VisualAnalysis Tutorials: Steel LRFD Beam Design
This example project will use the beam shown below to illustrate the process of designing a Steel Beam according to AISC LRFD provisions. The beam is an AISC W shape made of A992 Grade 50 steel. The member is part of a floor system so we will assume a floor slab provides continuous lateral bracing of the top flange.
Building the Model
Create the 20 ft long beam shown above. The left end of the beam is pinned while the right end of the beam is supported in the y-direction. If this is a "Space Frame' model (default setting) you will also need to fix the rotation about the X-axis, and displacement in the z-direction of the right node. Select any AISC Steel\W shape for the member cross section. Also make sure the material is set to ASTM A992 Grade 50. Apply a uniform dead load of -0.5 k/ft and a uniform live load of -0.7 k/ft to the member by selecting the member, bringing up the Right+Click context menu, and choosing "Apply Member Loads". Use the Load | Load Case Manager to select the ASCE 7 LRFD automatic building code combinations as well as Deflection Checks.
If you need help creating the model or applying the loads, please consult the VisualAnalysis User's Guide (Help | Contents).
Design Load Cases
The design software checks the load combinations you define in the Load Case Manager. You need to make sure you set up load combinations for the strength checks (LRFD in our example) and also the deflection checks. Each load combination generated from a Building Code will already be assigned a design category. Deflection load combinations are available from the Building Code list, or you may wish to set these up manually. IBC uses several different types of deflection checks, and these will appear in the Load Case Manager based on the service loads you have applied to your model.
Design Group Parameters
Switch to the Design View and make sure all the members are deselected by simply clicking in the model space. Design parameters are automatically set up for your beam if the "Auto-Group" feature is turned on under the Design Checks heading found on the Modify Tab. If this feature is turned off you can manually create a design group (to associate the member with a specific type of design). User specified design groups can be created by selecting one or more members to be grouped, right-click and choose Create Design Group.
Note: Sometimes you must manually create design groups. For example, if you wish to perform composite beam design in addition to non-composite design, you must manually remove members from the "Auto-Grouped" steel design group(s) and create separate design groups.
The parameters should appear as shown in the picture to the right, with the exception of the "Specification". Change this to "AISC LRFD 2005".
We are going to assume that a slab provides continuous bracing to the top flange of the beam. Change the default setting "Unbraced" to "Continuous" from the drop down box for Lateral Top bracing. Leave the bottom bracing option set to "Unbraced".
Let's assume that we need to meet an L/360 live load deflection limit and an L/240 dead load plus live load deflection limit in this problem. With this in mind, choose Member Span Ratio from the "Strong" drop down box. Specify 360 for the "L Only' ratio and 240 for the "D + L' ratio limit. Since we don't have any Snow or Wind loads, the "S or W' check will not be performed. The "Other' limit is used to check any load cases marked as serviceability cases that aren't "L only', "D + L', or "S or W'. Leave this ratio set to the default.
Size constraints allow you to limit the width and depth that the software considers in the design process. This can speed up the design process considerably because the design software can focus its effort on sections meeting the size constraints. For this example, we don't need to use any size constraints.
Designing the Member
Switch to the Result View to obtain analysis results for your member. Next, switch back to the design view to obtain design results for your member (Unity Checks). If you are happy with the results, you are finished! If you would like to investigate alternative sections, select the member and choose Design | Design Selected Group. The Steel Design Selection Dialog will be brought up with a list of acceptable W shapes and their respective utilizations. Choose a size and click OK.
After accepting the design, the unity value that appears in the design view should have a ~ in front of it indicating that it is a preliminary value based on the analysis results with the original member in place. To get the real updated unity value you must synchronize the design changes.
Synchronizing Design Change
By Synchronizing design changes, you change the model shape (the shape originally specified in the model) to the design shape (the shape chosen in the design view). To synchronize the design changes, select Design | Synchronize Design Changes. You will be prompted to re-analyze, select "Yes", and when it finishes re-analyzing go back to a Design View and review the unity check. It should no longer have the ~ in front of it, indicating that is a final unity value.
If the unity value is greater than 1.0 the member has failed and you need to reiterate the design process. The closer the unity value is to one the more efficient, but less conservative the member is.
Reporting Design Checks
To report the design checks, double-click on the member in the design view. A design report is created listing the checks performed. To view different design details, you can change the report type and many other properties by using the Modify Tab while in the Report View.