One Path, Four Handoffs
Load has exactly one destination on this job, and that is the slab edge. Glass is the first link in the chain and the least forgiving: a spanning membrane that takes wind across its face, delivers it to four edges, and passes its own weight down through two setting blocks. That is the whole of its structural duty. Anything more you ask of it — bridging a bracket that never got shimmed, holding a mullion that racked, absorbing a floor that deflected onto it — it answers with a corner crack or a failed edge seal, and it answers on somebody else's schedule.
Frames collect what the glass gives up. Transoms take the head and sill reactions and run them horizontally into the mullions; mullions gather those transom reactions plus their own tributary strip of wind, plus the dead weight of every unit hanging in their bay, and run all of it vertically to the anchors. The mullion is the only continuous vertical member in a stick system, and it does two unrelated jobs at once: bending under wind, and hanging under gravity.
Anchors turn an aluminium wall into part of the building. A pair of brackets per mullion is typical — one taking dead load plus wind, one taking wind only through a vertically slotted connection so the frame can grow, shrink and rotate without prying its own fixings. From the bracket, load enters an embed plate, a cast-in channel or a post-installed anchor, and from there the slab edge and the structural frame the engineer designed.
Faults on a curtain wall almost always trace back to one link being asked to do the next link's job. Track any defect backwards along that path — glass, gasket, transom, mullion, bracket, slab — and the cause usually sits one station upstream of where the water showed up.
One mullion line, taken apart
- Cover cap — the one component with no structural duty at all — it conceals the plate fasteners and carries the finish the client will actually look at
- Pressure plate and gaskets — clamps the unit against its wedges at a stated torque, and the gasket it compresses is ordered by the metre with splices allowed for Glazing Gasket/Wedge Linear Footage Calculator
- Insulating glass unit — spans between four edges and hands its own weight to two setting blocks; asked for anything more it answers with a corner crack Architectural Glass Weight Calculator
- Mullion — bends under wind and hangs under gravity at once, collecting half a bay of tributary area on each side plus every transom reaction Curtain Wall Mullion Tributary Dead Load Calculator
- Anchor bracket — three axes of adjustment absorb the slab's wander, and one bracket per mullion takes dead load while the other is slotted for wind only Curtain Wall Anchor Bracket Spacing Calculator
- Slab edge — the destination for every load in the wall, and almost never on the line the elevation was drawn against
Setting Out From the Building's Lines, Not Its Concrete
Control comes off the structural grid and a vertical datum, never off the finished edge of the slab you happen to be standing on. Transfer a plumb reference the full height of the elevation early — laser, plumb bob down a shaft, whatever the site allows — and mark a working benchmark at every floor. Bracket setting-out then references those marks, and the slab's own wander becomes a shimming problem rather than a geometry problem.
Module reconciliation is the next thing, and it wants doing on paper before it happens on steel. Take the as-built dimension between the two hard points of the elevation — a core wall, a corner column, a return — and divide it against the drawn module. The remainder tells you whether you have a clean run, a make-up bay you can absorb at the corner mullion, or a redistribution across every joint. Redistribution is cheap in the model and ruinous once units are cut.
Count everything twice against the elevation: standard modules, make-up modules, corner assemblies, vents, spandrel-only bays, and the intermediate transoms that exist only where a floor line lands. The bracket schedule, the gasket order and the sealant take-off all fall out of that count, so an error here multiplies down every trade sheet issued afterwards.
Reconciling the drawn module against the elevation you actually measured is the moment the count either closes or leaves you a make-up bay, and that answer has to exist before a single bracket or unit is ordered.
The total area of the building facade to be covered in unitized curtain wall.
The area of one standard unitized curtain wall panel — a 1.5 × 4 m unit is 6 m², a 5 × 13 ft unit is 65 sq ft.
Curtain wall panels needed
83 panels
They open the calculator with your figures already in it
Curtain Wall Panel Count Calculator: 83 panels — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Gives no spares, and a unitized panel is the one thing on a facade you cannot buy again quickly. Each unit is made to order with a lead time measured in weeks, so one damaged in transit, dropped off the hook, or broken by a following trade leaves an opening in a sealed building until a replacement is fabricated and shipped. Attrition units go on the first order or they are not available when they are needed.
- Counts panels, not the anchors they land on. Every unit hangs from cast-in embeds or slab-edge brackets that must be set, surveyed and shimmed to a tolerance a concrete frame rarely holds unaided, and it is that survey — not the panel quantity — that decides whether hanging starts on schedule. Out-of-tolerance edges are corrected with shims and extended anchors, neither of which this number orders.
Sweeping the Slab Edge Before Committing to a Bracket
Survey the edge condition floor by floor and record it — offset from the control line, top-of-slab level, and any local blowout, sag or rebar congestion at each mullion position. Concrete construction tolerances are governed by ACI 117, Specification for Tolerances for Concrete Construction and Materials, and structural steel erection by the AISC Code of Standard Practice for Steel Buildings and Bridges; neither promises a straight line. The wall has to be straight anyway, and that entire difference lives in the bracket.
Adjustability must exist in three axes at that connection: in and out to set the face plane, side to side to hit the module line, and up and down to set the datum. Slots with serrated or toothed washers give the first two; shim packs and levelling screws give the third. Once set, the connection has to be locked so it cannot creep — torqued through hardened washers, welded off, or pinned, exactly as the shop drawings and the connection detail require.
Post-installed anchors are the point at which field improvisation stops. Edge distance, embedment depth, hole cleaning and installation torque come from the anchor's own evaluation report and the engineer of record. A mullion landing over a congested edge gets a re-detailed bracket, not a relocated hole. Cast-in channels remove most of that argument, but only when they were surveyed — and the survey is the first thing to slip when the concrete crew is running ahead of the facade.
Log the interfaces below before ordering shims, because each one has a different owner and a different allowance.
| Interface | What it absorbs | Who sets the allowance |
|---|---|---|
| Slab edge to bracket | Concrete placement deviation and level variation | Structural drawings and the concrete tolerance standard |
| Bracket to mullion | Face plane, module line and datum adjustment | Curtain wall shop drawings and the system supplier |
| Mullion stack joint | Thermal growth and frame movement | System design, set against install temperature |
| Glass to frame | Edge clearance, face clearance and bite | Glazing details and the unit fabricator |
| Wall to adjacent construction | Building movement plus construction tolerance | Perimeter joint design, per ASTM C1193 |
The slab-edge survey turns into a bracket schedule here: quantity follows mullion spacing across the elevation, never an area figure lifted off the drawings.
The total run length of curtain wall requiring anchor attachment to the structure.
The maximum center-to-center spacing between anchor brackets, per the system's engineering.
Anchor brackets needed
27 brackets
- Bays along the run
- 26
- Bracket centres, as fixed
- 5 ft
They open the calculator with your figures already in it
Curtain Wall Anchor Bracket Spacing Calculator: 27 brackets — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The spacing is something you type in, not something this page checks. The same 40 m run returns 28 brackets at 1.5 m spacing and 15 brackets at 3.0 m spacing, and nothing in the arithmetic objects to either figure. Whether that spacing is adequate depends on the wind pressure acting over each bracket's tributary panel area, the dead weight hung on the bracket, and the rated capacity of the bracket and its fixing into the slab edge — none of which is calculated here.
- The result is one line of brackets along one run at one level. A curtain wall is normally tied back to the structure at every floor slab, so the building total is this figure multiplied by the number of anchorage levels: a 40 m (131 ft) perimeter over eight storeys is eight runs of 28, not 28 brackets in total.
- This counts anchor positions, not the hardware schedule. Many systems place a separate dead-load bearing bracket and a wind-load tie-back at each attachment point, and every bracket needs its own bolts, shims and packers, so the purchase list can run to two or more brackets and four or more fixings per position counted here.
- The run is divided by whatever spacing you enter, with no reconciliation against the mullion or panel grid. Brackets have to land on mullion centrelines, which means the spacing must equal the panel module or a whole multiple of it. Entering 1.5 m spacing on a 1.2 m panel module gives 28 brackets that mostly fall between mullions, where the 1.2 m module gives 35 that all land on one.
- The plus one assumes an open run with two free ends. On a perimeter that closes back on itself the final bracket is the first bracket, so the closed-loop count is one lower than shown: a 60 m (197 ft) closed perimeter at 1.5 m (5 ft) spacing returns 41 here where the loop has only 40 distinct positions.
What One Mullion Is Actually Holding
Tributary area decides everything about the anchor below it. An intermediate mullion carries half the bay on each side, floor to floor: vision units, spandrel units and their back pans, insulation, any shadow box, the transoms, and its own extrusion. Corner and jamb mullions carry less glass and often more torsion. A mullion beside a wide make-up bay carries more than the schedule assumed when that schedule was drawn for a repeating module.
Dead load goes to one anchor per mullion. Say that out loud to the crew, because the commonest field error on a stick system is bolting both brackets up solid, which turns the slotted wind anchor into a second dead-load point and locks the frame against thermal movement. The slot exists so the mullion can breathe; a bolt torqued down through the middle of it quietly converts a designed system into an undesigned one.
Wind is the other half of the mullion's life, and it is set by the governing code — in the United States by ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, as adopted and amended by the local building code, with corner and parapet zones taking substantially more than the field of the wall. Elevated zones show up in the shop drawings as heavier extrusions or closer anchor spacing, and those units are not interchangeable with field units even when the profile looks identical.
Before you accept a bracket, a shim stack or a weld length at the slab edge, you need the weight that one mullion is asking that connection to hold, because from here down every decision is sized by it.
The width of glass panel area that this mullion supports, typically half the spacing to each adjacent mullion.
The height of glass panel area that this mullion supports.
About 25 kg/m² (5.1 lb/sq ft) for 10 mm glass. The glass makeup's surface density, including any laminated or insulated glass layers.
Mullion tributary dead load
256 lb
They open the calculator with your figures already in it
Curtain Wall Mullion Tributary Dead Load Calculator: 256 lb — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- The answer is a mass — kilograms, or pounds once the imperial toggle is on — not a force. Convert it to a force before it enters any structural check, and do not paste it into the related mullion deflection calculator: that one asks for a horizontal wind line load per unit length, which is a different quantity from this vertical dead weight.
- Only the vertical dead weight of the glass is counted. Wind pressure, seismic and thermal movement, and any live or impact load sit outside this figure, and on most curtain wall mullions it is the wind bending case rather than the dead weight that sizes the section.
- The tributary-area split assumes equal panels centred on evenly spaced mullions and glazed on both sides. It does not hold at a building corner, at a jamb, or beside a door or opening where a mullion picks up glass on one side only, and it spreads the weight over an area when the glass actually lands on two setting blocks near the quarter points of the transom below.
- Nothing here checks where the weight goes once it reaches the mullion. The anchor, its bolts or embed, and the slab edge that receives it are a separate calculation, and whether the mullion is top-hung or bottom-bearing decides whether this dead load puts the section into tension or into compression, where buckling has to be checked as well.
- Tributary width is limited to 3 m (10 ft) and tributary height to 6 m (20 ft), capping the tributary area at 18 m² (194 sq ft). A mullion carrying glass through a two-storey atrium, or a module wider than 3 m, is outside those bounds — the field clamps the entry back to the limit and says so, and the load it then reports falls short of the real one in the same proportion.
Standing Frames, Splicing Them, Leaving Them Room
Mullions go up in floor-to-floor lengths and meet at a stack joint, sleeved so shear and wind pass through while the joint stays free to open and close. That gap is set to the temperature of the day it is installed — a joint closed up on a cold morning has nowhere to go in August, and a joint gapped generously in July can open past the sleeve's engagement in a hard frost. Shop drawings give a setting dimension against ambient temperature; use the actual reading, not the season.
Erect against string lines and a laser plane rather than off the previous mullion. Bay-to-bay error is invisible; six bays of the same error is a visible bow in the reflection, and glass reflections are the most unforgiving inspection tool on any elevation. Check plumb, face plane and module width at every mullion as it lands, before the transoms lock the geometry in.
Transoms set the horizontal line and the sightline. They arrive cut to length and should never be trimmed to suit a bay that is out — a shortened transom leaves the gasket short, and a forced transom pushes the mullion out of plumb and pre-loads the anchor. If a bay will not take its transom, the mullion spacing is wrong and the correction belongs upstream.
Protect the aluminium through all of it. Handling, storage and site protection of finished aluminium are covered by AAMA CW-10, Care and Handling of Architectural Aluminum from Shop to Site. Anodised and painted finishes mark permanently under mortar splash, weld spatter and alkaline runoff from fresh concrete, and nothing brings back an etched anodic coating.
The Glass Rides: Blocks, Clearances and Bite
Setting blocks carry the entire weight of the unit. They sit at the quarter points of the sill unless the fabricator's detail says otherwise, sized in length, width and hardness to the unit's weight and thickness. Two blocks, both in the same plane, both supporting the full thickness of the insulating unit so the outer lite is carried as well as the inner. A block short of the outer lite loads the edge seal in shear, and the edge seal is a moisture barrier, not a structural component.
Edge clearance, face clearance and bite are the three dimensions keeping glass out of contact with metal. Lose edge clearance and the unit bears hard on the frame at the first thermal cycle or the first bit of frame deflection; lose bite and the pressure plate has nothing to hold in a wind event. Glass product requirements for heat-treated, laminated and insulating units sit in ASTM C1048, ASTM C1172 and ASTM E2190 respectively, while the fabricator's own tolerance sheet governs the dimensions you actually glaze to.
Structurally glazed bays change the rules. Sealant there resists wind, not weight — dead load still goes to setting blocks and, where the detail calls for it, to a mechanical retention device. Structural silicone glazing practice is set out in ASTM C1401, Standard Guide for Structural Sealant Glazing, and structural glazing is generally a shop operation for sound reasons: cure conditions, substrate preparation and adhesion testing are all difficult to control from a hoist.
- Confirm the gutter, weeps and end dams in the bay are clear and undamaged before any glass goes in front of them.
- Set the interior gasket or tape run, cutting vertical lengths long and butting them hard into the horizontals so the joints stay in compression.
- Place setting blocks at the sill quarter points, checking each sits under both lites of the insulating unit.
- Land the unit onto the blocks with cups or a rig — never levering off the frame edge or dragging across the block.
- Fit the pressure plate and run fasteners from the centre outwards to the specified torque, seating the gasket evenly rather than crushing it at the ends.
- Snap covers only once the joint has been checked, since pulling a cover later to correct a gasket usually damages it.
Two blocks per unit sounds like nothing until a whole elevation of vision bays is counted, and a short box on the hoist stops glazing outright instead of merely slowing it.
The total number of glass lites or windows to be set.
The number of setting blocks placed beneath each lite.
Total setting blocks needed
80 setting blocks
They open the calculator with your figures already in it
Window Setting Block Count Calculator: 80 setting blocks — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The result is a count, not a size and not a load check. Setting block length, width and hardness are chosen from the weight and area of the lite and the bearing stress the block can carry, and a block narrower than the full glass edge will load only one pane of an insulating unit instead of the spacer and both panes; none of that is tested by multiplying lites by blocks, and the dimensions and durometer come from the glass manufacturer's glazing instructions.
- A single blocks-per-lite figure is applied to every lite in the batch, and the input accepts only two to four. A mixed order in which the oversized lites take four blocks and the rest take two must be run as two separate calculations and the totals added, and a lite whose manufacturer calls for more than four blocks cannot be entered at all. The total is an exact product, with no spare blocks added for units dropped, cut wrong or lost on site.
- Nothing here positions the blocks. The quarter-point placement the calculation assumes, the edge clearance the block sets between glass and frame, and the rule that a block must never sit across a weep hole or dam the glazing pocket are all outside the count, and a correct quantity fitted in the wrong place still traps water against the edge seal.
- Only the blocks that carry the lite on its bottom edge are counted. Side blocks and anti-walk blocks on operable sashes, top blocking, and the shims and spacers used to centre the glass in the pocket are separate items and are not included in this total.
- Block material is not considered anywhere in the calculation. An elastomer incompatible with the glazing sealant can stain the edge seal of an insulating unit or inhibit the cure of structural silicone, so the compatible compound has to be specified from the sealant manufacturer's compatibility data independently of the quantity.
Water Comes In; the Only Question Is Where It Leaves
A pressure-equalised system does not try to stop water at the outer face. It admits a little, drains it, and equalises the chamber behind the outer seal so there is no pressure difference driving the leak deeper. That logic rests on three continuous elements: the outer weather seal, the drained and vented cavity, and the inner air seal. Break any one and the assembly reverts to a face-sealed wall nobody designed.
Gutters at each transom collect what gets past the gaskets and route it to weeps in the mullion or out through the horizontal. Those routes need to be open, sloped as detailed, and closed with end dams that were actually installed rather than merely delivered. Drill a fixing through a gutter and you have built an internal downspout aimed at the ceiling below — a defect that surfaces months later, three floors from where it started.
Test the drainage before it disappears behind glass and back pans. A hose run into the gutter of a completed bay, watched from inside, catches unsealed end dams and blocked weeps while the correction is still a short job on an open frame. Once interior finishes are up, the same defect needs glass removed from a swing stage.
Spandrel zones are the thermal weak point and the condensation risk. Back pan seals, insulation continuity and thermal break alignment all have to agree with the vision-area detail, and the dew point must fall within the insulation rather than on the back of the pan. Interior humidity, climate zone and the assembly's thermal performance drive that outcome, and the governing energy code together with the project's thermal analysis decides what is acceptable.
Whether the dew point lands inside the insulation or on the back of the pan is arithmetic rather than judgement, and the spandrel is the bay where it gets decided.
The interior air design temperature used for the winter condensation check.
The exterior winter design temperature for the project location, typically a 97.5% or 99% design condition.
The overall U-factor of the spandrel/vision glass assembly being checked.
The interior air film coefficient, commonly taken as 8 W/m²K per standard convention.
The dew point of the interior air, based on its temperature and relative humidity.
Interior glass surface temperature
57.2 °F
The calculated interior glass surface temperature stays above the interior air's dew point shown below, so condensation is not predicted under these steady-state design conditions. This checks room-side vision/spandrel glass surface condensation risk only, per NFRC 500/AAMA 1503 steady-state methodology. Shadowbox CAVITY condensation — a distinct, common failure mode driven by trapped cavity moisture and solar vapor drive behind the spandrel panel — is NOT covered here and needs separate ventilation/vapor analysis per GANA/NGA guidance. No risk predicted under these conditions is not the same as none. The conditions are the ones you entered, and one surface is not the assembly.
- Interior air dew point
- 50 °F
They open the calculator with your figures already in it
Curtain Wall Spandrel Condensation Risk Checker: 57.25 °F — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Models conduction only. Warm indoor air pushed out through an imperfect air seal — a splice joint, an anchor pocket, a gasket that has taken a set — carries far more moisture into the assembly than diffusion does, and it condenses wherever it cools rather than on the face being checked. A wall that passes on paper can still run with water, or ice, inside the mullion, and the first sign of it is usually a stain at the head of the panel below.
- Uses the exterior air temperature, and the outer surface does not sit at it. On a clear, calm night the glass radiates to the sky and settles several degrees below the surrounding air, which drags the interior surface down with it — and those are exactly the nights condensation forms. A result that clears the dew point by a degree or two at the design air temperature has no margin left for the sky.
Movement the Wall Must Swallow Without Complaint
Three movements act on a completed elevation at once: thermal expansion along long aluminium runs, interstory drift under wind or seismic demand, and vertical movement of the structure itself as floors deflect under live load and concrete creeps. The wall accommodates all of them at joints — stack joints, slotted anchors, split mullions, the perimeter seal — and it accommodates none of them through the glass.
Head clearance under the slab above deserves particular attention on concrete frames. A slab that deflects onto the head of a curtain wall pushes vertical load into a member sized only for wind and its own hanging weight, and the first symptom is glass breakage in the bay beneath the longest span. Racking behaviour under drift is evaluated by AAMA 501.4, the recommended static test method for curtain wall and storefront systems subjected to seismic and wind induced interstory drift, and the design drift value comes from the structural engineer rather than the facade supplier.
Perimeter joints between the wall and adjacent construction — heads, jambs, sills, returns at cores — absorb construction tolerance and building movement together. Sealant joint design and application guidance sits in ASTM C1193, Standard Guide for Use of Joint Sealants, and joint width is a design output, not a field decision. A joint sized on site to whatever gap turned up will fail in extension or compression on the first full temperature cycle.
Proving It, Then Handing It Over
Field testing starts early, not at completion. Pick a representative bay among the first units installed and test it while the crew's habits can still be corrected: air leakage by ASTM E783, Standard Test Method for Field Measurement of Air Leakage Through Installed Exterior Windows and Doors, and water penetration by ASTM E1105, by uniform or cyclic static air pressure difference. AAMA 503, the voluntary specification for field testing of newly installed storefronts, curtain walls and sloped glazing systems, sets out how those field tests are applied and at what pressure relative to the specified design pressure.
Laboratory performance comes from the mock-up, tested under the exterior wall methods of AAMA 501 together with ASTM E330 for structural performance, ASTM E283 for air leakage and ASTM E331 for water penetration. Field results are held to a lower pressure than the lab, deliberately, and calling for a field test at full design pressure is a specification error that becomes a dispute. Read what the contract actually requires before the rig is booked.
Closeout is mostly evidence. Anchor torque and weld records, shim schedules by mullion, stack joint settings with the ambient temperature they were set at, drainage checks, gasket splice locations, and the test reports with their remediation. A wall with that file behind it survives a leak investigation years out; a wall without one gets its glass pulled while everybody argues about who to blame.
Take-off and first-fix checks
Everything below is sized by the load path — count the modules, weigh the mullion, then buy the connection.
- Bracket and shim schedule by mullion — Built from the slab-edge survey rather than the drawn edge; one line per anchor position, with the dead-load anchor identified.
- Module, corner and make-up bay count — Standard units, corner assemblies and make-up bays counted separately against the as-built elevation dimension.
- Setting blocks and edge spacers — Sized to unit weight and thickness, placed at the sill quarter points, supporting both lites of the insulating unit.
- Gasket and wedge runs — Vertical lengths cut long and compressed into the horizontals; order allowing for splices and offcuts.
- Stack joint setting record — Gap set against the ambient temperature reading on the day and logged floor by floor.
- Drainage proving — Gutters, end dams and weeps hose-tested bay by bay before glass and back pans close them in.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.