The American Institute of Steel Construction has just published Design Guide 25, titled Frame Design Using Web-Tapered Members. This publication details the design of web-tapered members, which are commonly used in the metal building industry. RAM Elements has had the capability to analyze and design these members since version 11.0 (v12.0 is the latest release); something no other software tool on the market is capable of doing!
Let's take a look at the features in RAM Elements V8i that incorporate this latest design guide:
Structural engineers at Bentley have created a brand new, state-of-the-art finite element used for the analysis of web-tapered members. The current formulation is a flexibility-based element in which the element internal equilibrium equations (axial, bending, and related shears) are always satisfied and an exact element stiffness matrix is derived. Thus, it only requires one element per member for linear analysis (unlike other programs that still segment in various prismatic members to approximate the taper). Shear deformations can be also included in the analysis for tapered members (the current implementation utilizes the concept of an “equivalent shear area” for considering transverse shear deformations). Otherwise, it is a typical 3D frame element with six degrees of freedom at each end (three translational and three rotational degrees of freedom). In addition, loads applied to the tapered elements are handled within flexibility-based formulation, thus accounting for their exact effects.
Update: You can read further about the finite element formulation in this blog post and white paper written by Bulent Alemdar, Senior Software Research Engineer with Bentley Systems.
The design can be carried out per AISC 360-05 or AISC 360-10 and—in both methods—ASD or LRFD. Additionally, we have added an option to include the MBMA (Metal Building Manufacturers Association) special considerations, which have been officially released by the AISC in association with the MBMA in Design Guide 25. The design and reports include the buckling strength multipliers, as defined in the design guide, for the different limit states. All the solved examples in the design guide have been added to the examples database installed with RAM Elements to show how to design properly a web-tapered member and that the results match those in the guide.
Web-tapered members can be analyized and designed using the following procedure.
Model your gabled rigid frame for a metal building or any other structure containing tapered members.
Select the appropriate members and on the Members | Sections spreadsheet, assign either
On the Members | Sections spreadsheet, specify the initial depth, d0, and final depth,dL, for each tapered member.
On the Members | Materials spreadsheet, specify the material as an appropriate steel grade for the tapered members.
(Optional) Repeat steps 2 through 4 as necessary.
Peform all other modeling functions just as you would for any other steel structure.
The program analyzes the model. Refer to the Information Panel for messages regarding the analysis.
The Design dialog box opens.
Select either the ASD or LRFD version of the AISC 360-05, AISI 01 design code and make sure the Use MBMA special considerations option is set to include the code checks of the new Design Guide.
Hint: Another nice feature for metal building design is that the cold-formed specification NASPEC 2001/04 is also included in the software (listed as AISI 01 in this dialog).
Set any other options as needed and click Design.
The program designs all members in the model. Refer to the Information Panel for messages regarding the design.
tool to obtain optimal thicknesses of the plates (web and flanges) used in the cross-section of the web-tapered members.
The example shown in this screencast can be downloaded on the Files tab, or by following this link:
Due to a requirement of Windows Vista and Windows 7, other examples of tapered members can be found in the C:\ProgramData\Bentley\Engineering\RAM Elements\Data\Samples\United States\Tapered member design\ folder.