Installing a Curtain Wall

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.

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.

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.

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.

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.

1. Confirm the gutter, weeps and end dams in the bay are clear and undamaged before any glass goes in front of them.
2. Set the interior gasket or tape run, cutting vertical lengths long and butting them hard into the horizontals so the joints stay in compression.
3. Place setting blocks at the sill quarter points, checking each sits under both lites of the insulating unit.
4. Land the unit onto the blocks with cups or a rig — never levering off the frame edge or dragging across the block.
5. 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.
6. Snap covers only once the joint has been checked, since pulling a cover later to correct a gasket usually damages it.

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.

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.