10-L.1              Scope

Steel structures checking according to ANSI/AISC N690-06 Specification for Safety-Related Steel Structures for Nuclear Facilities (September 20, 2006) in CivilFEM includes the checking of structures composed of welded or rolled shapes under axial forces, shear forces and bending moments in 3D.

In this version of the code, the specification committee unites both the ASD and LRFD methods, permitting the designer to choose between one or the other.

Both methods are based on the required strength of each structural member; depending on the method, this required strength is compared to a design strength or an allowable strength.

The calculations made by CivilFEM according to the following sections:

NB	Design requirements
ND	Design of members for tension.
NE	Design of members for compression.
NF	Design of members for flexure.
NG	Design of members for shear.
NH	Design of members for combined forces and torsion.

 

10-L.2              Checking Types

With CivilFEM it is possible to accomplish the following checking and analysis types:

·         Checking of sections subjected to:

Tension	ND
Flexure	NF
Shear Force	NG
Flexure and axial force	NH1
Bending plus axial force, shear & torsion	NH3.3

 

 

·         Buckling check:

Compression members subjected to flexure	NE3,NE7
Compression members subjected to flexure and torsion	NE4,NE7

 

10-L.3              Valid Element Types

The valid element types supported by CivilFEM are the following 2D and 3D ANSYS link and beam elements:

2D Link	LINK1
3D Link	LINK8
3D Link	LINK10
2D Beam	BEAM3
3D Beam	BEAM4
3D Tapered Unsymmetrical Beam	BEAM44
2D Tapered Elastic Unsymmetrical Beam	BEAM54
2D Plastic Beam	BEAM23
3D Thin-walled Beam	BEAM24
3D Elastic Straight Pipe	PIPE16
3D Plastic Straight Pipe	PIPE20
3D Finite Linear Strain Beam	BEAM188
3D Quadratic Linear Strain Beam	BEAM189

 

Moreover, it is possible to check solid sections captured from 2D or 3D models with a transversal cross section classified as “structural steel”.

 

10-L.4              Valid Cross-Section Types

The valid steel cross-sections checked by CivilFEM are as follows:

• All rolled shapes (I shapes, U or channel shapes, etc.) included in the program libraries (see the hot rolled shapes library and ~SSECLIB command)
• The following welded beams: I shapes, U or channel shapes, T shapes, box, equal and unequal legs angles and pipes. (~SSECDMS commands). These sections are considered as a generic shape.
• Structural steel sections defined by plates (command ~SSECPLT). These sections are considered as a generic shape.
• Shapes from solid sections captured from 2D or 3D models which transverse cross section is classified as “structural steel” (command ~SLDSEC).

 

The cross-sections considered in the N690-06 code depend on the type of checking:

 

 

10-L.5              Data and Results used by CivilFEM

CivilFEM works with the following groups of data and results for checking according to N690-06:

·         Data pertaining to sections: properties and dimensions of gross, net and effective sections, characteristics and dimensions of section plates.

·         Member properties.

·         Material properties.

·         Forces and moments over the sections.

·         Checking results.

10-L.5.1   Sections Data

N690-06 considers the following data set for the section:

·         Gross section data

·         Net section data

·         Effective section data

·         Data concerning the section and plates class.

Gross section data correspond to the nominal properties of the cross-section.

From the net section data, only the area is considered. This area is calculated by subtracting the holes for screws, rivets and other holes from the gross section area. The user should be aware that N690-06 indicates the diameter used to calculate the parameter AHOLES is greater than the real diameter (the total calculated area is introduced as the parameter AHOLES with the command ~SECMDF).

Effective section data and section and plates class data are obtained in the checking process according to chapter NB4 of the code. This chapter, classifies steel sections into three groups, compact, noncompacts and slender, depending upon the width-thickness ratio and some mandatory limits.

The N690-06 module takes the gross section data in user units and CivilFEM axis or section axis as initial data. The program calculates the effective section data and the class data, and stores them in CivilFEM’s results file, in user units and in CivilFEM or section axis. All these data can be listed and plotted with the ~PLLSSTL and ~PRSTL commands.

The section data used in N690-06 are shown in the following tables:

 

 

Table 10-L.5‑1 Common data for gross, net and effective sections

 

Table 10-L.5‑2 Gross section data

 

Table 10-L.5‑3 Net section data

* The section holes are introduced as a property at member level

 

The effective section depends upon the geometry of the section; thus, the effective section is calculated for each element and for each end of the element.

 

Table 10-L.5‑4 Net section data

 

Table 10-L.5‑5 Data referred to the section plates

 

10-L.5.2   Member Properties

For N690-06 the data set checked at member level is shown in the following table. The data is stored with the section data in user units and in the CivilFEM reference axis. (Parameters L, KXY, KXZ, KZ, CB, LB of ~MEMBPRO command).

 

Table 10-L.5‑6 Member Properties

 

10-L.5.3   Material Properties

For N690-06 checking, the following material properties are used:

 

Table 10-L.5‑7 Material properties

*th =thickness of plate

 

10-L.6              Checking Process

Steps necessary to conduct the different checks in CivilFEM are as follows:     

a)    Obtain material properties corresponding to the element, stored in the CivilFEM database and calculate the rest of the properties needed for checking:
Properties obtained from CivilFEM database: (command ~CFMP)

Modulus of Elasticity	E
Poisson’s ratio	n
Yield strength	Fy (th)
Ultimate strength	Fu (th)
Shear modulus	G
Thickness of corresponding plate	th

b)    Obtain the cross-section data corresponding to the element.

c)    Initiate the values of the plate reduction factors and other plate parameters to determine the section class.

d)    Perform a check of the section according to the type of external load.

e)    Results. In CivilFEM, checking results for each element end are grouped into alternatives in the results file .RCV, so that the user may access them by indicating the number of the alternative with the CivilFEM command ~CFSET.

The necessary data for each type of checking can be found in tables included in the corresponding sections in this manual.

10-L.6.1   Design Requirements.

10-L.6.1.1             Design for Strength Using Load and Resistance Factor Design (LRFD)

Design shall be performed in accordance with:

 

Where:

Ru	Required strength (LRFD).
Rn	Nominal strength.
Ф	Resistance factor.
ФRn	Design strength

 

10-L.6.1.2             Design for Strength Using Allowable Strength Design (ASD)

 

Design shall be performed in accordance with:

Where:

	Required strength (ASD)
	Nominal strength.
	Safety factor
	Allowable strength

 

10-L.6.2   General Processing of Sections. Section Class and Reduction Factors Calculation.

Steel sections are classified as compact, noncompact or slender-element sections. For a section to qualify as compact, its flanges must be continuously connected to the web or webs and the width-thickness ratios of its compression elements must not exceed the limiting width-thickness ratios l_p (see table NB4.1 of N690-06 Specification). If the width-thickness ratio of one or more compression elements exceeds l_p, but does not exceed l_r, the section will be noncompact. If the width-thickness ratio of any element exceeds l_r, (see table NB4.1 of N690-06 Specification), the section is referred to as a slender-element compression section.

Therefore, the code suggests different lambda values depending if the element is subjected to compression, flexure or compression plus flexure.

The section classification is the worst-case scenario of all the plates. Therefore, the class is calculated for each plate with the exception of pipe sections, which have their own calculation because their section cannot be decomposed into plates. This classification will take into account the following parameters:

a) Length of elements:

The program will define the element length (b or h) as the length of the plate (distance between the extreme points), except when otherwise specified.

b) Flange or web distinction:

To distinguish between flanges or webs, the program follows the criteria below:

Once the principal axis of bending is defined, the program will examine the section’s plates. Fields Pty and Ptz of plates indicate if they behave as flanges, webs or undefined, choosing the correct one for each axis. If undefined, the following criterion will be used to classify the plate as flange or web: if |Dy|<|Dz| (increments of end coordinates) and flexure is in the Y axis, it will be considered as a web; if not, it will be a flange. The reverse will hold true for flexure in the Z-axis.

·    Hot rolled steel shapes:

Section I and C:

The length of the plate h will be taken as the value d of the section dimensions.

Section Box:

The length of the plate will be taken as the width length minus three times the thickness.

 

10-L.6.2.1             Members Subjected to Compression

In order to check under compression, it is necessary to determine if the particular element is stiffened or unstiffened.

- For stiffened elements:

Plates with both ends fixed:      

Pipe: = 0.11

Box:               

 

- Unstiffened elements:

Plates not fixed at both ends (free-fixed or free-free):

                    

Angles:

Stem of tees:

 

10-L.6.2.2             Members Subjected to Bending

The bending check is only applicable to very specific sections. Therefore the slenderness factor is indicated for each section:

I-shaped sections and channels:

Rolled flanges:                                

Built-up flanges:                  

 

Webs:                                  

Pipe:                            

Box:                            

Tees:                        (stem)    (flanges)

 

10-L.6.3   Checking of Members for Tension   (Chapter ND)

The axial tension force must be taken as positive (if the tension force has a negative value, the element will not be checked)

Design tensile strength   and the allowable tensile strength , of tension members, shall be the lower value of :

a)    yielding in the gross section:

  = 0.90 (LRFD)     = 1.67 (ASD)

 

b)    rupture in the net section:

  = 0.75 (LRFD)     = 2.00 (ASD)

 

Being:

	Effective net area.
	Gross area.
	Minimum yield stress.
	Minimum tensile strength.

 

The effective net area will be taken as A_g – AHOLES. The user will need to enter the correct value for AHOLES (the code indicates that the diameter is 1/16^th in. (2 mm) greater than the real diameter).

Output results are written in the CivilFEM results file (.RCV) as an alternative.

Depending on the code, results stored for compression flexural buckling checking will be:

LRFD

, Design tensile strength for yielding in the gross section

 _, Design tensile strength for fracture in the net section

Design tensile strength of tension members (minimum of two f_t P_n  values)

CRT_TOT (LRFD) = Required tensile strength / (two   values)

 

ASD

_, Allowable tensile strength for yielding in the gross section  

, Allowable tensile strength for fracture in the net section

Allowable tensile strength of tension members (minimum of two  values)

CRT_TOT (ASD) = Required tensile strength /  (minimum of two values)

 

 

The design compressive strength, , and the allowable compressive strength,  , are determined as follows:

The nominal compressive strength,  , shall be the lowest value obtained according to the limit states of flexural buckling, torsional buckling and flexural-torsional buckling.

 (LRFD)     (ASD)

 

10-L.6.3.1             Compressive Strength for Flexural Buckling (NE3)

This type of check can be carried out for compact, noncompact, or slender sections. These three cases adhere to the following steps:

Nominal compressive strength,  :

(NE3-1)

(a) for

 

(b) for

          

Where:

	Gross area of member.
R	Governing radius of gyration about the buckling axis.
K	Effective length factor.
L	Unbraced length.
	Elastic critical buckling stress
Q	Slender reduction factor.. For compact or noncompact sections

Factor Q for compact and noncompact sections is always 1. Nevertheless, for slender sections, the value of Q has a particular procedure. Such procedure is described below:

Factor Q for slender sections:

For unstiffened plates, Qs must be calculated, and for stiffened plates, Qa must be determined. If these cases do not apply (box sections or angular sections, for example), a value of 1 will be taken.

For circular sections, there is a particular procedure of calculating Q. Such procedure is described below:

·                    For circular sections, Q is:

·        

 

Factor Q_s (NE7.1)

If there are several plates free, the value of Qs will be the largest value of the plates. The program will check the slenderness of the section in the following order:

·                    Angular

If
If

·                    Stem of T

If
If

·                    Rolled shapes

If
If

·                    Other sections

If
If

 

Where:

 

Factor Q_a (NE7.2)

The calculation of factor Q_a is an iterative process. Its procedure is the following:

1)    An initial value of Q equal to  is taken.

2)    With this value,  is calculated.

3)    This  value is taken to calculate .

4)    For elements with stiffened plates, the effective width  is calculated.

5)    With , the effective area is calculated.

6)    With the value of the effective area,  is calculated, and the process starts again.

·                    For a box section

If

·                    For other sections

If

 

If it is not within those limits,

With the  values for each plate, the part that does not contribute is subtracted from the area (where t is the plate thickness). Using this procedure, the effective area is calculated.

 

Finally, Q is calculated from  and , and  is obtained.

Output results are written in the CivilFEM results file (.RCV) as an alternative.

Results stored for compression flexural buckling checking are:

, _, , ,   and the total criterion, depending on design strength (LRFD) or allowable strength (ASD):

  = 0.85 (LRFD)    = 1.75 (ASD)

TOTAL CRITERION (LRFD) = Required compression strength /

           

TOTAL CRITERION (ASD) = Required compression strength /

 

 

 

 

10-L.6.3.2             Compressive Strength for Flexural-Torsional Buckling (NE4)

This type of check can be carried out for compact, noncompact, or slender sections. These three cases adhere to the following steps:

Nominal compressive strength,  :

 =         (NE4-1)

 

(a) for                        

 

(b) for            

=0.877 

Where:

	Gross area of member.
R	Governing radius of gyration about the buckling axis.
K	Effective length factor.
L	Unbraced length.
	Elastic critical buckling stress, computed below.
Q	Slender reduction factor. . For compact or noncompact sections Q=1.0.

 

For compact and noncompact sections, factor Q is 1. Nevertheless, for slender sections, the Q factor has a particular procedure of calculation. Such procedure is equal to the one previously described.

The elastic stress for critical torsional buckling or flexural-torsional buckling F_e is calculated as the lowest root of the following third degree equation, in which the axis have been changed to adapt to CivilFEM normal axis:

   (NE4-6)

Where:

	Effective length factor for torsional buckling.
G	Shear modulus (MPa).
	Warping constant (mm6).
J	Torsional constant (mm4).
	Moments of inertia about the principal axis (mm4).
	Coordinates of shear center with respect to the center of gravity (mm).

where:

A	Cross-sectional area of member.
l	Unbraced length.
	Effective length factor, in the z and y directions.
	Radii of gyration about the principal axes.
	Polar radius of gyration about the shear center.

In this formula, CivilFEM principal axes are used. If the CivilFEM axes are the principal axes ±5º sexagesimal, Ky and Kz are calculated with respect to the Y and Z-axes of CivilFEM. If this is not the case (angular shapes, for example) axes U and V will be used as principal axes, with U as the axis with higher inertia.

The torsional inertia (Ixx in CivilFEM, J in N690-06 Specification) is calculated for CivilFEM sections, but not for captured sections. Therefore the user will have to introduce this parameter in the mechanical properties of CivilFEM.

Output results are written in the CivilFEM results file (.RCV) as an alternative.

and the total criterion, depending on design strength (LRFD) or allowable strength (ASD):

  = 0.85 (LRFD)    = 1.75 (ASD)

TOTAL CRITERION (LRFD) =  Required compression strength /

           

TOTAL CRITERION (ASD) =  Required compression strength / (

10-L.6.4   Checking of Members for Flexure     (Chapter NF)

Chapter F is only applicable to members subject to simple bending about one principal axis.

10-L.6.4.1             Flexure Check

The design flexural strength,  and the allowable flexural strength,  , shall be determined as follows:

For all provisions:   = 0.90 (LRFD)       = 1.67 (ASD)

Where M_n is the lowest value of four checks according to sections F2 through F12:

a)    Yielding

b)    Lateral-torsional buckling

c)    Flange local buckling

d)    Web local buckling

The value of the nominal flexural strength taking into account the following considerations:

• For compact sections, if Lb < Lp only yielding of steel will be checked.
• For T sections, and other compact sections, only yielding and torsional buckling will be checked.
• The case of lateral-torsional buckling does not apply to sections loaded in the minor axis of inertia, nor to box or square sections.
• The case of lateral-torsional buckling only applies for sections with double symmetry and for channel and T sections. Therefore the rest of the sections will be checked for torsion plus combined loads and will not be checked under flexure.

For non-compact sections, the code contemplates the following cases (chapters NF6 through NF9):

 

 

 

 

 

 

 

 

 

Where:

 

 (positive sign if the stem is under tension, negative if it is under compression)

In T sections:

 stem in tension;  stem in compression.

For slender webs (NF5) the nominal flexural strength  is the minimum of the following checks:

• tension-flange yield
• compression flange buckling

The first check uses the following equation:

where:

	Section modulus with respect to tension flange.
	Yield strength of tension flange.

 

The second check uses the following formula:

where:

Output results are written in the CivilFEM results file (.RCV) as an alternative.

Results stored for flexural checking are (depends on LRFD and ASD load combinations):

 

CHK_YLD	Checked yielding limit state.
CHK_LTB	Checked Lateral-Torsional Buckling limit state
CHK_FLB	Checked Flange Local Buckling limit state
CHK_WLB	Checked Web Local Bucking limit state
CRT_YLD	Yielding limit state criterion
CRT_LTB	Lateral-Torsional Buckling limit state criterion
CRT_FLB	Flange Local Buckling limit state criterion
CRT_WLB	Web Local Bucking limit state criterion
MU	Required flexural strength
MP	Plastic bending moment

 

  = 0.90 (LRFD)   = 1.67 (ASD)       

TOTAL CRITERION (LRFD) =  MU /

TOTAL CRITERION (ASD) =  MU /

 

 

10-L.6.5   Checking of Members for Shear       (Chapter NG)

The design shear strength, , and the allowable shear strength,, shall be determined as follows:

For all provisions:
  = 0.90 (LRFD)    = 1.67 (ASD)

 

The nominal shear strength,  , of stiffened webs, according to the limit states of shear yielding and shear buckling, is:

For webs of rolled I-shaped members with:

  = 1.00 (LRFD)    = 1.50 (ASD)

 = 1.0 (web shear coefficient)

For webs of all other doubly symmetric shapes, singly symmetric shapes and channels C_v is determined as follows:

1. For

2. For             

3. For    

 

Where  is the overall depth times the web thickness.

We assume that there are not stiffeners, therefore web plate buckling coefficient, K_v will be calculated as a constant equal to 5.0.

Output results are written in the CivilFEM results file (.RCV) as an alternative.

Results stored for shear checking are (depends on LRFD and ASD load combinations):

 

AW	Web area
VN	Nominal shear strength

  = 0.90 (LRFD)    = 1.67 (ASD)

TOTAL CRITERION (LRFD) =  Required shear strength /

TOTAL CRITERION (ASD) =  Required shear strength / (/ )

 

10-L.6.6   Checking of Members for Combined Forces and Torsion (Chapter NH)

10-L.6.6.1             Checking of Members Subject to Flexure and Axial Tension / Compression (NH1)

For this check, it is necessary to first determine the value of Mn. This value comes into play with the checking of formulas. The value of Mn will be calculated in the same way as for members subjected to flexure; that is, the nominal flexural strength (M_n) is the minimum of four checks:

1.    Yielding

2.    Lateral-torsional buckling

3.    Flange local buckling

4.    Web local buckling

With the case of bending plus tension or bending plus compression, the interaction between flexure and axial force is limited by the following equations:

(a)  For

    (NH1-1a)

(b)  For

        (NH1-1b)

If the axial force is tension:

	Required tensile strength
	Available tensile strength:  (LRFD) or  (ASD)
	Required flexural strength.
	Available flexural strength: Design:  (LRFD) or Allowable:  / (ASD)
y	Strong axis bending.
z	Weak axis bending.
	Resistance factor for tension  (ND2)
	Resistance factor for flexure = 0.90
	Safety factor for tension  (ND2)
	Safety factor for flexure = 1.67

 

If the axial force is compression:

	Required compressive strength.
	Available compressive strength: Design:     (LRFD) or Allowable:   (ASD)
	Required flexural strength .
	Available flexural strength: Design:  (LRFD) or Allowable:  /  (ASD)
Y	Strong axis of bending.
Z	Weak axis of bending.
	Resistance factor for compression =0.85
	Resistance factor for flexure = 0.90
	Safety factor for compression =1.75
	Safety factor for flexure = 1.67

The following checks are performed by CivilFEM:

• Axial force and flexural buckling
• Bending moment Z direction
• Bending moment Y direction

If one of these checks do not meet the code requirements, it will not be possible to check the member under flexure plus tension / compression.

Output results are written in the CivilFEM results file (.RCV) as an alternative.

The results are the total results from tension or compression and bending checking (for both axes).

 

10-L.6.6.2             Checking of Members Subjected to Torsion, Flexure, Shear and/or Axial Force (NH3)

The design torsional strength,  , and the allowable torsional strength,  , shall be the lowest value according to the limit states of yielding under normal stress, shear yielding under shear stress, or buckling:

  = 0.90 (LRFD)    = 1.67 (ASD)

 

·         For the limit state of yielding, under normal stress:

  (LRFD)

  (ASD)

·         For the limit state of yielding, under shear stress:

  (LRFD)

  (ASD)

·         For the limit state of buckling:

 or   (LRFD)

 or   (ASD)

 

Where Fcr is calculated with an equivalent procedure as:

_-           Elements subjected to compression with flexural buckling, for the case of .

-          Elements subjected to compression with lateral-torsional buckling, for the case of .

 

Output results are written in the CivilFEM results file (.RCV) as an alternative.