Dear Designers

Now a days i m doing stress analysis for underground system. Is anydody can help me in putting the virtual anchor positions. Guide me asap.

With regards

Do not use "virtual anchor length" (VAL) to set the position of an anchor in your model. "Virtual anchor" is a length of buried pipe; it does not define a distance "to" something. If I have a length of bured pipe that has a straight run and no intermediate thrust points (e.g. a tee) longer than the VAL, then the two sides connected to this long run will not be influenced by each other - no "information" (i.e. load and displcament) from one side can get over to the other side. This all works because the accummulated axial restraint from friction sucks up all the (axial)load to the point where the pipe compresses rather than moves. Of course bending is knocked out of the run much sooner than the axial load.

So again, don't add an anchor to your end your model. Instead, in the CAESAR II approach, just run the pipe out and "bury" it. I recommmend that you do not simply run a pipe equal to the VAL. The CAESAR II modeler uses a flexible restraint (the K1) to model this friction load and the pipe has to move to accummulate the load. So the CAESAR II buried model will require more pipe length to build up this axial friction load and lock up the run.

Dear Diehl,

U helped me bit but my query was that " If my pipe moves from above ground to under ground as in case of gas pipeline, and our aim is to do the stress analysis for above ground system in that case where i have to finished the model generation. Is it at the point where there is a intersection between the A/G and U/G system or in the U/G side and if yes for the former case what will be the type of support and if yes for the later case what will be the location of support and type of support".

Kindly revert back asap.

With Regards

From the above ground point of view, to model the buried pipe section as a boundary condition you must consider the loads from the buried section and its effective stiffness at this termination. The displacement load will have the buried section pushing the above ground portion. The virtual anchor length can be used to calculate this displacement if its a straight run but any other thrust points such as the first underground elbow will complicate the calculation. Stiffness will be set by the bearing and axial response of the soil.

I would suggest you model some of the buried section and then model some more, if the numbers don't change much, then you probably have all the needed influence from the soil in your above ground model.