Significance of Global and local - forces & moments

Dear All,

There is so much to learn in CAESAR-II every day. Every time I generate output report with "restraint summary extended" for all forces, moments and displacements (Global) to submit, I wonder where global and local results are used. On the way, I found elements have local forces, moments and displacements according to there orientations.

Can anyone explain me the significance of these local and global results? Of course, they are useful for support structural designers, but will they consider local or global? If the equipment vendor got these local and global values from piping engineer, which one they use?

Yes, it is a very silly question to ask here, but I am confused while discussing with my colleague in a project and I want to confirm from you guys.

Thank you.

By and large, my structural engineer counterparts use global and convert X = South and Z = West into their local coordinates and normally I do not provide loads in local coordinates.

If I do, it's because the piping in question is angled arbitrarily off axis (e.g. 22 degrees from North to follow a road).

The problem with local restraints in CAESAR is that to get the sign convention correct, you must know each individual element orientation. One might surmise element directionality in the direction of increasing node numbers, but I've seen models where analysts went capriciously backwards, and the only way to know would be to compare against the global summary or have CAESAR.

As such, if I were a structural engineer and I received a local restraint summary, I'd consider it only useful for checking my math.

Hi VGA

I use the local restraint report more specifically for skewed nozzles/supports which are offset from the global plant axis and the equipment vendor/civil structural engineer is working in the local equipment/nozzle/support axis where there would be no offset of axis.

Thank you
Best Regards
Ryan Martin

Ryan,

I thought you might be interested to know that using the Nozzle Limit feature achieves a similar end, but may award you additional functionality.

It would be helpful to have also local displacements as I discover recently that I would need it.

Thank you all for your time and responses.
I understood that the locals' are for components which are not aligned with global axis as said by Ryan.M. And globals' are of course direct values acting with the same magnitude and direction on a particular node.

Thank you once again and cheers. This forum makes me feel that I am not alone.

Dan,

Agreed. If push really came to shove, I could (painfully) achieve that in Excel, but it would be a sometimes useful feature in CAESAR.

For what it's worth, here's that workthrough:

The math:

x1,z1 = Node "A" coordinate, global
x2,z2 = Node "B" coordinate, global
Lx = x2-x1 (also can be read directly from elements tab)
Lz = z2-z1 (also can be read directly from elements tab)
dx = displacement of node B, global, x
dz = displacement of node B, global, z
d = sqrt(dx^2+dz^2)

DL = lateral displacement of node B
DA = axial displacement of node B

DL = d cos(theta)
DA = d sin(theta)

where theta = 90+atan(dz/dx)-atan(Lz/Lx)

The spreadsheet:

Inside Excel, you'd need to copy+paste the elements tab, restraints tab, and export the displacements report. Convert the data from the elements tab. I'm unfamiliar with SI notation within CAESAR, but I believe in imperial you have to consider 3 possible notations... X ft., Y in and X ft. Y in, in CAESAR 2017 and in 2014 it's the same except "in" is instead "in."

You may benefit from text to columns methodology here, or you may use a combination of the following Excel functions to achieve your goal:

Clean()
Left()
Right()
Len()
Find()
Numbervalue()
If()

You will also need to copy+paste the restraint tab from CAESAR.

For elements and restraint tabs both, you'll need to form a column that combines node numbers into one longer string. I.E. element 10 to 20 will be element 1020. You achieve this with either Concatenate() or the "&" function.

Finally, you take your node number of a restraint in the output report, correlate it with a unique combination node identifier in the restraints report (e.g. support at node 20 belongs to element 1020), and then this unique identifier is correlated with the elements tab in order to obtain a directionality of the element it belongs to.

You perform this action using match() and index() functions in Excel.

Thank you.
I have some 18 lines at angles 0, 20, 40, .., 340 degrees so it would be quite a job to do. For now I do not need to be so precise so I will roughly estimate the displacements.

Anyway good to know.

And to beat that dead horse a little harder...

If using English units, here's the equation to convert lengths to fully utilizable values. Of course, B2 is where a given length or coordinate resides.

=IF(ISERROR(FIND(" ft.",B2)),NUMBERVALUE(LEFT(B2,LEN(B2)-3)),IF(ISERROR(FIND("in",B2)),NUMBERVALUE(LEFT(B2,FIND(" ft.",B2))),(ABS(NUMBERVALUE(LEFT(B2,FIND(" ft.",B2))))+NUMBERVALUE(LEFT(RIGHT(B2,LEN(B2)-FIND(" ft.",B2)-3),LEN(RIGHT(B2,LEN(B2)-FIND(" ft.",B2)-3))-3))/12)*NUMBERVALUE(LEFT(B2,FIND(" ft.",B2)))/ABS(NUMBERVALUE(LEFT(B2,FIND(" ft.",B2))))))

I'm going to rib the COADE folks about this one a little bit. CAESAR has an error in the code somewhere such that Y coordinates chops off the period at the end of "in." This started sometime between 2014 and 2017 releases.

I'm not certain of an easier or faster way to get useful lengths out of CAESAR's input editor into Excel.

There is also a utility on SmartSupport (Global to Local) that can be used to convert a single set of values (displacements or forces) from the global coordinate system to a local (element) system.

Michael - where is that period missing?

Richard,

Coordinates tab, Y coordinate column, following "in".

Dear Michael Fletcher and all,

Thank you for your patience in giving us detailed explanation. It helped me a lot.

Regards,
VGA.