Facade Movement and Tolerances

A facade, in the slender tower engineering Michael Stern concerns himself with, is treated as something that moves: the frame behind it shortens, creeps and deflects, the skin expands and contracts with the weather, and the whole tower sways, so every joint in a cladding system is sized to absorb motion that the structural code only partly describes. The rule is older than the problem. Deflection limits were written as fractions of a span for an era of short spans and heavy masonry, and the long spans, thin slabs and factory made skins of a modern tower have outgrown them. Below are the rule, the movements it misses, and the way joints, anchors and tests make up the difference.

What the code limits

The building code limits how far a beam or slab edge may deflect, but it does so by span. Structure magazine’s guidance on structure and enclosure explains that the International Building Code sets deflection limits in a table based on member length, intended to protect finishes, and that longer spans and newer framing have produced deflections many enclosure systems cannot take even when the table is satisfied.⁴ The material standards add their own figures: a serviceability limit for concrete, a limit for masonry that once carried an absolute cap of 0.3 inches until the 2008 edition dropped it, and steel guidance that defers to design guides on serviceability and facade attachment.⁴ The article’s own suggestion is an absolute number rather than a ratio, roughly 3/8 inch of differential deflection for infill walls and about half an inch where unitized curtain wall absorbs the movement at its stack joint.⁴

Cantilevered perimeters show why ratios fail. Each member of a cantilevered edge can meet its own span limit while the deflections add up at the tip, and it is the sum that the facade has to live with.⁴

Figure

How fast materials grow with heat

• Aluminum framing2
• Steel1
• Concrete1
• Clay masonry0.5

times the rate of concrete

What the frame does after the skin is hung

A tower is not finished when its cladding starts. Structure magazine’s list of the movements a structural engineer owes the facade designer runs: creep, slab edge deflection before the wall is installed, deflection under the wall’s own weight, live load deflection afterward, construction tolerances and the sway of wind and earthquake, with column shortening added for taller buildings because enclosure installation usually begins before the long term movement has happened.⁴ Even floors framed identically can deflect differently when one tenant chooses thick stone flooring and the next chooses carpet, or when one floor sits unfinished for years.⁴ WFM Media’s article on facade engineering for wind gives a similar list from the facade side, differential column shortening, story drift, racking, slab and beam edge deflection and vibration, and names the failure modes when coordination is poor: panel fallout, connection failure, glass rotating in its frame and joints that go irregular.¹

Creep is the slow one. A recladding story in Architectural Record describes a 1962 Manhattan apartment tower whose lightweight concrete frame kept shrinking for decades; the glazed brick had been laid without relieving angles, so the shortening frame pressed on the masonry until it bowed, spalled and let water in.²

Figure

What the frame does after the skin goes on

1. 01
   CreepConcrete keeps shortening and deflecting for years after it is cast, so a wall hung early rides a frame that has not stopped moving.
2. 02
   Dead loadThe slab edge sags under the weight of the units themselves as each floor of cladding is hung.
3. 03
   Live loadLater floor loads, furniture and finishes deflect spandrels further, often estimated at half the code live load.
4. 04
   Column shorteningTall columns compress and creep, lowering upper floors relative to the ground.
5. 05
   DriftWind and seismic sway shift one floor past the next, racking every panel in between.
6. 06
   ToleranceThe frame was never built exactly where the drawings put it, and the anchors absorb the difference.

What the skin does on its own

The facade moves without any help from the frame. Temperature changes the length of every material at a different rate; Structure magazine puts aluminum at roughly twice the rate of concrete or steel and four times that of clay masonry, which makes the thermal calculation fundamental to joint width in an aluminum curtain wall that sees the whole span of exterior temperature.⁴ Masonry veneers add a permanent change on top of the seasonal one, since fired clay expands over its life while concrete products contract.⁴ Metal skins are detailed for it directly. The Copper Development Association’s guidance for exterior copper pairs fixed cleats with sliding ones and adds expansion joints at intervals, so that a long run of sheet can lengthen in the sun and pull back at night without tearing its fastenings.⁷ Even a sealed spandrel cavity moves: a Perkins and Will study of shadow boxes recommends that back pans be attached to the framing with flexible material rather than rigidly, so heat driven expansion and pressure cycling do not deform them, and that the cavity be deep enough that the pan never touches the glass when wind bows it inward.⁶

Figure

The thermal cycle a mockup survives

complete hot and cold cycles applied to the whole specimen

3

held at each extreme once the temperature is reached

2 hours

low exterior temperature, with an optional 20 below for cold regions

0 F

high exterior temperature, with design wind loads applied during both extremes

180 F

Sway and drift

Lateral movement is the better defined half. Whole building drift limits, Structure magazine observes, are set clearly in the code and in ASCE 7 and are not subject to creep or long term effects, which is why the difficult coordination is usually vertical.⁴ The facade still has to rack. The Construction Specifier’s description of the AAMA 501 test sequence sets the drift test at a horizontal movement of 0.010 times the taller adjacent story height, cycled three times in each direction, with the allowance that a lite may crack but must stay in its opening and that seals may be repaired afterward; a second pass at 1.5 times the design displacement follows the structural overload test.³ Kora Studio describes standard unitized systems as able to absorb up to 75 millimeters of relative inter story displacement without glass breakage or loss of weather tightness, one reason the system is favored in seismic regions.⁸ WFM Media’s case study of a tower in Baku recounts the facade team agreeing slab edge deflection of 1.5 millimeters per meter under dead load with the structural consultant and sizing the unit joints to a drift limit of the story height divided by 300.¹

Tolerances before the first unit arrives

Movement is what happens after installation; tolerance is the misfit already built in by the time the units arrive. Kora Studio records the mismatch at the heart of every unitized job, a structural frame built to about 25 millimeters either way receiving a wall system designed to about 12, and concludes that the anchors need adjustment on all three axes or installation runs into trouble.⁸ Structure magazine makes the same point from the structural side: the facade is always set to a theoretical position regardless of where the frame ended up, movements before installation are absorbed by that adjustability, and tighter erection tolerances buy a more economical facade with smaller eccentricities at the anchors.⁴

Heavy cladding sharpens the stakes. Use Natural Stone’s guide to anchorage treats each stone panel as a beam spanning between its anchors, with flexural stress rising with the square of the span and falling with the square of the thickness, and names the three transition points where anchors fail: the kerf cut into the stone, the anchor itself, and its connection to the frame.⁵ Wind, thermal movement and seismic movement all travel through those points, and the guide urges that a cladding engineer be consulted at the start rather than after the details are drawn.⁵ The systems that carry this load are described in the entries on unitized curtain wall and metal cladding.

Figure

Two states a joint is checked against

	Serviceability	Ultimate
What it represents	Pressures the facade meets regularly through its life	The rare extreme event of the design life
Deflection	Must stay within limits	Some permanent deformation accepted
Joints and seals	Must not open; weather seal maintained	Must not let panels detach
Fixings	Hold without cracking or slipping	Hold; the load path stays intact
Typical ratio	Base case	About 1.5 to 3 times the serviceability pressure

Sizing the joint and proving it

Every one of these movements ends up in a joint width. Structure magazine points out that sizing horizontal sealant and curtain wall joints is usually outside the structural engineer’s scope, yet the engineer must supply the criteria, and that where floor stiffness differs, as between a stiff lowest level and the elevated framing over it, the joint has to take the whole of the elevated frame’s deflection rather than a share.⁴ Specifiers then split the demand in two. A specifier’s guide from Valmond and Gibson separates everyday wind, under which panels stay within their deflection limits, joints stay closed and seals keep working, from the once in a lifetime wind, under which a panel may bend permanently but nothing may come off.⁹

The proof is a mockup. The Construction Specifier’s sequence runs the specimen through leakage tests, structural loads, drift cycles and then thermal cycles between the extremes of a cold and a hot day with wind loads applied at both, repeating the leakage tests after each stage to show the joints still work once they have moved; the entry on performance mockups follows that sequence station by station.³