×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

moment distribution

moment distribution

moment distribution

(OP)
im designing a basement wall fixed on all four sides, with the load spanning horizontally.i've been told it is necessary to distribute the moments which i have calculated at the supports.
5m span- moment at support = 266.21 knm, moment at mid span= 133.11knm
7m span- moment at support = 499.9 knm, moment at mid span= 224.95 knm
How is the moment at the supports distributed? and how is the mid span moment affected?
Thanks!

RE: moment distribution

The problem with your question is that the corner moments are not fully developed by normal reinforcement methods.  Full moment connections in reinforced concrete structures are rarely achieved.  So to answer your question, I might design the walls for corner moments at midspan, and the midspan moments at the corners.  Or more conservatively, just assume all supports are pinned, and carry the reinforcement continuously throughout.

RE: moment distribution

Sasas2,

If the wall is fixed on four sides, the load spans both vertically and horizontally.  It is not a straightforward problem to determine the magnitude of moments at various positions on the wall, but there are several methods which could be used.  

You could find values from a book such as "Theory of Plates and Shells" by Timoshenko and Woinowsky-Krieger or you could analyze it using the Hillerborg Strip Method or Yield Line Analysis.

Be careful about assuming full fixity at the edges.  It probably will not occur on your structure.
 

BA

RE: moment distribution

(OP)
ok..thanks.i have calculated the moments using the hillerborg strip method and im designing reinforcement at two heights along the wall...im just not sure how to distribute the moments!

RE: moment distribution

A basement is just a small sized example of a "concrete tank".  PCA (Portland Cement Association) has a complete guide, including moment coefficients, called Rectangular Concrete Tanks.  It is appropriate for sanitary structures, i.e. water and wastewater -- with extra factors to limit cracking.  However, the moment coefficients in the book are very useful for all types of similar designs.  See if you can find a copy -- the rest of the design will be very easy then.

RE: moment distribution

Sasas2, don't you have a concrete design textbook that explains the calculations involved in moment redistribution? (there are 278000 links in google on the subject)

Sorry can't help on the calcs as I've never bothered designing a wall/slab with moment distribution.  If I was designing that wall I'd provide reo in it for both support cases (fully fixed and simple) and then nothing can go wrong.  

Good luck,
Paul

RE: moment distribution

Somewhere in my library, I have the PCA document for rectangular concrete tanks which gives moments in both directions at selected points based on an elastic analysis.  I could dig it up and provide it to you if you wish, but I really believe it is not the best way to approach the subject.  

It is better to arrive at a design based on sensible approximations.  If you are slightly over the required amount of reinforcement, who cares?  You understand how you arrived at the answer and could defend it in court if need be.

The title of this thread, "Moment Distribution" is really a misnomer.  Moment distribution refers to the Hardy Cross method of analyzing continuous beams or framed structures.  It does not address two way slabs or plates.

BA

RE: moment distribution

Yet I need to add, that given the current times, it is speedier and (should be more) accurate to use a model in FEM. Almost every program that has plates or solids should do that, and there is a number.

You see that I time and again insist in using this tools, and the main reason is that it is a pity that so simple (to use) tools are not yet more used. I think for myelf that the reason for that is that some engineers do not always feel cozy drawing the required model, but with autocad and whichever of the solid modelers to build such models (everything that is an extrusion) takes very little time.

In RISA 3D you import 3DFACE and LINE entites and place loads in a moment; I think to remember such is the case as well in SAP 2000 or you can build it. Autocad, Solidworks, Algor etc can take 3D solids (for the simple models made in 2 mins) in many formats and to apply simple loads or constraints is not much difficult. Hence I endear those timid in the matter to try, because the benefits are many.

RE: moment distribution

Moment redistribution is quite simple and is based on the fundamental principal of superposition.

Moments on continuous spans (even cranked continuous ones like this) the moment is made up of two components superimposed on each other.

The first component is the simply supported moment from the udl, or wl^2/8 which gives you the familiar parabolic distribution.

The second is the moment diagram from the end restraints which is the same as a simply supported member with a moment applied at each support. This gives a distribution that is an odd shaped rectangular distribution with a straight line between the end moment values.

The actual moment is one minus the other (as they are opposite sign.

Now when you redistribute moments you play around with the second component which correpondingly effects the mid span moment as well.

I hope this helps your understanding.

RE: moment distribution

(OP)
thanks guys!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources