last edits July 14
Protecting the Floor
The official report on the crash compiled by the Air Accidents Investigation Branch (AAIB) in 1989-90, explained the essence of this finding:
“[T]he lack of direct blast damage […] on most of the floor panel in the heavily distorted area suggested that this had been protected by, presumably, a piece of luggage. […] This supported the view that the item of baggage containing the IED had been positioned fairly close to the floor but not actually placed upon it.” This is the core of the dismissal of a bomb placed before 103A’s load. Those must touch “the floor,” and this one didn’t. To understand how much merit this really has, we must consider the container itself. It was a model LD3, with the following dimensions, per a Wikipedia page:
base width / overall width - 61.5 / 79 inches
depth - 60.4 inches
height - 64 inches 
a sloped floor, as I tend to do. The distinction may be important, as the blast seems to have been centered just on or against the lowest aft corner of that sloped panel, just above the arrow attached to “W” here.
The author of appendix F, the AAIB’s Mr. Thomas Claiden, was questioned at camp Zeist trial in 2000. Mr. Claiden apparently has no explosives training or expertise, but is an engineer who knows about aircraft failures, and has some common sense it seems. He makes one firm, science-based distinction in discussing explosion height or anything about the floor. One one hand there are physical deformations – warping, bending, fractures, tears, and broken welds. On the other hand, there are explosives-type damage “or what I consider to be blast effects” like cratering, blackening, signs of explosive chemical contact. The distinction is very important for his findings and the elimination of Heathrow as the point-of-origin. 
Figure 9 of the AAIB report’s appendix F, at left, shows a photo Claiden took of two vertical-ish members – one due vertical, the other sloping up at a 45 degree angle along the outboard floor panel. Where they meet, or did meet before being blasted apart, in a V shape is at the bottom of the container, where sloping floor meets level floor, and it’s here the black, corroded look of blast damage is most visible. Claiden’s main focus is the absence of pitting on the inboard faces of the vertical bar, and I concur this tends to indicate a blast origin a bit outboard (right) of that line.
Now, in the above photo, imagine a line running from where the members meet towards the camera. This line would mark the edge between the level and sloped floors, a line followed by the outboard edge of the main floor panel. This was found to shows signs of trauma, its frame warped downward chaotically and broken near the aft side (right here), and “severely blast damaged,” meaning to Claiden unprotected by any luggage.
The Shiny Stuff
In talking with the Zeist Court, Mr. Claiden truly seems to relish using and explaining non-expert language to discuss the deformation of the floor panel. “Sandbagging,” for example, is an effect “very much as if a very heavy sandbag had been dropped on it.” Other parts appeared “guillotined” “concertinaed,” or “dished,” which is similar to sandbagged and joins warping, tearing, and the like, all of which to him imply physical force but not explosives contact. After discussing these distortions of the main floor panel, he further explained:
"There's one other aspect, I thought, to it, and that is the surface -- condition of the surface of the -- the upper surface of the floor panel in that area, which was very different, again, in character to the edge member, in that it was still – I won't say smooth and polished, but it's normal for a baggage container, because they do get heavily scratched, and they are dirty. But essentially it was smooth surface, shiny, with that proviso, and just did not have the appearance of having been pitted, blackened, eroded, or -- or burnt, for want of a better word." 
"[T]he first thought that came to mind -- and it's very difficult to prove it from that alone, or prove it, indeed -- but the first thought was that that surface had been protected, and I presumed by a piece of baggage, part of a piece of baggage, or whatever, but something that did not allow the direct effects of an explosion to actually impinge upon that surface." 
"Now, had it been in a suitcase placed directly on the floor, I think, I would have expected to seen more blackening, pitting, and shattering damage of the floor, and I don't see that." 
Considering the sharp divide between intact but distorted floor panel and blasted frame, Claiden reasonably surmised, as I hope I've related with the illustration:
I agreed with Claiden's finding the blast center was outboard of the vertical beam, and this means it was outboard of the level floor he's obsessed with. So what would be directly underneath this outboard explosion? How did that patch of floor fare?
The Other Floor
If the overhanging bag in the arrangement pictured above were slid any further over, It would come to rest against the sloped floor nearest the hull, a section that Claiden spent far less time studying. According to the diagram of figure F13, below, it was indeed right up against that portion of floor, with little room for anything beneath it.
damage to a neighboring container, AVN 7511. While my own rough measurements, based on container proportions, are a bit different, the placement of this ominous brown Samsonite seems fairly close to where the damage suggests, and in turn not far from where Mr. Bedford had seen TWO just like it.
“If we can now consider the outboard side of the overhang of the container,” the Crown's Mr. Turnbull led the witness, “we would be looking at the two panels ... one sloping and the other vertical.” He correctly noted “these would be the two panels which would be the nearest parts of the container to the aircraft skin,” especially the lower panel/floor. It would thus be the best possible spot for a bomb to be rested.
The horizontal member separating the upper and lower panels is present, but broken about 1/4 in from the aft end – remarkably similar to the base edge member’s failure point. As for the lower panel beneath that, not much can be seen. The rounded section hanging down at left is from the upper face, and the actual remaining sloped floor is a little hard to see, about 40% of it blow out and crumpled forward, wiggling off to the left here. Designated AI 84, Claiden explained the fragment:
"A Well, this is folded back with two significant bends. At the edge, which is torn or fractured, there are some very tight bends, and in the context of what we are looking at here, fairly ragged edges.
Q And which direction is it bent in?
A Well, to me it looks as if it's moved -- the fractured end has moved from aft to forwards. And in doing so, I think it's come up against a bit of the aircraft structure. There's an imprint there of one of the structural elements of the aircraft.
Q If we revisit figure 3.5, do we see that this part of the panel is depicted as taking up a little under half of what would have been the original panel?
"Q Was any of the panel which would have occupied the right-hand side of the diagram recovered?For some reason, that doesn’t convince this blogger of anything in particular except that this chunk was probably NOT from a spot maybe eight inches from the center of detonation. AI 100 remains an open mystery - the transcripts reveal no secrets as to why this oddball chunk was drawn in right there. And I'm at a loss for an explanation myself.
A I think the straightforward answer is no, but there was one fragment we did recover, which was small, and it's labelled AI 100, which I had strong suspicions may have come from that part, or indeed the panel above. Because those areas were missing, it was very difficult, if not impossible, to be absolutely positive.
Q Was there any other part of panel to fracture-match it to?
A No. Well, there was the edge. I did look, but it didn't appear to fit.
Q Yes. So that's a small portion, described in your report as AI 100, about which you had suspicions but were not able to take those further?
A Yes. The suspicions were generated from -- really from the shape and the size of the fragment. As you can see, generally, from the panels on this reconstruction, they tend to be fairly large sections, damaged, bent, whatever, that tear a small, roughly circular -- it wasn't circular, but very roughly circular."
All in all Thomas Claiden’s Appendix F is a decent and informative study, credentials or not. It does support well its claim the lower floor was protected, and doesn’t itself seem to read anything into this conclusion.
 Air Accidents Investigation Branch “Report no. 2/90 of the accident … at Lockerbie…” Signed Charles, M.M. Inspector of Accidents, Department of Transport, July 1990
Appendix F: Baggage Container examination, reconstruction, and relationship to the aircraft structure.
 Wikipedia: Unit Load Device http://en.wikipedia.org/wiki/Unit_Load_Device
 Testimony of Thomas Claiden – witness no. 317, May 25, 30 2000. Trial transcripts, day 10 pp1340-1381, day 11 pp1518-1627
 Claiden testimony, day 10 p 1442-43
 Claiden testimony, day 11, pp1510-11
 ibid. p 1512
 ibid. p 1515
 ibid. pp 1516-17