Use the auto-flanger tool if you want to acheive very unrealistic flanges, very quickly. The auto-flanger tool places flanges near the end of each an every support beam you have already created, which is almost correct, but not entirely correct.
The reason supports have connections is to do with manufacturing capabilities and transport capabilities. A transport lorry can only carry a length of beam that is 35ft long for example, so every 35ft along the langth of your supports a flang should appear. But also, wherever there is a connection between supports and one of the supports comes off at an angle, the support coming off at an angle must have a flange near the connection point.
Most people wouldn't bother going into that much detail though, as it's not all that important unless you're pretty anal like me
so in truth the auto-flanger generally will suffice.