives you the message that the applied load is zero. But this is not an error on the SQ case.

Hope this helps.

RE: ETABS - Sequential Construccion Case - ERROR - faros3000 - 12-19-2009 11:32 PM

http://forum.civilea.com/post-18243.html#pid18243




You are right in modeling your floor slab using membrane elements (that´s one of the biggest differences of Etabs with relation to other software, or at least used to be). Let´s just fix forthe following discussion the following hipothesis: all floors are modeled as membrane elements, and all walls (vertical area elements) are modelled as shells.

Then, you still have to work on both the CONSTRAINTS and INTERNAL MESHING of your system. Have a look at he following images.

This is a building located in a highly prone-earthquake zone. The vertical and lateral resisting system consist of both masonry and RC walls.

Then, the lateral deformation of the building using a modelling technique similar to yours. Take a look at the gaps that form between adjacent vertical panels, as well as with floors andbeams, showing the poor deformation compatibility obtained. Obvoiusly, this will have a great effect on the distribution of forces in the structural elements. [Modelling note: floor aremembrane-type elements, walls are shell-type elements]



And finally, a fine refined internal meshing and propper constraint assignment. Take a look at the compatibility of deformations. [Modelling note: floor are membrane-type elements, wallsare shell-type elements]

Also, in the past membrane elements only could have 4-edges and be defined by 4-nodes. Not anymore. But be careful if you want to use more edges/nodes, you may also end up loosingload paths. Take time to review the presentations of CSI software outlining the scope of this software:

http://www.comp-engineering.com/technical_papers.htm

I think you should take a deeper look at understanding the basics of constraints and internal meshing, for both membrane and shell elements in Etabs.

Here you´ll find a good way to start in the fist paper listed: "Mesh Transitioning..."

http://www.csiberkeley.com/support_technical_papers.html

Enjoy!!

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ETABS - Sequential Construccion Case - ERROR - Printable Version