Steel Design
Requires: Design Level
VisualAnalysis checks and designs hot rolled steel members (beams, columns, braces, etc.) according to specifications listed below.
- AISC 360-10 ASD & LRFD
- AISC 360-05 ASD & LRFD
- CSA S16-14 LRFD
Composite beams can be checked using the AISC specifications. Please note that you must use appropriate load combinations (Strength for LRFD, Allowable for ASD) to obtain design checks.
Steel Design Quick Links
- Assumptions and Limitations
- Capabilities
- Torsion Design
- Parameters
- Report Options
- Report Organization
- Report Notation
- Report Notes and Warnings
- References
Steel Assumptions and Limitations
Design Forces
All forces and moments are assumed to act about the principal axes of a member's cross-section. Similarly, shear loads are assumed to pass through the shear center so that no torsional moments will be present in the results. Axial loads are assumed to pass through the centroid of a member so that no moments are present in a first order analysis of an axially loaded member. It is up to you to watch for and check any second-order effects or other conditions that violate assumptions made in the analysis or design.
Steel 'h' Approximation
AISC defines h as "the clear distance between flanges less the fillet or corner radius". If a shape is not in the shape database or does not store the k dimension, then we assume h = D - 3*tf.
Span Length Assumptions
VisualAnalysis looks at a member-element's length to determine the span length which in turn affects unbraced length. For a Combined Member this is the total length of the member, regardless of internal nodes or connecting elements. For members designed according to the AISC specifications, this might also effect the determination of Cb. For members designed according to CSA specifications, this might effect the determination of w_{2} and k.
If your member span is longer than the element span, you may want to use the Combined Member feature in VisualAnalysis to help make the design software smarter. Alternately, you can specify an unbraced length directly and override the calculated values for Cb.
Tapered Members
There is no support for tapered members. Only prismatic members will be checked. You could 'check' a tapered member by splitting it into many short prismatic pieces, and setting a larger unbraced length on each section, but you cannot 'design' (search for solutions) to such a member, unless you grouped each piece into its own separate design group. This is not very practical as you need about 100 pieces to be as accurate as a single depth-tapered member using the built-in taper feature.
Custom Shapes
VisualAnalysis will not design or check built-up shapes (channels on wide flanges) or shapes that do not fall into the normal AISC or CSA categories (W, HSS, 2L, etc.). There are also limits on member dimensions. Deep beams (plate girders) are not checked for example. For unusual shapes, you can use the stress check feature to get preliminary checks from VisualAnalysis. You can also use IES ShapeBuilder to get stress distributions on more complex shapes.
CSA Limitations
Due to a lack of specific information in the CSA manual, bending checks for Tee and Angle sections are carried out according to the AISC LRFD Specification requirements. Class 4 shapes are not supported with the "steel" design groups, you can use the AISI Cold-formed steel design for any thin-walled/cold-formed shapes. Also the AISC definition for Cb is used in place of the w_{2}(omega) used in the CSA, with a 2.5 limit.
AISC 2005 Limitations
Equation G6-2a, using Lv for a shear check is not evaluated, G6-2b is used always. This is conservative.
Steel Capabilities
Shape Types
VisualAnalysis will check and design AISC and CSA shapes in the IES Shape Database, along with any shape in the shape database that supports one of the 'property sets' required by steel design. This allows British Steel shapes in the IES Shape Database to be supported, but only for AISC or CSA checks. There is no support for non-AISC/CSA specifications or codes. You may customize the IES Shape Database to include whole libraries of custom, foreign, or legacy shapes. Use IES ShapeBuilder to customize the IES Shape Database.
In addition to the database shapes, VisualAnalysis will check the Standard Parametric shapes created directly in VisualAnalysis. These include: rectangles, squares, I-beams, channels, tees, single angles, round and pipe sections and zees.
Composite beam design is tested for Wide Flange or I-Beam type shapes only, though other shapes may check OK.
Member Forces
VisualAnalysiss designs members for axial forces, shears, bending moments, torsional moments, and deflections. Checks are automatically made whenever a demand is present in the results. Where appropriate, both the strong and weak axes of the member are checked in addition to any interaction requirements.
All member forces are with respect to the member element's local axes, which are assumed principal. For asymmetric shapes these axes won't align with the geometric axes.
Seismic Compactness
Member elements can be checked against the seismically compact classification set forth in AISC's Seismic Provisions. The classifiaction depends on the type of lateral-force-resisting system being used and the type of forces present in the member. The user must decide if a particular member is required to be seismically compact.
Torsion Design
Design Specification
All torsion checks are based on the provisions of AISC. The Canadian steel code (CSA) does not provide specific requirements for torsion design (see CSA section 14.10). The general requirements given in CSA are satisfied using the more detailed requirements given in AISC 360 and AISC Design Guide 9
Torsion Check Types
The following limit states are checked in VisualAnalysis during the torsion design process:
- Torsional shear stresses are compared to the capacities from AISC equations H3-1 through H3-5 for closed shapes and from AISC equation H3-8 for open shapes.
- Torsional normal stresses are compared to the capacity from AISC equation H3-7.
- Combined Stresses with Torsion are evaluated using the equations described below.
The limit state described by AISC equation H3-9 is not checked as there is no guidance for calculating Fcr in the specification.
Torsion Design Levels
VisualAnalysis provides two levels of torsion design.
- Limited Torsion Design - When shapes with an open cross-section are twisted they tend to warp. If such a member is designed in VisualAnalysis without specifying idealized torsional boundary conditions warping is neglected. When warping is neglected the entire torsional moment is assumed to be resisted by uniform or Saint Venant shear stresses. Neglecting warping can result in incorrectly low unity values in some cases and incorrectly high unity values in others. If the limited torsion design is used for a member subject to warping, design done by VisualAnalysis is incomplete. The complete design would need to be done outside of the software.
- Advanced Torsion Design - The advanced torsion analysis uses a numerical solution process to solve the differential equation associated with warping (see AISC's Design Guide #9 for a summary of torsion theory). The solution is based on idealized boundary conditions provided by the user, and the internal torsion distribution calculated during the finite element analysis. The solution determines the angle of rotation along the length of the member and the first two derivatives of the rotation. Using this information warping normal stresses, warping shear stresses, and uniform shear stress can be calculated for a given shape. Advanced torsion design is based on warping section properties for shape types (i.e. I-Beam or Channel) these calculated section properties may differ from published values. Advanced torsion design is only available in the advanced level of VisualAnalysis.
When closed shapes are designed, limited and advanced torsion processes will produce similar results. While AISC only explicitly requires the combination of shear and normal stresses for HSS cross-sections, VisualAnalysis conservatively calculates them for all cross sections--even though they are unlikely to control--because it makes sense to do so from a mechanics viewpoint.
Torsion Boundary Conditions
Torsion Boundary Conditions are idealized as one of the following during design in VisualAnalysis.
The options listed above are idealized boundary conditions. Boundary conditions between these options cannot be considered in VisualAnalysis. Furthermore, the idealized boundary condition used for design may conflict with the boundary condition used during analysis. For example one member's twisting support may be provided by the bending stiffness of another member it frames into. In this scenario, twisting is restrained at the end and torsional moments can develop, but the rotation may still be greater than zero at the end. If during design the ends of the beam are idealized as torsionally pinned the design software will force the rotation at the ends to be zero when solving the differential equation. Sound engineering judgment must be used when selecting the appropriate boundary conditions. In some cases none of the idealized boundary conditions provided by VisualAnalysis will be appropriate, and the design will need to be completed outside of the software.
Stress Superposition
When combining stresses for torsion design, VisualAnalysis conservatively combines the maximum stresses regardless of where they occur on the cross-section. This results in conservative unity values, but is not unreasonably conservative for most shapes and loadings.
Combined Forces with Torsion
VisualAnalysis uses the following two equations to calculate unity values for the interaction of torsion with flexure and axial forces.
"sv" denotes Saint Venant's stress
"w" denotes a stress from warping
Fn,sv = 0.6*Fy for open shapes and is defined by AISC H3 for closed shapes.
Fn,w = 0.6*Fy
fT = 0.9
The equation shown above is in LRFD form, the ASD version is similar.
The combined equations are based on AISC H3-6 and Design Guide #9 section 4.7. Combining the demands in the manner shown allows both open and closed cross sections to be checked with the same interaction equation. The AISC specification does not explicitly dictate how demands should be combined with torsion for open shapes.
The equation used to check combined stresses with torsion is more conservative than the combined equations for axial and bending found in sections H1 and H2 of AISC. Because of this AISC does not require the combined equation with torsion to be checked unless the torsion unity by itself is greater than 0.20. In VisualAnalysis a similar threshold of 0.10 is used for open shapes.
Twisting Stiffness: Analysis vs. Design
The finite element formulation for beams in VisualAnalysis does not consider warping. Warping actually has a significant impact on a beam element's twisting stiffness. Because of this, the rotations calculated during the advanced torsion design process might vary significantly from the rotations calculated during the finite element analysis.
The advanced torsion design process uses the internal torsion distribution calculated during the finite element analysis. For an indeterminate problem, this distribution may have been different if warping stiffness had been considered during the finite element analysis. In light of this, the advanced torsion design may be performed using a torsion distribution that is incompatible with the more accurate torsional stiffness used in the design calculations. Judgment is needed to determine when the incompatibility is significant or unconservative.
Segmented Members
Torsion boundary conditions are applied at the end of each member element. As a result, segmented members must be combined before the advanced torsion design will work. The software will not stop you from designing a chain of individual member elements. However each element will be designed on its own and the torsion boundary conditions will be applied at the interior nodes; as a result the design results will be completely wrong.
Steel Parameters
Note: All of the parameters described below are edited/selected on the Modify Tab of the Project Manager while in the Design View.
Specification: Choose between ASD and LRFD code provisions.
Overstrength: Designs member for higher seismic forces based on user specified Overstrength factors and IBC Seismic Design Category.
Seismic Compactness: Choose weather a member should be checked for seismic compactness, and the type of lateral-force-resisting system.
Shape Category: Chooses shape category from which the VisualAnalysis "Design" feature selects shapes to optimize design.
Disable Checks: Prevents design checks from being performed on the selected design group. Allows you to speed up design checks and focus on targeted areas of larger models.
Bracing Parameters:
By default, member unbraced length is assumed to be the member-element length between nodes. See Bracing in VisualAnalysis for a complete description of bracing member elements for design checks. The lateral top/bottom unbraced length is used for Chapter F flexure checks. Strong unbraced length is used for Chapter E compression checks.
Deflections Parameters:
Deflections may be specified in a variety of ways. Specified Deflections criteria does affect unity checks. You must also include "deflection" load combinations in the Load Case Manager to obtain deflection checks. More information is available on the Design Concepts.
Axial Parameters:
See Design Parameters.
Constraints Parameters:
Options under this heading allow you to specify depth and width limitations on the members chosen when the VisualAnalysis "Design" feature selects shapes to optimize design.
Overrides Parameters:
Overrides Require: Advanced Level
Fy: This option allows you to override the yield stress used for both checking and design.
Cb: This option allows you to override the AISC bending coefficient.
Steel Report Options
The steel design report includes a number of optional components. You may control what information is included by double-clicking on the report after it is generated. By default the report echoes the design parameters and shows only the controlling checks for each member and load case. If you wish to investigate results further, you may expand the report to show every check made at every offset of every member in every load case.
Steel Report Organization
Reports have two primary parts: Parameters and Checks. The parameter section presents the information you entered in the parameters dialog. It is grouped and formatted for easy reading. If the group has a valid design shape, there will be some additional information about that shape attached to the end of the parameter section. (If the group does not have a valid design each member might be a different shape, so details about the shape are not reported.)
Checks are organized in the form of tables. The layout of columns in a typical report table is as follows:
Column Name | Description |
---|---|
Name | The member's name as in VisualAnalysis |
Section | Only in member reports, the cross section (e.g. W8x10). |
Load Combination | Forces and stresses are obtained from strength load cases while deflection checks utilize the service case numbers. A load case may have both a strength number AND a serviceability number, that may differ. |
Offset | This is the distance from the 'start' end of the member. The number and locations of offsets are as defined in the performance settings in VisualAnalysis |
Demand, Capacity | These columns vary depending on the type of check, but they represent code stipulated values. In most cases these are used directly in the unity check, but there are some special cases where the unity checks also include intermediate values, or values not reported. |
Code Reference | The controlling equations or provisions from the specification. For example, "F1-8" refers to this equation number in the ASD or LRFD manual. A code reference like "AE3-3" refers to equation (A-E3-3) in Appendix E of the manual. Sometimes there are additional references given, like "AB5-6" to indicate a reduction for slender compression elements (Q < 1). |
Unity Ratio | The unity check value for this particular member, load case, and offset. Unity checks are calculated as the absolute value of an actual stress divided by an allowable stress [ASD] or as the ultimate force (factored) divided by [F x (the nominal force)] [LRFD]. Check values of less than 0.005 are not reported to help reduce report size. |
Steel Report Notation
The steel reports attempt to use a notation similar to that used in the ASD and LRFD specifications. We have not used subscripts however, so a parameter that appears as L_{r} in the specification will appear as simply Lr in a VisualAnalysis report. The other major difference in notation is the use of section axes coordinates z and y in place of the AISC coordinates x and y.
Accordingly: fa, fb, fv are actual axial, bending and shear stresses, respectively. Similarly, Fa, Fb, Fv represent allowable axial, bending and shear stresses, respectively. P is an axial force, M is a moment and V is a shear.
Greek characters are used in the LRFD reports, using the Symbol font installed on most Windows systems. Where this is not possible (dialog boxes and other on-screen controls), the Greek characters are spelled out: phi, lambda, etc.
Steel Report Notes and Warnings
By default, a given check table will only display the worst-case unity check for a given member and load case. You may view the complete set of checks by clicking on the table and choosing Table Properties from the context menu. Then make adjustments in the dialog.
In some cases there will be notes and or warnings displayed either in a check table or at the bottom of the report. These are designed to be self-explanatory.
Steel References
VisualAnalysis was written to conform to the following specifications:
- Specification for Structural Steel Buildings, ANSI/AISC 360-10, June 22, 2010, American Institute of Steel Construction (AISC).
- Specification for Structural Steel Buildings, ANSI/AISC 360-05, March 9, 2005, American Institute of Steel Construction (AISC).
- CSA S16-14 Design of Steel Structures, CSA Group, June 2014, ISBN 978-1-77139-355-3.
Additional References:
- Manual of Steel Construction, 14th Edition, 2010, AISC.
- Manual of Steel Construction, 13th Edition, 2005, AISC.
- Manual of Steel Construction, Load & Resistance Factor Design, Volume 1, Structural Members, Specifications & Codes, 2^{nd} Edition, 1994, AISC.
- Flexural Strength of WT Sections", Duane S. Ellifritt, et. Al., AISC Engineering Journal, 2^{nd} Qtr 1992. pp. 67-74. (With special thanks to Mr. Blake Haley for his kind assistance with WT design.)
- Design Strength of Concentrically Loaded Single Angle Struts", A. Zurieck, AISC Engineering Journal, 1^{st} Qtr 1993. pp. 17-21.
- Steel Structures: Design and Behavior, Emphasizing Load and Resistance Factor Design, 4^{th} Edition., Charles Salmon & E. Johnson, Harper Collins College Publishers, 1996.
- Design of Steel Structures, 3rd Edition, Edwin Henry Gaylord, et. Al., McGraw-Hill, Inc., 1992.
- Steel Design Handbook: LRFD Method, Akbar R. Tamboli, editor, McGraw-Hill, Inc., 1997.