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ISO 19902 (first edition, published 12 DEC 2007) Standard checks the structural strength and stability of tubular members. This standard is applied to cylindrical members with thickness > =6mm, ratio D / t < 120 and Yield < 500 MPa.

http://www.iso.org/iso/catalogue_detail.htm?csnumber=27507

Standard support calculations for 2 sections: Standard Circular Tube and Nastran Tube.

To add ISO 19902 standard execute Standard - ISO 19902 from the main menu. Alternatively, use Standard context menu:

It is possible to edit Safety Factors and Selection.

Safety Factors are set to Standard default values. It is no necessary to change them.

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

By default selection is set to 2 sections: Standard Circular Tube and Nastran Tube. If not all circular members should be checked then selection can be modified.

It is possible to create combined standard using ISO 19902 and AISC 360-10. Circular tubes will be checked by ISO 19902 and following shapes: I, C, T, L and rectangular tubes by AISC 360-10. To create combined standard check option Use AISC 360-10 for non-tubular shapes and press Define to fill settings for AISC 360-10 standard.

Options explanation is in AISC 360-10 chapter.

Unit system is used for conversion units in check calculations. By default MKS (Meter/Kilogram/Second) system is used.

In validation of requirements units conversion is used:

Press to change the unit system.

Formulas of ISO 19902 standard use Yield Stress from 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:

Create Tables and Plots for Loads - creates standard tables and plots for Overall Check for selected Loads.

And also in calculation of Allowable Bending Stress:

Press to change the unit system.

When all settings are defined press OK to create the standard.

Standard Contain 7 checks: 1 and 2 geometrical pipe dimensions and limits, 3-6 - stress checks for different direction including combine check, 7 - overall check (results from check 3-6 and maximum over them) and Joint Check.

The result of Overall check is Utilization Factor = Stress / Allowable Stress. If Utilization Factor < 1 check is OK.

Joint Check ISO is a part of ISO 19902 (1st, Dec 2007) standard but is available for all standard types. To create new Joint check perform Checks - Add - Joint Check ISO in standard:

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 Connection ID and press Navigate 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.

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 Braces Overlapping - open window to set the overlapping brace. Is enabled only if one brace overlaps other one in connection:

Set Overlapped (Yes) / (No) - set to the selected braces if they are overlapped.

Press to open check table for calculations:

__Information type:__

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.)

__Table Settings:__

Show only joints that fail (value >) - set maximum value of failure;

Parameter that fails - select a parameter to compare with failure value. It is enabled if the option above is turned on.

Sort by parameter - Sort connections by ascending absolute value of the selected parameter.

__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, select Connections to be included in the table, set number format and press Fill Table 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, select connections and press to preview results:

Note: Results are displayed only for braces. Chord values are set to 0.

Press to open an expand flow table for check:

__Display Options:__

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

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

Show 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.

Select Connections and press Fill Table to display the results.

- clear all calculation for loads;

- set if brace is critical, Ub (1 by default) and partial resistance coefficient (1.17 by default) to critical braces:

Select Is Critical and press Apply to set if all selected braces are critical or non-critical.

Fill Ub and press Apply to set Ub parameter to all selected braces.

Fill Resistance factor and press Apply to set factor to all selected braces.

Note: Ub and Resistance factor are applied only to critical braces.

- set if use can calculations (section 14.3.5) and effective length:

Select Use Can calculations and press Apply to set if formula 14.3-11 will be applied to all selected braces.

Fill Effective Length and press Apply to set the value to all selected braces.

Note: Effective Length is applied only to braces that Use Can calculations.

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

According to 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 ISO 19902 - First Edition 2007-12-01.

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°

τ ≤ 1.0

*f*_{y} ≤ 500N/mm^{2}

*gT* > -1.2γ

*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. 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. Percentage of each gap is calculated using forces of the connection.

__Basic Joint Strength.__

Joint axial and bending capacities shall satisfy following equations:

where

P_{d} - is the design value of the joint axial strength, in force units;

M_{d} - is the design value of the joint bending moment strength, in moment units;

Υ_{Rj} - is the partial resistance factor for tubular joints, Upsilon;_{Rj} = 1,05 ;

Strengths for simple tubular joints are calculated by following formulas:

where

P_{uj} - representative joint axial strength, in force units;

M_{uj} - representative joint bending model strength, in moment units;

f_{y} - representative yield strength of the chord member at the joint (SMYS or 0,8 of the tensile strength, if less), in stress units;

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

d - brace outside diameter;

θ - included angle between brace and chord;

Q_{u} - strength factor;

Q_{f} - chord force factor;

__Strength factor Qu__

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

where

f_{y,b} - representative yield strength of the brace at the intersection of the chord, in stress units;

t - brace wall thickness at the intersection of the chord;

For -2.0 < ^{g}/_{T} < 2.0, gap factor Q_{g} is calculated as linear interpolation between formulas 14.3-7 and 14.3-8:

Note: g - 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}__

where

λ = 0,03 for brace axial stress

= 0,045 for brace in-plane bending stress

= 0,021 for brace out-of-plane bending stress

q_{A} parameter is calculated by the following formula:

where

P_{c} - axial force in the chord member from factored actions;

M_{c} - bending moment in the chord member from factored actions;

P_{y} - representative axial strength due to yielding of the chord member not taking account of buckling, in force units:

P_{y} = A*f_{y}

f_{y} - representative yield strength of the chord member, in stress units;

A - cross-sectional area of the chord or chord can at the brace intersection;

M_{p} - representative plastic moment strength of the chord member;

M_{p} = W_{e}* f_{y},

- plastic modulus;

D - chord member diameter;

T - chord member thickness;

ϒ_{Rq} - partial resistance factor for yield strength, ϒ_{Rq} = 1,05;

ipb - in-plane bending;

pb - out-of-plane bending;

C_{1} and C_{2} coefficients are taken from the table:

q_{A} parameter is calculated for each chord member from the left and right side of intersection of a respective brace. Furthermore, axial forces are calculated for each brace type separately:

Maximum of left and right side chord members are taken into account:

Q_{faxial} = Q_{faxial}(K joint)* brace percentage(K joint) + Q_{faxial}(Y joint)* brace percentage (Y joint) + Q_{faxial}(Cross joint)* brace percentage (Cross joint)

Q_{fipb} = 1- 0.045 * q^{2}_{A}(bending)

Q_{fopb} = 1- 0.021 * q^{2}_{A}(bending)

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

where

P_{uj}- joint axial strength, in force units;

P_{uj,c} - is the value P_{uj,c} from Equation (14.3-1), based on the chord geometrical and material properties, including Q_{f} calculated from chord can properties and dimensions;

Note: r cannot be taken greater than 1;

L_{c} - effective total length;

Y_{n} - the lesser chord member thickness on either side of the joint;

T_{c} - chord can thickness;

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 - can diameter;

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

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

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

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

__Strength check__

Each brace in a joint that is subjected to an axial force or a bending moment alone, or to an axial force combined with bending moments, shall be designed to satisfy the following conditions:

for all joints except those identified as non-critical

where

U_{j} - joint utilization;

P_{B} - axial force in the brace member from factored actions;

M_{B} - bending moment in the brace member from factored actions;

P_{D} - design value of the joint axial strength (see 14.3.2);

M_{D} - design value of the joint bending moment strength (see 14.3.2);

ipb - in-plane bending;

opb - out-of-plane bending;

U_{b} - brace utilization. Can be set manually to critical joints. Default value is

Υ_{zj} - extra partial resistance factor. Can be set manually to critical joints. Default value is 1.17;

__Overlapping joints__

The strength of 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.

b) Section 14.3.5 (can calculations) does not apply to overlapping joints.

Shear capacity = f_{y} * effective area /√3 - 1.05)

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

Area_{1} = 2 * p_{1}* t_{1} - area of the through brace;
Area_{2} = 2*(p_{2}-q) * t_{2} - area of the overlapping brace;

where

t_{1} - thickness of the through brace;

t_{2} - thickness of the overlapping brace;

p_{1} = d_{1}/sin(θ_{1});

p_{2} = d_{2}/sin(θ_{2});

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

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

q - overlapping distance (negative gap);

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

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

Shear UC = Applied shear force / Shear capacity

c) If axial forces in the overlapping and through braces have the same sign (both in compression or both in tension), the check of the intersection strength of the through brace on the chord shall use the combined axial force representing the force in the through brace plus the portion of the overlapping brace force(s). The portion of the overlapping brace force may be calculated from the ratio of the cross-sectional area of the brace that bears onto the through brace to the full area of the overlapping brace.

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 / P_{uj};

P_{uj} - joint axial capacity from formula (14.3-1);

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 intersection strength of the through brace on the chord. This combined moment shall account for the sign of the moments.

M_{d1}, M_{d2} - respective in-plane and out-of-plane bending moments of the through and overlapping brace;

Modified moment UC= (Modified ipb moment / M_{uj}(ipb))^{2}+Modified opb moment / M_{uj} (opb)

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

Note: Part e) and f) are not implemented.

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

__Case1:__

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