On 29-Apr-2005, "J. Clarke" wrote:

Where you run into the kind of problem you describe is when a strong but
brittle material is substituted for a weaker but more ductile material.

*NOT* necessarily true.

Please provide a case in which replacing a weak ductile material with a
strong equally ductile material results in the kind of failure you
describe. Please include the analysis.

You've made two different statements here.

If something is stronger, the fact that it's ductile doesn't necessarily make
it ok. You can change the load regime without entering the point where ductile
failure of the replaced element comes into play.

Buildings of the kind where an individual would be installing or removing a
beam are not typically under "dynamic stresses" to any significant extent
unless you want to count wind loading. If you want to talk skyscrapers
it's another story, but they typically have little or no wood in the
structure.

I've done dynamic analyses of some pretty small structures. They don't have
to be skyscrapers to have dynamic loading problems.

How can it "result in excessive transfer of stress" if the loading is the
same?

The problem is that the loading isn't necessarily the same. Just because the
design load is the same, doesn't mean that the load in use is the same.
If someone overloads a properly designed building element, they will see
precursors of failure. If the element is overdesigned, those precursors
(e.g. excess deflection) don't show up. Proper design means that you get
a warning if you have overloaded the structure.

Show us how to make a house fall down by making the headers too strong and
maybe someone will listen.

The poster said that overdesign is never a problem. No mention of headers.

Mike