Building Movement Joints and BIM

Computer modeling allows greater visualization, functionality, and design success in creating buildings that are allowed to move safely.
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Sponsored by Nystrom, Inc.
Peter J. Arsenault, FAIA, NCARB, LEED-AP

Control joints (also called construction joints). This is one of the most common types of joints and is typically described as an intentional line or break that is created in the surface of a material. Its role is to encourage and direct anticipated cracking in an orderly manner instead of random, uncontrolled cracking. Control joints are almost always used in concrete floor and wall surfaces which are prone to cracking due to shrinkage during curing (see Figure 3). They may also be used in masonry construction to absorb other forces. In any case, they typically are spaced fairly close together meaning that the amount of force that each control joint is absorbing is relatively small. As such, they do not usually fully penetrate all layers of a structure, rather are cut about 25 percent or so into the depth of the material. The spacing of control joints across a smooth plane such as a wall, floor, or roof surface will vary and depend on the material used as shown in Table 2.

Control joint pattern in concrete floor slab showing diamond shaped isolation joints around columns

Illustration: Nystrom, Inc.

 

Recommended control joint spacing for various materials
Table 2

Materials

Control Joint Spacing

Concrete Slabs on grade

24 times slab thickness - rectangular sections should restrict longer side to 1.5 times shorter side

Concrete exterior walls

Every 20 feet

Type I Concrete masonry walls

 

Unreinforced:

The lesser of 40 feet or twice the wall height

Reinforced:

The lesser of 50 feet or three times the wall height

Type II Concrete masonry walls

 

Unreinforced:

The lesser of 20 feet or twice the wall height

Reinforced:

The lesser of 25 feet or three times the wall height

Stucco Walls

10 feet

Some general "rules of thumb" are shown, but should be verified for particular projects based on specific project conditions.

Courtesy of Nystrom, Inc. ; compiled from various data.

Structure/enclosure joints (sometimes called isolation joints). Any place in a building where a non-structural component meets a structural component the potential exists for an unwanted transfer of forces. Hence, a full-depth joint or separation between these components is needed. Examples of this type of joint would include the top of a non-load bearing partition that connects to the underside of a structural floor slab or a non-structural exterior wall or spandrel panel that connects to a structural frame. When the different components are comprised of different materials, the rate of expansion or contraction will be different even though they are exposed to the same conditions. Hence the separation is intended to allow the resulting differences in movement instead of transferring a structural force onto a non-structural component. This might be accomplished by the use of a combination of rigid and flexible anchors along with gaps between the sections of non-structural components to allow each piece to float or move independently from the other. In some cases, such as a concrete slab on grade adjacent to a concrete foundation wall, a separation is warranted even though the materials are the same. The reason here is due to the fact that the two components will settle (vertical displacement) differently due to the different loading conditions that each are subject to-the wall carrying significantly more weight in the form of structural building loads while the slab on grade is typically carrying only its own weight and any live loads. The differences between the forces transferred to the soil could cause different settlement conditions over time, causing cracking or failure if not isolated. Around columns, it is common to use diamond-shaped isolation joints that meet with concrete control joints to cleanly address multiple forces.

 

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Originally published in Architectural Record
Originally published in November 2010

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