Okay, let me see if I can understand this argument. I'm very confused, and I hate being confused, even more than I hate soundling like an idiot, so I'm going to risk sounding like an idiot in order to try to sort this out.
There's some Very Important Number, let's call it for the moment the "VIN Load," which represents the load a building floor is rated for, according some Very Important Piece Of Paper which is created somewhere in the course of erecting a big honking building.
Mr. Urich is claiming that the VIN Load represents the most the building can withstand without Bad Things Happening. So that, if the VIN Load is 100 lbs, then the building can support 100 lbs. and if you put 101 lbs there, it's likely to have a boo-boo. Therefore it's forbidden to put more than 25 lbs. in the building, in order to leave a fourfold safety factor.
(BTW, I'm not so confused that I don't understand that I should be saying pounds per square foot, not just pounds; I'm using plain pounds to simplify the phrasing of the examples.)
Mr. Mackey is claiming that the VIN Load represents the load that the building is designed and expected to carry in normal expected use (or perhaps, at the high end of an estimated range of loads for various expected uses). Therefore, if the VIN Load is 100 lbs, you can put 100 lbs there, and no boo-boo will happen because if the Important Piece Of Paper says 100 lbs, the building must be constructed to actually support considerably more than that (though that safety factor is not necessarily x4).
Is this an accurate summary?
I'm one of the duellists, and I don't work on structures, so everyone, please feel free to go back and check my work.
Here's how I understand it:
There is some conservatism built into what you've called the VIN Load, which I would call the "design load." The conservatism arises because building code has some standard values for things like flooring and estimated use by occupants, and these values are naturally rounded up. You can't set up a test office and measure that it weighs 91 lb / ft
2, and then use that figure in your calculations -- you must use the agreed-upon value of 100 lb / ft
2. That sort of thing.
However, there is
no way the design load is conservative by a factor of 4.
The structure is built so that its "rated capacity" is equal to or greater than the design load. The rating means that, according to code, the structure is certified to carry that load, not just that calculations show it's theoretically possible. In order to get this certification, you have to include additional "safety factors." In the case of the WTC, this is a complex function because you're adding many loads together, each has its own safety margin requirement, and there are other aggregating factors such as "live load reduction" that are designed to reduce the requirements where multiple safety factors pile up and give you an excessively conservative structure.
So, in other words, ultimate strength of the structure > rated capacity = design load > actual load.
Approximately speaking, ultimate strength = (safety factor) * rated capacity = (safety factor) * design load = (safety factor) * (actual load + weight padding).
The design considerations for the WTC are handled in NIST NCSTAR1-1, which describes the design load and safety factor calculations. However, it is important to note that NIST actually calculates the true load of the structure in a completely different fashion, as seen in Chapter 4 of NIST NCSTAR1-2A. The structure "as built" does not always match the plan, which is why NIST found that some (very few) structural members of the WTC had demand-to-capacity ratios of greater than 1, i.e. they were eating up some of the safety factor described above in day to day usage. It happens.
You can't get too carried away with DCR, though, because of overlapping requirements. For example, one of the primary considerations for the Tower design was wind load. Some structural elements were sized for the wind, and thus without wind, just standing upright, these structural elements had quite a bit more capacity than needed. Thus the DCR for other elements is sometimes very low, like 0.3. It depends on what elements you're talking about.
My primary argument against
Gregory Urich, however, is that if we follow his numbers and accept his assumptions (which I don't), the results he gets badly fail the "sniff test." He would have us believe the WTC was so over-conservative that I question if it could have handled any occupants at all. I don't think he'd even be able to finish the structure within his computed weight limit, let alone furnish and actually use it.
Any comments and substitutions of proper terms of art from structural engineers, architects, and the like are welcome.
