• Or so I thought. Someone asked me recently if I knew how large a piece of glass needed to be to resist a certain wind load. I don’t know why I remembered I had it, but a chart from the 1970 UBC used to be what was used to figure out if glass was strong enough for wind loads.To use the chart, you found where the the thickness of the glass shown along the top row intersected with the wind load column on the left. That number was the maximum square footage of glass allowed for that wind load. Example: ¼-inch glass, 40psf wind load = 27 sq.ft. maximum glass lite size, regardless of the height and width. These values represented a “fresh off the float line” strength of annealed glass.

    Referring to another table, you could then increase the 27-square-feet allowable lite size by incorporating it into an insulated unit (27 x 1.5 = 40.5 sq.ft.), heat strengthening it (27 x 2 = 54 sq.ft) or tempering it (27 x 4 = 108 sq.ft). The factors could be combined, such that a heat strengthened, insulated unit could be, at 40 psf, 27 x 1.5 x 2 = 81 sq.ft. That’s assuming, too, if you heat treat one lite, both lites in the IGU had to be heat treated one way or another.

    These charts were developed primarily by glass manufacturers. Easy, right? Not so fast, kemosabe!

    The good folks at ASTM came along and developed ASTM E1300 “Standard Practice for Determining Load Resistance of Glass in Buildings,” which addressed more factors. According to Bill Lingnell, the old UBC charts didn’t take into account weatherability, and they used a statistical probability method following a normal distribution curve. By contrast, ASTM E1300 was developed using a failure prediction method, and also accounted for glass weathering. The IBC since has generally adopted ASTM E1300 as the glass strength standard.

    Currently, there are a lot of companies offering ASTM E1300 software that will confirm the glass construction – insulated or monolithic, heat treated or not, laminated, etc. – and whether it will work for a given wind load.

    Some programs will also calculate how much the glass deflects. If you’ve ever been in a completed building, and the occupants call in with a complaint that “the glass is about to fall out,” you know about this part of the discussion. Occupants aren’t used to glass that moves like a trampoline under certain exterior wind conditions. Glass deflects equally when annealed, heat strengthened or tempered. It takes approximately four times the force to break a tempered lite compared to one that was annealed, but both still deflect the same under the same load.

    Some architects ask about limiting the glass deflection to 1 inch. This is a human comfort thing, as there’s nothing in the codes and/or in the glazing industry that limits deflection. In most instances, the means to limit deflection on larger lites or in areas with greater wind load is to increase the glass thickness. Most framing systems don’t easily accommodate varying glass thicknesses (i.e. 1” IGU in typical vision areas, 1¼” thick in higher wind load zones). Vision and spandrel glass thickness differences used to be a common practice, but have virtually disappeared due primarily to energy constraints. The software, too, will allow one lite to be different than the other, be it tempered, heat strengthened, or laminated while the other lite stays annealed. The UBC charts didn’t do that.

    But that’s all the software will tell you. It won’t reveal whether the glass needs to be tempered, or if safety glazing is needed because it’s in a door, is adjacent to a door, or because it is adjacent to a walking surface where no handrail or horizontal is present. It also won’t tell you that safety glazing is required if the sill is lower than a certain height off the floor, or if there’s a horizontal or handrail 3’-0” to 3’-4” off the floor so that if the glass breaks, someone won’t fall through the opening.

    Software also won’t tell you if the glass should be heat treated at inside corners for sun-facing surfaces. Glass doesn’t always allow all of the sun’s energy to pass through it, some is reflected, and if that reflection is onto another glass surface, the lite on that adjacent surface may need to handle more than just direct thermal energy. Reflective metal panels may have reflective effects on adjacent glass, also.

    Other factors that affect glass substrate selection or make-up are reflective coatings and tintings, to name a few.

    The lesson here is you can trust software, just know what it’s doing, and more importantly what it’s not telling you. Although many of the old rules don’t work as well as they used to, one that still holds true is: when in doubt, call the glass manufacturer. Their tech staffs are able to help with these types of issues. It’s nice to have a second set of eyes to help get your project right, both in the estimating and execution phases.

    Lastly, PLEASE NOTE: Any comments on this blog that deal with my age, especially from Greg Carney, will be deleted with prejudice!

    Tags: , ,

  • I’m not good at music trivia, and I won’t claim to have a favorite song. My car radio  has buttons set to country and classical stations, news, oldies, easy listening, and one or two stations the kids liked –  heaven knows why. Yet, when it comes to the Beatles, I can’t remember a time I haven’t been able to sing all the words to their songs. Now, here’s a building that takes to heart their song “I’ll Follow the Sun.”

    This is an instance of something on paper looking good, but in practicality, can they make the building change orientation?  And conceptually, as the article points out, having solar panels on some sort of “moveable mounts” makes perfect sense, as the sun’s never in the same position throughout the day and seasons.

    Based on the geometry, fixed solar panels typically are set to an angle perpendicular to the sun at solar noon of the spring or fall equinox. At that fixed position, they are 100-percent efficient. But,  as the sun moves east to west and up and down in attitude during the course of its journey across the heavens, its rays will be perpendicular to a fixed panel for brief moments, possibly only twice a year, depending on how they’re set. And when the rays aren’t perpendicular, the panel’s efficiency drops off.

    Curved or parabolic reflectors attempt to overcome that inefficiency by capturing and/or focusing more rays to a collector. Most of these collectors change their angle in relation to the horizon or to the travel east to west, thus offsetting that part of the sun’s journey.

    But an entire building that twists to follow the sun? The structure would have to be substantial given the load transfers, both for the building’s weight and wind loads at the staggered floors. Maybe those end walls shown in gray in some of the conceptual drawings are just pure structure, with no way of permitting even the smallest of windows.

    I guess it could work to twist the whole of the tower east to west and returning it to the east to face the sunrise in the morning. But, buildings are heavy. To get the whole of this new “Twilt” building to twist, can you imagine the machinery to move it, let alone the size of the turntable the building would be constructed on (basically the foundation so that it wouldn’t tip over, also), and the force required to start that rotation?

    Additionally, the solar panels themselves would be cantilevered off balconies or extended slab edges, which could then be mechanically moved to orient them to the sun’s attitude, much like the parabolic collectors mentioned above. In combination with the turntable, this could make the venture credible.

    But realistic?  Is there a developer out there prepared to spend this kind of mullah, and are there users, be they office or condominium buyers, willing to pay the rent or purchase price?  That’s obviously a decision way above my pay grade, and it’ll be interesting to see if this turns into reality.

    An idea just occurred to me. Maybe the “death ray” buildings we’ve recently read about are missing the mark, and with a little work could be made to serve as productive solar collectors and not death sources from the sky. Maybe the architects could design curved surfaces into buildings and focus the sun’s energy on a focal point on the ground that can collect the rays and turn them into electricity. It might require land and the collector might have to move as the focal point of the sun’s rays change.

    I’d love to see how that concept would play out in the shape of a building. Wouldn’t you?

    Until then, as the Beatles sang, “And now the time has come, and so … I must go.”




  • The Council on Tall Buildings and Urban Habitat recently added a new category for the skyscraper heights they recognize as the world’s tallest: “vanity height.” It seems with the rash of buildings claiming to be the tallest, the Council wanted to distinguish between buildings that have the highest occupied floors and the ones that get their height advantage by adding antennas or other unusable space, hence the term “vanity height.” Has there ever been a more appropriate title?

    The Burj Khalifa, at 828 meters (2,717 feet), is slated to be surpassed as the world’s tallest building next April.  Ground has just been broken on the Sky City Tower in Changsha, China, that will be 10 meters (34 feet) taller than the Burj.

    Did anybody catch the duration of construction for that one? Promise, no typos follow: July 2013 through April 2014. In less than 10 months, they want to construct an 838-meter tall tower. Do you find that hard to believe?

    By comparison, the Empire State Building (EBS), built in the 1930s, was 381-meters (1,250 feet) tall; an antenna added in 1950 extended that by 62 meters (203 feet), for a total height of 443 meters (1,453 feet). The EBS was erected at a rate of 4.5 floors a week. On most towers, erecting a floor a week today is pretty typical. Now do it 4.5 times as fast. Unbelievable! Excavation started in January 1930, actual construction started on March 17, 1930, and the ribbon-cutting happened on May 1, 1931, 15 months later. It’s mind-boggling such a building can be erected that fast.

    And, this before computer drafting, scheduling or even electronic calculators were available to help plan and execute the schedule. The architect only needed two months to draw the plans. They used a previous design as the basis of the EBS, which gave them a little bit of an advantage, but come on, only two months?

    There must have been a lot of preconstruction meetings to talk about planning, sequencing, and execution. While the construction only took 15 months, the planning effort must have been equally as long, right? Take out your daddy’s hand tools, Grandpop’s slide rule, your great grand-daddy’s drafting board and construct such a building today ‘cause that’s how it was done. I’d like to know how much was actually fabricated prior to the start of construction. Just on the structural steel alone, they had to be two-three months or more out in front of the field crews.

    I guess it’s all these “modern conveniences” that allow such a project as the China tower to even pass from conception to an actual project, with everyone chomping on the bit to get going. I wonder how long the original schedule was and how it was decided to shorten it to 10 months.

    As an aside, can you imagine what would have happened (in the case of the EBS) or will happen (in the ChinaTower) if one of the subs falls behind in their work?

    It wouldn’t surprise me to learn that the curtainwall is already fabricated and a floor can be erected in one day that we used to think of being done in a week. Fabricating 60-80 frames a day ramped up to say 150-200 a day, or more.

    With the pressure to be on time, the penalty (and hopefully the reward) clauses for being late (or ahead of schedule) must be enough to overcome the risk. Based on what little is known from my vantage point, is there any way fabrication didn’t at least start a year ago, or more?

    There can’t be anything except prefabricated, site-assembled work, like unitized curtainwall, for all the building systems, including structural, mechanical and electrical; even plumbing modules could help conceivably shorten durations. Ship to the site, set it in place and connect it all together. Can’t wait for welding, can’t wait for concrete to cure. Is there a better/cheaper and, most notably, faster way of construction?

    Or are there so many crews planned to be on site at any one time to handle a site built installation? The necessary equipment to move manpower around the tower must be staggering. I heard that moving crews on the Burj Khalifa could take up to an hour near the end of construction, to move from the ground to getting to an assigned work station, and that, to facilitate that, work starting times were staggered so that everybody wasn’t showing up at the same time. That kind of attention to detail, to moving people and materials around the site, must be so well thought-out and planned for in advance. It can’t be possible to meet such a schedule and not have thought out every little thing, can it?

    Stay tuned, we’re in for a heck of a ride if the developers here ever get this wild of a hair to bring these methods to the States. The paradigm shifts are enough to go looking for the aspirin bottle just thinking about it. Or will everyone be saying, “No problems, just opportunities and challenges?” That would have to be the mindset, right?

USGlass Magazine

USGlass Magazine