Truss Redesign

I installed trusses in my pole barn that cannot support a dead load on the bottom chord.. I would like to install a ceiling of 12 inches of blow in insulation and metal under that. Trusses are 8ft on center with bottom purlins 4ft on center. More can be added. I would like to beef up the bottom chord of the truss to support the weight. They are scissor trusses. The pole barn is 30x48.