Originally Posted by TheArchitect
Of course it turned out that 600% had been plucked out of the air, and you've since started using 100% instead. It's a funny old game, eh?
Your working hypothesis is that the load from the facade and floor failures should have redistributed to the core via the hat trusses, and that the core should have been able to take these increased loads because of the safety factors.
One of the first things we have to understand is what we mean by safety factor: the ratio of the breaking stress of a structure to the estimated maximum stress in ordinary use.
However it is important to understand that the loads/forces found on a complex structure such as WTC may be acting in any number of ways; for example horizontal bending or overturning moments, or gravity loads.
In the same vein, there is a difference between the safety factor of an individual part of a structure andthe structure as a whole. It is perfectly possible for a steel beam to have a high factor of safety against (say) buckling under vertical (gravity) loads but a different figure for other forces.
To claim a figure of 600%, or 200%, or 50% as a global safety factor therefore just betrays how little you understand the subject. 600% against what? Which kind of forces? How are they acting? We can therefore only really understand the performance of the structure through global modelling.
Unlike Jazz, NIST actually do this - and not once, but three times. They look at the original design calculations, modern design calculations, and then a more forgiving global model of their own. The latter tells us that:
- Core columns in WTC typically had a Demand to Capacity Ratio (DCR) of 0.83 with around 10.6% of components exceeding design capacity under normal conditions.
- Hat Truss Columns had a typical DCR of 0.59, with 14.3% exceeding design capacity (some by DCRs of up to 1.95).
Other elements of the trusses had lower DCRs, however a truss is only as strong as the weakest member so we can set these to one side.
Now the section you quote Jazz, from page 133 of the PDF file, doesn't model the structural performance or indeed individual components. It's actually just a note of the NYC and Port authority design codes. So they don't actually prove anything very much.
So where does this take us:
Well, Hat Trusses may have had up to 0.4 "spare DCR" however this is in respect of design loads. The purpose of the hatt trusses was not to cantilever loads from envelope to carr, but even if it was then it certainly wasn't capable of taking double the design load.
Quote:
But even if it had been, the INTACT core only had some 0.17 "spare" DCR. that's only about 20%, and we need to bear in mind that some of the core structure already exceeded calculated capacity.
Now Jazz, you then CONTINUE to overlook some fundamental issues here:
1. We know that about a third of the core columns in each building were badly damaged or completely severed, further reducing the ability of the core to accept any loads.
2. Further damage would have occured to core structures during the collapse from both debris impacts and from dislodgement/failure at floor connections.
3. Despite this, we also know that some of the lower cores stood for 15 to 20 seconds following the main collapse.
So, in summary:
- Your 600% claim for structural safety factor of the core is rubbish
- Your reworked figure of 200% is rubbish
- Your claim that the hat trusses had sufficient spare capacity is rubbish
- Your claim that the core should have held the loads is rubbish.
