Elements of structural steel members may be incorporated into the model of the piping system through the use of the structural steel modeler. These elements may vary from a single member (such as a saddle, welded to the pipe), to a complete restraint structure, to a complete model of the building steel. The steel may be modeled in order to include the stiffness of the structure in the analysis, or in order to calculate the loads on the steel for stress analysis of these members (which can be done through the AISC unity check, accessible from CAESAR li s main menu).
In general, constructing a structural model is very similar to constructing a piping model — the same information is required: geometric layout, element cross sections, material parameters, boundary conditions, and loading. Internally, the calculations made are identical for the structure as for the pipe. The only major differences (aside from the actual process of problem coding) are:
Almost all joints between piping elements are assumed to be fixed connections (i.e., all three forces and all three moments are transferred between adjacent piping elements). In steel structures, the connections may transfer only selected loads between adjacent elements, depending upon the actual joint construction. For example, the clip angle joint shown in Figure 3-109 is one of the most common connection types — it is assumed to transmit forces, but no moments. Moment connections require welding of the flanges of the wide flange, since moments are carried in the flanges and shear is carried in the web. Therefore the user must be careful to accurately model the internal connections between members. (Note that the default connection provides full fixity.)
Structural members perform very similarly to pipe elements except that the structural elements are not symmetric about their member axis; in fact, structural elements are usually weak for loading in one direction and strong in another. This means that the local orientation of the element is very important. Element orientation is specified through the use of the ANGLE parameter, which specifies the angle which the element is rotated away from its "standard" orientation. In CAESAR II's structural modeler, "standard" orientation is defined as follows:
a) for elements running in the horizontal plane (beams), the element's weak axis coincides with the global Y-axis
b) for elements running in the vertical plane (columns), the element's strong axis coincides with the global Z-axis
c) for elements running in a skewed plane (bracing), the projection of the element's weak axis on the vertical plane coincides with the global Y-axis
Correct orientation of elements can be easily checked by using the structural modeler's PLOT command. CAESAR II's structural modeler uses keyword input, which may be entered interactively or through a file. Commonly used keywords are listed below:
Where more data is required for a command, the program prompts the user for it. The program includes over 900 standard steel shapes, the properties of which may be accessed by name. Three databases are available—the 1977 AISC, 1989 AISC, and the 1991 German (DIN) standards. The user may enter the specific cross-sectional parameters for non-standard shapes.
A sample problem, showing coding of a pipe rack, is shown in Figure 3-110. The accompanying input illustrates the use the most common keywords to define a model.
Note that a node point (1020) must be placed along a structural member whenever an intersection with the piping system is to occur. The structure is included in the piping problem by entering the file name through an option of the Kaux menu. The pipe is then attached to the structure by using the attachment node point on the structure as the restraint CNODE.
Many structures (such as building frames or continuous racks) have a high degree of repeatability. The user can take advantage of this through the use of node and element generation commands. For example, the large structure shown in Figure 3-111 can be created using generation commands. The process is to first define one corner node, then fill a single column line of nodes, then sweep the line of nodes out into an area pattern, and finally sweep the area pattern up into a volume pattern of nodes, each step of which takes a single command. Elements can then be generated in a similar manner.
The entire structure can be entered using only 14 commands to define the geometry, cross-sectional properties, material properties, and boundary conditions:
Modeling And Analysis Of The Piping System
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