From:	Mark Smith
Sent:	Tuesday, July 26, 2011 3:08 PM
To:	Peter Titus; Ali Zolfaghari
Cc:	Lawrence E. Dudek; Bob Simmons
Subject:	RE: pedestal calc

Pete,

For question (1) below, your answer did not include the machine torque 
(effects of the TF legs).
Is some percentage of the machine torque applied to the pedestal 
analysis?
If so, how much and what model (figure)?

Also, you mentioned (below) 233 MPa for the maximum stress. 
Executive summary page 6 shows 135 MPa, page 19 figure 9.2.-5 
shows 233MPa.
Please clarify.

Lastly, I found various articles for tubular structure's weld stress 
concentration factors (SCF) based on steel, oil rigs, etc.
The SCF's can be between 3-10, give or take a few, depending upon 
connection parameters.

So, I think it prudent not to use the girth weld SCF =1.

We need more margin. 


Mark Smith

From: Peter Titus  
Sent: Monday, July 25, 2011 2:18 PM 
To: Mark Smith 
Cc: Ali Zolfaghari 
Subject: RE: pedestal calc

 

 
From: Mark Smith 
Sent: Mon 7/25/2011 12:54 PM 
To: Peter Titus; Ali Zolfaghari 
Cc: Lawrence E. Dudek; Bob Simmons 
Subject: pedestal calc
Hi Pete.
 
I have a few questions for the pedestal calculation.
 
(1) What figure has the results for the simultaneously applied loads?
 
Section 9 has some of the results from the global model in which the 
vacuum loads, dead weight, Centerstack casing normal operating 
thermal expansion loads, TF Expansion Loads, and all loads from the 
PF coils for the given equilibrium, are applied. I have another global 
model run in which the bent spoke lower lid was straightened out. These 
may produce different pedestal stresses - will update as soon as I have 
access to Andrei's computer.
 
Also, in the executive summary (calc page 6), no stress concentration 
factors are used for the welds.
This seems generous. Typically, you have used a factor of 4.
(2) Is no stress factor within AWS standards for full penetration welds?
 
The factor of 4 is for fillet welds. In AISC and ASME a full penetration 
girth weld is assumed to have a stress concentration factor of 1.0. These 
pipe welds will have a stress concentration factor above 1.0, even 
though they are modeled as being full penetraition,  because their 
intrersections are at some angle - but the FEA should include some 
measure of this - at least in so far as the mesh density can capture it. - 
This is why I keep emphasizing  that the Thompson scattering port 
reinforcements should have full penetration welds, so that you can claim 
that the FEA represents the stress. Rigorously the  mesh density should 
be fine enough to resolve the stress concentration. The peak loading 
(Lorentz-Deadweight) in the pedestal is compressive which doesn't 
propagate fatigue cracks. 
 
Note, scenario 79 has been used for the worst applied torque.
Based on the design point sheet, there is about a factor of 1.5 between 
sc 79 and sc 21 torque.
Assuming linear behavior, sc 79 would scale to about 202 MPa. 
 
I am not sure how much global torques appear in the pedestal - 
especially with the stiffer spoked lid. Figure 9.2.5 is for scenario 21 and is 
described as having the largest torque found. The local Tresca stress for 
this EQ, at the intersection of the Vee and middle flanges is 233 MPa. 
This includes the dead weight and all other loads - but is for the bent 
spoke lid. 
 
A modest weld stress factor (1.48 vs 4) puts the pedestal above the 300 
MPa weld allowable.
 
Given the reported stress is 233 Tresca, and much of this is 
compressive, I think weld fatigue will not be a problem for the Vee Pipe 
Pedestal. 
 
 
 
Mark Smith
Mechanical Engineer
Princeton Plasma Physics Laboratory
P.O. Box 451
Princeton, NJ 08543-0451
(609) 243-2778