D1.J.8.1 Joint Checking
The program only checks simple joints and overlapping joints formed between circular hollow section members. Any other type of joint within the structure or joint cans will not be considered for API joint checks. Other types of joints (such as grouted joints, joints with ring stiffeners, etc.) are not considered.
The API code states in Cl. 4.2.1 that the value of yield stress of the chord member to be used in the calculation of the joint capacity should be limited to 0.8 times the tensile strength of the chord for materials with a yield stress less than or equal to 500 MPa.
The yield stress to be used in the joint capacity checks value is specified in the joint data file ( filename .PUN). For every joint, the value specified in the FYLD column will be used as the yield strength to be used for the joint capacity checks. When the file is created for the first time by the program, a default value of 36 ksi is used for all joints. The value used for each joint check will also be reported in the output file.
Minimum Joint Capacity
Clause 4.2.3 of the code specifies a minimum capacity for any joint as follows:
The connections at the ends of a member should develop the strength required by the design loads, but should not be less than 50% of the effective strength of the member. The effective strength is defined as the buckling load for a compression member or the yield load for members in tension. You, however, must ensure that this condition is satisfied even if the joint strength indicates a PASS status.
The program checks to see if the capacity of a joint as calculated by the methods in the code satisfies this requirement. If not the program issues a warning to that effect and marks the joint as FAILED. The program calculates the axial and/or bending moment capacities of the joint and reports the load/capacity ratio for each condition. The program also reports a "critical ratio" along with the condition that induces this ratio. Note that the maximum among the various individual ratios will be reported as the ‘critical ratio’. The program also reports a PASS/FAIL status for the joint.
Refer to D1.J.2.6 Simple Joints: Capacity Checks for details of capacity checks performed.
Clause of 4.2.4 of the API code essentially classifies a joint into one of the three basic types: K, X, and Y. Joint classification is the process whereby the axial load in a given brace is subdivided into its K, X, and Y components corresponding to the three joint types. A joint—as considered in the code—is the connection between a "chord" and a "brace" that are in the same plane. The program considers any two members to be in the same plane if they lie in planes that are within ±15 degrees of each other. The classification of a joint can also be a mixture of any of the basic types mentioned above. Once the classification of a joint has been identified, the capacity of that joint is then evaluated per Section 4.3 of the code.
The program automatically identifies the joints in a structure and identifies the chord and the brace members. The program applies the ±15° rule to determine the members in a plane and then determines the joint as being the intersection point of these members. Since a joint is between a chord and a brace member, the program considers two members at a time and then proceeds to identify the chord and the brace member at that joint. The program assumes the member with the larger diameter among the two members as the chord member and the other is considered as the brace. If both members have the same diameter, the chord is assumed to be the member with the thicker wall. If both the diameter and thickness of the members are identical, the program will assume the most horizontal member to be the chord. To be automatically considered as a chord member, the member has to be continuous across the joint. The user can always edit the joint data file (*.PUN) to add or delete new BRACE-CHORD joints.
The chord and brace member numbers (from the STAAD input file) are saved under the CHORD and BRACE columns in the filename .PUN file.
When the joint data file (.PUN) is created by the program, a default joint Class Y is assumed for the initial joint checks. This is indicated by the K, X, and Y column values being set to 0, 0, and 1 respectively. Since the API code allows for a mixed joint classification, you must manually vary the contribution factors for K, X, and Y joint classes for a given joint. For example, if a joint is to be 25% K, 25% X, and 50% Y, then you must assign K column value of 0.25, X column value of 0.25, and a Y column value of 0.50 for that joint. The program will verify that the supplied contributions sum to 1.0.
If the joint has a gap (i.e., a K-GAP joint), the gap distance (in inches) must be supplied in the GAP column. The value to be provided will be the actual gap between the brace members at the joint. An overlap can be specified by setting the gap to a negative value. The overlapping brace in this case can then be indicated by specifying the member number at the OBRACE (Overlapping brace) column in the data file.
Clause 4.4 of the API discusses overlapping joints. Checks for overlapping joints will be performed as described Section 22A.2.6. The difference will be in that the gap value, g, will be taken as negative in evaluating the various factors.
If the axial loads in the overlapping brace and the through brace have the same sign, the axial load in the through brace will be increased to allow for the loads in the overlapping brace. This will be achieved by allowing a portion of the overlapping brace load equal to the proportion of the overlapping brace area to be added to the axial load in the through brace.