I am going to make a 16ft gate for a field, the outside frame is going to be 40mm box and the bars across the middle are going to be maybe 30 x 6mm flat bars. Would they be strong enough or would I be better using 30mm box for the bars in the middle. There is going to be just the one bar down the middle and it is going to be 40 x 6mm flat bar
Something similar to this
Something similar to this