Nios II to SRAM via Tri-State Bridge

Are you using the SoPC builder component "IDT71V416"? If so, your problem is that the component is only for a 32-bit wide interface. That component was used specifically for 4 development boards that used two of the IC's in parallel. What you need to do is create a custom tri-state component with a 16-bit wide data bus. No you don't need to write any VHDL. You just need to let SoPC builder know that your component is 16 bits wide.