Tuesday, February 12, 2013

Modeling of Slab Column joint


In structural engineering, the Flat Slab is identified as a slab which is supported generally without beams by columns with or without column heads. However, I have understood that Flat Slab designs are not quite popular in Sri Lanka, may be owing to its complexity in design, detail and construction. But things what I am going to discuss here are equally important in any other column slab design problem such as pile cap etc.


Generally the slab can be analyzed by dividing it in to longitudinal and transverse frames consisting of columns and strips of slab. This is identified as Equivalent Frame method and frames are analyzed by moment distribution. However these manual methods are things for the past. More sophisticated and user friendly Finite Element Methods (FEM) and software are widely used to solve the problem.



Plate mesh and the columns : STADD Pro





3D view of the Model



However it can be seen  in FEM that the support moments in slab column joint are sometimes unacceptable  owing to its unrealistic high values. The anomaly is varying as accordance with span, column sizes and slab thicknesses.







Longitudinal Bending Moment Diagram





Transverse Bending Moment Diagram









However , most FE software offers  'peak smoothing' techniques to addresst this problems.

In General, the number of elements used in a FEM model can greatly affect the accuracy of the solution. As the number of elements, or the fineness of the mesh, is increased, the accuracy of the model increases as well. However in this particular Slab Column problem finer mesh would deteriorate the quality of the results more.




Deflection pattern : scale is adjusted



The solution for this problem  lies on the  realistic modeling of boundary conditions  of the slab.


In this regard, I usually  adopt two methods;

  1.) As the first method, Slab may modeled as a plate/shell element while column may modeled as a three dimensional solid object.  By doing so, we get closer to the real world scenario of the slab column joint. However analysis of this model may be time consuming and required higher degree of computer resources.

2.) In the second method, the flat plate is modeled as a plane of finite elements, and columns are simplified to “pin” or “fixed” supports applied at the elevation of the plane of the slab. Sometimes these supports are modeled as springs with a finite elastic stiffness to improve the behavior of the model.

In this method it is essential to connect nodes rigidly which lies within the area of the column section to the end of the frame member. This technique is otherwise referred to as a master-slave technique, where the end of the frame member column is the master joint, and the joints in the slab within the column area are slaves.

This technique offers an advantage over the use of three dimensional solids with respect to computational efficiency  and  Analyzing time of the model.






I hope this will help you in FEM modeling.




22 comments:

  1. This is interesting Yasela. Have you tried applying your technique to pile caps? I've found in the past that the assumption of a rigid foundation to manually analyse pile groups can give rise to unrealistically high pile axial forces as the distance from the centroid of the group increases.
    As I understand the true distribution of pile forces tends to be more uniform, due to the flexibility of the pile cap.
    My efforts to model pile groups using SAP2000 were not successful, for reasons you have stated here. Even when the slab's stiffness is increased by several orders, I found that the effect of the pile on the slab moment reduces very quickly when moving away from the pile.
    I will try your method when I get the chance.

    ReplyDelete
  2. Yasela SumanasingheFebruary 14, 2013 at 1:46 PM

    Thanks Asanka ,


    Yea I have applied these techniques in Pile cap modeling and I have found that it works well. As my understanding, we assume that foundation is rigid in comparison to the soil and that the pressure distribution is linear. For smaller spans this particular assumption is satisfactory. However generally I choose adequate thickness for pile cap, so that it behave rigidly, as well as it withstand against the punching shear failure owing to high axial forces of piles. For laterally loaded pile groups in bridges simply I use Tomlinson equations ( W/n+Wex/(x1+x2….+xn)) and I have found no contradictions of it compared to FEM results.

    ReplyDelete
  3. Hello friends, its great piece of writing regarding tutoringand fully defined, keep it up all the time.


    Also visit my web-site: exercises

    ReplyDelete
  4. I am extremely inspired along with your writing abilities as neatly as with the
    structure for your blog. Is this a paid subject or did you customize it your self?
    Either way stay up the nice high quality writing, it is uncommon to peer a great weblog like this one
    today..

    My web-site: guild wars 2 gold

    ReplyDelete
  5. Wow that was strange. I just wrote an really
    long comment but after I clicked submit my comment didn't appear. Grrrr... well I'm not writing all that over again.
    Anyway, just wanted to say superb blog!

    Feel free to visit my weblog: Work From Home Opportunities

    ReplyDelete
  6. My partner and I stumbled over here from a different website
    and thought I might check things out. I like what I see so i am just following you.
    Look forward to looking into your web page yet again.

    my webpage mazda rx 8 review

    ReplyDelete
  7. Hey there are using Wordpress for your blog platform?
    I'm new to the blog world but I'm trying to get started and set up my own.
    Do you need any html coding knowledge to make your own blog?
    Any help would be really appreciated!

    Look at my web blog ... raspberry ketone

    ReplyDelete
  8. I blog often and I seriously appreciate your content. The
    article has truly peaked my interest. I am going to bookmark
    your site and keep checking for new details about once per week.
    I opted in for your RSS feed too.

    Here is my web page - raspberry Ketones Diet

    ReplyDelete
  9. Amazing! Its genuіnely amazіng pοst, I havе gοt much clear iԁea on the topiс of from this
    piecе of writing.

    Hеre is my page - Http://ashfun.com/WXULeif/info/

    ReplyDelete
  10. It is easy in my opinion to get a aνerage е-cig
    and decide they're not for you without having researching and seeking different items. I started together with E-Lites and they are alright but not excellent. I found all of them hard to draw/drag about and you had to change the method you light up to get a satisfying puff associated with smoke. In any case this isn't
    а reviеw οf E-Lites іtѕ critiquing thе GreenSmoke so hеre goes:


    Alsо visit my blog - green smoke coupons

    ReplyDelete
  11. Keep on working, great job!

    Look at my web blog teen porn

    ReplyDelete
  12. I love what you guys tend to be up too. Such clever work and coverage!

    Keep up the good works guys I've added you guys to my personal blogroll.

    Feel free to visit my weblog: xxxpornovideo.net

    ReplyDelete
  13. Professional Design Services is Ocean County & Monmouth County Engineering Firm L.L.C's Our staff provide the expertise necessary to prepare construction documents for the site work and assists the client in obtaining bids and negotiating proposals for these elements.

    ReplyDelete
  14. Hello,
    These sort of collapses occur due to reduction in lateral pressure in foundation and

    surrounding soil. The main cause is that engineers ignore the nearby excavations.
    Civil Engineer should have vast knowledge and understanding of the subject to handle such

    issues.
    I would suggest http://cescientist.com/ask-a-scientist . It is a service that can answer

    your every question/query about Civil Engineering.
    Thanks

    ReplyDelete
  15. The Bluetooth modules provide a platform to quickly and safely go into production with competitive and advanced features.

    ReplyDelete
  16. civil engineering is always a fun just viist http://www.iamcivilengineer.com

    ReplyDelete
  17. knowledgeable information presented in the post, thanks for sharing such a great post.

    Construction Quality

    ReplyDelete
  18. For more information regarding civil engineering info please visit
    http://www.allabout-civil.tk/

    ReplyDelete
  19. i am having fun reading your blog realy.....greatttt

    ReplyDelete
  20. This comment has been removed by the author.

    ReplyDelete
  21. Thanks for suggesting this great information with us.Describing of way is very good.

    Gas Strainer

    ReplyDelete
  22. Great post shared here.Thanks for sharing.Steel fibres for concrete can improve toughness- by preventing crack propogation from micro-cracks to macro-cracks.For more details,contact Precision Drawell.

    ReplyDelete