	Applies To
	Product(s):	AutoPIPE,
	Version(s):	All
	Area:	Report
	Original Author:	Bentley Technical Support Group

Logged Nov 2014, AutoPIPE V8i 09.06.01.10

Problem:

Can you please explain the forces and moments, why in the middle of a segment the magnitude of the forces flip from (-)ve to (+)ve at the same node point.

Solution:

Please see the following AutoPIPE help section:

Help > Contents> Contents Tab> Reference Information> Results Interpretation> Pipe Forces and Moments: Global Option, read this entire document.

As mentioned in the document:

- In order to interpret the meaning of pipe forces and moments it is important to understand the cross section on which the forces act.
- consistent procedure to define the cross section
- all pipe segments have a segment direction
- at any point there will be two cross sections to consider, namely the - side, which is immediately before the point, and the + side, which is immediately after the point
- the cross section for pipe force output is obtained by removing all of the segment up to that point. The pipe forces and moments act on the exposed cross section
- positive forces and moments act on the cross section in the positive X, Y and Z directions

With that said, let us consider the following example:

Cantilever beam made up of 1 segment (A), Origin point = A00 segment direction (+)ve to the right (A01, A02), and finally a Force = +1000 acting on the

Example #1, first off consider the restraint reaction as a force (Red arrow facing left on A00 ). For determination of Global Forces and Moment for the pipe, the Segment Direction is very important.

Consider Segment A, and suppose we want to obtain the forces and moments acting on the cross section of the pipe at node A01. What you will basically do is that moving in the segment direction from start point (A00), remove all the previous points leading up to A01 (Boxed in Red rectangle and light red fill). So now you will have an exposed section of the pipe available to you at node A01. Now for that remaining piece to remain in equilibrium, a force of magnitude 1000 acting in the negative X direction would be required (Green arrow). This is your Global Force reported by AutoPIPE for A01.

Example #2, now suppose the segment direction was reversed, as seen below on Segment B:

Similarly for segment B, suppose we want to find Global forces and moments acting on B01. Moving in the direction of the segment from start point (B00), remove all the previous points leading up to B01 (Boxed in Red rectangle and light red fill). You will have an exposed section of the pipe available at node B01 (You will need to look left). For the remaining piece to remain in equilibrium, a force of magnitude 1000 acting in the positive X direction would be required (Green arrow). This is your Global Force reported by AutoPIPE for B01.

Example #3, now suppose the the cases above were combined where these example was composed of 2 segments Segment A origin was at A00 progressing to the Right at A01, and Segment B origin was B00 progressing to the left joining segment A at node point A02. as seen below:

Furthermore suppose we want to find Global forces and moments acting on A01. Looking at the (-)ve side of A01 from segment A point of view the force is -1000, However, looking at the (-)ve side of A01 from Segment B point of view the Force is +1000. Thus giving the affect of the force changing at the same point but in reference to how AutoPIPE calculates the forces and moments, the results are correct.

If you have concerns about how the forces and moments are calculated in this 3rd example, reverse one of the segments (A or B but not both), so that both segments are progressing in the same direction. Then the calculated forces will have similar signs.

What is driving the sign convention change at A00 and A01?

Example:

Per the following example problem, answer these 2 basic questions:

Why are all of the reported forces at A00 and A01 +ve?

Why are the anchor restraint loads all -ve for A00, however all +ve for A01?

Model details:

Press = 300 PSI

Temp = 150 Deg F

Amb. Temp = 70 deg F

Analysis Set:

Pressure Stiff= P1

Hot modulus = T1,

Pressure Extent. = Yes,

Ignore Friction GR = Yes.

Give piping arrangement and support system below with respective reported information:

Question:

What is driving the sign convention for the forces at A00 and A01, why are all of the forces +ve?

Answer:

In this particular case, with this piping arrangement, support system, loading, segment direction (Segment started at A00 and ended at A01), and Results Interpretation for Pipe Forces and Moments: Global Option. All of the results are +ve.

To further hep answer this question, ask yourself; what side of the node point are the anchors on when the free body diagram is applied for calculating Forces and moments?

Note, review AutoPIPE help... Help > Contents> Contents> Reference Information> Results Interpretation> Pipe Forces and Moments: Global Option.

Now knowing these details of how forces and moments are calculated, plus segment direction, and understanding Anchor A00 is located on the upstream side while Anchor A01 is located on the downstream side of their respective node points.. This is very important information. The calculated forces report are on the downstream side, unless the values are different, then 2 sets of results (+ve and -ve) are printed at each node point. Again what is driving the sign convention for the forces at A00 and A01? For A00, due to the laws of equilibrium, the upstream results are equal and opposite of the downstream side results. Conversely, Anchor A01 is located on the downstream side, thus the reported forces and moments will match both values and direction.

There are 3 good exercises that should aid with understanding the results and drive home the concept of haw the results are reported in AutoPIPE:

1. Animate load case, see animate shape below:

From this vantage point see how the piping is responding to load case GT1P1. Now it is easy to understand what is happening at the anchors. Per this load case, thermal and pressure loadings is pushing the elbow A05 piping in the -X-dir, thus causing the -ve loading on downstream side of Anchor node point A00. Conversely, thermal and pressure loadings is pushing the piping against the upstream side of anchor node point A01 (+X-dir), thus causing the elbow displacement at A03.

2. Reverse Segment command (Home > Operations > Segments > Reverse Segment(s)),

Segment now starts at A01 and progresses towards A00.

Again, notice how all of the results are now equal and opposite that of the original segment direction. All of the forces results are -ve, and the restraint load still have opposite signed values.

3. Rotate the entire model 180 about A00, and review the results

Note, if the model was rotated 180 deg then you will see the same values but in opposite direction, see image below (note, vertical direction is not rotated, therefore vertical results has the same directional sign as before rotation).

Question:

What is driving the sign convention for the restraint loads at A00 and A01?, Why are the anchor restraint loads all -ve for A00, however all +ve for A01?

Answer:

Very similar to the concepts mentioned above, ask yourself, what side of the node point are the anchor on when the free body diagram is applied? Next, determine the segment direction....etc...

Again, the reason for the signed results is based upon how the model is constructed, applied loads, etc.. In this case, GrT1P1 is applying a -ve X-dir load on the Anchor at node point A00 and exerting a +ve X-dir load on the Anchor at node point A01

05. Forces and Moments output Results appear to have both (+)ve and (-)ve at the same point in AutoP

Problem:

Solution:

See Also