Accommodating Movement in Building Envelope Materials

I S S U E 4/2017 VOLUME 34 NUMBER 4 Journal of architectural technology published by Hoffmann Architects, Inc., specialists in the rehabilitation of building exteriors. Accommodating Movement in Building Envelope Materials Richard P. Kadlubowski, AIA and Christopher M. DeRosa, AIA, PE W e may think of the building are anticipated, the design should ac- envelope as an inanimate object, but, commodate each material’s propensity really, its components can be quite for expansion and contraction, and the mobile. Building materials grow, shrink, building enclosure must allow for dif- shift, bulge, deform, and elongate in fering – and, often, opposing – move- response to stresses and fluctuations ment of adjoining materials. in the environment, and these dimen- Calculations and detailing for seem- sional changes often impose strain on ingly small movements in the building adjacent elements. envelope may seem cumbersome and Where the forces of movement are time-consuming, however the forces not foreseen during design and con- that develop from these deformations struction, evidence of the struggle will can be immense. Neglecting to include emerge, in the form of cracks, spalls, adequate expansion joints, control displacement, broken glass, warped joints, bond breaks, flexible anchorage, metal, and, eventually, breakdown of slip planes, and other means to allow the assembly. for changes in dimension will ultimate- Failure to anticipate and allow for ly expend more time and money in movement in building materials com- remediation than a prudent approach pels imparted stresses to find their would require at the outset. Where own path to release, which is nearly al- a building owner is faced with the ways an undesirable one. Not only are unfortunate discovery that such prac- cracks unsightly, they also open path- tices were not employed at the time ways for moisture penetration into the of construction, all is not lost. There building enclosure, which compounds are rehabilitative measures that can be the problem as materials swell or cor- taken after the fact, but unless the un- If expansion joints are not provided, the rode, placing further outward pressure derlying issue of restrained movement building will make its own. on adjoining components. is addressed, cosmetic fixes to cracked masonry or displaced stone will do To design for movement in the build- little to keep ahead of the problem. ing envelope is to identify the properties of the materials used in construc- With attention to the causes of tion, as well as the environmental and movement, including fluctuations in siting conditions of the building, and to ambient temperature and moisture, develop a design that either minimizes applied loads, chemical interactions, or allows for such movement. For and materials’ propensity to expand or instance, where dimensional changes shrink over time, the general behavior Richard P. Kadlubowski, AIA, Senior Vice President and Director, Architecture, manages Hoffmann Architects’ Virginia office, where he diagnoses and treats distress in building enclosures. Christopher M. DeRosa, AIA, PE, Project Architect, brings both architecture and structural engineering to the challenges of building movement. JOURNAL of each element in relation to others to 100°F, for which the panel would From then on, it expands as it absorbs can be predicted. To allow for factors increase in length 0.0000098 in./in./°F moisture from the atmosphere and that might impact as-built conditions, a x 120 in. x 100°F, which is 0.1176 in., precipitation. Drying the brick at nor- conservative approach to incorporat- or just under 1/8 in. Although this mal temperatures will not reverse this ing fluidity in the building envelope de- number might seem small enough to expansion process, which is greatest sign should limit movement if possible be negligible, restraining the copper in the first few weeks after firing but and accommodate material move- sheet by hard fastening it at both ends continues at a slower rate for years. ment, without imposing restraint. would lead to bowing and buckling as Just how much a given brick will ex- the temperature rises and the cop- pand depends mainly on raw materi- Types of Movement per expands, and, as the temperature als, but firing temperature is also a When designing for movement of drops, the shrinking metal would strain factor. A brick fired at lower tempera- building enclosure materials, architects at the fastening points, potentially tear- tures will expand more than the same and engineers must consider dimen- ing itself apart. brick fired at higher temperatures. sional changes that can Concrete shrinks as it cures, COEFFICIENTS OF LINEAR THERMAL EXPANSION arise from the effects of the due to the natural moisture environment, including tem- MATERIAL AVERAGE COEFFICIENT loss that occurs soon after the perature and moisture varia- OF LINEAR EXPANSION concrete is cast. As with brick, (x10-6 IN./IN./°F) tions, as well as elastic and the initial dimensional change inelastic deformation from BRICK 4.0 of concrete is irreversible and applied loads and volume CONCRETE (STRUCTURAL) 5.5 tends to be significantly greater changes due to chemical in- CONCRETE (NORMAL POURED) 8.0 than any ensuing moisture- teraction. Designers should related volume fluctuations. also take into account the STAINLESS STEEL 5.5 TO 9.6 The degree to which con- tendency of a given material TYPICAL STEEL 7.3 crete will shrink during curing to shrink or expand over its COPPER 9.8 depends on the properties service life, and the ways in ALUMINUM 12.8 of the concrete mix, including which neighboring materi- GLASS (PLATE) 5.0 water-cement ratio, cement als with opposing volume composition, and aggregate changes might pull or push GLASS (HARDENED) 3.3 type, as well as size of the con- against each other. crete unit, curing conditions, and size Temperature Movement Moisture Movement and placement of embedded reinforc- Changes in temperature cause most Porous building materials such as brick ing steel. building materials to expand and and concrete expand as they absorb Wood is the building material most contract. Unless the movement is water and contract as they dry. Like subject to dimensional changes due to restrained, the changes are generally temperature movement, moisture moisture. As part of the commercial reversible. As the temperature rises, deformation is generally reversible, seasoning process, the wood shrinks materials tend to expand, then con- except in the case of the initial shrink- as moisture content diminishes from tract again as the temperature drops. age or expansion that takes place the initial fiber saturation point, until How great these volume changes are with some building materials. Often reaching equilibrium with the environ- depends on the type of material and considerably greater than subsequent ment. After this initial shrinkage, chang- may be expressed in terms of coeffi- reversible moisture-related dimension ing moisture content in the wood will cient of thermal expansion (see table). changes, expansion or contraction due continue to cause the wood to swell Projected thermal movement is the to natural aging can yield significant and shrink. The degree of shrinkage product of the coefficient of thermal internal stresses that may manifest as differs in the radial, tangential, and expansion, the overall temperature cracks, spalls, open joints, and leaks as longitudinal directions, with the latter change, and the length of the compo- restrained elements struggle to release typically so small as to be considered nent. As an example, take a 10-foot built-up pressure. negligible from a design standpoint. sheet of copper roofing at a building Brick is smallest in size after exiting the This swelling and shrinkage can ulti- site with a temperature swing of up kiln, when it is the driest it will ever be. mately cause the lumber to crack. 2 VOLUME 34 NUMBER 4 Chemical Action In the presence of moisture, chemical reactions can take place within or between building materials that impact the volume and integrity of the component. Corrosion occurs when ferrous metals are exposed to moisture, whether through direct contact with liquid water or via the condensation of Shear cracks riddle the exterior wall of At this garage, movement-related cracking this urban parking structure. worsened as embedded steel corroded. ambient humidity. As steel corrodes, it expands; the volume of rust is greater concrete. applied during erection of the struc- than that of the original steel from ture, wood and concrete members which it formed. The pressure of the A variety of other chemical reactions begin to sag permanently by a small swelling steel is then transferred to the can also cause dimensional changes amount, eventually stabilizing after the surrounding concrete, brick masonry, and deformation in building materi- first several years of the life of the glazing, or other material, causing als, which place sufficient strain on the building. Concrete, a seemingly solid movement and cracking. components to cause cracking and threaten the structural integrity of the mass, has some fluid properties, which Carbonation takes place when hy- assembly. cause deck slabs to thin and expand drated Portland cement in concrete in length. Other building materials, like reacts with carbon dioxide in the air. Structural Deflection brick, exhibit only negligible creep in Although the reaction leads to an In response to the variety of forces response to applied loads. However, if increase in the mass of the concrete, acting on the building envelope, the attached rigidly to a wood or concrete it also causes a reduction in volume. components will undergo deforma- frame, masonry may sustain stresses Not only does carbonation lead to tion and movement. In addition to from movement of the adjoining concrete shrinkage, it also reduces the the dead loads (forces that remain member. alkalinity of concrete, making the em- relatively constant, i.e. the self-weight bedded reinforcing more susceptible of the building envelope) and live loads Settlement to corrosion.

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