All in Agreement: Bend the Rules
November 1, 2008
Place a wooden pencil between two points and apply downward pressure until it breaks. Now move the two support points closer so that the span is lessened and then apply pressure until it breaks. Repeat this process until you can’t break the pencil.
Depending upon your strength or the tool you’re using to apply pressure, you will come to the point where the two support points are touching, therefore it becomes impossible to snap the pencil, but you could crush it.
Dimensional lumber used as floor joists are normally installed with the crown up because as weight is added the “crown” allows the floor joist to “flatten.” The size of a structural steel beam spanning between two columns while holding up the floor above is determined by calculating all the loads imposed on the beam.
The pencil test tells us that as weight is applied to the pencil it bends and if enough weight is added, it bends until it cracks or snaps in half. The same is true with just about any building material. If enough force is applied to a floor, roof or wall it will move.
Is a full height non-load bearing wall really non-load bearing? What if the roof or floor it is framed under moves up or down such as is the case of a heavy snow load? The weight of a deep snow on a rooftop may cause the roof to deflect downward which applies a load to the non-load bearing wall. In most cases the weight of the roof coming down transfers to the floors below the roof which in turn causes lower floors to deflect or move.
The good news is that today’s engineers know that buildings “deflect” to some degree and they design buildings to accommodate deflection/movement.
HEAD OF WALL CONDITIONS: (HW-D)
Head of wall conditions is where the top track of a wall attaches to the structure above. A top track can attach to a concrete, metal or wood floor or roof deck. Engineers have found that floors and roofs move up and down making it critical that full height head of wall conditions allow for this deflection.
Another way to think about full height walls and deflection is to think of the pencil test in a vertical situation. If a pencil is stood on end between two points and downward pressure is applied the pencil will deflect or bend. If enough pressure is applied the pencil will break meaning it failed to withstand the load.
The point of most head of wall conditions is to allow the floor or roof to move up or down without putting pressure on the stud below the track or lifting high enough to allow the top track and stud to disconnect.
Where things get tricky in head of wall deflection is when firestopping for fire ratings or caulking is added to achieve smoke and sound ratings. Firestopping material and other types of caulking are limited in how far they can extend and compress. Extension is the distance the firestop or caulking can be pulled apart (lift) before it fails and compression is the distance the material can compress until it fails.
COMPRESSION AND EXTENSION
UL (Underwriters Laboratory) is a private testing company not unlike Interteck Testing Services. The 18th Edition of the Gypsum Association Fire Resistance Design Manual includes listings of 16 different private fire testing companies and 20 different sound testing companies.
UL 2079 requires joint systems with movement capabilities to be cycled through the intended range of movement prior to the fire test. The movement cycling is intended to demonstrate the compatibility between the individual components of the joint system and the fire resistive assembly. Joint systems are required to be cycled through the intended movement range 500 or 100 complete movement cycles at a minimum rate of 1, 10, or 30 cycles per minute prior to the fire test per volume 2A of the UL Design page 1429.
According to UL 2079 there are four types of joint systems: floor to floor, wall to wall, floor to wall and head of wall. There is also a fifth type known as bottom of wall. Our goal is to focus on head of wall otherwise known as HW-D conditions.
Head of wall conditions are tested until they fail. Forces such as fire, smoke, and movement are all added to determine the wall’s limitations. In most cases plans or specifications indicate the amount of deflection required in HW-D conditions. For ease of math purposes let’s suppose that all interior non-load bearing walls require 1 inch of deflection per the plan or spec.
When firestopping or caulking is added to the head of wall, testing agencies include a percentage of how far the firestop or caulking material can compress or extend before it fails. For example, UL Test HW-D-0091 allows 25 percent of the joint width for compression and extension. If you do the math, 25 percent of 0 feet, 1 inch equals 1/4 inch. This means that the head of wall can move up 1/4 inch or down 1/4 inch for a total of 1/2 inch. It’s common for an architect to indicate the amount of deflection required and to include tested systems such as HW-D-0091 which appears to allow 1 inch of deflection. The key word is “appears” in this case. Just because the test was done with a 1-inch nominal joint does not mean the head of wall will deflect 1 inch.
Compression and extension determines deflection when firestopping or caulking material is added to the head of wall system. There are many HW-D tests that do not allow 100 percent compression and extension. Some allow 12, 15, 20, 25, 33, or 50 percent compression and extension. If a job requires 1-inch deflection and the test allows for 50 percent compression and extension, the head of wall system will give you a true 1-inch deflection. If not as in the case above you only get 1/4 inch plus 1/4 inch for a total of 1/2 inch.
SOUND CAULKING HW-D
Oftentimes plans and specs call out for example, 1/2- or 1-inch deflection, but do not call out a tested HW-D system. In these cases I’ve noticed that the wall type schedule, specifications or details indicate the addition of sound caulking to the top/bottom of wall to reduce sound transmission. The problem is that in most cases the sound caulking is shown filling the gap at the top and bottom of the drywall.
Let’s say that at the time a full height wall is completed the gap at the top of the wall and the bottom of the wall ended up being exactly 1/2 inch. The drywall contractor then buys acoustical caulk and fills the gaps.
Here are a few problems with filling the gap. First, applying caulk to a cut edge of drywall or on a floor that is covered with drywall dust does not provide the best adhesion. Now picture in your mind the roof-floor deflecting downward due to snow load. What happens to the sound caulking? It compresses. When the snow melts the roof should come back up. What happens to the sound caulking? It extends. The point is that sound caulking in a “gap” is not going to compress and then come back. You would hope that once it compresses it would stay compressed, however, when one floor moves down other areas of the floor move upward including floors below.
LIFE SAFETY-SOUND TRANSMISSION
Thinking back to Sept. 11 and the courageous firemen who lost their lives in the World Trade Center brings me back to a harsh reality. People live and work in the buildings we build and head of wall construction should be considered life safety.
Firestopping gives people time. Time is the critical factor in a fire and the more time a person has before the fire spreads the better the odds of survival.
Wall and ceiling contractors shouldn’t hesitate to tell the GC that they cannot install MEP work that would make it impossible to properly firestop a head of wall condition. When it can’t be done correctly the only solution is to have the obstruction removed.
THE LAND OF DISNEY
On our way to Disneyland last month I was pretty excited. As we rode the tram to the park I was telling my wife all about my memories of the park. I explained that it was an inspiration to me because of the landscaping, cleanliness, maintenance, and the fact that everything looked perfect.
The last time I was there was 20-something years ago and I was really shocked to arrive to find that it’s worn out. It’s not clean, the landscape is not maintained to the standard it once was. Buildings and railings have not been painted in years. I saw one street cleaner in the entire day. Fake plants were dirty and dusty. Trash cans were full on a slow day. The food? Gross. Double gross on the price. It’s nowhere close to cutting edge in any regard these days. I wasn’t having visions of pirates or flying things. I was envisioning really big bulldozers, although the parking garage is nice. I guess cutting edge creativity costs a lot of money and why spend money when people are willing to accept less?
We all get accustomed to our environment. If the duct work goes up ahead of us and we can’t firestop our head of walls, we often take an “if they don’t care, I don’t care” attitude. The same applies to deflection and sound caulking.
When it comes to life safety, deflection and sound control, we can’t live in the land of fantasy. Tomorrow land on the other hand is making sure we do things right.
Remember: Teamwork begins with a fair contract. W&C