STAAD LIMITATION 5

[sigma1525]

I have a model with about 65000 ellements and I am having dificulty assigning the plate thicknesses for all of them.

Does anybody know what is the maximun number of elements or joints the program can handle?

BENTLEY is out for lunch.

Thanks

 

[Ron]

Check your documentation. I would guess that you are approaching the limits...even though you might have an "unlimited" license.

 

[ToadJones]

can I ask what you are modeling?

 

[Gumpmaster]

Can you assign the thickness in the input file. It may just be too much for the GUI to handle.

 

[delagina]

if it takes some time just to assign thickness, it will take forever once you do the run analysis.

i say just increase the plate size to lessen the number of plates

 

[sigma1525]

ToadJones: A 90" pipe with water resting on a 120 degrees steel saddle. Need to check local stresses at the support.

Gumpaster: Good advise but did not work.

Delagina: You are correct it takes about 15 minutes to run it. Redusing the quantity of element makes sence.

Note: Running it with STARDYNE it takes less than 5 minutes but I am not comfortable with STARDYNE.

Any comments about STARDYNE V STAAD analysis will be appreciated?

Thank you all

 

[JLNJ]

65,000 elements? How many miles long is the pipe?

 

[sigma1525]

65,000 miles

 

[ToadJones]

I dont get it....if you have selected the "plates cursor" then all you have to do is click "CTRL + A" like in any other windows program and it will select all the plate elements. Then simply assign the properties.

As a side note, be careful checking local stresses in STAAD with plates and the normal supports found in STAAD. STAAD sees the support as a "knife edge" and the local stress will be ridiculously high and inaccurate.

 

[sigma1525]

ToadJOnes

I dont have the problem anymore because I increased the element sizes, therefore, reduced their qunatity by almost 50%.

I agree with your comment about "knife edge" but in this case my model includes the 120 degrees 3/4" thick pipe saddle with stiffeners etc. and that is why I had so many elements. Therefore, STAAD should see a uniform bearing surface 24" wide at the bottom 3rd or 120 degrees. The local stresses at the edge of the saddle are high but not what I expected, considering this pipe ovehangs (Cantilevers) over the support (saddle) by more than 10 ft.

Is there something I do not see here?

NOTE: Roarks discusses this problem but they are limiting it to R/t <50. In this case 46/0.5 = 92...?

Once again thanks for your help

 

[TLHS]

I've tried to use STAAD finite element for piping attachments before, but the way it works means that your boundary values tend to be unnecessarily high. The program is fine for things like vessels where you don't have much in the way of point loads or abrupt geometry changes (or where you're accounting for those things in other way), or things like concrete where you're looking at less abrupt loading changes over a larger distance, but I won't use it for details in thin walled plate structures.

STAAD uses a plate system where it sees each element as an infinitely thin plate. This is a fine system, but you end up with localized issues where they intersect if moments are applied or otherwise created by the loading. You'll also have issues when you're supporting things on plate edges or welds, because they end up having no real bearing/connection area so you don't see the advantages you should expect by increasing the thickness of component plates of your attachments.

For example, let's say you've got a quarter inch pipe, 30 inches in diameter and you weld a quarter inch thick plate to it that protrudes from the pipe at 90 degrees as a cantilever. The face of the plate is in the same plane as the pipe cross section. Apply a force to the face of that protruding plate and you'll end up with a cantilever with a shear and a moment at the pipe face. STAAD sees the moment only as a pure moment at the base and works through the problem. Now, say that result has localized stresses that are too high within the first quarter inch close to the connection point. You may decide that you want to spread out the forces over a larger area and increase the size of the connecting plate to one inch in thickness with the assumption you'll get enough advantage in the increased moment arm at the base of that connection to make the attachment work, since it's four times thicker and your stresses were only a problem very locally. However, if you run it you should find that the stresses are effectively the same, except for any redistribution due to the change in element stiffness. The model still sees the connection points as finite points rather than being able to account for the finite area. It will be a pure moment transferring into a point/series of points rather than a pair of couple forces (tension at one weld and a distributed compression along the bearing surface or at the other weld depending on how you think the connection works)You have this same issue when you apply point or line loads perpendicular to the pipe face. The actual thickness of the attachments won't affect the way the load gets applied.

Also, you may want to watch out. I don't know what your actual attachment looks like, but if Roarke's has a model for a thick walled pipe but not for a thin walled pipe, it's probably because that loading style could result in buckling of the pipe wall under certain conditions. If so, it might be a failure that won't get picked up in a finite element analysis either.

I've run into a couple of other issues using STAAD for this application but didn't really bother working out exactly what the issues were, since by then it just seemed like it wasn't worth it. I'll still sometimes use it to figure out how far down a pipe localized effects from an attachment continue before they become negligible, but that's about it.

On the plus side, it does seem that using STAAD is at least conservative, if not necessarily all that accurate.

I hope this made some sense.

 

[sigma1525]

TLHS

It does. Thank you.

Can you recommend an alternative method or software. Are you or anyone you know willing to take a shot at it for the right $'s obviously. (Not sure if I can say this in this forum....?)

Once again thanks

 

[TLHS]

I've worked out methods for the two or three specific scenarios I've run into by using combinations of information from Roark's, the pipe code for the pipe being used, some basic mechanics and a ridiculous amount of reading to understand how the pipe works. Unfortunately, I don't think I can generalize it all that well from the specific cases I've investigated.

Piping attachments are actually surprisingly tricky and often seem to get the "We've done it this way for years!" treatment with nobody actually understanding how they work.

If you've seriously got an 8 foot diameter pipe with a half inch wall, all I can really suggest is that I very much suspect that you'll require some sort of full encirclement ring to maintain stiffness at the support. If your pipe goes out of round it's no longer acting like the constant cross section beam you're likely assuming to get the support loads.

You may want to look into ring girder design. It may be the sort of thing that fits for this problem, plus I believe there are some well established design procedures, although I've never designed one myself. The only book I personally have that mentions them, though is "Tubular Steel Structures: Theory and Design" put out by Lincoln Electric. There's about three pages on saddles and ring girders, but it's likely fairly out of date. I've got something that references "AWWA M11" as having ring girder design information, so possibly look into that standard.

I'd be interested in hearing what you come up with at the end of all this. Take the above with a grain of salt. Although I've done some reading I'm certainly not an expert and have only approached this type of problem a couple of times.

 

[TLHS]

Just to add to that, I tracked down a couple of pages of AWWA M11 and it has information on the design of pipe saddles and ring girders, and how to check the stresses on the pipes in both those situations. So yeah, it'd be a useful document to look into.

Parts are actually word for word the same as the Licoln Electric book I have by Troitsky, so I presume either the author contributed to the AWWA document or the AWWA used him as a source.