Function Accommodating Movement

A BUILDING is never at rest. Its movements, though seemingly small, are extremely powerful and can cause irreparable damage unless the building is detailed to accommodate them. There are a number of sources of movement in buildings that the detailer must keep in mind:

Temperature movement is caused by the expansion and contraction of building materials with rising and falling temperatures.

Moisture movement occurs in porous materials such as wood, plaster, masonry, and concrete. These materials swell as they absorb moisture from water or humid air and shrink as they dry.

Phase change movement accompanies a change in the physical state of a material. The phase change movement that is of primary interest to the detailer is the expansion of water as it freezes.

Chemical change movement takes place in certain construction materials as they cure or age. Gypsum plaster expands slightly as it changes from a slurry to a solid. Solvent-release coatings and sealants shrink as they cure. Reinforcing bars that rust expand and can crack the masonry or concrete in which they are embedded.

Structural deflections always accompany changes in the loads on a building. Beams, slabs, trusses, and arches sag more as thev are loaded more heavily, and less as their loads are reduced. Columns grow shorter as loads are applied to them. Wind and seismic loads flex and rack exterior wall components, and move buildings laterally by substantial amounts.

S:ructural creep is characteristic of wood and concrete, both of which sag permanently by a small amount during the first several years of a building's life and then stabilize.

Foundation settlement occurs when the soil beneath a building deflects or creeps under loading. All foundations settle; if the set-dement is small and is uniform across the entire building, little movement occurs within the components of the building itself. If settlement is nonuniform from one wall or column to another, considerable movement must be accommodated.

We can predict, often with impressive accuracy, the magnitude of movement that will occur from each of these sources: Temperature movement can be quantified rather precisely using the expected range of temperature difference and the coefficient of thermal expansion of the material (page 36). Moisture movement cannot be quantified with such-precision, but we can predict it accurately enough to prevent it from causing damage to a building pp. 77, 78). Phase change and chemical change movements can be estimated with varying degrees of accuracy. Struc-rural deflections are computed very closely using standard engineering techniques, and structural creep can be quantified to within manageable limits. A gcotechnical or foundation engineer can provide enough data on expected levels of foundation movement to guide the detailer.

In detailing a building, we concede that most movements are unprcventable and are caused by forces so large that we cannot restrain them. Instead we provide movement joints between building components at such intervals and in such configurations that the movements can be absorbed without harm in these joints. If we did not provide movement joints, the forces that cause movement in a building would create their own joints by cracking and crushing components until the building's internal stresses were relieved. At best, the result would be unsightly; at worst, the result would be a leaky, unstable, unsafe building.

The detail patterns that relate to accommodating movement in buildings arc associated with several simple strategies. The first of these is to manufacture and configure building materials in ways that minimize their tendency to move in undesirable ways. Its associated patterns are:

Seasoning and Curing (page 76) Vertical-grain lumber (page 78) Equalizing Cross Grain (page 80) Relieved Back (page 82) Foundation Below Frost Line (page 83)

A second strategy is to separate building elements that are likely to move at different rates and in different ways. Its m patterns are:

Structure/Enclosure Joint (page 84) Abutment Joint (page 86)

A third strategy is to divide large building surfaces that are likely to crack, crush, or buckle into smaller units of such a size that the likelihood of such failures is greatly reduced. This leads to the following patterns:

Expansion Joint (page 87) Control Joint (page 89) Sliding Joint (page 92)

A fourth and final strategy is to divide a large building, especially one with a complex geometry, into two or more geometrically simple buildings, each of a size and compactness such that we can reasonably expect it to move as a unit in response to such large forces as foundation settlement and seismic accelerations. This leads to the pattern

Building Separation Joint (page 93)

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