[Strawbale] PEX, Concrete Slab, Fear of Death (by contractor )

[Matt Chase]

Hi All

In regards to comments made that the ceiling systems of residential 
construction are able to withstand SUV's parked door to door, I 
highly doubt it, and could confirm my doubts by some quick 
calculations for the present building.  However, the issue of 
importance here is not whether a building could withstand a load 
like that, but whether it would be allowed to.  

Simply put, the approach to structural design is to determine the 
expected loads on the structure, multiply them by a factor of safety 
to increase them and then to determine the resistance of the 
proposed structure to various types of loads (gravity - vertical, 
wind, earthquake - lateral etc.) and decrease them by a factor of 
safety.  At the end of this process, the resistance must be greater 
than the loads.  Typically, engineers try to bring these values as 
close together as possible by reducing the resistance of the 
structure, since the loads cannot typically be changed.  This uses 
less material than a stronger structure, obviously beneficial for 
the bank account and for those concerned with material usage for 
other reasons.  So, if you decide to increase the loads acting on 
your structure, you usually don't have a lot of room to play with, 
and in general, need to beef up the structure.  Again, this isn't to 
say that if you increased the loading on a structure designed for 
less load than it is now being forced to take, that it will 
definitely collapse.  Structural design is all a matter of 
probability - set so high that collapse is unlikely even in the 
event of extreme, unexpected events.  However, you [hopefully] 
aren't going to convince any engineer or building official to let 
you build a structure in which the factored loads exceed the 
factored resistance.

So, filling an attic with water in bottles will require a stronger 
structure.

Cheers,


Matthew Chase
MSc. Candidate
Biosystems Engineering,
University of Manitoba
Winnipeg
(204) 237-5315
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