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D12.A.7 Tubular Joint Check, NPD 3.5

For pipe members, punching shear capacity is checked in accordance with the NPD sections 3.5.1 to 3.5.2, except The chord is defined as the member with the greater diameter in the joint. If the diameters are the same the program selects the member with the greater thickness of the two. The chord members must be collinear by 5 degrees.

The punching shear run sequence is performed in two steps. The program will first identify all tubular joints and classify them as T type joints (TRACK99). The joints to be checked will be listed in a file specified in the CODE NPD parameter list, below called GEOM1. This file is used as input in the second run. The file is an editable ACSII file saved under the file name given in the CODE NPD parameter. The TRACK parameter is then set to 98 which directs the program to read from the file GEOM1 file and use it as input to the second run, i.e., the joint capacity checking. The program will check the capacity for both chord members entering the joint. The local y and z moments will be transformed into the plane defined by the joint itself and the far end joints of the brace and chord, defined as in- and out-of plane moments.

The ASCII file should be edited to reflect the correct classification of the joints, gap, can or stub dimensions, yield stress and other geometric options if required. The program will not change the brace or chord definition if this is changed or modified in the input file GEOM1. See Appendix A page xx for GEOM1 example file.

Joint classification parameters in the file GEOM1 are:

  • KO K joint overlapped
  • KG K joint with gap
  • TY T or Y joint
  • X X joint

Input example for the classification run.


Static strength of tubular joints

The basic consideration is the chord strength. The required chord wall thickness shall be

determined when the other dimensions are given.

The following symbols are used:

  • T = Cord wall thickness
  • t = Brace wall thickness
  • R = Outer radius of chord
  • r = Outer radius of brace
  • Θ = Angel between chord and considered brace
  • D = Outer diameter of chord
  • d = Outer diameter of brace
  • a = Gap (clear distance) between considered brace and nearest load-carrying brace measured along chord outer surface
  • ß = r/R
  • g = R/T
  • g = a/D
  • fy = Yield stress
  • Qf = Factor
  • Qg = See table 6.1
  • Qu = See table 6.1
  • Qßd = See table 6.1
  • N = Design axial force in brace
  • MIP = Design in-plane bending moment in brace
  • MOP = Design out-of plane bending moment in brace
  • Nk = Characteristic axial load capacity of brace (as governed by the chord strength)
  • MOPk = Characteristic out-of-plane bending moment capacity of brace (as governed by the chord strength)
  • σax = Design axial stress in chord
  • σIP = Design in-plane bending stress in chord
  • σOP = Design out-of-plane bending stress in chord

This section gives design formulae for simple tubular joints without overlap and without gussets, diaphragms or stiffeners. Tubular joints in a space frame structure shall satisfy:

N N k / γ m


N k = Q u Q f f y T 2 sin Θ

Qu is given in Table 6.1 and Qf is a factor to account for the nominal longitudinal stress in the chord.

Qf = 1.0 - 0.03γA2

A 2 = σ a x 2 + σ I P 2 + σ O P 2 0.64 f y 2

Table 1. Values for Qu
Type of joint and geometry Type of load in brace member
  Axial In-plane bending Out-of-plane bending
T and Y 2.5 + 19β 5.0√(γ)β 3.2/(1-0.81β)
X (2.7 + 13β)Qβ    
K 0.90(2+21β)Qβ    

For β > 0.6, Qβ = 0.3/[β(1 - 0.833β)]

For β ≤ 0.6, Qβ = 1.0

For γ ≤ 20, Qg = 1.8 -

For γ > 20, Qg = 1.8 - 4g

but in no case shall Qg be taken as less than 1.0.

When β ≥ 0.9, Qf is set to 1.0. This is also applicable for moment loading. For cases with tension in the chord, Qf is set to 1.0. This is also applicable for moment loading.

The brace end moments shall be accounted for in the following cases:

  1. Out-of-plane bending moment when β > 0.85
  2. When the brace acts as a cantilever
  3. When the rotational stiffness of the connection is considered in the determination of effective buckling length, and / or the structural coefficient γmk = 1.00 for the beam-column design of the brace or chord. See Section 3.1.3.

The characteristic capacity of the brace subjected to in-plane bending moment shall be determined by:

M I P k = Q u Q f d f y T 2 sin Θ

Where Qu is given in Table 6.1 and

Qf = 1.0 - 0.045γA2

The characteristic capacity of the brace subjected to out-of-plane bending moment shall be determined by:

M O P k = Q u Q f d f y T 2 sin Θ

Where Qu is given in Table 6.1 and

Qf = 1.0 - 0.021γA2

For combined axial and bending loads in the brace, the following interaction equation should be satisfied:

N N k + ( M I P M I P k ) 2 + M O P M O P k 1 γ m

For overlapping tubular joints without gussets, diaphragms, or stiffeners, the total load component normal to the chord, NN, shall not exceed

N N = N k γ m l 1 l sin Θ + 2 f y t w l 2 3 γ m

where (see NPD fig. 3.10)

  • ll = circumference for that portion of the brace in contact with the chord (actual length)
  • l = circumference of brace contact with chord, neglecting presence of overlap
  • Nk = characteristic axial load capacity of brace
  • tw = the lesser of the throat thickness of the overlapping weld or the thickness t of the thinner brace
  • l2 = length as shown in NPD fig. 3.10

The above formula for the capacity of overlapping joints is valid only for K joints, where compression in a brace is essentially balanced by tension in brace(s) in the same side of the joint.