hi,
Radio Access Barrier is used to setup a connection (dedicated resource) between an RNC and a NodeB. RAB is one of the RRC features. So, an RNC is going to establish RAB when it going to receive/send a PS/CS signaling/data connection from/to a NodeB.
If you would like to calculate the BHCA for an RNC, you need to consider all Signaling (LA, SHA,etc) and PS/CS traffic (voice call, vedio streaming, etc) genetated between the RNC and the NodeB.