November 2005The misunderstood expansion joint can sometimes be one of the last things considered when designing a building. But a few small steps early on can save a lot of headaches in the long run. When considering a joint-system installation, some factors to think about are movement, fire rating, new materials and designs, design trick to cover joints and installing accessories. Let’s start with movement.
The Joint’s a’Rockin’…Understanding Movement
The first and most-important requirement of an expansion joint is the movement it allows. What does this mean? Movement is usually described two ways: as a unit of measure (2 inches) or a percentage (50 percent). Obviously with one of these callouts on the plan, you can calculate the other. For example, when a 4-inch joint is required to have 50 percent movement, that means the joint will need to compress 2 inches and expand 2 inches. This type of movement is called out as +/–2", which leaves little interpretation.
There are a few who would argue that in the above example, the +/–2" callout for the 4-inch gap is 100 percent movement. This is due to a +2" and –2" movement, which gives you a total of 4 inches of movement equaling the total joint width. However, this is not typical. With a 100 percent callout, which is fairly standard, a 4-inch joint would compress 4 inches and expand 4 inches in total.
The most common or standard movements are 25 percent, 50 percent or 100 percent. So, if you have the option, is one movement better than the other? In terms of price and available options, yes. For example, the structural engineer has determined 2 inches of movement is needed at a particular location. We can take this movement and design for a 2-inch structure gap that will require 100 percent movement or a 4-inch gap with 50 percent movement. Either one of these examples will give you the 2 inches of required movement. This is a fairly generic example, but figure the lower the percentage of movement, the more options for joint covers you will have. With a higher percentage of movement, the systems become more complex to move that dramatically and because of that, the price of the system and the installation problems go up exponentially.
Saving on Fire Barriers
This leads us into fire barrier systems, which are one of the most important, expensive and mislabeled components of an expansion joint. There are two main types of fire barriers: mineral wool, which resembles a type of insulation, and fire blankets. Each type has its place in the industry.
First, mineral wool is inexpensive in comparison to blanket, and makes it fairly easy to get a higher fire rating. However, it cannot meet a hose-stream test for vertical studded applications—at least not yet. Mineral wool also has a relatively small movement capability due to the fact that it needs to be highly dense, since it is held in place by friction. This means that for the majority of applications, mineral wool is only good for thermal movement. This ties back to movement—smaller movement means less expense for the fire barrier.
On the other hand, fire blankets can be installed in almost any application, and they have better movement capability than the mineral wool. Almost all fire blankets come standard at 50 percent movement. This option is a requirement once you get into large joint widths or large movement needs. They are hose-stream approved for studded walls due to the fact that if the studs start to lose their integrity, the fire barrier will not fall out since it is fastened to the studs rather than friction held. The only major downside to fire blanket over mineral wool is the overall price. Therefore, fire barrier should be called out very accurately on plans to maintain a reasonable budget.
New Systems Mean Greater Options
The last decade has ushered in a new era of visually pleasing systems made of better materials that have replaced outdated, heavy, over-engineered joint covers. One major change has been in expansion-joint mounting system and methods.
Many recessed-mount systems (system needs a block-out in the slab) no longer require concreted anchoring. They have been replaced by an easier expansion bolt hardware style. Popular seal systems have also come a long way in types of materials used to make up the seal. A dual durometer seal is a great example of a major improvement—a hard material is used for the barbed part of the seal, which allows it to "click” into place, while a softer material is used for the rest of the seal. This allows for greater movement in the seal.
Joints have been slow to follow the construction trend for cleaner, lighter, more aesthetically pleasing products. However, many new systems are now being designed to hide the joint almost in its entirety. Many systems are now using the surrounding finishes in conjunction with the expansion joint system to hide the joint or blend it into the interior design.
Design Tricks Can Also Help Save Money
Reducing the visible part of the joint path can be taken a step further in the proper design of a building. Expansion joints can be easily hidden between walls along the path, but be very careful with this approach as fire codes may come into play. For instance, a chase wall may "hide” the joint, but there is still the need to provide adequate fire protection along the joint … and how can an inspector determine proper materials and installation if those materials are inside the wall?
Also, in certain cases, a finish can be used to cover an expansion joint. For example, a ceiling tile condition can absorb some movement and reduce or eliminate the need for a joint system. This also holds true for a half-inch expansion joint not expecting a lot of movement with a carpeted finish.
Watch Out for Accessories
Sometimes, even with the best intentions and plans, the design and installation of the building finish accessories can be the biggest problem to foresee or overcome. For example, something as simple as a doorway or a window in the joint path, without the proper design, can cause a big problem when you get the movement you design for.
It is also important to consider finish accessories and their proximity to expansion joints. Accessories include such items as countertops, cabinets, handrails, lockers and toilet partitions. Now, you might be thinking, "Who in their right mind would mount a cabinet over a joint?” and that would be an honest response. I’ve seen it done, though, and often the accessory ends up pulling out of the wall once movement is introduced.
To wrap up, if you want to avoid having to take a lot of your favorite pain reliever, it’s important to approach expansion joints and cover systems with the following mindsets:
- Have a clear understanding and agreement on the movement designed and specified.
- Select the appropriate fire barrier system to save on cost while achieving the needed rating.
- Research new joint systems and materials to find the best fit and avoid heavy, over-engineered products.
- Remember that design and finish tricks can easily hide joints.
- Avoid installing accessories in close proximity to joints.
About the Author
Matt Scheeler is a senior estimator for InPro Corporation, which is headquartered in Muskego, Wis.