A reinforced structural assembly is provided including a hollow structure having a cavity with a hole. A bag is inserted into the cavity using a tube. The bag has an end with an opening extending out of the cavity through the hole. A device injects a structural foam such as a two-part epoxy into the bag. The bag is closed and the end of the bag is inserted into the cavity. A cap is installed over the hole. The foam expands to fill the cavity but is contained within the bag thereby providing localized reinforcement in the desired area.