Our company has pipe support design specifications. I don't know of any guidelines requiring end caps, its just good engineering practice to have them. Our dummy leg specification requires line size diameter dummy legs up to 3" pipe, for larger line sizes the dummy leg is typically 1/2 the line size plus 1 size larger (ie a 4" line size requires a 3" dummy leg. Many of the end caps in our plant are plate and actually help stiffen the pipe. Also the plates are over sized (pipe diameter plus 1") to prevent the support from slipping off the I-beam. Being in South Texas I appreciate the end caps because we have a problem with bees during the summer and open ended pipes make a real nice home for them. Hope this helps.