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VisualAnalysis Tutorials: Double-Chord Wood Trusses

Project Description:

This tutorial will illustrate three ways to overcome modeling limitations associated with using doubled-members, such as in double-chorded trusses. VisualAnalysis does not have built-in support for "doubled" member shapes.

Truss dimensions resemble that of a "Roof Truss" found under the Create Tab of the Project Manager. It is 40ft in length, 10 ft tall, has no overhang, and consists of four panels. The left support of the truss is fixed and the right support is pinned. All members will be standard dresses 2 X 4 dimension lumber. The design loads will be 0.5 K loads at the top joints of the truss.

Be sure to set the structure type to a "Space Truss" for this tutorial.

Method 1: Model Half of the Truss

In order to determine the axial stresses that truss members will experience, we simply can model half of the truss under half of the load. All chord members will be modeled as SS2x4 elements and all interior members will be modeled as rectangular elements that are 0.75" in width and 3.5" in depth. This will ensure proper load paths through truss elements.

Using the "Roof Truss" standard found under the Create Tab, model the truss with the given parameters listed above.

This model provides correct axial stress values. For axial forces of interior truss members, remember to multiply the displayed force resultants by two as only half of the member is represented in the model. Also, if the self weight of the chords is substantial, take into account that only half of the chord weight is represented.

Method 2: Create a Custom Shape

Another option is to create a single custom shape that behaves like the effective cross-section of the double-chords shown on the right.

First, using the "Roof Truss" standard found under the Create Tab, draw the truss with the given parameters.

Select all the interior members of the truss and change their shape to NDS Wood | Dim. Lumber (Std. Dressed) | SS2x4.

Select all of the chord members and under the Source drop menu of the Modify tab, click "<Add Custom "Blob'>". Note that in this instance a Standard Parametric rectangle might be the easier choice, but blobs work for any shape.



A dialog box will appear allowing you to input parameters pertaining to the custom cross-section. Finding the cross-sectional properties of your customized shapes, whether by hand or using IES ShapeBuilder software is entirely up to you. The more parameters that are input into the custom dialog box, the more accurate the results.

You will notice that for certain applications, such as a truss structure type consisting of two-force members, not all parameters are needed to accurately portray member behavior (members in a truss structure type do not carry substantial shear or bending moment).

This model provides correct axial stress values. For axial forces of chord members, remember to divide the displayed force resultants by two as the custom member is representing both members of the double-chord.



Method 3: Draw a 3D model

Sketching the whole double truss in full 3D model space may be the best modeling consideration, though it is the more complicated modeling procedure, making use of fictitious "Rigid-Link" elements.