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RE: wind load / curtain wall

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Window and curtain wall design gets very tricky.

1. Unless you have a wind tunnel test on your structure you need to use the default values from ASCE 7 for the various wall zones on your structure.

2.  The suction is different than the windward loading.

3. Once the loads are determined then you need to use the section modulus not the moment of inertia. It will be different in suction vs the windward load. Most mullions are not symmetric.

4. Don't use the composite value of the section modulus, and know the properties of the material (presumably aluminum). You can use each mullion section, but they will not act compositely through the thermo-brake material. This is a common mistake because the manufacturers seldom account for the differences in transferring horizontal shear due to temperature.

5. Be mindful of the capacity of the Rivnuts or whatever connector they use. They often list allowable tension and shear values. I have seen one engineer attempt to use the upset value. All of the aluminum Rivnuts had to be replace with stainless steel rivnuts.

I would just performance spec. the window assembly to resist the winds per ASCE 7. The window mullion manufacturer will design it, and have a structural engineer verify the design. Require an engineers seal on the design calcs.

I have seen good structural engineers get this one wrong.

Harold Sprague

From: "Mueller, John" <jmueller(--nospam--at)>
Reply-To: <seaint(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: wind load / curtain wall
Date: Thu, 29 Sep 2005 12:48:21 -0400

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

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...


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