ASME B31.3 Appendix D, Table D300 provides formulas to calculate SIF for fittings and branch connections etc. For Welding Tee, the SIF (out-of-plane for example) i= 0.9/h2/3 where h= 4.4*T/r2. In order to use these formulas legitimately, two geometry rules have also been given as shown on the same Table. These rules address requirements on crotch radius and crotch thickness. i.e. 1) rx>=1/8Db, 2) Tc>=1.5 T. If these two geometry rules are not met, note (11) shall apply; which will change h from h1= 4.4*T/r2 to h2=T/r2. Subsequently, the SIF, i2 will be increased to (4.4)^2/3 or 2.685 times the i1.

Caesar II assumes that all welding tees meet those geometry rules and has been using formulas as shown on Table instead of Notes (11) for calculating SIF. In other word, the default is h1= 4.4*T/r2 as shown on Table, and program provides no option for h2=T/r2 as shown on Notes (11). Stress engineer has to correct the stresses at welding tee manually as required only if he(she) knows what kind of welding tees are in the piping system. Piping specification group has been instructed to put these geometry rules in piping specification, so that default formula (h= 4.4*T/r2 ) in Caesar could be validated.

ASME B31.3b-2001 Addenda has been issued on August 31,2001 with Mandatory date of March 1, 2002. This issuance made revision on this subject as follows:

1. h= 4.4*T/r2 with geometry rules (rx>=1/8Db and Tc>=1.5 T) has been moved from face of Table to Note (11).
2. h= 3.1*T/r2 is the revised formula for calculating h, as shown on face of Table 300, without any geometry rule restrictions.

Now, the question is how will Caesar II build in these revised rules in the program? Will formula as shown on face of Table (h= 3.1*T/r2 ) still be the default? or formula as shown on Note (11) be the default? or, options will be provided for user's choice.

You stated:


It is true that CAESAR II determines "h" using 4.4*T/r2, which assumes the fitting meets the dimensional requirements. In previous editions of the code, Note 11 stated that if these dimensional requirements were not met, then "h" should be computed as T/r2. <em><font color="#0000ff">Now, here is the part where the engineer needs to apply engineering judgement. You have to determine whether or not your fitting meets the dimensional requirements. If it does, fine, leave things alone. If it does not, then change the fitting type to an unreinforced tee. "h" will be computed as T/r2 and you satisfy the requirements of Note 11.</font></em>

Previous editions of the code assumed the fittings met the dimensional requirements, by defining "h" as 4.4*T/r2. The new edition, assumes the fittings don't meet the dimensional requirements, and define "h" as 3.1*T/r2. (In the new edition, Note 11 now provides 4.4*T/r2 if you do meet the dimensional requirements.)

The issue here is: does your fitting meet the dimensional requirements? In most instances, you will not know, unless you cut one in half and take measurements. You the engineer still have a decision to make!

You asked:


Currently our thinking is:

• Activate the necessary input fields for these fittings so users can specify the crotch radius and thickness. This will allow CAESAR II to determine if the dimensional requirements are met, and use the proper computation for "h".
• For instances where users leave the crotch radius and thickness blank, the software will now default to using either 4.4 or 3.1 in computing "h".
• The determination (on using 4.4 or 3.1) will be made based on a new configuration switch. This serves two purposes: (1) It allows the software to operate the same as previous versions for QA and benchmarking, and (2) it allows users to set a default assumption regarding whether or not their fittings meet the dimensional requirements.

We believe this covers all situations. <em><font color="#0000ff">However, as stated above, the engineer must still decide whether or not his fittings meet the dimensional requirements. CAESAR II is just a tool, it won't make decisions for you.</font></em>