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RE: wind load / curtain wall

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Subject: wind load / curtain wall
From: "Mueller, John" <jmueller(--nospam--at)pcf-p.com>
To: <seaint(--nospam--at)seaint.org>

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I'm an architect, not an engineer... So please forgive my basic
question...

Would someone point me to a reference for how to use wind-load
information to determine the moment of intertia required for a mullion,
assuming a normal rectangular mullion/muntin pattern?

thank you...

John.
---------------------------------------------------

John,
Let me be sure I understand the questions. You have a glass curtain wall and your concern is that you will need to design a mullion between panes of glass to resist wind loads?

Since you have not described the mullion and I assume it is not a window casing set up against traditional wood posts or columns, then you will rely upon the window manufacturer to design the window frame - including the mullions. I say this as the framing will provide the rough opening, but there will need to be room to set and sqare the glass frame.

However, if you have to design a "post" to resist the wind loads against the glass then you might want to treat the post as a conventional simply supported beam and use the wind force (based on wind load calculations normal to the building in your geographical region) as a uniform load from both sides of the mullion. If you have a glass window on both sides, the mullion will take the tributary width of the glass on both sides multiplied by the force in pounds per square foot against the surface of the glass (and mullion) and design the mulion for a uniform load where the largest moment will occur in the middle (mid-height) where bending occurs.
With this said (I've never been asked to design one before so possibly those who work on taller buildings with curtain walls can respond) I would be more concerned with deflection of the mullion due to bending than whether or not ou are exceeding the allowable required moment of inertia of the material you use. The allowable deflection can be greater than what the glass needs to resist cracking or breaking. So stiffness is the most important issue here and how it ties into the material you use to create a mullion.

I hope this helps. Local wind loads can be obtained from your building official and the code is pretty straight forward for calculating the pressure against the glass to be used for the mullion design.

Dennis S. Wish, PE


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