Tubular Joints

Tubular joints are not true parts of a model since they inherit the majority of their properties from the elements which intersect to form the joint. The concept of a joint is really only used in checking the codes which are applicable to joints and in computing fatigue at the intersection of the elements forming the joint.

If stiffeners are associated with either of the two chord classes, then they will be used in computing the "crushing" of the joint, in computing the code check for the joint, and in computing the stress concentration factor for the joint. The number of stiffeners will be the sum of the number for both chord segments. For situations where two classes define the chord with different stiffener attributes, then the properties of the last one encountered will be used. Here, the stiffeners are "smeared" over the effective length (or true brace footprint of the joint). The SCFs computed by whichever method are reduced according to "Lloyd's Register of Shipping Recommended Parametric Stress Concentration Factors for Ring_Stiffened Tubular Joints".

In MOSES, there are three commands which one can use to override the default behavior of the program in dealing with joint: &REP_SELECT, JOINT, and JNT_PROP. The first of these can be used to set defaults used when joint structural results are obtained, the second is used to produce the joint structural results themselves, and the last is used to override default properties for specific joints. Any option applicable to the JOINT command is also valid for &REP_SELECT. Here, we will be be discussing a subset of the options available for these two commands and providing a framework for those applicable to the JNT_PROP command.

The options of interest here for the JOINT and &REP_SELECT commands are:

     -SN, CURVE, TYPE, S(1), N(1), ..... S(n), N(n)

     -MIN_SCF, MIN_SCF

     -SCF, SCF_TYPE, FRACHOL, MAXCHOL

and the form of the JNT_PROP command, which is available either during an INMODEL or in the MEDIT menu is:

     JNT_PROP, :JSEL, -OPTIONS

where the available options are:

     -JNTCLASS, PRCK, PRCT, PRCX

     -MIN_SCF, MIN_SCF

     -BBC_MUL, MULT

     -EFF_CHD_LEN, ECHD_LEN

     -LEN_FACTOR, FRACHOL

     -MAX_CHD_LEN, MAXCHOL

     -CHD_FIXITY, CHD_FIX

Notice that many of these options can be used on all three commands. The hierarchy is: things used on &REP_SELECT serve as basic defaults. Things used on JOINT override things specified on &REP_SELECT, but things defined with JNT_PROP override both of the other settings. Things specified on JOINT define how, in general, the joint is to be checked, and the JNT_PROP command is used when the general settings are not applicable to specific joints. On the JNT_PROP command, :JSEL is a selector (selection criteria, or name which may contain wild characters) of the joints for which the options are applicable. In general, things discussed here deal with four categories: joint classification, joint crushing parameters, stress concentration parameters, and defining and selecting SN curves used for fatigue.

MOSES automatically classes joints based on the load path. In some rare cases, one may wish to override the automatic joint classification. Since one normally will want to override a few joints, the way to do this is with the -JNTCLASS option of JNT_PROP command. Here, PRCK, PRCT, and PRCX are the percentages of K, T, and X joint types used to classify a joint. When this option is used, all joints selected by :JSEL will use the specified classification for all load cases.

In Joint Crushing MOSES treats the joint as a two dimensional ring. Two basic assumptions here are that the braces do not alter the stresses in the ring and that an effective length of the chord is used to distribute the load. The -BBC_MUL option allows a factor of the bending stress under a brace to be used. In other words, if one believes that the brace will prevent any bending stress in the chord under its footprint, then he should specify a value for 0 with the -BBC_MUL option. The opposite, conservative, view is that the brace has no effect in the bending of the chord where one specifies a value of 1 for MULT. Of course, one can specify any value between these two. The -EFF_CHD_LEN option is used to change the effective chord length for Joint Crushing from the default behavior computed according to API RP2A for both joints with and without rings.

For a discussion of the -SN option, see the sections on defining and associating SN curves. The -MIN_SCF, -SCF, -LEN_FACTOR, -MAX_CHD_LEN, and -CHD_FIXITY options modify the computation of stress concentration factors. The first is available with all the commands considered here, -SCF is valid only for -REP_SELECT and JOINT, and -LEN_FACTOR -MAX_CHD_LEN and -CHD_FIXITY are available only for JNT_PROP. The -MIN_SCF option defines a minimum value for SCFs. Any computed SCF smaller than MIN_SCF will be replaced with MIN_SCF. The default value form MIN_SCF is zero. If a stress concentration factor was not defined for a beam, then one will be automatically computed based on the joint classification. This computation is controlled by the -SCF option. The valid values for SCF_TYPE are K&S, API, MARSHALL, and EFTHYMIOU. With the automatic computation of SCFs, two values are computed: one for the root of the weld on the brace side, and one for the root of the weld on the chord side. The two additional parameters FRACHOL and MAXCHOL are used with SCF_TYPEs other than API or MARSHALL. With these computations, the SCFs depend upon the length of the chord. FRACHOL is a fraction of the actual chord length which will be used. The default value of FRACHOL is one. MAXCHOL is the maximum length which will be used and its default value is "infinite". Suppose that the chord length was 200. If FRACHOL is set to .5 and MAXCHOL is 50, the program will first set the length to be used to .5*200. It will then take the maximum of 100 and 50 and use this value in the computation. Notice that the values of FRACHOL and MAXCHOL can also be defined with the JNT_PROP command, but different options are used. The -CHD_FIXITY, option can only be used with the JNT_PROP command. And it defines the "chord flexibility" CHD_FIX. This is a measure of the bending support at the ends of the chord. It should be a number between .5 and 1. The first of these corresponds to the chord ends being pinned and the later fixed. By default, a value of .7 will be used.