January 27, 2016 § Leave a comment
Daley Plaza? Federal Center? For me the civic space that defines the city is the Lakefront Trail, especially in the morning. Curbed just linked to the video above, showing an intrepid biker hyper-lapsing the just-renovated section through Lincoln Park. When the weather is good (i.e., better than now), a morning run up the lake is my standard routine when I’m in town and it never fails to be a great mashup of the city and its residents. Glad this section is re-opened–for a couple of years now it’s been a pothole- or orange fence-dodge and this should make the bike/jogger/stroller ballet that much more enjoyable…
January 20, 2016 § Leave a comment
Brick and concrete in Thursday’s Big and Tall class, and in brushing up on my Vitruvius this passage comes up:
17. The laws of the state forbid that walls abutting on public property should be more than a foot and a half thick. The other walls are built of the same thickness in order to save space. Now brick walls, unless two or three bricks thick, cannot support more than one story; certainly not if they are only a foot and a half in thickness. But with the present importance of the city and the unlimited numbers of its population, it is necessary to increase the number of dwelling-places indefinitely. Consequently, as the ground floors could not admit of so great a number living in the city, the nature of the case has made it necessary to find relief by making the buildings high. In these tall piles reared with piers of stone, walls of burnt brick, and partitions of rubble work, and provided with floor after floor, the upper stories can be partitioned off into rooms to very great advantage. The accommodations within the city walls being thus multiplied as a result of the many floors high in the air, the Roman people easily find excellent places in which to live.
18. It has now been explained how limitations of building space necessarily forbid the employment of brick walls within the city.
The translation here seems kind of weird, but the comment at the start of paragraph 18 makes it clear that Vitruvius’ objection is to walls of unfired brick, which was only half as strong as fired brick. If that’s the case, the point here is that unfired brick requires piers that are too large to meet the city’s building code.
Here’s the same argument, nearly 2000 years later, for why brick isn’t a suitable material for a tall urban building:
“The carrying strength of street walls for masonry construction is the carrying strength of the piers; that is, that part of these walls measured between the windows, and this fact limits the height of masonry buildings. The horizontal area of the piers either takes floor space or window area, and both alternatives are objectionable…. The lower floor piers are seven feet thick, in spite of using vitirified brick and cement mortar in their construction, figured to carry eighteen tons per square foot, dead and live load. The windows of these buildings might be better if they were wider, and the floor space taken in the lower floors by the walls is very valuable.”
Corydon T. Purdy, “The Evolution of High Building Construction.” Journal of the Western Society of Engineers, XXXVII, no. 4. August, 1932. 204-205. Note the “in spite of using vitrified brick,” then the strongest available.
January 11, 2016 § 1 Comment
Oh, here’s a doozy…The Trans-Canada highway has been closed this morning because of a bridge failure over the Nipigon River in Ontario. Not, as you can see, the most spectacular failure, but pretty tough to get the family sedan over that rather abrupt change in level.
Two SCI-TECH principles come immediately to mind.
First, it’s pretty clear what happened. Although no one’s saying anything yet, you can see that the failure happened at an expansion joint (note the ‘teeth’ drooping from the edge of the deck). The only thing authorities have mentioned so far is that they’re blaming “extreme cold conditions” for the failure. Concrete’s coefficient of thermal expansion is a pretty benign 14-ish 10-6/°C, about the same as steel, which is one of the reasons we use steel and not, say, aluminum for reinforcement. But it got down to -24°C in Thunder Bay last night, or a good 45°C colder than room temperature. The bridge is listed as spanning 252 meters. Assuming that’s measured at 21°C, at its coldest last night the bridge would have shrunk by a solid 15.8 centimeters from its design length (14 cm/cm x 10-6/°C x 252m x 45°C), or about 6 inches. Thermal expansion (or, in this case, contraction) is a powerful force, and it’s easy to imagine that amount of pull fracturing whatever pins were holding that joint together. You can see, too, the results of diagonal tension members in a cable-stayed bridge–the deck is not only pulled up, it’s also pulled back since the deck suddenly became a very skinny horizontal column. It’s buckling, but very gently.
Second point: Redundancy and resilience. The Trans-Canada is apparently the only automotive route between the eastern and western halves of the country. So today, if you want to drive from Toronto to Vancouver, you’re going to be stuck going around the southern edge of the Great Lakes. The Nipigon River Bridge is being replaced by a double-span, which would provide the kind of backup you’d expect in a system so reliant on a single node. But, of course, that backup would have been subject to the same forces of thermal contraction, and if both spans had been designed to the same standard, they could both have ended up looking like this.
Anyway, to any neighbors to the north enjoying a leisurely drive through the Midwest today, wave and smile, and enjoy the extended tour of the U.S., brought to you in this case by the laws of physics…
Update, 12 January 2016:
“A multimillion-dollar bridge on the Trans-Canada Highway, the sole east-west route across part of northern Ontario, has partially reopened after sustaining serious damage over the weekend, provincial officials said Monday.
“The Ontario Provincial Police and the Ministry of Transportation confirmed that one lane of the Nipigon River Bridge has reopened.
“A statement from Transportation Minister Steven Del Duca said the lane is available to cars and regular-weight transport trucks, but that engineers are still working to determine whether it can sustain the weight of oversized trucks.”