Friday, March 30, 2007

Detail of the Week: Expand your horizons...and your roof.

Buildings are big, some of them really big. When a building is big enough, it needs expansion joints to allow the building to expand when it gets really warm and contract when it gets really cold. It sounds like not much of a deal, perhaps, but when you have two 35-foot long steel beams each trying to expand one inch into each other, finishes can crack or warp or worse, the structure can deflect and cause structural failure. I found some photos here recently of an expansion joint system in the roof of a one-story building with a concrete structure and precast concrete exterior.

Here's a photo of the expansion joint coming up to the exterior wall where two precast concrete panels come together.

The expansion joint itself is a product that the architect specifies and the contractor buys and installs. They come in a pretty wide range of shapes and sizes, depending on where you want to put it and how wide the expansion joint is (the more movement that's expected, the wider the joint--this joint is about 2 inches wide). This joint is in the open on a roof, so it looks like a thick rubber half-of-a-tube with a piece on each side that the contractor attaches to wood blocking on top of metal studs. The wood blocking/metal studs are the box underneath the expansion joint bump. The contractor then puts a rubber-polymer roofing material over it (well call it an EPDM sheet in da biz), seals it down with an adhesive, and covers all edges and gaps with a sealant. An expansion joint has to run from one exterior wall to another exterior wall. This one goes to a space between two exterior panels and all the way down to the ground on the other side. You can see the tan-orange sealant on top of the panel gap. On the other side, there's a piece of spongy material adhered into the gap between the panels going down the face of the building.

When a building has a couple of wings or big chunks coming together, the expansion joints have to intersect. This is not always perty. However, this contractor did a really nice job of bringing the three wings of this building together.

This expansion joint also runs to a clearstory, which is the name for a part of a roof that pops up above the rest to get some natural light into a building. You see it hit right behind the column on the end of the clearstory. See the gap in the yellow sheathing on the column and the sheathing on the soffit (the horizontal parts of the roof)? The expansion joint is wrapping up around the column and running along the high side of this angled roof.

How can this be? Doesn't the roof structure have to be connected to some beams or something? Well, sure. The roof joists are attached firmly to the beams on the low side of the clearstory. However, because the expansion joint runs along the high side of this clearstory, the joists sit on Teflon pads. One pad is attached to the joist, and the other is attached to the beam holding it up.

The Teflon pads are the thin light-colored wafers between the rusty-looking steel plates. The upside-down U-shaped piece of steel on top allows the joists to sit at an angle.

Here's the thing about expansion joints: pretty much finish needs an expansion joint. The brick face on a building needs an expansion joint at least every 200 feet (if I recall correctly), and even drywall needs very small, pretty expansion joints, called control joints, in wide expanses of walls.

The little lines in that drywall are control joints. Here, they're being used for aesthetics, to separate different colors in this decorative soffit. The control joint is a little piece of V-shaped plastic about 1/4-of-an-inch wide with little flanges that the drywall contractor muds into the drywall on each side of the joint. It allows the drywall to expand and contract so it doesn't crack and look skanky.

Have I expanded your horizons with my expansion joints? Good. Now go have a good weekend.


BaxterWatch said...

great minds.

And welders? Hell no. There is nothing left to weld TO. What you can't see in that photo is the inner metal stack has eaten through to the outter shell. We're going to have to replace the whole thing, including expansion joints, which I suspect are harboring highly acidic ash.

have a great weekend.

faded said...

2 observations: The first is from a maintenance point of view. The second is from a structural point of view.

The first thing to break on a roof are the bendy parts. Inspect the joints regularly after they are installed. The contractor did a nice job on the ones in the picture. This makes them easy to inspect. Also don't let anyone step on them. Bad things happen especially after the joint ages.

I had the pleasure(?) of working in a one story 225,000 sq ft factory when the roof failed. The owners had ignored it for 25 years and then we had a cold snap with temps around 12 F at night for two days then temps around 75 F during the day on the third and fourth day. Every expansion joint in the roof split. On the fifth day it rained about 4 inches on 4 hours.

This experience lends new meaning to the architectural idea of blending a sense of interior and exterior space.

The second item is a structural question about the roof detail. Based on the detail it appears that the roof is sitting on the Teflon pads creating a compression only joint. It is free to slide as required. What happens when wind gets under the overhang or comes up the side of the building and over the parapet and creates a low pressure area on the roof? Is there a chance that the clerestory roof will lift? I would guess this building is probably somewhere on the great plains where super cell thunderstorms occur. Wind speeds could get to 80 or 120 MPH cresting the edge of the roof.

Mile High Pixie said...

Bax: Acidic ash? Sounds like a great name for a rock group! :-P

Faded: The gal I got this photo from said the building was designed to a Class C Exposure for 100 mph winds (so you were right--this was built in an open area: good call!) I'm not sure how the structural engineer planned for uplift on this side of the roof, but I'll have to ask. Her struct eng was one that I've worked with in the past. Good question!

Anonymous said...

Looks like the plate is bolted to the beam. The bolts go through a slotted hole in the beam and a round hole in the plate that's welded to the curved steel. The slots in the beam allow the bolts (which are stationary against the plate) to slide one way or the other during expansion or contraction.