Mockups, Shop Drawings and Submittals
The submittal, the rule under which nothing on a building is fabricated until a drawing of how it will be made and a sample of what it will look like have been approved, is the quiet discipline behind every facade on the kind of building Michael Stern writes about, and it holds because a mistake caught on paper costs almost nothing while the same mistake caught on the wall costs a facade. The detailing practice AMC Engineer calls the curtain wall shop drawing package the longest lead and highest risk submittal on most tall buildings, the one place where the contractor’s engineer designs part of the building and where a single missed millimetre of thermal movement shows up as a cracked seal three summers later.1 What follows sets out the rule, where it came from, and the record of what it has caught.
The rule
A shop drawing is not a copy of the architect’s drawing. AMC Engineer draws the line: the architect’s facade drawings establish what the envelope must be, the module, the sightlines, the performance criteria and the look, while the shop drawing establishes how one named system, a particular profile series, glass makeup and anchor, delivers that result on this structure with its actual tolerances, and the two documents come from different hands and carry different contractual weight.1 Because the set is reviewed and stamped before anything is extruded or cut, facade contractors often call it the approval package, as distinct from the internal fabrication drawings that follow.1
Two authors, one wall
The rule exists because responsibility is split. Structure magazine’s account has the design, fabrication and erection of facade systems usually subcontracted to a specialty contractor whose team includes manufacturers, erectors, detailers and its own engineer, and that the project’s design team delegates the system and its connections to that engineer within performance requirements, loads and deflection limits fixed in the specification.2 The industry documents it reviews, from the precast institute’s handbooks to the steel institute’s design guide on facade attachments, agree on the failure mode: in the words of the Council of American Structural Engineers that the article quotes, the primary failure on projects with a specialty engineer is the lack of coordination and delineation of responsibility.2 Who designs the clip that holds a panel to a slab edge is settled, or left dangerously open, in the contract documents long before the submittal arrives.
The submittal cycle for a facade
- Embeds firstEmbed and bracket layouts are issued as an early separate package, approved before the floor they sit in is poured.
- Approval setThe facade contractor submits elevations, details, panel schedules and sealed calculations for review.
- Three reviewersThe architect reviews design and performance, the structural engineer checks the delegated calculations, and finish samples run their own loop.
- Two cyclesA second round of review is normal on curtain wall before the set is stamped.
- Fabrication drawingsInternal shop drawings follow approval, and glass and aluminium are ordered against them.
The embed that must be released first
The sequence has a hard deadline inside it. AMC Engineer describes the connection between facade and frame as the single most consequential detail in the package, carrying wind, self weight, seismic demand and thermal movement at once, and usually the first thing released: embed and bracket layouts go out as a separate early deliverable, a few sheets per floor, and must be approved and in the hands of the structural contractor before the relevant floor is poured.1 Miss that window and the remedy is post fixed anchors that have to be re engineered.1 It is also why a unitized package reads differently from a stick built one, dominated by panel schedules and reconciliation of the as built anchor survey rather than by site setting out; the systems themselves are compared under unitized curtain wall.1
The sample
The second half of the rule is the mockup, and the smallest version of it is a training exercise. LearnGlazing built one two storeys tall on a timber frame to teach project managers their own product, and its sequence mirrors the trade’s: the drafting team produced shop drawings as the blueprint, a takeoff listed every part, the frame went up stick by stick on one side and as a preassembled ladder on the other, wind load and dead load anchors were set with the steel coated against contact with the aluminium, gaskets and foam closed the system, and the glass went in from the bottom with the weep holes aligned.3
The visual mockup
On a real building the first mockup is for the eye. The architects Payette describe reviewing one for a Cambridge research institute, built on site to confirm colours and materials and to coordinate the transitions between stone base, timber and glass curtain walls, perforated screens, metal fascia and soffits, where several subcontractors met for the first time; the fascia went a shade darker and the stone base type changed before the mockup was finished, because, as the firm put it, there is something to be said for seeing it all together at full scale.4 The New Zealand facade maker INSOL adds the reason a visual mockup stands on the building’s own elevation: placed at the designed orientation and height it shows how the facade reacts to daylight and to viewing angle, and it has caught unintended consequences, from a shadow cast across a London footpath to the whistling of glass fins on a Manchester tower, that no drawing would have.5
The mockup for the eye against the mockup for the chamber
| For the eye | For the chamber | |
|---|---|---|
| Question | Does it look right, in this light, at this scale? | Does it keep out air and water and stand up to wind and movement? |
| Where it stands | On site, on the elevation it is meant for, so daylight falls on it as it will | In a laboratory chamber or on a test frame |
| Who gathers round it | Owner, architect and the trades whose work meets there | The facade contractor, the consultant and the testing agency |
| What it settles | Colour, texture, joints and transitions between trades | Anchorage, gaskets, drainage and the system's structural margin |
| Price of a change | A shade darker or a different stone, before fabrication | A redesigned connection, before thousands of units are made |
The performance mockup, briefly
The second mockup is for the chamber. The precast maker BPDL separates the two: the visual mockup is a draft of the facade’s layout, finishes and materials, while the performance mockup is built for testing, made in the factory by the normal production method so it is as close as possible to the real casting, and used to confirm air and water tightness, resistance to thermal movement and condensation and stability under structural movement, on New York projects among others.6 Glass Magazine calls a preconstruction laboratory mockup relatively inexpensive insurance, and insists on a true to size specimen built with the job’s own anchorage, glazing, sealants, gaskets and splices, two storeys by two bays as a rule and better with a corner in it, because one bay one storey high proves nothing.7 The tests themselves, and what they measure, are covered in facade mockups and testing.
Where the cheap errors live
The price of an error rises with every stage it passes. The Construction Specifier sets out the laboratory sequence that finds the dear ones early: a preload to half the design pressure, air infiltration at 300 pascals, static water at 720 pascals for fifteen minutes, dynamic water under a propeller, deflection under positive and negative load with limits of span over 175, storey drift cycles, three thermal cycles between minus 18 and 82 degrees, and finally an overload to one and a half times the design pressure with no breakage or permanent damage allowed.8 Each test that fails in the chamber is a redesign on paper; each that would have failed on the building is a replacement in the field.
A laboratory mockup test sequence
- PreloadHalf the design wind pressure, held ten seconds, then the wall and anchors inspected.
- AirAir leakage measured under a set pressure across the specimen.
- WaterStatic water spray under pressure, then a dynamic test with a propeller driving the rain.
- LoadPositive and negative design pressures applied while deflections are read.
- Movement and heatStorey drift cycles and thermal cycles from well below freezing to above 80 C, with the leakage and water checks run again after each.
- OverloadOne and a half times the design load, with no breakage or permanent damage allowed.
What the record shows
Three cases fix the scale. A fascia colour changed a shade at a Cambridge visual mockup cost a conversation.4 A missed embed window costs post fixed anchors and the engineering to justify them.1 And Building Design + Construction records the case that justifies the whole rule: a 22.7 by 12 metre mockup of a unitized stainless steel clad curtain wall for an office tower in Asia, built by the factory that would make every module and shipped to Florida for testing, passed its air, water and drift tests and then lost four frames to the ground when its anchorage failed under a negative load of 2.5 kilopascals, the suction a storm would have exerted on the real facade, leading to a redesign of the connections before production began.9 The ledger reads a shade of paint, a set of anchors, and a facade.
