MOVEMENT JOINTS IN BUILDINGS
structures that can move independently of one another. Building separation joints can be classiÞ ed into three types: volume-change joints, settlement joints, and seismic separation joints.
- Large-scale effects of expansion and contraction caused by temperature and moisture are relieved by volume-change joints. These are generally placed at horizontal or vertical discontinuities in the massing of the building, where cracking would be most likely to occur. They are also located at intervals of 150 to 200 feet (40Ð60 m) in very long buildings, the exact dimension depending on the nature of the materials and the rate at which dimensional changes occur.
- Settlement joints are designed to avoid distress caused by different rates of anticipated foundation settlement between different portions of a building, as between a high-rise tower and a connected low-rise wing, or between portions of a building that bear on different soils or have different types of foundations.
- Seismic separation joints are used to divide a geometrically complex building into smaller units that can move independently of one another during an earthquake. (Buildings in seismic zones should also be detailed with structure/enclosure joints that permit the frame of the building to deform during an earthquake without damaging brittle cladding or partition elements.)
Building separation joints are created by constructing independent structures on either side of the plane of the joint, usually with entirely separate foundations, columns, and slabs . Each of these independent structures is small enough and compact enough in its geometry that it is reasonable to believe that it will move as a unit in response to the forces that are expected to act upon it.
Detailing Movement Joints:
The first imperative in detailing a movement joint is to determine what type or types of movement the joint must be designed to accommodate. This is not always simple. Often the same joint is called upon to perform simultaneously in several of the ways that are outlined hereÑas a volumechange joint, a settlement joint, and a seismic joint, for example. A joint in a composite masonry wall may serve as both an expansion joint for the brick facing and a control joint for the concrete masonry backup. Once the function or functions of a joint have been determined, the expected character and magnitude of motion can be estimated with the aid of standard technical reference works, and the joint can then be designed accordingly.
It is important that any structural materials that would restrict movement be discontinued at a movement joint. Reinforcing bars or welded wire fabric should not extend through a control joint. Expanded metal lath is interrupted at control joints in plaster or stucco. The primary loadbearing frame of a building is interrupted at building separation joints. At the same time, it is often important to detail a movement joint so that it will maintain a critical alignment of one sort or another. several expansion and control joints that use interlocking masonry units or hard rubber gaskets to avoid outof- plane movement across the joint. In concrete slabs, smooth, greased, closely spaced steel dowels are often inserted across a control joint at the midheight of the slab; these permit the joint to open up while ensuring that the slab will remain at the same level on both sides of the joint. The curtain wall mullion in allows for movement along one axis.
while maintaining alignment along the two other axes. Joints must be designed to stop the passage of heat, air, water, light, sound, and Þ re. Some must carry trafÞ c, as in the case of joints in ß oors or bridge pavements. All must be durable and maintainable, while simultaneouslyadjusting to movement and maintaining an acceptable appearance.Each joint must be detailed toallow for the expected direction andextent of movement: Some joints willhave to operate only in a pushÐpullmanner, while others are expected to accommodate a shearing motion and even a twisting motion as well. The exterior joint closure is usually obtained by means of a bellows of metal or synthetic rubber.
A perusal of manufacturersÕ catalogs classiÞ ed under Section 07 95 00, Expansion Control, of the CSI/CSC MasterFormat system will reveal hundreds of different joint closure devices for every purpose. Every designer of buildings must develop a sure sense of where movement joints are needed in buildings and a feel for how to design them. This is neither quick nor easy to do, for the topic is large and complex, and authoritative reference material is widely scattered. Numerous buildings are built each year by designers who have not acquired this intuition. Many of these buildings are Þ lle with cracks even before they have been completed. This brief essay and the related illustrations throughout the book are intended to create an awareness of the problem of movement in buildings and to establish a logical framework that the reader can fill in with more detailed information over time.