We are building a structure out of 2″schedule 40) PVC over a partically built house we then want to put tarps over it. The problem we are having is that some of the connections will stay but some seem to pop off. We put together two 20′ pipes then cut in the middle and put in a 4 way cross fitting then we bend it into an arch anchored to the floor of the house and one up by the roof line . The arch is about 15ft. off the ground. any explainations? Any suggestions would be helpful.