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VisualAnalysis Tutorials: Wood Truss Member Design

Project Description:


Check to see if the five foot long 2x4 truss member shown below satisfies NDS ASD requirements.

 

Building the Model

The member is a 5 foot long 2x4. It is made of mechanically graded (MSR) Douglas-Fir Larch with species and commercial grade 1800f-1.5E.

If you need help creating the model please consult the VisualAnalysis User's Guide (Help | Contents).

Loading

For the purposes of this example we will assume that a separate analysis was run on a complete truss and it was found that the member needs to carry an axial load of 500 lb. For the purposes of this example, we will assume the axial load is dead load and that no self-weight needs to be included.

Apply the 500 lb axial load as shown above in a dead load case. Make sure that the self-weight is not included in the case by going to the "Load Case Manager | Service Cases Tab" and Click the "Modify" button in the bottom left corner of the Dialog when the Dead Load case is selected. If the radio button next to "Don't add the structure weight to this load case" is not selected, choose it now. Click OK. Back in the "Load Case Manager" go to the "Load Combinations" tab and choose ASCE7-05 ASD for the Automatic Building Code Combinations. Click Close.

Information on applying loads is available in the VisualAnalysis User's Guide (Help | Contents).

Set Up Design Parameters

Switch to Design View. Select the member in the design view to make its design parameters active for editing in the Modify tab of the Project Manager and change them as described below.

Wood:

Design the member as a column. For the Shape Category, select Dim. Lumber (Std. Dressed).

Bracing:

We will assume that the member is unbraced in all directions. We will assume that the member is part of a braced system in both the y and z-directions. With this in mind, leave the Sidesway y and Sidesway z options set to No.

Columns:

The ky value refers to the effective length factor for buckling in a plane created by the member's local x-axis and local z-axis. In other words, it is the effective length factor for buckling in which the member kicks out in its local z-direction. The ky value is used to calculate the slenderness ratio ky*ly/ry.

The kz value refers to the effective length factor for buckling in a plane created by the member's local x-axis and local y-axis. In other words, it is the effective length factor for buckling in which the member kicks out in its local y-direction. The kz value is used to calculate the slenderness ratio kz*lz/rz.

The default setting for the k factors is to let VisualAnalysis calculate them automatically using the relative rigidity of members framing into a joint and the nodal support conditions. Since the member is part of a truss (i.e. the member's moments are released at both ends), the k-factors should be approximately 1.0. Since no members frame into the joints except the truss member itself and the nodal supports are pins, the automatic calculation should yield k factors that are correctly close to 1.0. With this in mind, do not override the k factors.

Size Constraints:

We are only interested in getting a unity value for the member in this case so we don't need to restrict the size.

Configuration:

Leave the Repeat Member and Has Bolt Holes boxes unchecked.

C-Factors:

Leave the Temperature Range at T<=100 F. Also leave the High Moisture Content, Pressure Treated, Qualifies for Buckling Stiffness Factor, and Qualifies for Incise Factor boxes unchecked.

Overrides:

We don't want to override any of the automatically calculated factors so leave these parameters unchanged.

Once the design group is set up, analyze the model.

Verifying Capacity

Once the model has been analyzed, switch to a design view and take a look at the unity value for the member to determine if it is adequate. If the unity value is less then one the member has passed all of the design checks.