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This page contains frequently asked questions related to *cables*. *On this page:* {toc} h1. General FAQ h2. How can I find initial cable geometry/length for more complicated loading, such as when various point loads are applied along the cable? {open-question} h2. Do I need to assign load representing the initial conditions to a cable whose geometry was determined using the Cable Geometry form? *Expanded Question*: When the cable geometry form is used to determine the initial cable layout and length, I believe that one of the main objectives is to specify something like "initial tightness" of the cable. Would it be then correct to say that, in most of the cases, I would want to apply loads (such as DEAD + TARGET loads in the [Cable Watch & Learn video|videos:Cables video]) that will cause or match this tightness? Also, is correct to say that I would typically want to run subsequent nonlinear analysis to analyze for additional external loads (such as WIND loads in the Cable Watch & Learn video)? *Answer*: Generally, yes. There is a relationship between initial shape and tension in the cable that depends on the cable length, the loading, and the flexibility of the supports. The cable drawing tool can calculate this for a single cable with fixed supports under distributed load. Full analysis must be used for cable systems, cables connected to flexible structures, or other loads. The best approach to be used depends on what you know and how the structure is to be built. If the cable length is known, simple nonlinear analysis is adequate. Otherwise, iteration on initial geometry or target force loading may be needed. In any case, once the initial shape and tension is determined for the basic built structure, further analysis is needed for other loads, such as wind. Nonlinear analysis should be used whenever possible. The Watch & Learn video covers the basics quite well. h2. In normal static linear analysis, is the cable stiffness considered in calculations or not? [Nonlinear analysis] is required to get meaningful results with the cable element. Linear analysis can be performed, but they should always use the stiffness from the end of a nonlinear static analysis case in which all cables are loaded by their self weight or other transverse loads. For cases where no transverse load is present on a slack cable elements, the program will internally assume a very small self-weight load in order to obtain a unique shape. However, it is better if you apply a realistic load for this purpose. h2. Why do I see nonzero moment diagram on the cables in my model? You may be modeling the cable using straight frame objects. If this is the case, please uncheck the "Model Cable Using Straight Frame Option" on the "Cable Geometry" form to model the cable as a catenary.page is devoted to frequently asked questions (FAQ) related to cables. |
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Modeling
How are cables imported from AutoCAD?
ANSWER: AutoCAD lines, and therefore cables which are drawn as lines, are imported into CSI Software as frame objects. Cables may be drawn over these frame objects, then frames may either be deleted or assigned null properties.
As an alternative, one cable may be drawn to activate the tendon connectivity table, then interactive database editing is available to copy the connectivity-frame data into the connectivity-cable table to create cables within the model.
Large Displacement should be included in analysis when cables are modeled using frame objects.
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Why are the cables of my model returning nonzero moment values?
ANSWER: It is possible that the cables were modeled using straight frame objects. Frame objects generate a bending response. If moment values are returned, cables should be modeled as catenary elements by unchecking the Model Cable Using Straight Frame option on the Cable Geometry menu.
Why does the cable shape calculator generate an asymmetrical profile for multiple segments?
ANSWER: Once load is applied to the original geometry, a symmetric shape should result.
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Question 1: How is initial cable length and geometry calculated when multiple point loads are applied? Answer 1: OPEN QUESTION |
Loading
Should I apply a load which represents initial cable conditions before running subsequent nonlinear analyses?
ANSWER: Yes, in most cases, it is a good practice to assign a load which represents initial cable conditions. This is done using the Cable Geometry menu. Initial cable tension is dependent upon initial geometry and applied loading. Given distributed loading on a single cable with fixed supports, the cable drawing tool can calculate this tension. Full analysis must be implemented for systems of cables, cables connected to flexible structures, and conditions in which additional loads are applied. The best approach depends on what information is known and how the structure is to be built. If cable length is known, a simple nonlinear analysis procedure should be sufficient. Otherwise, initial geometry iteration or target-force loading may be necessary.
Once the initial shape and tension of a cable is determined, further analysis is necessary when additional loading is applied. Nonlinear analysis is also recommended. Video 16 of the SAP2000 Watch & Learn video series provides excellent instruction on this topic.
How can I display the wind load which is applied to cables?
ANSWER: The wind load which acts on cables may be displayed by selecting Display > Show Load Assign > Frame/Cable/Tendon. To assign wind load to cable objects, the auto load pattern should be defined using the Exposure from Frame Object option.
Formulation
Is cable stiffness considered during static linear analysis?
ANSWER: Nonlinear analysis is necessary to generate meaningful results which concern cable behavior. Linear analysis may be performed, though cable stiffness values should be taken from a nonlinear static-analysis load case in which cables are subjected to all loading conditions, including self-weight, transverse, etc.
When a cable with slack is not subjected to any form of transverse loading, a negligible self-weight is imposed to enable the formulation. However, it is best to apply this self-weight load manually such that it is of realistic magnitude.
See Also
- Vertical cables article