Abutment joints. In building situations where two different types of construction meet or abut each other, a full depth joint is needed. This is common in cases where, for example, a framed wall system abuts a brick or masonry wall system. Each system can be expected to move at different rates due to the same or different forces that each one is subjected to. Hence, the separation is made wide enough (typically 1/2 inch or less) to allow the movement based on the size of the abutting systems and the spacing of other types of joints in each system. Abutment joints are also needed wherever new construction abuts with older construction, even if the materials are the same. New masonry mortar will typically shrink as it sets, so if it is tied in or interleaved with existing mortar, it will cause cracking and breaking. Instead, a continuous, flexible separation that completely separates the old from the new will prevent this from happening. Regardless of material or construction types, abutment joints are also appropriate wherever a building portion that is heated or cooled abuts a portion that is not similarly heated or cooled such as a warehouse or canopy. The different temperature conditions will result in different amounts of thermal expansion and contraction which need to be accounted for.

Building separation joints. In large buildings or buildings with multiple wings, towers, or other distinct sections, particular attention needs to be paid to separating these sections to avoid the transfer of forces from one section to another. While these building separation joints are sometimes referred to as expansion joints, in reality they are intended to separate a large building into smaller discreet sections that can act independently from one another (see Figure 4). In so doing, they can then handle a variety of movement types beyond the large-scale thermal expansion and contraction described above. Other forces such as vertical displacement or settlement of foundations, large-scale material shrinkage or creep, and seismic activity can be accommodated by using this design approach of creating structurally separate building portions. In these cases, the full three dimensional section needs to be separated such that a continuous break is formed along walls, floors, roof, and other building elements. Each section needs a distinct structural system on either side of this continuous break, such that, for example, two columns and beams may be required where only one would otherwise be provided. The structural systems must not span across the joint or the purpose of the joint is lost.

Building separation joints need to occur wherever there is a change in building mass or to separate large masses into smaller sections. These joints need to penetrate the entire 360 degree profile of the separations as shown in the BIM illustration on the right. Illustration: Nystrom, Inc.

 

Given the multipurpose nature of these joints, they should be considered early in the design process by all disciplines involved including architects, structural engineers, soils engineers, seismic consultants, etc. The locations of building separation joints will in large part be determined by the final architectural massing of the building and may be a consideration in that process. Keep in mind that sections that can be identified vertically (e.g., a tower adjacent to a lower section) are as important as sections that can be identified horizontally (e.g., wings off of a central core). As with pure expansion joints, large sections over 150 to 200 feet long will require additional joints to be located in concert with the rest of the building design.

Expansion joints. This term has been used somewhat generically to refer to all types of building joints. However, it is more appropriately used to describe only full depth joints that are larger (wider) than the other joints described above with the intent of allowing movement specifically due to thermal expansion and contraction. In walls, floors, ceilings, and roofs, they should serve as open, full-depth seams that can open and close slightly to allow thermal movement in adjacent materials. Expansion joints may also be appropriate at locations of structural weakness defined as anyplace a geometric change occurs in a wall or other surface. This would include corners, changes in heights, changes in width across a surface (such as around large door or window openings), or other significant geometrical changes. Note that some of these same conditions may be treated with multiple control joints if the relative size of the change is small or if enough multiple joints are used (as in around closely spaced multiple window openings). But full-depth expansion joints will still be needed at all major geometric changes or over long sections of surfaces. Unbroken lengths of masonry surfaces should generally not exceed 125 feet without an expansion joint while concrete and steel surfaces can usually go up to 200 feet before needing an expansion joint(see Figure 5). These distances and the width of the joints are calculable from an engineering standpoint and may be adjusted based on other factors such as total temperature swing exposure or building shape (i.e. rectangular vs. non rectangular). It is the prerogative of the designers to choose the best locations and spacing of these joints consistent with the overall design and appearance of the building.

Expansion joint locations can be dictated by the size or shape of a building-or both. Illustration: Nystrom, Inc.