Reccommended reading for anyone having to deal with circus rigging/safety is the FEDEC guide: http://www.fedec.eu/wp-content/uploads/2013/11/m8en.pdf
Assuming that your steel girder is safe, it should not be too difficult to connect a silk safely. It is easy to get hold of slings that allow working load of 1 ton, and by wrapping it around the girder so that you have 2 loops coming down you almost double that to 2 tons. However, indeed sliding is a problem because this can wear out the equipment. If it is difficult to install a completely static (steel) connector, an easier solution might be a second security sling. Connect to the carabiner on the silk with a bit of slack, so that if the first sling should ever break the second sling (which should not have worn out since there is no tension on it) it will hold.
Make sure that your carabiner/shackle/swivel also has a high workload. Only steel is acceptible, aluminium climbing gear is not made for regular tension (you fall once and throw them out) and can break without warning, whereas steel will visibly bend.
Check your equipment from time to time using your own common sense and good judgement. Risks are real and can be prevented!
I am not a rigger.
by Daniel Simu, in response to this post 2015-09-23 11:37:10