A heat recovery steam generator (“HRSG”) includes a first casing having an interior enclosing at least one duct for gas to flow therein along a gas flow axis. Each duct is defined by duct defining members that are spaced apart from each other and extend into the interior of the first casing. A plurality of stiffening members are elongated along the gas flow axis. Each of the stiffening members are positioned between two of the duct defining members. The stiffening members and duct defining members have a substantially similar coefficient of thermal expansion.