moment distribution 1

[Sasas2]

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!

 

[hokie66]

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.

 

[BAretired]

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

 

[Sasas2]

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!

 

[ChasWaughSE]

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.

 

[pwht1]

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

 

[BAretired]

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

 

[ishvaaag]

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.

 

[csd72]

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.

 

[Sasas2]

thanks guys!