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# How to model a compression bar

• Select the bar(s) you want to define as compression bar(s).
• Click on . This will set t he bending moment My’ and Mz’ free.
• Select ‘Function’ next to ‘Nx’ and click on
• Compression is negative, tension is positive. So define a function for which applies: if F<0 (= compression) then the bar can resist it (=fixed) and when if F>0 (= tension) the bar can’t resist is (= free).
But because ‘Free’ is the equivalent of 0kN, it’s recommended to enter a very small value. Calculating with 0kN migth give iteration problems. The definition of ‘small’ depends on the model. For some models it will be 0,1 kN/m, for others it will be 1kN/m. Choose a value and calculate the structure. If it calculates, divide the value for example by 2 and try again. If it doesn’t calculate, multiply the value by 10 and try again. You’ll see that after a few attemps, the tensile force in the compression bars will be neglectable in comparison to the compression force.
• Select the function for the other end of the bar.

Note: the countarpart of a compression bar is a tie rod.