Abstract: A flange connection for the ends of two pieces of square ducting has eight pieces of open-sided box girder, four of which are slipped out and fixed onto each of the two ends forming frames thereon. The two frames are then bolted together.For producing the desired sealing force along any two paired of the two frames, anglepieces at the frames' corners have nosepieces running out against each other so that, on screwing up the bolts tight, the middle part of the two girders in a pair are levered and bowed out towards each other for producing said sealing force. Each of the bolts is placed through a hole running through and have two legs on each anglepiece and through two wall parts of each of said box girders.