assume that any stresses are tensile. When I get $20k to do a stair
rail, I'll be less conservative. Again with the flaws - there is no way
to economically specify maximum flaw size, so I use values for allowable
stress from the glass industry. Finally, point loads aren't part of the
guardrail standard for the building code, only pressures (50lbs over 1
ft^2), unless it is used at the rail itself, in which case you can apply
a point load in FEM and see the stress contours.
My detailed experience is with fused silica and BK-7 glass for ports in
space flight canisters for shuttle missions. Somebody else did the
actual fracture study, so I don't have the hands on program experience,
but had to know the hows and whys in order to design the interface and
test/certify the windows for flight. We probably had $80-100k in
material and services per unit. Can't quite justify those numbers for a
stair rail, even in the Architecture building at Tech. ;-)
Jordan
Christopher Wright wrote:
>
> On Mar 12, 2009, at 11:07 AM, Thor Tandy wrote:
>
>> I suggested rational analysis because we have a standard that has
>> taken the statistical data and created an equation to determine a
>> reference pressure allowed on the glass. We then apply a variety of
>> factors for such things as duration, tempered, laminated etc. With
>> that I can then do an FEA and check that I am below the von Mises
>> values ... that's also why I suggested care with the boundary
>> conditions because that's where the imperfections will become
>> significant.
> I'm real curious about a couple of things--
> Why do you use the von Mises stress as a criterion, rather than
> maximum (algebraic) principle stress? Glass is much stronger in
> compression than in tension, and the von Mises stress carries no sign.
>
> How do you get away without considering some sort of fracture physics,
> like LEFM, in considering flaws?
>
> Also you mentioned only pressure, but the real bugaboo with glass is
> point loading, either at supports or from impact loads.
>
> My experience with glass is dated, going back to the 60's when people
> were looking to use it in submersibles. Glass and ceramic spheres
> (more accurately paired and properly mated hemispheres) were generally
> acknowledged to be great for resisting external pressure, but everyone
> got really nervous about impact loads and local high loads from
> supports and mis-mated interfaces. We had an instrument housing made
> from glass hemispheres that simply vanished one day during a fairly
> shallow dive. One second the gyro was working fine; the next it was
> dead. Only an inconvenience at that point, but it was always a
> consideration in discussions of monolithic glass viewports.
>
> Christopher Wright P.E. |"They couldn't hit an elephant at
> chrisw@skypoint.com | this distance" (last words of Gen.
> .......................................| John Sedgwick, Spotsylvania
> 1864)
> http://www.skypoint.com/members/chrisw/
>
>
>
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