The pilot landed on a runway that was scheduled to be closed about 9 minutes after he arrived. A NOTAM had been issued about the closure. The pilot was aware of the NOTAM and confirmed the runway was still open by communicating over the airport’s common traffic advisory frequency before he landed. The pilot said that when he was on short final approach to the runway, the airplane struck the top cross-arms of an unlit airport runway closure marker. The pilot was subsequently able to land safely. The pilot said he was unable to see the marker as he approached the runway due to the nose-high flight attitude of his airplane on approach. The impact with the marker resulted in substantial damage to the airplane’s wing spar. A representative of the engineering company contracted by the airport acknowledged to the Federal Aviation Administration that their employees had moved the marker in front of the runway’s threshold prior to it being closed because they thought they had permission to do so by the airport; however, the person or entity who had granted that permission was never determined.