How the two differ in kind
A curtain wall mullion is the vertical member between glazed panels, and it does two jobs that are checked separately.
DEAD LOAD is the weight it carries: the glass either side of it, the spandrel panels, and its own weight, delivered down the mullion and into the anchors that connect it to the structure at each floor. It is a genuine check — the anchors and their brackets have to take it, and the mullion has to carry it without buckling — and it is rarely what sizes the section.
WIND DEFLECTION is what does. The mullion spans between floors and resists the wind pressure acting on the tributary width of glass either side of it, and the governing criterion is not strength but how far it BENDS. Glass is brittle, and the edge seals, gaskets and structural silicone that hold it tolerate only limited movement of the frame around them — so deflection limits for glazed elements are tighter than for ordinary structural members, expressed as a fraction of the span with an absolute cap. Those limits are what the section is chosen against.
The consequence people underestimate is how steeply deflection scales with SPAN. Bending deflection grows far faster than linearly with the length of the member, so a modest increase in floor-to-floor height requires a disproportionately deeper mullion to hold the same limit — which is why facade depth and storey height are linked, and why a change in floor heights late in a design has a facade consequence the architecture did not anticipate.
Two further points decide real designs. Wind pressure is NOT uniform across a facade: corners and the top of a building see substantially higher local pressures, so the mullions there are checked against a higher load. And the ANCHOR must accommodate movement — the building and the facade move differently, and a bracket that resists that movement transfers load into the glass.
The factors that actually differ
Show
| Dead load | Wind deflection | |
|---|---|---|
| What it is | The weight of glazing carried by the mullion to its anchors. | How far the mullion bends under the wind pressure on the glass either side of it. |
| Which usually governs | Rarely. It sizes the anchors more often than the section. | Almost always — the deflection limit for a glazed element is what chooses the section. |
| The criterion | Strength and stability, plus the anchor's capacity. | DEFLECTION, as a fraction of span with an absolute cap, because glass seals tolerate little movement. |
| Effect of span | Proportional — a taller mullion carries a little more of its own weight. | Steep. Deflection grows far faster than linearly with span, so storey height drives section depth. |
| Distribution across the facade | Uniform, by panel size. | Not uniform. Corners and the top of the building see higher local pressures. |
| What it loads | The anchor brackets and, through them, the floor edge. | The mullion itself, and — if the seals cannot take the movement — the glass. |
| Movement accommodation | The anchor takes the weight and must still allow differential movement between facade and structure. | The glazing pocket and gaskets accommodate the frame's bending. |
| Failure | Anchor overload or bracket distortion at the floor edge. | Seal failure, gasket extrusion and, at the extreme, glass breakage. |
| Design consequence | Bracket and embed design at each floor. | Section depth, and therefore the facade's sightlines and its cost. |
| Both required | Yes, and they size different components. | Yes. |
Which one, and when
Choose dead load when…
- Designing the anchors and brackets at each floor, which is what the weight actually loads.
- Heavy glazing — thick insulating units, laminated glass, spandrel panels with backing.
- Where the floor edge condition is constrained and the bracket has to be designed around it.
- Checking the mullion's own stability under the accumulated weight over a tall panel.
Choose wind deflection when…
- Choosing the mullion section — this is the check that decides it.
- Where the storey height is large, since deflection scales steeply with span.
- At corners and at the top of the building, where local wind pressures are highest.
- Where the glazing system's seals or structural silicone set a tight movement tolerance.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why does deflection rather than strength govern?
- Because what the mullion is holding cannot tolerate movement. A steel or aluminium member bending well within its strength can still bend far enough to distress the glazing it carries: the glass is brittle and does not flex with the frame, so the frame's curvature has to be absorbed by the gaskets, the edge seals or the structural silicone in the glazing pocket, all of which have limited capacity for it. Exceed that and the seals extrude, water gets in, and in the extreme the glass is stressed at its edges. Deflection limits for glazed elements are therefore tighter than for general structure and are usually stated as a fraction of the span with an absolute cap — the cap being the one that governs on longer spans, since a proportional limit alone would permit a large absolute movement.
- Why does storey height matter so much?
- Because bending deflection scales with a high power of the span, so a small increase in the mullion's length produces a disproportionately large increase in how far it bends under the same pressure. Holding the same deflection limit therefore needs a substantially stiffer section, and stiffness in a bending member comes mainly from DEPTH. The practical consequence is that a facade's mullion depth is driven by the floor-to-floor height more than by anything else, and that a late change to the storey height — a raised floor added, a services zone deepened — has a facade consequence the architecture may not have priced. It is also why facades on tall storey heights use deeper sections, split mullions, or intermediate horizontal support to reduce the effective span.
- Is wind pressure the same across a facade?
- No, and treating it as uniform under-designs exactly the parts most likely to fail. Air flowing around a building accelerates and separates at the corners and over the roof, producing local suction substantially higher than the pressure acting on the middle of a windward face — which is why codes divide a facade into zones with different design pressures, with the corner and edge zones the highest. The consequence for a curtain wall is that the mullions and the glass at the corners and near the top are checked against a higher load than those in the field, and are frequently a different specification. Local pressure also matters for the anchors and for the glazing itself: a unit that is adequate in the field may need a thicker make-up in a corner zone.
- What does the anchor have to do besides carry weight?
- Accommodate movement, in several directions, without transferring load into the facade. The building's structure deflects under load and moves over time; the facade expands and contracts with temperature; and the two do so independently. An anchor that resists that differential movement forces it into the mullion and the glass, which is a load neither was designed for. So the standard arrangement carries the dead load at one anchor per mullion — a fixed point — and lets the others take wind load while permitting vertical movement, usually through slotted connections. It also has to accommodate construction tolerance, since the concrete edge is never exactly where the drawing says, which is why curtain wall brackets have three-dimensional adjustment and why setting them out is a survey exercise.
- How is the tributary width worked out?
- As half the glass panel either side of the mullion, since the glass spans horizontally between mullions and delivers its wind load to the two it sits between. So a mullion carries the pressure acting over a width equal to half a panel on each side, over its full span between floors — and that area times the design pressure is the load producing the bending. Two consequences follow. Wider glass panels put more load on each mullion, so a design that opens up the glazing module needs a deeper section even at the same storey height. And a mullion at the end of a run, with glass on one side only, carries half the tributary width and can be a lighter section — although in practice it frequently has to resist a different condition at the return or the corner.
- What is a split mullion?
- A mullion made of two interlocking halves rather than one section, used where the facade has to accommodate movement or where units are installed from one side. In a unitised curtain wall, each unit carries half a mullion at each vertical edge, and adjacent units interlock to form the complete member when they are set — which is what allows a whole storey-height unit to be lifted into place and engaged with its neighbour rather than being assembled in situ. The split also creates a movement joint at every unit, which accommodates thermal and structural movement across the facade. Structurally the two halves act together for wind load through their interlock, and the design has to account for how completely they do so, which is a property of the system rather than an assumption.
- Does the glass contribute stiffness?
- In a conventional captured system, essentially not — the glass sits in a pocket with gaskets and is free to move relative to the frame, so it delivers its wind load to the mullions and contributes nothing to their stiffness. In a structural silicone glazed facade the bond is continuous and there is some composite action, but design practice does not generally rely on it: the silicone is engineered to transfer the wind load from the glass to the frame, and crediting it with stiffening the frame as well would depend on a bond whose long-term behaviour is deliberately conservatively treated. So the mullion is designed as though it carries the load alone, which is the safe assumption and the one that keeps the deflection check meaningful.
- What happens if a mullion deflects too far?
- The failures are at the seals rather than at the member, and they are progressive. Gaskets extrude or roll out of their channels as the frame bends around them, opening a path for water and air; edge seals on insulating units are stressed and eventually fail, which fogs the unit and is the common visible symptom years later; and structural silicone is worked beyond its movement capability. At the extreme the glass itself is stressed at its edges and can break, usually from an edge flaw. None of that looks like a structural failure — the mullion is intact and the facade is standing — which is why excessive deflection is diagnosed as a water leak or a seal defect rather than as a stiffness problem, and why the deflection limit is the criterion rather than an advisory figure.