It is well known that the sum of two z-ideals in C(X) is either C(X) or a z-ideal. The main aim of this paper is to study the sum of STRONGLY z-ideals in RL, the ring of real-valued continuous functions on a frame L. For every ideal I in RL, we introduce the biggest STRONGLY z-ideal included in I AND the smallest STRONGLY z-ideal containing I, denoted by Isz AND Isz, respectively. We study some properties of Isz AND Isz: Also, it is observed that the sum of any family of minimal PRIME ideals in the ring RL is either RL or a PRIME STRONGLY z-ideal in RL. In particular, we show that the sum of two PRIME ideals in RL which are not chains is a PRIME STRONGLY z-ideal.