tutorial 5 braced frame

7/28/2019 Tutorial 5 Braced Frame

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VisualAnalysis Tutorial 5: Braced Frame Tutorial Examples • 1

Tutorial 5: Braced Frame

Project Description:

This example will use the following frame to model lateral wind loading,

patterned live loading, and temperature loading. Additionally, momentreleases will be required on the cross bracing.

Braced Frame Geometry

Setting up the Project:

When you start up VA you are presented with the VisualAnalysis Startup Wizard. Choose Cancel in this wizard.

FEATURES:Generate Copies

Moving MembersRenaming MembersName Filter Graphical Results



2 • Tutorial Examples VisualAnalysis Tutorial 5: Braced Frame

VisualAnalysis Startup Wizard

A new project automatically generated for you. Make sure the project is a plane frame (Edit |Project Information), set the unit style to US ft-lb (Edit | project Units), and name the project"Braced Frame" (Edit | Project Information).

Basic Level Users: The Basic Level of VA does not support the Generate Copies command or the Rename command. If you are using the Basic Level you will have to graphically sketch themodel and rename the members individually.

Modeling the Structure:To begin draw a 32 ft column member and specify it as a W12x87. Note you may have to zoomout so that you can draw the full 32 ft. This is easily done by spinning your mouse wheel,pressing Ctrl <->, or using the menu commands. Select this column and select Model |Generate Copies. Use a rectangular pattern and generate 3 copies in the X direction spaced at20 ft.




Since the middle bay is spaced at 15 ft we will use the move command to correct our model.Select the two right most columns and choose Model | Move. Specify Move Selected Items and

–5 ft in the X direction.




You can select multipleitems by holding down theCtrl key while clicking onthem.

Note, the –10 ft in the

Generate Copies command has the effect of moving inthe –Y direction.

When we generated copiesin the –Y direction it automatically split and connected our beams and columns.

Now draw in the top-level beams and specify them as W14x30. After drawing in these three, select them and using the Generate Copiescommand on the Model menu, make two copies in the Y direction at –10 ft.When you finish, the model should look like the following.

We now need to add our cross members to the middle bay. Specify thesemembers as L3-1/2x3-1/2x3/8. When you draw these members VA willpresent you with some “Member crossing options”. This is one of the fewcases where we want to select the “Leave unconnected, good for X-bracing”option. We do not expect any force transfer to exist between the bracemembers. We also need to be sure to release the moments on thesemembers. To do this, select the members and under Connections and Typeon the Project Manager, change it to “Simple Connect”. A frame structure,as we selected at the beginning, assumes by default that all of the

connections are moment connections. When this is not the case, themoment must be "released" so that it behaves simply connected.

Lastly, make the column base supports fixed.

Cleaning up the Model

Up until now, in both this tutorial and previous tutorials, we have neglected a good namingconvention. With simple models this is usually not so important but as our models get morecomplex, it is crucial. It is a matter of preference as to when you name your elements. Somepeople like to name them as they go. Alternatively, you can use the Rename command and do itwhen your model geometry is essentially finished.

For this tutorial select all of the beam elements and go to Model | Rename. Change the nameprefix to "BM".




Similarly, select the column members and rename them with the COL prefix and select the crossbracing and rename them with the XBRC prefix. As we will see, this will make loading andviewing operations much easier later on.

Final Model Geometry

Loading the Structure:

If you started with the Auto-Generated project you may already have a number of service loadcases created depending on your customization settings. Please either edit your existing servicecases or newly create the following service load cases.

• Dead Loads, include the structure self-weight

• Live Loads, Pattern 1

• Live Loads, Pattern 2

• Wind Loads

• Temperature Loads

We now need to apply loads to the structure within the corresponding service load case. This isdone by selecting a particular load case from the drop down menu on the status bar. Thecurrently selected service load case is the one the loads are applied in.




Dead Load Service Case

We want to apply a 500 lb/ft uniform load in the global -Y direction to all the beams. Make surethe Dead Loads service case is selected in the status bar at the bottom.

• On the Filter Tab of the Project Manager, under Members, locatethe Name Filter field and type in BM and press Enter on the

keyboard. This will isolate all beam elements based on the namingconvention we set up.

• To select all of the beams, just hold down the Ctrl + Shift keys andclick on one of the beams. Choose Load | New Members Loads.

• On the Type tab, specify the Load Type as Uniformly distributedand the Global direction, Force Y. Move to the Magnitude tab andspecify the load as –500 lb/ft, then click OK. (A minus signindicates the negative direction of that specified. i.e. Force –Y)

Live Loads Pattern 1 Service Case

We now want to apply a 2000 lb/ft LL in a similar manner as above. Switch the service case toLive Loads Pattern 1 in the status bar.

• With the BM name filter still on, select the members as shown below and apply the 2000 lb/ftload.

Live Load Pattern 1 loading with the BM name filter on.




Live Loads Pattern 2 Service Case

Don't forget to switch to theLive Loads Pattern 2 service case in the statusbar.

Again, apply a 2000 lb/ft load, but to the beams shown below for Pattern 2.Note that instead of using the Load menu, after selecting the desiredmembers, you can right click and choose Apply Member Loads from thecontext menu that appears.

Live Load Pattern 2 loading with the BM name filter on.

Wind Loads Service Case

Don't forget to switch to theWind Loads service case inthe status bar.

We now want to apply our wind load to the left column. To remove the BMname filter that is still on, go back to the Name Filter field, delete the BM, andhit the Enter key. You should now see all of your structure again. The windload is to be a linearly distributed load varying from 0 lb/ft to 160 lb/ft. Toapply this over numerous members the load needs to be specified for each othe three column segments.

• Select the bottom column and choose Apply Member Loads, on the Type tab select Linearlydistributed and change the direction to Global Force X. Specify the magnitudes as 0 lb/ft to 60lb/ft. (160 lb/ft / 32 ft = 5 lb/ft per foot of column.)

•

Select the middle column and apply the wind load this time ranging from 60 lb/ft to 110 lb/ft.• Lastly, select the top column and apply the wind load ranging from 110 lb/ft to 160 lb/ft. Your

model should look like the following.




Wind Loads

Temperature Load Service Case*

For this tutorial we will say that some kind of temperature differential is taking place in the exterior columns. Switch to the Temperature Loads service case and apply a 5 deg-F/ft temperaturegradient to the exterior columns. Note that this is done the same way as the member loads, justa different Load Type is selected.

*Basic Level Users: The Basic Level of VA does not support temperature loadings. Just ignorethis loading scenario and continue with the tutorial.

Temperature Gradient of 5 deg-F/ft applied to exterior columns.




Analysis & Results:

Now that we have our model created and loaded, we should check our model for errors beforesetting the analysis options and actually analyzing the structure. Select Analyze | Check Modelfor Errors to make sure there are no problems. If something looks like a potential problem, it willdirect you to where to start looking. With a good model, we should set our analysis options. Goto Analyze | Analysis Options and uncheck Calculate dynamic mode shapes. Leave the Static

Analysis setting at Calculate both 1st and 2nd order results. Now select Analyze | Analyze Now or click the lightning bolt button on the toolbar. You can view the results for each of the servicecases by switching them in the status bar while in a result view.

Deflected shape due to Wind Loads

Note that the deflected shape VA shows is amplified. This is controlled on the Filter tab of the

Project Manager. Sometimes the amplification makes the deflected shape look strange. Anexample of this is in the dead load case, the cross bracing looks like it has deflected a significantamount but when you actually check its deflection it is on the order of 0.2 inches.

Similar to checking our model for errors before analyzing, we should also validate our analysisresults before reporting and working with our results. Use Analyze | Result Validation Checks to check the results. This will bring up a report summarizing the basic checks VA has made tovalidate correct results.

The Result Validation Checks check translations in the model to verify that the small deflectionsassumed in a linear analysis are not violated. The validation checks also report the applied loadsand reactions to make sure the statics balance. Lastly, the report also performs basic materialstrength checks. As you can see in the report above, we have an issue with translation andstress in the temperature load case and further analysis and design work is needed.




There are a number of ways to view the results for the various load cases. In the 2D Tutorial welooked at some graphical reports. Again, as in Tutorial 4, the Report These Results option isavailable and will give you Nodal Displacements, Nodal Reactions, and Member Internal Forces.In this tutorial we will look at member graphs. These are useful if you need to view the shear,moment, or displacement along a specific member. In the Wind Loads case, select the top leftbeam, right-click and choose Graph Member from the context menu.

Member Graph for the top left beam in the Wind Loads service case.

tutorial 5 braced frame

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