# Karamba shows compression on cables

Hey guys, I have a structure which is spanned by cables. When I analyse it with Karamba the beam view component shows only compression in the cables. Actually there must be tension, right? Can someone have a quick look at my definition?

tension compression problem.gh (708.8 KB)

Your pretension is a positive elongation? So you’re increasing the length which creates a compressive force when resisted? Practically the easiest way to pretensio a cable is shortening it by a certain predefined amount (of course, there are also tools to pretension with force control).

How many minutes do you expect a “quick look” to take? How many people will take that quick look and what is the total of all their times combined? Yes, there must be tension in cables, not compression.

Ah okay the pretenion has to be negative, thanks a lot it is working correct! Sorry I am not very familiar wit Karamba

But I don’t understand, the initial strain component says elongation is positive…?

You are imposing a positive strain on your cables. You are forcing the cable to get longer. If that elongation is inhibited a compressive internal stress starts to build up. Imagine a bar between to fixed supports, if you’re imposing a elongation (e.g. by heating it up), a compressive stress will build up. Prestressing a cable is usually the opposite: you pull it against some fixed supports, then clamp it again making it effectively shorter. Look up how cable prestresses are applied in practice (e.g. Suspa DSI or Pfeiffer)

Thank you very much for that explanation!