Field NotesAugust 14th, 2020 | Category: Architects' Guide to Glass and Metal
Door Openings in Building Entrances
By Chuck Knickerbocker
If you’re a fan of optical illusions, visuals that give the illusion of differing from reality, you probably are a fan of M.C. Escher. His drawings morph from page to page. From stairways that people walk on sideways to water that pours up and down in a continuous loop through the channels circling the page, his impossible constructions are a sight to behold. Optical illusions now also have their own YouTube category on how to draw them (“How to Draw an Impossible Trident,” for one).
Entries into buildings get a lot of attention, and working through their design can sometimes feel like a 3D optical illusion. Entries function as a statement that designers are trying to set for the whole of the project. They could include utilizing polished metals, heavy glass doors that are as tall as we can possibly get them, custom hardware, a revolving door or two and more. In a word, they often exude the glitz the owner’s looking for as an entrance into their property. But, it’s not the doors or door frames that cause the challenge. It’s the curtainwall frame around it that can lead to heartburn.
Just as I started to write this, one such project raised its head—a 32-foot-tall glass wall running the perimeter of a lobby. In terms of the typical frame members, that’s easy: employing one of our steel curtainwalls or others will take a hot rolled section and clad it with aluminum or stainless steel. There are no issues there. The specified glass on this project is an insulating glass unit with two laminated lites weighing close to 15 pounds per square feet, which did add to the overall challenge, however.
To make way for the entrances, the designers created two openings, 19 feet wide by 10 feet tall, for the doors, which is where the real fun begins for the curtainwall framing: trying to transfer the wall loads around that opening. We fight the good fight, and we can find a solution, but will it be acceptable from a design perspective?
Because of the spacing of the vertical mullions, the desire to use pairs of doors or because the diameter of a revolving door and its placement in and out in the curtainwall often requires it, the verticals no longer run the full height of the wall. All of the windload of any mullion not running the full height, plus the contributory deadload weight of the wall that mullion is carrying, gets transferred into the 19-foot-long door header horizontal. That then has to be transferred through that horizontal into the jambs on either side of the opening.
The typical mullion, which is roughly 2 ½ inches by 11 inches, would work for that door header horizontal for windload only, even at 19 feet long. Heck, if it works for windload on a tall 32-foot wall, 19 feet is a piece of cake. While this is not necessarily always true—the mullion has a much wider contributory area when it’s a header—it was in this case.
But, when that typical mullion was laid on its side and required to carry the deadload weight of the wall above the door opening, the deflection calc’d out at almost 4 inches over that 19-foot length. We all know that isn’t going to work. Any guesses as to the final size of the member required? It ended up being a HSS 12 by 8 by 5/8 inches to get the deflection under 1/8 inch.
One architect recently suggested hanging the curtainwall from the top. Yes, that can be done, but there’s one minor and one major fault in that logic. Minor: the movement joint has to be at the wall’s sill detail. Live load deflection, thermal expansion/contraction will all show up at the base of that wall. We can make that work, if you can live with the wider joint between the sill horizontal and the structure below. Major: the doors and the walls in which they are mounted should all be deadloaded to the same floor. Hanging a curtainwall 32 feet above from where the weight of the door is supported on the ground floor is just asking for trouble. I haven’t come across a door header or hardware that can handle the kind of differential movement we could be talking about in these scenarios. In this situation, it might be necessary to employ a secondary frame supporting the doors inside the opening for the wall. The wall never directly supports the doors. That adds sightline issues that architects probably aren’t going to be happy with.
Next time, please give some consideration for the framing around those door openings, and run as many vertical mullions down to the ground floor as possible. A wide entrance looks good on paper, but it’s not easy to pull off. If you need a double door, or a series of double doors, cut off one mullion at the header, but let every other mullion run to the base of the wall. In our example above, a 19-foot span is a lot harder to frame than if that opening could have been two 9-foot-6-inch by 10-foot openings. In the example above, it would have made it possible to use the 2 ½-inch by 11-inch typical vertical as the door header.
Please let me know if you’ve come across other ways to solve this, or if you have a magic wand to make door openings easier to frame. You could be instantly qualified for a job and take my place!
Thanks for reading.
Chuck Knickerbocker is the curtainwall manager for Technical Glass Products (TGP), a supplier of fire-rated glass and framing systems, along with specialty architectural glazing products. With more than 35 years of curtainwall experience, he has worked successfully with numerous architects, building owners and subcontractors from development of schematic design through installation. He can be contacted via email at email@example.com.
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