Design of Horizontal Mullions For Gravity Load

[RFreund]

When designing a horizontal mullion for gravity load (which is typically the self weight of the glass pane above), is it typical to assume the glass spans to the end setting blocks?

Seems like a reasonable assumption. The glass pane will be very stiff so any deflection will cause the glass pane to span to the end supports (i.e. the setting blocks). Similar to our beam on beam discussion a few weeks back.

Thanks.

EIT

http://www.HowToEngineer.com

 

[racookpe1978]

Glass is really a "liquid crystal" and it WILL slowly settle over time. (Years/decades) And the heavier the pane of glass, the faster it is deform structurally into a bowed shape holding water/ice/snow -> which causes (slightly) faster ponding loads.

But right at the edges of a square/rectangle pane of glass, its movement of the edges of the glass will depend more on the seal itself: As soon as the rubber deforms - above and below the glass, you will start getting leaks at the edges.

 

[dhengr]

Rfreund:
I think you’ve got the right idea. The mullions will be loaded primarily in shear, by the two point loads out near the ends of the mullion, near the stiles/jambs. The trick is to size the length, durometer (hardness) and locations of the setting blocks so as not to damage the glass due to these concentrated loads, and a slight amount of deflection of the mullion.

 

[XR250]

The glass will be significantly stiffer than the mullion - no matter what size you use. As such, the load will be concentrated near the ends (as Dhengr said). I would size it for L/600 for the dead load of the glass just to cover yourself.

 

[Ron]

dhengr has it.

Also...why are you doing this? Storefront and curtain wall manufacturers have this down to a fine art. Is this a custom application or are you doing calcs for product approval by government agency.....just curious.

 

[RFreund]

Thanks for all the replies. In this instance I am reviewing this fine art. But we are starting to get into more facade work. Which is pretty interesting stuff. The requirement comes from the design build situation by the Architect.
I think in the past shop drawings were submitted for storefront / curtain walls but typically they are not required to be stamped and/or calcs are not requested. However now calcs are starting to be requested (or maybe they always have, I'm probably to green to know the situation). It seems as though the typical process is -> There are some drafters who provide drafting services to the curtain wall supplier. They create the shop drawings based on they desired system and designate "anchor by others". Then if calcs are requested the engineer comes along and provides calcs for member design and anchorage, in some sort of calculation report.

I was looking at two different submittal. One is 2 pages of calcs, the other was 85 pages. Funny how different we can approach things. The detailed calcs limited deflection to 1/8" or 1/4 the glass bite. However they applied point loads located at the end setting blocks. As we have discussed.

EIT

http://www.HowToEngineer.com

 

[canwesteng]

Re: Glass as a liquid. Actually a common misconception. It is a solid and will behave elastically.

http://math.ucr.edu/home/baez/physics/General/Glass/glass.html

 

[hokie66]

Be careful about assuming that large panels of glass will be supported only at their ends, as that is not always where they put the blocks.

 

[Ron]

RF...not sure of your location, but some states require a product approval process. In my state (Florida), any product such as this must have testing and engineering to validate the properties of systems claimed by the manufacturer. As an example, each manufacturer must submit an approval package to the Florida Department of Building Codes and Standards. That package must bear the signature and seal of a licensed professional engineer. Most of the glazing packages are pretty similar with plans of the systems, cross sections of the members, etc.

Take the system designation and go into Florida's product approval search system and see if the system you have is approved. If so, you'll find links to the engineering backup.

Here's the link to the Florida Product Approval system....

Florida Product Approval

 

[RFreund]

@Ron, Interesting thanks.

@Hokie - good point. To clarify though, as part of the submital they designated that the location of the blocks and anchors. In this particular case they were (supposed) to be placed at 1/8 points.

EIT

http://www.HowToEngineer.com