{"id":5806,"date":"2020-09-25T12:15:22","date_gmt":"2020-09-25T12:15:22","guid":{"rendered":"https:\/\/buildsoftsupport.com\/?post_type=ht_kb&p=5806"},"modified":"2024-08-21T09:59:20","modified_gmt":"2024-08-21T09:59:20","slug":"why-can-a-cable-not-be-modelled","status":"publish","type":"ht_kb","link":"https:\/\/support.buildsoft.eu\/es\/knowledge-base\/why-can-a-cable-not-be-modelled\/","title":{"rendered":"Why can a cable not be modelled"},"content":{"rendered":"

In order to understand why it is not possible to model a cable in Diamonds, you should know the difference between a cable and a beam. Well, we’ve added a small example to illustrate the difference.<\/p>\n

A wet sweater (5kg \u2245 0,05kN) is hanging on a steel string which spans 5m. What would happen if we’d consider the the string as a beam and as a cable?\"\"<\/p>\n

If we would consider the string as a pinned beam, the bending moment would be<\/p>\n

\n

  <\/span>   <\/span>\"\[\frac{F\cdot l}{4} = \frac{0,05kN \cdot 5m}{4} = 0,0625 kNm\]\"<\/p>\n<\/p>\n

That seems small. But the string is only 5 mm in diameter (section modulus W<\/em> = 12,27 mm\u00b3). This means that in order to carry the weight of the sweater, the string should withstand the stress \"\sigma\" of<\/p>\n

\n

  <\/span>   <\/span>\"\[\frac{M}{W} = \frac{0,0625 kNm}{12,27 mm^3} = 5093 N/mm^2\]\"<\/p>\n<\/p>\n

Only 20 times more than S235 steel… We wonder were that gigantic strength is hidden. And yet it works\u2026<\/p>\n

The above example is a typical linear approach<\/strong> to a problem. Perhaps you noticed that we avoided the topic ‘deflection’. In the beam-approach the deflection \"\delta\" would be<\/p>\n

\n

  <\/span>   <\/span>\"\[\frac{F \cdot l^{3}}{48 \cdot E \cdot I_y } = \frac{0,05kN \cdot (5m)^{3}}{48 \cdot 210 000 N/mm^2 \cdot 30,68mm^4} = 20,2 m\]\"<\/p>\n<\/p>\n

Think how much the string would have to elongate to deform that much (5m string deflecting 20m?!?). This question brings us closer to understanding nonlinear geometry<\/strong>\u2026 where large deformations theory is being used.<\/p>\n

The string cannot elongate 20m as linear approach suggests, but it will elongate some for sure. This elongation will cause a tensile force in the string. In turn, this tensile forse will stabilize the structure, reducing the deflections. Using literature, we can approach the tensile force and resulting stresses, deformations:<\/p>\n

\n

  <\/span>   <\/span>\"\[\theta =\left ( \frac{F}{E \cdot A} \right )^{1/3}=1,316°\]\"<\/p>\n<\/p>\n

\n

  <\/span>   <\/span>\"\[N = \frac{F}{2 \cdot tan(\theta)} = 1,088kN\]\"<\/p>\n<\/p>\n

\n

  <\/span>   <\/span>\"\[\delta = sin(\theta) \cdot \frac{l}{2} = 57mm\]\"<\/p>\n<\/p>\n

\n

  <\/span>   <\/span>\"\[\sigma=\frac{P \cdot cos(\theta)}{A} =55,4 N/mm^2\]\"<\/p>\n<\/p>\n

It is more to believe the string will deform 57mm under a tensile stress of 55,4 N\/mm\u00b2, instead 20m under a stress of 5093 N\/mm\u00b2.<\/p>\n

Conclusion<\/h2>\n
    \n
  • A cable does not behave like a beam.<\/li>\n
  • Cables cannot be modelled in Diamonds. Not with a beam, nor with a tie rod.
    \nThere is no element in Diamonds supporting the tensile forces needed to describe the cable-behaviour. Diamonds is not suitable for large deformation theory.<\/li>\n
  • Even though tensile forces arise in tie rods, you cannot load tie rods perpendicular to their axis in Diamonds. Diamonds will simply refuse to calculate the model.<\/li>\n<\/ul>\n

    (Source cable formula’s: Roark’s Formulas for Stress and Strain, ISBN 0-07-072542-X)<\/p>\n","protected":false},"excerpt":{"rendered":"

    In order to understand why it is not possible to model a cable in Diamonds, you should know the difference between a cable and a beam. Well, we’ve added a small example to illustrate the difference. A wet sweater (5kg \u2245 0,05kN) is hanging on a steel string which spans 5m. What…<\/p>\n","protected":false},"author":3,"comment_status":"closed","ping_status":"closed","template":"","format":"standard","meta":[],"ht-kb-category":[1273],"ht-kb-tag":[549,525,510,515,524,526,509,550,511,512,517,527,514,513,528,529,516],"_links":{"self":[{"href":"https:\/\/support.buildsoft.eu\/es\/wp-json\/wp\/v2\/ht-kb\/5806"}],"collection":[{"href":"https:\/\/support.buildsoft.eu\/es\/wp-json\/wp\/v2\/ht-kb"}],"about":[{"href":"https:\/\/support.buildsoft.eu\/es\/wp-json\/wp\/v2\/types\/ht_kb"}],"author":[{"embeddable":true,"href":"https:\/\/support.buildsoft.eu\/es\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/support.buildsoft.eu\/es\/wp-json\/wp\/v2\/comments?post=5806"}],"version-history":[{"count":3,"href":"https:\/\/support.buildsoft.eu\/es\/wp-json\/wp\/v2\/ht-kb\/5806\/revisions"}],"predecessor-version":[{"id":13378,"href":"https:\/\/support.buildsoft.eu\/es\/wp-json\/wp\/v2\/ht-kb\/5806\/revisions\/13378"}],"wp:attachment":[{"href":"https:\/\/support.buildsoft.eu\/es\/wp-json\/wp\/v2\/media?parent=5806"}],"wp:term":[{"taxonomy":"ht_kb_category","embeddable":true,"href":"https:\/\/support.buildsoft.eu\/es\/wp-json\/wp\/v2\/ht-kb-category?post=5806"},{"taxonomy":"ht_kb_tag","embeddable":true,"href":"https:\/\/support.buildsoft.eu\/es\/wp-json\/wp\/v2\/ht-kb-tag?post=5806"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}