We may think of the building envelope as an inanimate object, but in reality its components can be quite mobile. Building materials grow, shrink, shift, bulge, deform, and elongate in response to stresses and fluctuations in the environment, and these dimensional changes often impose strain on adjacent elements.
Where the forces of movement are not foreseen during design and construction, evidence of the struggle will emerge, in the form of cracks, spalls, displacement, broken glass, warped metal, and, eventually, breakdown of the assembly.
Failure to anticipate and allow for movement in building materials compels imparted stresses to fi nd their own path to release, which is nearly always an undesirable one. Not only are cracks unsightly, they also open pathways for moisture penetration into the building enclosure, which compounds the problem as materials swell or corrode, placing further outward pressure on adjoining components.
To design for movement in the building envelope is to identify the properties of the materials used in construction, as well as the environmental and siting conditions of the building, and to develop a design that either minimizes or allows for such movement.
After you have read and studied the text, you should be able to:
• Identify causes of dimensional changes in building envelope elements.
• Describe strategies for accommodating movement in the building enclosure.
• Distinguish among types of movement joints, including construction joints, control joints, and expansion joints.
• Explain the effects of differential movement in building envelope materials and discuss methods for permitting independent movement.