Structural Steel Example #3 - Estimate the stiffness components of a structure - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
12

Estimate the X, Y, and Z stiffness of the structure at the point 1000. In general, the stiffness of a three-dimensional structure, condensed down to the stiffness of a single point, must be represented by a 6´6 stiffness matrix. Initially, you can estimate only the on-diagonal, translational stiffnesses.

Specify the structural input data for Structural Steel Example #3

  1. Click File > New from the CAESAR II main window ribbon.

  2. Select Structural Input as the file type, and enter the file name as SUPP3 (for the purposes of this example).

  3. Navigate and select the CAESAR II data folder, and click OK.

    The software displays the Units Selection page.

  4. Click Next to accept ENGLISH,FIL, the default value for current units.

    The software displays the Vertical Axis Selection page.

  5. Verify that the vertical axis is set to the y-axis, and then click Next.

    The software displays the Material Selection page.

  6. Click Next in the Material Selection dialog box to use default material properties.

    The software displays the Cross Section Selection page.

  7. Specify the two cross sections, Section ID 1 as W12X65 and Section ID 2 as W10X22, and then click Next.

    The software displays the Model Definition Method Selection page.

    Select Method 1 - Element Definitions (the default setting) to use the element dimension (Edim) method of input. Then, click Finish.

    The software opens the CAESAR II Structural Input dialog box, where you can interactively input data. Click the arrow on each line in the modeler to expand or condense the information.

  8. Use the interactive input processor to input the following commands.

    You can also import these commands by inputting them in a text editor and then importing the .str file into the model. For more information, see Structural Steel Example #2 - Design a support to limit nozzle loads.

    UNIT ENGLISH.FIL

    VERTICAL=Y

    MATID 1 YM=29E6 POIS=0.3 G=11.6E6 DENS=0.283

    SECID 1 W12X65

    SECID 2 W10X22

    ; Preceding entries completed by opening dialog

    ; Columns have strong axis in Z (Default is X)

    ANGLE=90

    ; Generate all columns

    EDIM FROM=5 TO=10 BY=5 LAST=20 DY=12-

    EDIM 25 30 BY=5 LAST=40 DY=12-

    EDIM 45 50 BY=5 LAST=60 DY=12-

    EDIM 65 70 BY=5 LAST=80 DY=12-

    ; Beam orientation is standard

    ANGLE=0

    ; Set the default Section ID to 2

    DEF SECID=2

    ; Beams are pinned, both ends are free to rotate

    BEAM FIX

    ; Define most beams

    EDIM 10 30 5 LAST=35 DZ=-14-

    EDIM 30 50 5 LAST=60 DX=-10-

    EDIM 50 70 5 LAST=80 DZ=14-

    EDIM 70 10 5 LAST=20 DX=10-

    ; Node 1000 will be fixed in rotation

    BEAM FIX FAXIAL FSHRSTR FSHRWEAK TAXIAL TBNDSTR TBNDWEAK TSHRSTR TSHRWEAK TTORS

    ; Add midpoint 1000 on top beam

    EDIM 20 1000 DZ=-7-

    EDIM 40 1000 DZ=7-

    ; Define anchors at the bottom of each column

    FIX 5 65 BY=20 ALL

    ; Set representative loads

    LOAD 1000 FX=0000 FY=10000 FZ=10000

  9. After you enter all model data, the SUPP3 structural model appears as follows:

  10. When you are satisfied that the model has been entered properly, click File > Save to check and save the model.

    CAESAR II checks the input. If no fatal errors are found, the software writes the CAESAR II Execution files. The model may now be used in a piping analysis or analyzed by itself. For the purposes of this example the model will be analyzed by itself.

  11. Click OK, and then close the CAESAR II Structural Modeler Input dialog box.

  12. Return to the CAESAR II main window.

Analyze the structural model for Structural Steel Example #3

The structural input processor generates multiple lists you can use for documentation and checking.

  1. With the SUPP3 job still open in the CAESAR II main window, click Analysis > Piping > Static Analysis on the ribbon.

    The software displays the Static Analysis dialog box.

  2. Replace the Weight (W) load in Load Case 1 (L1) with F1 (the applied loads).

    You can type FI directly in the Load Cases box.

    From this point, structural steel analysis is performed just like a piping analysis. Output from a structural analysis is comprised of displacements, forces, and moments.

  3. Click Run the Analysis , and then click OK.

    The software displays the Static Analysis dialog box.

  4. Select the F1 load case in the Load Cases Analyzed box and Displacements and Global Element Forces in the Standard Reports.

  5. Click Add and then Finish to generate the reports.

The Displacements and Global Element Forces reports for the F1 load case displays as follows.

Notice that the structure is stiffer in the X direction, even though the Z dimension is greater, due to the orientation of the columns. The Global Element Forces report (which displays forces and moments) is particularly interesting because all beams have pinned ends. Most of the beams carry no load. This is because the transfer of the load to the beams in this model is due to rotations at the column ends, and not translations.

By adding cross-braces you can eliminate this problem and cause the beams to pick up more of the load. The 1000 end node of the elements from 20-1000 and from 40-1000 carries a moment because it is not a pinned end connection. The 1000 end node is just a point at midspan for the application of the load.

Kx = 10,000 lb. / 7.0909 in. = 1410 lb./in
Ky = 10,000 lb. / 0.2828 in. = 35360 lb./in.
Kz = 10,000 lb. / 25.7434 in. = 388 lb./in.