General Mechanical

Concrete Block – Longitudinal Stress Analysis

• usha_civil
Subscriber

Hi

Would like to plot longitudinal stress graph (using ansys workbench) for a concrete block fixed at one end and subjected to axial compressive load at another end. I created path at the centre of the block and plotted the graph. The stress should be uniform throughout the length. But I am unable to plot the graph properly. Need guidance.

Thanks a lot

• peteroznewman
Subscriber

Does this video help?

Here is another one, but go to 11:50 to see the Path done.

• usha_civil
Subscriber

Thanks for the video. Really helpful. Got some ideas. Will implement what I have learnt from the video and will get back to you, if I have any query.

Thanks once again Peter

• usha_civil
Subscriber

Hi

I have attached my stress graph for your reference. My query is "Normal stress along longitudinal axis must be in bell shaped curve with max at the centre." I am unable to get the plot properly. Can you guide me?

Thanks

• peteroznewman
Subscriber

You can attach zip files to your posts as long as the file size is < 120 MB.

• usha_civil
Subscriber

Attaching the file.

The equivalent stress is compressive along central longitudianl axis and the normal stress should be tensile. But I am getting both graphs similar.

• peteroznewman
Subscriber

To get a uniform stress along your compressive column from the base to the top, delete the Fixed Support and use three displacement boundary conditions. One displacement BC on the base has Z=0 with X and Y free. A second displacement BC on one X face has X=0 and Y and Z free.  A third displacement BC on one Y face has Y=0 and X and Z free.

This is what you would do to create a 1/4 symmetry model of your column where you would slice the column into half in each dimension of its cross section.

If you don't care about the centerline of your column shifting very slightly due to Poisson's ratio expanding material out under load, you can just apply it to two sides of your column and not slice the column at the center planes.

• usha_civil
Subscriber

Thanks for the guidance. I got the answer for compressive stress along central longitudinal axis. But normal stress along the same axis should be tensile and should be minimum at the ends and max at the centre. The graph should be like a bell shaped curve. The result is different. Kindly guide me regarding the normal stress along central longitudinal axis.

May I know why I can not use fixed support? I am considering it as cantilever beam.

• peteroznewman
Subscriber

My previous post described how to get a uniform compressive stress in the column, with no stress in any other direction.

Here is an illustration of the support at the bottom of the column. The first figure is an unloaded column, the second figure is a loaded column that is compressed by the load and expands laterally. If the column is on rollers, the face is free to expand by the Poisson's ratio of the material without picking up any lateral stress.

If you used a fixed support, the material wants to expand, but cannot due to the fixed support and so develops lateral stress.

If you want a cantilever beam, it is perfectly fine to have a fixed support.

I don't understand why you say this:

"But normal stress along the same axis should be tensile and should be minimum at the ends and max at the centre. The graph should be like a bell shaped curve."

Can you provide a reference that explains it?

• usha_civil
Subscriber

As you mentioned in the previous post, when one end of a beam is fixed and an axial load is applied at the other end, lateral stress will be developed. I would say, compression will take place in the direction of loading and expansion (ie. tension) will take place in lateral direction. The compressive stress in the direction of loading will be uniform whereas the tensile stress in the direction perpendicular to the direction of loading will be zero at the ends and max at the centre. So if we plot graph for the compressive stress along the direction of loading (longitudinal direction), it should be a straight line. The graph for the tensile stress along the longitudinal direction should be a curve with zero at the ends and max value at the centre.

• peteroznewman
Subscriber

You don't have the correct concept for how lateral stress develops in an axially compressed column.

If the ends of the column are prevented from expanding as shown in your last figure, that causes a compressive stress in the Y direction near the ends, but the midpoint of the column length where the material is free to expand has no stress in the Y direction as shown in the figure below. You don't need tension to expand laterally.

I can plot the profile of the Stress in the Y direction along the center line from the center to the end.

The stress in the X direction is uniformly compressed near the midpoint of the column length, then increases and decreases as it reaches the fixed ends along the centerline.

I can plot the profile of the Stress in the X direction along the center line from the center to the end.

There is only a tiny portion of the column that goes into tension, and that is the very short portion of the free side of the column adjacent to the fixed end that is being compressed.

• usha_civil
Subscriber

Thanks a lot for your guidance and explanation. Really very helpful. Still I am not convinced about your statement "You don't need tension to expand laterally". But surely think about it.

Thanks once again as you have guided me patiently, though I didn't explain my part clearly to you.

• peteroznewman
Subscriber

If you edit the model attached to my last post and change the Displacement support from Y=0 to Y is Free, you will have effectively put rollers on the end. Now you will see the perfectly uniform compressive stress along the X axis and zero stress along the Y axis.

Think about it this way. Stress = Force/Area. Tensile stress means a force is pulling on an area. Compressive stress means a force is pushing on an area. Poisson's ratio causes the material to expand laterally but there is no force doing that, it is a material property. If you wrap a narrow band around the column center, then compress it, the column will expand into the band, which will push back, creating a force in the Y direction and causing compressive stress in the Y direction. This is similar to what is happening at the end support when Y=0 is included.