Federal Roof Crush Standard
After many years of delay, NHTSA, the Federal agency responsible for highway vehicle safety has come up with a new strength regulation for (some) highway vehicle roofs.
For vehicles of 6,000 lbs
or less gross vehicle weight, the new strength to weight ratio-SWR- is 3, this
up from 1.5 for the old standard.
For vehicles between 6000 and 10,000 lbs, the required SWR is 1.5.
Basically it’s the same old test, test plate forced against the roof until 5 “ of deflection is achieved, the applied force must at least equal that defined by the SWR. In other words a structure is allowed to fail catastrophically under an arbitrary load. As long as the load is sustained for a distance of 5”, the structure is deemed safe or adequate or something. The protocol is ridiculous from an engineering point of view, an arbitrary static test for a structure that is impact loaded in real world accidents.
But the new standard
probably has some value, especially since the effective SWR’s are
likely to be 4:! or even higher given the manufacturing variations that must be
accommodated in the design phase. The biggest inadequacy may be that the new
regulation does not cover heavy trucks. The industry itself assumed the
responsibility for a heavy truck standard about 1980 but never did anything.
There is some evidence that if the new regulation for the 10,000 lb class might
be beneficial for big rigs weighing up to 80,000 lbs, if they were covered by
it; even while it probably won’t be of any benefit for the trucks it is
intended to cover-mostly large pickups and light city delivery vehicles.