PTB-4-2013 Eample E4.3.7 mystery variables

[rww88]

The ASME Code Section VIII Division 1 solution method presented in Example E4.3.7 of PTB-4-2003 provides and equation for the combined force factor f₂ that consists of variables F_S [axial force] and M_S [net section bending moment] for which I am unable to locate a specific reference to in Part UG or mandatory Appendix 1. I am particularly interested in any specific Code reference to the underlying reasoning for the numerical sign convention assigned to these loads in this example.

 

[pdiculous963]

This is simply a strength of materials calculation. As noted in the Code f[sub]2[/sub] is the axial load per unit width of circumference. The axial load is a combination of the axial force and the bending moment. The first half of the equation is converting the force into force per unit width by dividing by the circumference. The second term is converting the bending moment into force per unit width by dividing by the section modulus/thickness.

For the numerical sign convention, the bending moment causes compression on one side of the tower and tension on the other. Therefore, a plus or minus is used to determine the maximum and minimum loadings. It is important to check the negative moment as if the junction will be in compression under pressure, the Code requires a more detailed analysis.

 

[XL83NL]

Be aware PTB-4 has been revised and republished in 2012 and 2013.

 

[rww88]

Thanks for the quick responses. I have the 2013 edition, simply made a typo. Referencing back to my original query, I must assume then the Code has used PTB-4 to introduce and define the two variables F_s and M_s as well as the combined load equation, as opposed to Part UG or Appendix 1-5? In Figure E4.3.7 would the axial forces be acting downward? Would the net bending moments be acting counterclockwise? Or is the signage and directions even of relevance?

 

[TGS4]

The Code absolutely will NOT use PTB-4 to do anything, particularly something as important as introduce new variables. The response provided by pdiculous963 it's also the response that I would give.

Remember that the Code is not a design guide. Good engineering practice must be followed to comply with the rules. That's what you are seeing in PTB-4.

 

[pdiculous963]

As the Code applies to a wide range of vessels with unlimited loading scenarios, the Code does not define F[sub]s[/sub] and M[sub]s[/sub]. The designer is expected to use engineering principles to determine the unit forces f[sub]1[/sub] and f[sub]2[/sub]. The author of the document arbitrarily used F[sub]s[/sub] and M[sub]s[/sub] to designate axial force and overturning moment for the equations.

The axial forces are due to weight of the tower, so yes they act downward. The bending moment is likely due to wind load, so it would act as an overturning moment.

 

[rww88]

Thanks again. Please accept my apology for my poor choice of words. It was certainly not my intention to imply that the purpose of PTB-4 is, or should be, anything beyond its title of "Section VIII Division 1 Example Problem Manual".