03/03/2026
FE 4.3: ULS vs. SLS: Why "Bending" is as Bad as "Breaking"?
[Mullion School: Facade Fundamentals 4.3]
We have our "Wind Load" (Lesson 4.2). We apply this force to our mullion (the aluminum beam) in our analysis software. Our first check shows: "It doesn't break!"
So, we are finished?
No.
A façade engineer must check for two distinct types of failure, known as "Limit States."
1. Ultimate Limit State (ULS) - Strength
• The Question: "Will it break?"
• The Goal: This is about Safety. We check if the stress (like Bending Moment) inside the mullion or anchor exceeds the strength (capacity) of the material.
• The Check: Actual Stress < Material Capacity. If this check passes, the system will not collapse, and no one gets hurt.
2. Serviceability Limit State (SLS) - Deflection
• The Question: "Will it bend too much?"
• The Goal: This is about Functionality and Comfort. The mullion might be strong enough not to break, but it is not stiff (rigid) enough to work properly.
• The Check: Actual Deflection < Deflection Limit.
The "Deflection Limit" is a rule set by building codes. A common rule is L/175 (where "L" is the span, or height, of the mullion). For a 3500mm mullion, the maximum allowable "bend" or "flex" is 3500 / 175 = 20mm.
Why is Bending (Deflection) a "Failure"?
If your mullion deflects 30mm (passing ULS but failing SLS), your façade has failed in three critical ways:
1. Structural Failure (Glass): The rigid glass plate is held in the frame "pocket." When the mullion flexes 30mm, it can "disengage" (pop out) the glass, or worse, put a "point load" on the glass edge, causing it to shatter.
2. Weathertightness Failure (Water): Our waterproofing (Lesson 3) relies on gaskets being "compressed." When the mullion flexes outward, it "de-compresses" the gasket, opening a "gap." Air and water are then sucked directly into the building.
3. Occupant Comfort Failure (Perception): The people inside don't know the ULS calculation is fine. They just see the windows "flexing," "vibrating," and "creaking" in the wind. They will "perceive" the building as unsafe.
Conclusion:
A successful façade design must pass both ULS and SLS checks. It is not enough for a mullion to be "strong." It must also be "stiff." If your design fails the L/175 deflection limit, you must make it stiffer (e.g., use a "deeper" mullion profile or add steel reinforcement).
In our next post (4.4): The mullions are designed. But how do we attach them? We explore the art of "Anchors" and how they manage the building's own secret movements.