We have stairs going down to finished basement that are carpeted on top but open underneath. There are 2 stringers on each side. When walking up or down the stairs they flex (bounce) a lot. I have tried screwing the risers into the treads. That helped a little. My question is what method would be best to strengthen the existing strings. Should I sister a 2x8 to the side stringers, adding a center stringer would be tough due to the carpet on the topside, or maybe some type of steel brace under the stringers running the entire length. I have to have to stairs open underneath so putting a support from the underside of stairs to floor wont work, Help.