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fazal_mahmud_siddique
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PostPosted: Wed Oct 24, 2012 5:28 pm    Post subject: etab model checking Reply with quote

Respected SEFIAN,
               I am a fresher of ETABs. I am grateful to those experience sefian who has taken part their posting in numerous point. Lots have
requested to get a example .EDB file for learning and gaining confidence.smilarly, I am suffering from  some of same doubt. So, I have uploaded one .EDB and .$et file (both for static and response spectrum)for a G+5 RCC building having a lift and stair case  and sloping roof. The building is taken identical for simplicity . It has a slopping roof at top floor in the rear end panel . I modeled it and analyzed both  for static analysis and  dynamic analysis (response spectrum).  Both analysis file are same except static seismic load and response spectrum  case.  Esteemed  sefian are requested to go through it and clear the point of my doubt.


MODELING ASPECT:
(i)stair case ( Waist slab is provided 150 mm thick , assigned as “WAIST” of thickness  ½ x 150=75 mm as shell, remaining 75 mm is  
    provided as dead load UDL)along with floor finish of  1.00KN/m2 i.e 0.075x25+1=2.875 kn/m2 because I want to consider the  
    stiffness of waist slab for half thickness.
(ii) Lift as wall of 200mm thick. Assigned as shell.
(iii) Slopping roof portion.          (same style as stair case slab)  assigned as shell
(iv) shear wall   (Assignment of “Pier” and “spandrel”)
(v) all other Hz floor slab assigned as “ membrane” of thickness 125 mm.

ANALYSIS ASPECT:
                 Meshing option:
(i) stair case ( Meshing procedure)
(ii) Lift  shear wall    (Meshing procedure)          ( If my process is incorrect please brief so that  can follow)
(i)     Slopping roof portion.  (Meshing procedure)
Doubt:
1.     Meshing Area: I am meshing the area element as below
Selected the Shell/AREA /Wall element to be meshed.
Then gone to Assign >Shell/Area > Area object mesh option >
Floor Meshing option:
Checked Auto mesh object into structural element (under this head)
Checked Mesh at Beam and other meshing line
Checked Mesh at wall and ramp edge(if wall and ramp is there in the model,if not unchecked)
Checked Mesh at visible grid
Checked Further Sub divided oot mesh with max size of say 0.5 m (for finer meshing)


Checked Ramp and Wall Meshing (if Ramp ,stair waist,Wall is in the Model)for sloping roof
Checkd Sub divide into say 5 vertical and 8 horizontal (the ramp/waist was not meshed until it was done)


2.     In one pdf tutorial by CSi (old one) it was suggested that
Select area object to be meshed then go to Edit then check the option
Mesh Quads/Triangles into say 5 by say 8 Area
Or by using check intersect at visible grid
check point object in edge
Check Intersect with selected line object
****Which one we should follow (1)step or (2) step
In the second procedure shell (slab panel) was defragmented and become difficult to modify or select the slab panel ,more over it takes lot of time to extrude, zoom etc.



          Live load reduction:
          live load reduction is kept off as I am not confident about it, if it is to be performed how it can be done , is the following procedure is  
                              correct. Can we do this for live load reduction
Going  to Option Reduce live load/user defined and putting the table as below -
     
No.FL                            Reduction in %
T                                          10
5                                           20
4                                           30
3                                           40
2                                           50
1                                           50
Base                                     50

          
Pattern Live Load:
          Pattern live load is not intended as it not done as per IS 456-2000. So the pattern live load factor is kept  “0”

          Load combination:
          Given manually specially for Seismic loading i.e. (1.2 DL + 1.2 Reduced Live Load +- 1.2 Seismic X or Y)
                             i.e. (1.2 DL + 1.2 *0.00 , 0.25 0r 0.50 +- 1.2 Seismic X or Y)= i.e. (1.2 DL + 0.0, 0.30 or 0.60 +- 1.2 Seismic X or Y)depending upon the
                            categories of LL whether it is  0.75 KN/m2 on super roof  or 1.50 KN/m2 for roof slab (0.0 *LL1) , 2.00KN or 3.00/m2 (0.25*LL2),and for         
                            above 3.00Kn/m2 (0.50*LL3)
                              I am not confirmed whether auto load combination does it or not.(according to categories of Live Load)

          Seismic (STATIC):
          Programme time period is used. Can user defined as per code” (in filled brick wall) 0.09H/√B can be used though all beams are not loaded  
                             by brick wall.  which one is preferable sir, I went through SEFI  general discussion, different viewed differently, but what will be the real  
                             solution.
                             
          Seismic (DYNAMIC):
          Response spectrum analysis gives much less result without enhancing it by proper scale factor:
Scaling of dynamic base shear
As per clause number 7.8.2 of IS 1893(Part 1) :2002 If we generate earthquake loads by response spectrum method, the design base shear (Vb’) shall be compared with a base shear (VB) calculated by using a fundamental period Ta, where Ta is as per clause 7.6 where Vb’ is less than VB, all the response quantities (Member forces, displacements, story forces, story shears and base reactions) shall be multiplied by VB / Vb’

Can we do like this:
First we run analysis by static seismic load by giving user defined time period (Fundamental period  
        Txs= 0.09H/√X for” EQX”            ( for in filled brick wall)
And Tys= 0.09H/√Y for “EQY”
and note down the storey shear for the EQX and EQY load cases.
Also note down the Time period for “Modal analysis”  for 1st Mode and 2nd Mode Ie. “Txm” and “Tym”

Can we take average time period Tx= ½(Txs +Txm ) and  Ty= ½(Tys +Tym ) which will be used for “static analysis for base shear.  (   rather than Txs and Tys as it scales more than 2 times)

Then we run analysis for response spectrum and noted the storey shear for it. If it is less than the static case then we enhabnce the scale (0.981) by    VBx ( response spec) / Vbx (static case) *0.981 and VBy ( response spec) / Vby (static case) *0.981 respectively. And final analysis is done. I have got U1 - 1.561x.981=1.531 and U2-1.535x.981=1.506 in this model

Please comment on the fundamental time period sir: (i) Tx= 0.09H/√X or 0.075 *H0.75  or First mode time period for modal analysis.and direction for U1 and U2 ( generally it is observed that the Higher Time period is found in first mode along short direction of the building)

          Static Load Cases : (static analysis)


Load                          Type                Self wt multiplier          auto lateral load
DEAD         
SELF)                                                     --------    DEAD                                    ------     1
Area                                                  ( SUPER IMPOSED DEAD)                   ……….    0
Wall                                                  ( SUPER IMPOSED DEAD)                   ……….    0                    
LIVE1    ( > 0.75,1.50 KN/m2     ……….…  REDUCIBLE LIVE                         -----     0         (for floor slab)
LIVE2    (= 0.75,1.50 KN/m2)   …………    REDUCIBLE LIVE                            ------  0        ( For roof and super roof)
LIVE3    (>3.00 KN/m2)   .……                  REDUCIBLE LIVE                             ------  0       (For stair, Machine slab of lift)    
EQX      (in case of static analysis)  ----         QUAKE                                          …… 0 --(1893-2002) applied for  (X +y  eccent)
EQY      (in case of static analysis)  ----         QUAKE                                         .…… 0 --(1893-2002) applied for  (Y + X eccent)

             Note: TIME PERIOD : Programme Calculated.



Response spectrum
     Functions for IS 1893-2002
Response spectrum Cases as
U1= (spec1 ) = 0.981 (I*g/2R),   as scale factor) and U2 (spec2)= 0.981 (scale factor) CQC –Modal combination and SRSS –Directional combination. Damping- 0.05

Mass Source
(i)     From Load option selected

DEAD                                        Multiplier          Remarks
DEAD                      1     
Wall(SIDL)                1          
AREA (SIDL)                                     1
LIVE1                                      0.25  %       for floor  Live load of 2.0/m2          
LIVE2                                     0.00  %       for super roof live load 0.75 KN/m2 and roof floor 1.50KN/m2
LIVE3                                      0.50  %        for (Live load >3.00 Kn/m2)

And Checked both option (i) Include lateral mass oly
                                              (ii)Lump lateral mass at storey levels



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Jeet_mbm
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PostPosted: Fri Oct 26, 2012 10:10 am    Post subject: Re: etab model checking Reply with quote

Dear Er Fazal,

     I appreciate you for your effort to model such building efficiently, being a fresher of ETABs. I would like to discuss some of my observation as below


1.      Please explain you view is detail why you model waist slab as 75 mm shell instead of 150 mm as you already model that in actual inclined plane.

2.     As per ETAB manual we should name a same Pier label to whole vertical wall along the height. Instead of using different labels for each wall at storey level.

3.      Regarding two methods of meshing, I would say both these method are conceptually different but ultimately result is same. In other word by your first method (through Assign menu) you are meshing a main object keeping your main object as intact. While in other method (Through Edit Menu) you are dividing the main object into smaller size objects.

4.     In live load reduction—ETAB do that correctly. Make a correction in your user defined table (order of floors )as following
No.Floors                            Reduction in %
1                                        10
2                                           20
3                                           30
4                                           40
5                                           50
6                                           50
7                                           50

5.     Which Pattern live load you are talking about ?

6.     In general cases ETAB do load combinations rightly as per IS code method. But it would be advisable to make your own load combination. That will give you better understand at post analysis stage.

7.     For Time period of structure it would be advisable to follow user defined time period depending upon type of structure (With infill or without infill). To consider particular structure, having very less infill walls, as infill structure or bare frame is again design engineer’s decision.  From my side it would be conservative to use “With infill” time period in such case.

8.      In general if your structure is regular and uniform then you will find approximately same time period as by codel “bare frame” formula and first mode time period.  In case of building with infill you will find that first mode time period is too high as compared to codel “Infill brick” formula. The reason behind in your model you have not modeled infill walls and soil medium (flexibility) etc.

Hope you have found some suitable explanation for your doubts.

With Regards

Jitendra Sharma
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fazal_mahmud_siddique
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PostPosted: Mon Oct 29, 2012 5:26 pm    Post subject: ETAB Model checking Reply with quote

Respected Sir,
I am really happy and grateful for your kind valuable view and advice regarding my doubt in ETABS and pay my respect for checking the etab sample model. It definitely encouraged and help  me a lot towards learning of “etabs” Sir.

At the same time , I would like to thank and my heartfelt gratitude to my respected Sir and reputed Sefian Mr. Rahul Leslie who gave me the real boost up  and valuable clarification and advice and trips regarding  my quarries sparing his valuable time. I will never forget his useful help to me.

Sir, I am forwarding herewith my views and request to comment and elaboration as below.

1.     Please explain you view is detail why you model waist slab as 75 mm shell instead of 150 mm as you already model that in actual inclined plane.

Sir, I observed when the floor horizontal slab are modeled assigning it as “ Membrane “ of reqd. thickness, It was found  that full load transferred to the beams which gives higher result for BM, Shear force. Accordingly, high reinforcement. That means slab is not pertaining any rigidity to the frame.  When I assigned slab as “shell” it shows much lesser BM,SF in the related beam . Accordingly, lesser reinforcement. Perhaps this was because slabs pertaining rigidity.  To be in the conservative side I used “membrane” in case of Horizontal floor slab.
As I know sloping roof or slab cannot be assigned as “membrane” but as “Shell”. If I assign full thickness of sloping slab/waist slab  it will give much lesser result. So I thought that at least the rigidity of half of actual thickness provided may be considered to get the desirable Moment ,SF and Reinforcement . Hence, I did it experimentally. But I am not all confirmed whether my procedure is correct or incorrect . I am not also confirmed whether sloping roof/waist slab is transferring its load to the beam properly or not while meshing it in my way.

Please give your view and correct me Sir.

2.     As per ETAB manual we should name a same Pier label to whole vertical wall along the height. Instead of using different labels for each wall at storey level.

Sir, It was my ignorance, I am grateful to you for your kind advice and making correction in this regard.

3.     Regarding two methods of meshing, I would say both these method are conceptually different but ultimately result is same. In other word by your first method (through Assign menu) you are meshing a main object keeping your main object as intact. While in other method (Through Edit Menu) you are dividing the main object into smaller size objects.

Thank you very much Sir for your kind clarification regarding the two ways of meshing procedure.

4.     In live load reduction—ETAB do that correctly. Make a correction in your user defined table (order of floors )as following
No.Floors                            Reduction in %
1                                           10
2                                           20
3                                           30
4                                           40
5                                           50
6                                           50
7                                           50

Thank you very much Sir. Sir, I have cleared myself that Etabs does the live load reduction in Column (axial Force only) in Indian Code also.
But , my doubt is whether No of stories upported as in the Table automatically understand 1 as (floor/slab which support 1 number slab above i.e floor immediate beneath the roof slab or 2nd slab from Top for which reduction factor is taken 10%, again  2 as floor/slab which support two numbers slabs  above or 3rd. slab from Top for which Reduction factor is taken 20%) and so on upto reduction factor 50%

Please give an understanding and your valuable comments Sir.

4.     Which Pattern live load you are talking about ?

Sir, As per IS 456:2000 clause 31.5.2.3 when the Live Load (L.L.) exceeds 3/4 Dead Load (D.L.),

a)     maximum positive moment near mid span of a  panel may be assumed to occur when three  quarters of the full design live load is on the panel and on alternate panels;
b)      and maximum negative moment in the slab at a support may be assumed to occur when three quarters of the full design live load is on the adjacent panels only.
    
Cl  3-1.5.2.4 In no case shall design moments be taken to be less than those occurring with full design live load on all panels.

When a structure is uniformly loaded with Live Load relief will be provided for moments due to redistribution of moments amongst all the spans. Larger moments will occur when some spans are loaded
Figure below is showing a continuous beam with Loading Pattern





    








So, I want to know, does ETABS carryout analysis for Pattern Loads as per the instruction of the IS 456:2000 clause mentioned above .

Whether pattern live load factor to be put as “0.75” or “0” in etabs



I have gone through the 'Concrete Frame Design Manual' that comes with ETABS and found a  lines which I am not able to interpret.

"The pattern loading is approximately, but conservatively, performed in the program automatically.


5.     In general cases ETAB do load combinations rightly as per IS code method. But it would be advisable to make your own load combination. That will give you better understand at post analysis stage.

  Thank you very much Sir.

6.     For Time period of structure it would be advisable to follow user defined time period depending upon type of structure (With infill or without infill). To consider particular structure, having very less infill walls, as infill structure or bare frame is again design engineer’s decision.  From my side it would be conservative to use “With infill” time period in such case.

Thank you very much Sir.

7.     In general if your structure is regular and uniform then you will find approximately same time period as by codel “bare frame” formula and first mode time period.  In case of building with infill you will find that first mode time period is too high as compared to codel “Infill brick” formula. The reason behind in your model you have not modeled infill walls and soil medium (flexibility) etc.

Thank you very much Sir.

I want to know one more thing Sir. How Earth pressure in case of Basement is applied in Etabs Sir. I have not find any option of applying pressure of Linearly varying in Etabs  Sir. Please elaborate briefly so that I can follow Sir, I will be highly grateful to you and obliged.

With Regards

F.M.Siddique
Guwahati, Assam
fazalarju@gmail.com



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PostPosted: Thu Nov 01, 2012 5:07 am    Post subject: Re: ETAB Model checking Reply with quote

Dear Er. F.M.Siddique Ji

Thank you very much for your kind words. We all are on same journey of learning. In such journey role of good team is very important and we all should be thankful to SEFI Admin and SEFI Senior members to lead such dynamic team. Thanks to SEFI.

Regarding ETAB, I would say that your observation for shell and membrane is absolutely right. Modeling of particular slab as shell or membrane shall be finalized at DBR stage itself. It is matter of idealization (approx) in analysis, which designer should agree/understand beforehand because these approximations affect overall chemistry between different structural members. For example if we model particular slab as shell then one should ensure that proper load get transfer to respective beams by help of proper meshing.

For inclined shell members I have to check the behavior. But I would say first go with full thickness of waist slab and check the behavior. Otherwise your approximation is also looks ok.

Live load reduction.

As you know live load reduction is only done in vertical members. In ETAB if we say storey 2 column beams the columns which supports the storey 2 slab ( column between storey1 slab and storey2 slab). I think this clears your doubt !

Pattern load .

Give me some time, I will reply on this in detail. Right now I have very limited understanding for patter load in ETAB. We can request RAHUL JI for his understanding regarding this.

Earthpressure.

To apply this you have to mesh the retaining wall in vertical plane so that u can apply idealized pressure in that respective small shell element. (Please see the attached model. Don’t go with other consideration in attached model. its only for your reference for Earth pressure EP).

Hope I m able to address some of your issues.

With Regards

Jitendra Sharma



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PostPosted: Sun Nov 04, 2012 5:19 pm    Post subject: etab model checking Reply with quote

Sir,
I am waiting for your kind reply regarding Pattern live load in Etab
Moreover ,I request kindly to have a look particularly in the Lift portion ie shear wall. which give much support reaction. I doubt something missing in the modelling and design particularly in the shear wall in my earlier model.PLEASE POINT OUT.
with regards
F.M.Siddique
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PostPosted: Mon Nov 05, 2012 10:31 am    Post subject: Reply with quote

Dear sir,

Following are my observations
1) Building 1st mode is in torsion plz add some stiffness.
2) No live load reduction is considered.

plz find attached file



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PostPosted: Tue Nov 06, 2012 4:07 pm    Post subject: Etab Model checking Reply with quote

Dear Sir,
Your observation have taught me a new finding i.e. torsion. But I am ignorant how to detect limiting torsion.
(i)Whether it can be found out as numerical output from show table.
I animated  the modal under different mode. I observed the torsional effect in the 1st. mode by eye estimation. It seems not to move straight way (to and fro) in X axis direction but with some rotational movement. But in 2nd. Mode its movement (to and fro) seems somewhat  straight way in the Y axis direction.  
(ii)Centre of Mass and centre of rigidity have also shown a considerable differences. Does it reflect the occurrence of torsion in the building.
Is it the sole way to find out the torsional effect.
To give stiffness to the building in the direction of torsion takes place, can we provide Shear wall across that direction. In Y direction in case of  my model or we can introduce other thing to give stiffness. Please give your valuable views in this regards.
Thank you very much for uploading the model showing correction for Live load reduction.
With regards
F.M.Siddique
Guwahati
Assam
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PostPosted: Tue Nov 06, 2012 4:22 pm    Post subject: Reply with quote

Dear sir,
Please send me .$et or .e2k text file because i have lower version of Etabs.

Thanks.
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PostPosted: Wed Nov 07, 2012 4:48 am    Post subject: Reply with quote

Dear sir,

First thing is it necessary to perform dynamic analysis?
only then the modal analysis or ritz analysis is effective.

however it is advisable to reduce the eccentricity between center of mass and rigidity.

in analysis out put plz check the co-ordinates by then we can judge the direction where the stiffness to be added. one small tip usually  torsion can be controlled by adding shear walls at the outer boundary.

please do check the dynamic mass participation ratio both x and y direction.


Rgds
Manohar
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PostPosted: Wed Nov 07, 2012 4:49 am    Post subject: Reply with quote

sir another note it is preferable to have 3rd mode as torsion as far as possible, but 1st mode shall not be.
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