ETABS 2016 Tutorial: Trusses

Transcription

1 ETABS 2016 Tutorial: Trusses Below is a tutorial that was organized for educational purposes at Christian Brothers University only. The procedure of analysis in ETABS 2016 is similar to that of ETABS v9. Example Determine the member forces for the Pratt truss shown below. All members are made from the same material and have the same cross-section properties. Solution Step One: Open ETABS. Step Two: Select New Model.

2 Step Three: Select the Initialization Options. For this example, we will select Use Built-in Settings. Note that for our problem, we have SI (Metric) units. This option is found under Display Units.

3 Step Four: Specify a grid spacing and story height (based on your problem). Note that the Z- coordinate defines the gravity direction. This is specified by the Story Dimensions. Working in the xz plane, we will specify 7 grid lines in the x-direction, with a spacing of 3 m and 4 stories, with a story height of 2 m. You may work in either the 2D or 3D window. Let us work in the 2D window for this example. In the 2D window, change the view to elevation 1. This will take us to the xz elevation view.

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5 Step Five: Define material properties. Modify the material properties by making the mass and weight per unit volume zero (since this is a truss example, we will assume the material weight is zero so as to not induce unwanted shears and moments). Let s modify the A992Fy50 Steel.

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7 Step Six: Define frame sections. For this example, we are given that all members have the same cross-section properties. Because of this, we can select any section, so long as we apply it to all members. To illustrate adding a new section, let us select a circular steel rod and use this for all of our members. To do this, first select General under the Special set of selections. Let us use the default diameter of 25 mm. Note that we can define as many new sections as we need for a given problem. We will name this 1A.

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9 Step Seven: Now we shall draw our members. First, draw the bottom truss chord (as one continuous member). Make sure you select the frame section 1A that you defined earlier. This is selected in the Properties of Object tab. Next, divide the bottom chord it into three equal segments. To do this, select the member, click Edit, Edit Frames, then Divide Frames. For this problem, divide the bottom cord into 6 frame objects.

10 Notice that ETABS automatically placed external pin connections at the two new joints. We need to change these to internal hinges. Select the five interior points we want to change and click the Assign button. Change the joint restraints to a single joint. If you click on the check mark button, you can turn off the invisibility option and see the joints.

11 Draw the other truss members. Remember to use the frame section you defined earlier.

12 Note that for this example, we have a roller at the right support. Change the existing pin to a roller by selecting Assign, then Joint, then Restraints. Now, since this is a truss, we must release all of the internal moments since trusses do not support internal moments. The default for ETABS is a frame. That is, when you draw a structure in ETABS, it automatically assumes a frame structure. In order to release the moments and analyze this as a truss, first select all of the members. Then, under the Assign menu, select Frame and Releases/Partial Fixity. Select Moment 22, Moment 33 and Torsion and set all selections to zero. (ETABS will only allow one of the Torsion fixities to be set to zero.)

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14 Step Eight: Next, we shall apply our loads. Select the joints where the loads are to be applied. Under the Assign menu, select Joint Loads and Force. For now, we will only consider one load combination and we will consider it to be a dead load. Step Nine: Now we run our model by selecting the play button. The model will then show the deformed shape of the truss.

15 To determine the member forces, select the button shown below. Compression is color-coded as red and tension is color-coded yellow on the axial force diagram.

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17 We can also determine the displacement of a joint by right clicking on a desired joint.