Robert Klara gives the background to the crash in *The Hidden White House: Harry Truman and the Reconstruction of America’s Most Famous Residence, * pp. 63-65.
https://books.google.com/books?id=6VdLQkUoCSAC&pg=PA63
"One of the cracked beams on the White House's second floor happened to run beneath Margaret Truman's sitting room, a pretty little retreat with a marble fireplace and walls of robin's egg blue. That June, Margaret had left not one but *two* pianos in that room. The First Daughter, who'd launched a singing career the previous March when she soloed with the Detroit Symphony, practiced on her Steinway religiously. But she was fond of four-hand compositions, too--accompanied by her father or by her friend Annette.
"The assistant usher later would recall the First Daughter's pretty voice, which still was echoing in his head. "Mr. West, do you think we could roll the little spinet piano into my sitting room just for tonight? Annette Wright is coming over to spend the night and we'd like to try a duet."
"*Just* for the night? No. Housemen had trundled the Gulbransen upright in and out of Margaret's room for years now, fitting it snugly alongside the baby grand--and adding perhaps as much as a thousand pounds to the beam's burden below. No apparent harm had come of it--except on one recent night.
"Annette and Margaret had been deep into a classical duet when, sight-reading their sheet music, they came to a movement marked with an "*ff*"--fortissimo. The girls had let it rip, leaning hard into the keyboards and hammering at the ivories with all the might in their young fingers. Suddenly, Margaret had felt vaguely dizzy. It had almost seemed like the floor beneath them was rising and falling in time to the music. It had taken the girls a moment to realize that, in fact, that was exactly what was happening. "The two young ladies and the pianos were, for all intents and purposes, bouncing up and down rhythmically on an impromptu springboard--the split beam beneath them," Purves later explained.
"It had been a warning sign, and a big one at that. Had the warning been heeded, the two pianos would never have been left in Margaret's sitting room that summer. But they had been left there. And now, in June of 1948, the house could stand it no more.
"Exactly what happened next will probably never be known, for the versions varied widely and took on new colors with each telling. This much is certain: Straining under the enormous weight of two pianos, a portion of the sitting room's dry-rotted flooring split, opening a rift big enough to swallow one of the pianos' legs. Down it went--plunging through the parquetry, through the subfloor, and punching through the plaster ceiling. It must have made quite a noise, that falling piano--230 strings opening up in a hellish chorus as the instrument staggered to its knees, then the dead thud of hardwood on hardwood. In the room below, a shower of splinters and plaster dust fell twenty feet to the floor.
"Margaret would later remember that it was the spinet that took the plunge. Nevertheless, for the rest of his life the venerable Theodore E. Steinway loved telling the story of how he took a phone call from the White House not long after this incident. It was a young Margaret Truman on the line, asking for a man who could come over to rescue a piano..."