There are a number of things to consider when specifying expansions joints, but before we get into the specifics, let’s review the details on why building joints are so important.

Why do we Need Building Joints?

A building can move in any direction—expanding, contracting, or sliding along the X and Y axis as well as up and down along the Z axis. Joints are needed to relieve this movement where there are long expanses and where the structure would crash into another fixed or independently moving structure.

What Type of Movement is the Building Subject to and How Much?
The type of movement a building is subject to impacts the joint. A standard expansion joint system allows for a range of motion of ±25 percent of the nominal joint width. For example, a 102-mm (4-in.) nominal expansion joint system will support a gap dimension of 76 to 127 mm (3 to 5 in.). An expansion joint specified to seismic standards supports movement of ±50 percent or, in this example, a gap of 51 to 152 mm (2 to 6 in.).

It is recommended that you form a close relationship with the structural engineer to be able to share ideas and ensure you are both on the same wavelength. It is also very important to remember that nominal joint size is slotted in the building and it expands and contracts from this size. However, you are designing to minimum and maximum sizes, so your joint needs to cover the worst case.

Joint Performance Considerations

Joint systems basically close off the gaps in a building system and still allow it to move while permitting persons to move about freely inside without tripping or slipping, and also maintaining the ability to carry the floor loads that have to be moved or rolled over it. The following are seven things to consider when specifying expansion joints from both an internal and external aspect. You should choose joints that:

1. Minimize slipping or tripping hazards over the joint.
2. Support necessary floor loads.
3. Maintain fire separation integrity.
4. Maintain sound separation integrity
5. Maintain thermal separation integrity.
6. Maintain moisture barrier integrity.
7. Provide acceptable continuation of the finish.

In-depth Information About Building Movement

There are various types of building movements caused by the different forces to which a building is exposed. These forces, which have a bearing on joint integrity, include:

• Thermal expansion and contraction caused by seasonal temperature changes
• Moisture (rain/snow/humidity)
• Sway caused by wind
• Differential movement (seismic events, etc.)

Be assured that all these types of movements occur in every building and you should consider if the magnitude of the movement poses a threat to the structure or integrity of the finishes used to cover the structure?

Thermal Expansion and Contraction

This is a simple linear equation: Large temperature swings and expansive materials like steel and longer lengths create conditions for expansion and contraction. Understanding the thermal sensitivity and coefficient of expansion of the structural material is key to determining movement needs. Glass and wood generally have a lower coefficient of thermal expansion than metals. Question: Will your building be exposed to the full climatic range of temperature swing due to loss of power or off-hours energy conservation? If so, then the full climatic temperature swing should be calculated.

Moisture

Porous materials are prone to expansion (swell) and contraction (shrink) from moisture (rain, vapor, etc.). The swell and evaporation of water in building materials may impact the integrity of the structure over time. Freezing water inside structural components is of particular concern in colder climates as this causes significant strain on structural materials. Parking garages are examples of movement under sidewalks and drives.  They have considerable support structures beneath the surface. Moisture is mostly prevalent in gymnasium floors as architects will manage for moisture in all other applications. Wood fibers expand when wet—they get fatter, not longer. Wood expands in direct proportion to its moisture content. Different species of wood are more or less susceptible to expansion due to the varying structure of the grain. 500 inches of wood gym floor with an expansion coefficient of .0037 and a moisture change of 3% will expand 5.55 inches.

Wind

Wind on the face of a long expanse can cause the building to move perpendicular to the thermal expansion and contraction resulting in differential motion. Some joints are not capable of moving in two directions. (Use your hands flat to the ground to demonstration motion in two directions—side to side and towards and away from you.)

Remember it is important to take into account that all of these elements: heat, moisture and wind have an effect on the amount your building moves. Therefore, the kind of joints you specify will play a key role to helping minimize tripping hazards, provide support, maintain separation integrity for fire, sound, thermal and moisture barriers, and provide an acceptable continuation of the finish.  

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About Nystrom

^{Since 1948, Nystrom has been the go-to manufacturer of specialty building products offering a wide range of floor, roof, wall, ceiling and stair access products that create safer, more accessible buildings. Nystrom is also proud to offer hassle-free services like access to technical experts, REVIT tools for BIM, HPDs for LEED v4, 3-part specifications and AIA continuing education courses.}

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