# Norsok N004 (Rev. 3, 2013)

Norsok N004 (Rev. 3, February 2013) Standard - specifies structural strength and stability requirements for steel tubular members. This standard is applied to the cylindrical members with thickness ≥ 6mm, ratio D / t < 120 and Yield < 500 MPa.

Standard supports calculations for circular tubes shapes only.

To add Norsok standard execute

- from the main menu. Alternatively, use the context menu:It is possible to edit Safety Factors and Selection.

Safety Factors are set to the Standard default values. It is not necessary to change them. Material Safety Factor is calculated according to the section 6.3.7 by default but it is possible to set a constant value.

According to the calculation procedure, Beam Length for Y and Z direction is required. Data from Beam Member Finders is used automatically:

If beam members are not recognized press .

By default, the selection is set to Circular Tube Shapes. If not all circular members should be checked, then the selection can be modified.

Formulas of Norsok N004 standard use Yield Stress from the materials. It is important to set this value for each material. Standard check if all Yield Stress is not equal to zero:

If count > 0, then press to edit Yield Stress and Tensile Strength:

When all settings are defined press

to create the standard.The Standard contains the following checks: 1 and 2 geometrical pipe dimensions and limits, 3 - Material Factor (if material factor is calculated according to the section 6.3.7), 4-7 - forces checks for different directions including combine check, 8 - overall check (results from check 4-7 and maximum over them), 9 - Joint Check.

The Result of Overall check is Utilization Factor = Force / Allowable Force If Utilization Factor < 1, the check is OK.

## Joint Check

Joint Check Norsok is a part of Norsok N004 (Rev.3, Feb 2013) standard. To create new Joint check perform

- - from the context menu:Maximum distance from joint nodes of one connection on the chord - include joints that are formed by multiple nodes:

It is possible to set Custom distance or use distance that equals D/4 where D - chord diameter so that each connection has its own distance to check.

Angle between braces treated as in one plane - defines allowable angle between braces of one connection that are located in different planes:

Chord maximum curvature angle - Not always chord elements form a straight line. To include inclination in a chord default angle = 3 degree is used.

Force Tolerance - a type of brace depends on axial forces of members of connection. This option defines allowable tolerance for the total force that is used in calculations of brace type.

Calculate all braces as TY - Joint type is based on loading. This option will ignore loading and set all joint types for a brace to TY (100%).

Set

and press to display connection in the table.Joint check is based on connections. Each connection is a set of elements near joint node (see Joints Finder). Connections consists of Chord and braces. Brace contains only 1 element with ID defined in brackets (#). Connection can contain braces from the both sides of the chord. Information is displayed in second brackets: U- upper braces; L- lower braces;

Press to find all connections of the model. Window with connections that are recommended to be checked manually will be displayed:

Connection has to be checked when:

- D1 = D2 = d3 = d4, T1 = T2 = t3 = t4

- D1 = D2 = d4, T1 = T2 = t4, d3 > D1

- D1 = D2 = d3 = d4, T1 = T2 = t4, t3 > T1 according to the following image

- Add new connection manually:

- edit selected connection;

- preview highlight selected connection;

- preview only selected connection;

- remove selected connection;

- plot all connections values in colors + Labels with IDs;

- show information about all connections;

- hide information of all connections.

#### Edit Connection

Pick Chord elements and braces (each brace consist of 1 element) and press to preview connection properties.

- swap chord and brace selections

Press to preview and highlight connection in Femap.

Set

- open window to set the overlapping brace. Is enabled only if one brace overlaps other one in connection:- set to the selected braces if they are overlapped.

Press to open check table for calculations:

__Show results:__

Utilization factors only - axial and bending capacities, combined load, overlapping and can capacities;

Intermediate Results - geometrical factors, brace forces, allowable stresses + utilization factors;

All results - Intermediate results + calculation factors (chord/brace dimensions, fiber stresses, effective length, Qa, overlapping percentage, etc.)

__Display Parameters in:__

Rows - display parameters in rows and connections info in column headers.

Columns - display parameters in column headers and connections information in rows.

Filter and sort results using Filter / Sort By control.

__Table build type:__

Parameters in rows - each parameter values are in one row for all connections;

Parameters in columns - each parameter values are in one column for all connections.

Pick Load, define selection to be included in the table, and press

to display the results.- Plot Axial force and Projected force of all braces of selected connections:

- Plot gaps of all braces of selected connections:

- Plot braces types of selected connections:

Press to open criteria window:

Set plot options, define seleciton and and press to preview results:

Press to display the plot.

display plot with all the settings;

- display plot with current view settings (ignore view from plot options);

Note: The results are displayed only for braces.

__Show:__

Load Results - display results for each selected load per brace.

Min/Max - display only minimum and maximum results among the loads for each brace.

Load Results and Min/Max - combines two previous options.

Skip rows without maximum - do not display loads that do not contain any extreme results among all parameters.

Connections are taken from selection (one element is enough to include connection).

Press

to display the results.- clear all calculation for loads;

- set if use can calculations (section 6.4.3.5) and effective length:

Select

and press to set if formula 6.56 will be applied to all selected braces.Fill

and press to set the value to all salected braces.Note: is applied only to the braces that Use Can Calculations.

Gaps depend on the Load and are calculated only between the braces with different axial forces (tension and compression). Otherwise the gap = 0. According to the standard, brace types are defined by the following rules:

According to the standard, joint types are defined by following rules:

Joint type is based on type of loading. By checking if forces of connection are balanced joint types are classified on K, TY, and X (Cross).

K - tension and compression loads are balanced.

TY - tension or compression load goes as a shear force in a chord.

X (Cross) - Connection has to contain braces from the both sides to check on a cross joint. If balanced forces of all braces on one side and balanced forces of all braces of other side are equal then all braces are classified as X (Cross).

Interpolation - the order of joint type recognition is following: K -> X (Cross) -> TY. Each brace can have all 3 types of the joint type taken as a percentage of the axial load of a brace to a summation of all braces loads.

Note: If brace has 0 force joint type for such brace is set to TY.

Mathematical background

All calculations are performed in accordance to Norsok N004 (Rev.3, Feb 2013).

Nomenclature and geometric parameters that are used in results:

The validity ranges of connection parameters:

0.2 ≤ β ≤ 1.0

10 ≤ γ ≤ 50

30° ≤ θ ≤ 90°

g/D ≥ -0.6 (for K joints)

*f*_{y} ≤ 500N/mm^{2}

*f*_{y} - chord allowable static stress = Min(yield stress, tensile strength * 0.8).

For each brace connected to the chord, connection elements of the chord are taken into account. The minimum allowable stress is taken if elements are of different materials.

Note: If material yield stress > 500 Mpa - allowable static stress is taken equal to material yield stress.

**Gap calculations.** . The gap is a minimum distance between two differently loaded braces (tension and compression) measured on a shell of the chord.

Note: Overlapping braces can contain negative gap if they are loaded in a different way.

Note: It is possible that brace can have two or more gaps to considerate. a percentage of each gap is calculated using the forces of the connection.

**Basic Joint Strength.**

Joint axial and bending capacities shall satisfy the following equations:

where

N_{Rd} - the joint design axial resistance;

M_{Rd} - the joint design bending moment resistance;

f_{y} - the yield strength of the chord member at the joint;

ϒ_{M} = 1.15;

T - the chord wall thickness at the intersection with the brace;

d - the brace outside diameter;

θ - included angle between the brace and the chord;

Q_{u} - the strength factor;

Q_{f} - the chord force factor;

**Strength factor Qu**

Q_{u} is the strength factor which varies with the joint and the action type:

where

f_{y,b} - the yield strength of brace;

f_{y,c} - the yield strength of chord;

t - the brace wall thickness;

For -0.05 ≤ ^{g}/_{D} < 0.05, the gap factor Q_{g} is calculated as linear interpolation between the limiting values of the above expressions:

Note: g - the total gap of the brace. From the following picture g = 0.023076 * 0.1362 + 0.196152 * 0.8638 = 0.172579;

__Chord force factor Q _{f}__

σ_{a,Sd} - the design axial stress in chord, positive in tension;

σ_{my,Sd} - the design in-plane bending stress in chord, positive for compression;

f_{y} - the - yield strength;

C_{1},C_{2},C_{3} - the coefficients depending on the joint and the load type:

The average of the chord loads and bending moments on either side of the brace intersection should be used in the formulas (6.54), (6.55). The chord thickness at the joint should be used in the above calculations.

Q_{f total} = Q_{fK} * brace percentage K + Q_{fTY} + brace percentage TY + Q_{fX} * brace percentage X -for the axial loading;

Extra joint axial capacity calculations are performed to the connections that contain increased thickness of the chord. The axial strength is calculated by following formula:

where

N_{can,RD} - N_{RD} from the equation (6.52) based on the chord can geometric and material properties including Q_f calculated with respect to the chord can;

T_{n} - the nominal chord member thickness;

T_{c} - the chord can thickness;

Note: r cannot be taken greater than 1;

Effective length L is calculated for each brace separately. It is a minimum distance from the end of can till the point of intersection of chord and brace multiplied on 2.

T_{c} ≥ T nominal;

L_{1},L_{2} ≤ 1.25 * D. If L_{1} and L_{2} and L_{2} exceed 1.25 * D distance, can will not be recognized;

D - the can diameter;

L = 2 * L_{1} - the effective length for the left brace;

L = 2 * L_{3} = 2 * L_{4} - the effective length for the middle brace;

L = 2 * L_{2} - the effective length for the right brace.

Note: This section is applied to the connections with cans. If the brace L_{c} 0 section is not applied. If the brace is overlapping the section is not applied.

**Strength check**

Joint resistance shall satisfy following interaction equation for axial force and/or bending moments in the brace:

for all joints, except those, that identified as non-critical

where

N_{Sd} - the design axial force in the brace member;

N_{Rd} - the joint design axial resistance;

M_{y,Sd} - the - design in-plane bending moment in the brace member;

M_{z,Sd} - the design out-of-plane bending moment in the brace member;

M_{y,Rd} - the design in-plane bending resistance;

M_{z,Rd} - the design out-of-plane bending resistance;

**Overlapping joints**

The strength of the joints that have in-plane overlap involving two or more braces may be determined using the requirements for simple joints defined in 14.3, with the following exceptions and additions.

a) Shearing of the brace parallel to the chord face is a potential failure mode and shall be checked.

Shear capacity = fy * effective area /√3 - 1.05)

Effective area is the total area of two braces that overlap:

Area1 = 2 * p_{1}* t_{1} - the area of the through brace;

Area2 = 2*(p_{2}-q) * t_{2} - the area of the overlapping brace;

where

t_{1} - the thickness of the through brace;

t_{2} - the thickness of the overlapping brace;

p_{1} = d_{1}/sin(θ_{1});

p_{2} = d_{2}/sin(θ_{2});

d_{1}, d_{1} - the original diameter of the through and overlapping braces respectively;

θ_{1}, θ_{2} - the inclination of the through and overlapping braces respectively to the chord;

q - the overlapping distance (negative gap);

Applied shear force is taken as the summation of forces, perpendicular to the chord of the through and the overlapping braces;

Shear UC (Ultimate Capacity) is calculated as the relation of Applied shear force to shear capacity:

Shear UC = Applied shear force / Shear capacity

b) Section 6.4.3.5 (can calculations) does not apply to overlapping joints.

c) If axial forces in the overlapping and through braces have the same sign, the combined axial force representing that in the through brace plus a portion of the overlapping brace forces should be used to check the through brace intersection capacity. The portion of the overlapping brace force can be calculated as ratio of cross sectional area of the brace that bears onto the through brace to the full area.

Modified axial force= P_{d1}+P_{d2} * ov;

where

P_{d1} - the axial force of the through brace perpendicular to the chord.

P_{d2} - the axial force of the overlapping brace perpendicular to the chord.

ov - overlapping percentage,

ov=q/p*100%;

Modified axial UC= Modified axial force / N_{Rd};

N_{Rd} - the joint axial capacity from the formula (6.52);

d) For both in-plane or out-of-plane moments, the combined moments on the overlapping and through braces shall be used to check the through brace intersection capacity. This combined moment shall account for the sign of the moments.

M_{y,Sd1}, M_{y,Sd2}, M_{z,Sd1}, M_{z,Sd2} - the respective in-plane and out-of-plane bending moments of the through and overlapping brace;

Modified moment UC= (Modified ipb moment / M_{y,Rd})^{2}+Modified opb moment / M_{z,Rd}

Modified axial and moment UC=Modified axial UC+Modified moment UC

### Particular cases

Sometimes models cannot represent real situations. For such cases calculations are updated to get the worst possible result according to the parameters:

**Case1:**

Sometimes models cannot represent real situations. For such cases calculations are updated to get the worst possible result according to the parameters:

When Chord is formed by elements with different properties around the joint node and D1<>D2, D = min(D1, D2); T = min(T1, T2) are considered for calculations.

__Case 2:__

D1=D2, D1<D3. For such case D3 is recognized as chord as it has bigger diameter. Naturally pipe of bigger diameter cannot be welded to smaller. Such connections are recommended to be checked.

__Case 3:__

a)

D1 = D2 = D3 = D4;

T1 = T2 = T3; T4 >T4;

When all diameters of connection are equal, thicknesses are compared. Element with thickness = T4 is recognized as chord.

b) In case when:

D1 = D2 = D3 = D4;

T1 = T2 = T3 = T4;

When all elements of connection are of the same dimensions, chord is recognized as pair of elements that form straight line. If any pair that matches conditions is found, random element will be recognized as a chord.