KD (Knock-down) design rack is a popular design on the market in order to win the maximum loading quantity inside containers. The sea freight is going crazy on the shipping market all over the world this year so that saving the shipping space is a main challege for most of shippers . 
Bolt and nut are being used to fix the upright frame quickly while we can still guarantee the enough loading capacity by using the right spare parts accordingly.
The loading quantity can get 50% increasing withine one 40'HQ when we use such kind of KD rack comparing with the old frame welded together.
That is a market trend currently.