Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Point supported glass design

[Subject Prev][Subject Next][Thread Prev][Thread Next]
I look at the glass from a minimum tensile stress, and for simplicity 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(--nospam--at)skypoint.com   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/members/chrisw/



******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
** This email was sent to you via Structural Engineers* Association of Southern California (SEAOSC) server. To* subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
* Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you* send to the list is public domain and may be re-posted* without your permission. Make sure you visit our web* site at: http://www.seaint.org******* ****** ****** ****** ******* ****** ****** ********

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
* Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********