even if the curtain is subjected to strong wind in order to facilitate shortening the bar so as to allow it to slide easily along the guide 33a and so as to facilitate reinsertion into the slideway the ends of the bars may be telescopic as shown in fig9 for the bar 20 a sleeve 47 is mounted on the end of the tube 12 either by being force fitted thereon or else by means of a pin 18 which simultaneously serves to hold the inside end of the spring 19 which urges a piston 41 outwards the piston having a function equivalent to that of the endpiece 15 of the bar shown in fig3 unlike the previous example the end of the bar need not be flexible in order to enable the bar to escape from its slideways under the effect of an abnormal force the bar must be flexible overall or else it must include a zone e g a central zone which is particularly flexible however once such a bar has escaped from the slideway it can clearly be seen that it can be returned thereto by pushing the piston 41 into the sleeve 47 e g by hand or else while the curtain is being raised by virtue of a suitable guide ramp onto which the piston is directed during curtain raising while the bar is being raised the piston 41 is thrust by the ramp onto the tube 12 and is guided to the edge of the slideway at this moment the spring 19 is free to expand and the piston is reengaged in the slideway it is then no longer.