punching shear

June 25, 2015 § 3 Comments

Rookery Masonry Sub Structure (3)That beautiful shot to the left there is from the basement of the Rookery, courtesy of CAF docent Claudia Winkler.  On a recent basement tour their group noticed the sloping walls of the brick pier and wondered whether this was, in fact, one of the infamous pyramidal footings that supported many skyscrapers of the 1880s.  Chicago Skyscrapers says otherwise–the Rookery is supported on grillage foundations.  What gives?

Glad y’all asked, because this is an important problem in bearing foundation design.  Above ground, engineers work hard to collect loads from floors and direct them into columns and piers.  This is important, because the columns’ slenderness is the only thing that makes a skyscraper work–you can’t rent out structural area, so you have to find ways to condense loads from the floors above (two-dimensional) into elements that take up minimal floor space (columns are one-dimensional).  All well and good, and there’s a good story about the “skeletalization” of brick structures into piers and then into wrought iron and eventually steel columns.

There’s a problem, though, when the columns get skinny.  A skinny column can actually support a fair bit of weight, but the connection between the column and the floor is subject to all kinds of intense stresses as the floor loads are redirected into the column.  In particular, the column wants to “punch” through the floor due to the shear inherent in the geometry of the problem.  Think of pushing a pencil through cardboard–that’s what a thick, brick pier ‘feels’ like.  If you try it with a nail, you can see the problem of a skinnier steel column.

salvadori columnThat’s Mario Salvadori’s sketch of the problem, along with the most common solution.  To prevent columns from punching through slabs, we typically try to maximize the (take a deep breath) interface area between the column and the slab.  In other words, project the plan of the column through the depth of the slab.  This is the area of material that will resist the shear forces at work–for a round column and a flat slab it will be a cylinder with the radius of the column and the depth of the slab.  We can increase this area, and thus spread the stress out, by doing one of two things.  We can make the slab deeper, which increases the height of the interface area (the height of the cylinder, e.g.) or we can increase the circumference of the column and thus the perimeter of the interface area.

mushroom capHere’s how that was typically done in 1910s concrete construction.  You can see that the top of each column has two ‘additions.’  There’s a ‘drop panel” that’s roughly square tucked up against the slab–this essentially makes the slab deeper around the column.  And there’s a cone-shaped “mushroom cap” that spreads out the cross-sectional area of the column.  Both of these are local modifications to the geometry of the slab or the column–they put additional depth or area only in the areas that they’re needed.  As a result, the connection is able to spread the shear forces out over a much longer and deeper perimeter.  Instead of the interface area being a cylinder whose surface area is the circumference of the column times the depth of the slab, it’s the area of a cylinder whose surface area is the circumference of the top of the cone and the depth of the drop panel.  In other words, lots more.

Right, so what does this have to do with foundations?

Chicago engineers faced exactly the opposite of the slab problem below ground, in that they were trying to take the condensed loads in columns and spread them out over a wide area of clay soil–in other words, they were trying to turn the one-dimensional loads in the column back into the two-dimensional loads of the slab in order to distribute them from a point load (which would have sheared right through Chicago’s weak, wet clay) into an area load.

grillageThe solution–eventually–was to literally treat the foundation pads as a network of joists and beams just like the ironwork supporting the floors above.  Here’s Birkmire’s drawing of a typical “grillage” foundation, (which in Chicago was often built of rejected steel rails, not the beams shown here).  Steel’s bending capacity meant that the rails could easily take the bending loads (for those of you keeping score, these work like double cantilevers in reverse), but the punching shear remained a problem that was solved by the trapezoidal column base you can see between the column proper and the top row of steel beams.  This is totally analogous to the mushroom caps above.

CAF Docent talk Feb 2015.001But before steel came in to the picture, engineers had to spread these loads out through materials–limestone and brick–that weren’t so good in bending.  Thus the “pyramid” foundation, basically a big pile of rocks or bricks that very gradually spread the column loads out over the required area.  The problems with these were twofold: they were heavy, which put even more load onto the poor soil below, and they took up room–either in the basement where service space for newfangled technologies like elevators and generators was becoming more and more important, or below the surface, which required extra excavation and therefore expense.  The grillage foundation solved both of these.

So.  The Rookery’s foundations are, absolutely, grillage foundations–here’s Engineering and Building Record reporting on the building’s completion in 1888:

The construction of the foundation is as follows: Under the pier is laid a homogeneous bed of concrete seventeen inches thick. On top of this steel rails are laid quite close together and about two feet shorter than the width of the foundation. On top of these rails is laid a second tier in the opposite direction but standing back at the sides about three feet each way. Above these is a third row of beams which is kept back to about the outer lines of the piers above on the sides though projecting on the ends; and finally there is a fourth row of beams which occupies a spaces a little larger than the area of the pier. These beams are bedded and surrounded with cement, and by reason of their being so thoroughly interlocked, form as it were a solid mass of steel enabling the foundations to spread out as quickly as they do without any defection of the beams, and thus spread the entire weight of the piers over the area of the lowest footing course.”

rookery planSo what’s going on above?  The Rookery is a hybrid structure–most of its structure consists of iron columns but there are also four massive cores that housed the building’s fireproof safe deposit boxes in the corners of the courtyard.  I’m guessing that the picture above was taken at the base of one of these, where the masonry pier has to be supported on steel grillage foundations.  If that’s the case, it would make sense that the “column base” for the walls would actually be made out of brick, spreading the (really heavy) loads of these four cores out over the steel grills below.  You can see that the shape of the brick “spread” in Claudia’s photograph is pretty close to the shape of the column cap in Birkmire’s drawing.

But it was also common for (expensive) iron columns to sit on (cheaper) brick piers in basements, where even if space was at a premium it certainly wasn’t as valuable as the space on the rental floors above–so this could also be a brick pier supporting iron columns above and resting on iron or steel grills below.  This would also explain the “column base.”

Either way, punching shear remains an issue that engineers deal with all the time.  It’s easier to handle today with high-strength reinforcing steel, but you can still see evidence of this issue in slabs with drop panels, waffle slabs that are filled in around column supports, and in concrete girders that get just a bit deeper as they approach columns.  Even Nervi found ways to cope with the problem…rather elegantly:

INTERIOR

606

June 11, 2015 § 2 Comments

Still slightly abuzz from last week…in addition to the Congress, we got to see a major stretch of the Riverwalk open to pedestrian traffic and Henry Ives Cobb’s 1895 Chicago Athletic Club open as the city’s latest chicest hotel (it looks amazing).

But we did just miss what might be the best thing to happen to joggers, bikers, and pedestrians of all types on the north side.  The 606, the city’s answer to New York’s High Line, opened just as we were finishing things up on Sunday.  Using an old elevated freight railway (about 3 of 46 miles of elevated, grade-crossing-free track constructed in the early 20th century), the city has built a linear park that does a nice transect from west of Milwaukee Avenue all the way to the lakefront.  And, sure enough, here’s cyclist and hyperlapse cinematographer Steven Vance recording the entire thing:

Bicycling west to east on the Bloomingdale Trail from Steven Vance on Vimeo.

Can’t wait to get back and try this out…

5icch–done!

June 8, 2015 § Leave a comment

IMG_1820Well, it’s not exactly the UN, but that is Construction History royalty there, debating the future of the field at our Scientific Committee lunch this week.  Five triennial meetings along, the international field seems to have some momentum, but as Santiago Huerta pointed out in his closing keynote, any pure historical research in these days of vanishing funding and tight budgets is a challenge.

All the more reason to appreciate the roughly 300 delegates who showed up, from points as far afield as Japan, Brazil, Turkey, and the Near North Side.  The city cooperated, with exactly one June rainstorm the entire week, and cool weather for walking tours on Friday–we showed off Mies, Frank Lloyd Wright, the River’s bridges, downtown skyscrapers, two construction sites, and as mentioned earlier a brilliant canopy tour of 1890s terra cotta being restored.  I got to show off skyscraper history with WJE preservation engineer Ed Gerns, a rare treat for me and (I hope) informative for a good crowd of 30 or so.

IMG_1822Our keynotes were spectacular–UCLA’s Stella Nair on ‘experimental archaeology’ in South America that tries to understand how stone carving techniques enabled massive construction there, James Campbell on libraries, brick, and staircases, Santiago’s stocktaking on the discipline, and SOM”s Bill Baker on whether or not Frank Lloyd Wright’s Mile High Tower would have been feasible or not (spoiler alert: not).  Baker’s talk was followed by a reception at SOM’s Chicago office, really one of the week’s highlights as they put out a huge range of models and drawings for us to gawk at over our wine and canapes.  And SOM’s generosity was matched by the Builder’s Association of Chicago, which sponsored a cocktail evening and presentations by the leaders of some of the city’s longest-lived family contracting firms–a genuinely historic evening and great to see international historians mingling with some of the city’s biggest construction names (thanks especially to Mary Brush of Brush Architects for organizing this…)

IMG_1827And, plenty of good paper sessions.  The range of topics at these Congresses gets more and more astonishing every time, and there are always happy surprises.  The history of thermal insulation in postwar Belgium?  Sure thing–and absolutely fascinating.  Dante Bini’s pneumatic concrete formwork, which built a vacation home for Michelangelo Antonioni, among others?  Astonishing (and, frankly, mystifying…I still can’t figure out how these actually stood up).  Contracting in India in the 1950s, the construction process of Lina Bo Bardi’s Sao Paolo Museum of Art (on the bucket list), and George Romney’s role in HUD’s “Operation Breakthrough” housing initiative in the 1970s? Those were all in one session…And that’s just what I managed to attend.  With six parallel sessions each day, the range and number of papers were frankly daunting.

IMG_1816And we did manage to get out of the Palmer House once or twice.  Here’s my Iowa State colleague Rob Whitehead and I treating keynote speaker James Campbell and his Cambridge University colleague to an elegant meal at one of the city’s finer dining establishments–Bucktown’s Arturo’s Tacos.  We like to treat our international guests properly.

IMG_1826It was about eight years ago that the founding CHSA membership–Brian Bowen, John Ochsendorf, Don Friedman, and I first mentioned hosting the International Congress in the U.S.  Chicago was the only city we ever even considered, and the process of organizing and putting this on has been more of a joy than any of us might admit.  We treated ourselves to one final, celebratory lunch on the way out of town, toasting our Congress manager Melanie Feerst for her tireless work to actually get things organized and in place.  We have a huge list of people to thank–a local organizing committee that made hundreds of delegates feel absolutely welcome in Chicago, the Chicago Architecture Foundation for helping organize some of our tours, a scientific committee that reviewed over 500 abstracts, local volunteers, sponsors, and of course attendees who were willing to fly thousands of miles to take part in the discussions.

Santiago is right that it’s an uphill battle to keep an admittedly abstruse discipline going these days, but it’s hard not to be optimistic after a week like this.  The number of students, young faculty, and professionals who put together great research and great stories all left us feeling like there remains plenty of promise in CH.

Next year, the American branch will have its biennial meeting in Austin, Texas in May.  And at the conference’s close we were thrilled to announce that 6ICCH will be held in 2018 in Brussels.  See many of you there…!

5icch on site

June 7, 2015 § Leave a comment

 That’s Mark Kuberski, one of Chicago’s premier facade restoration specialists, pointing out blocks of terra cotta from William Le Baron Jenney’s New York Life building on tour day this week.  He and WJE engineer Rachel Will got a couple dozen of our delegates up on the work platform above Monroe Street on Friday for what was one of this week’s highlights.  Amazing to see this stuff up close–identification numbers from the assembly process, and even fingerprints in the originals.

  And the replacement stuff is equally interesting.  Most of the original blocks are being replaced, but where new pieces are required there’s a painstaking process to re-mold elements that are faithful to the original but rendered in more durable and stable material.
Great stuff, and a good moment of introducing international folks to some of the city’s best… 

 

5icch–We Built Chicago program tonight

June 5, 2015 § Leave a comment

IMG_1772The 5th International Congress on Construction History is partner to an historic event tonight…”We Built Chicago” will feature representatives from some of the Chicago construction industry’s most storied firms–Pepper, Berglund, McHugh, and Ozinga among them–discussing their companies histories and contributions to the city’s development.  Mary Brush from Brush Architects and Marvin Levine from the Levine Company–both tireless supporters of Construction History in the city and of this week’s Congress–will moderate.  And a reception afterwards promises a good chance for international scholars and local heroes to meet, share a drink, and find out more about each others’ worlds.

5:00-8:00pm  School of the Art Institute Ballroom, 112 S. Michigan Ave.

Free and open to the public (no ticket needed, despite the rather nice one shown…) –more details at 5icch.org

Many thanks to the Builders Association of Chicago for sponsoring the event and the School of the Art Institute’s Historic Preservation Program for donating spac

5icch–day one

June 4, 2015 § 1 Comment

  so…Swiss insulation products, Renzo Piano’s early patents, Expo 58 structures, precast Brazilian factories, dinner at the Girl and the Goat, a lakeshore run and Experimental Archaeology with UCLA superstar Stella Nair’s keynote this morning.

Are we off to a good start?  We are off to a good start.  We’ve doubled the coffee allowance per diem for the rest of the conference, too…

5icch–building, construction, work

June 3, 2015 § Leave a comment

prudential train yardIt’s kind of hard to imagine, but the little day counter on this page has finally ticked down to zero and the 5th International Congress on Construction History is underway.  Lots of folks to thank who have made this possible and who have put in endless hours over the past six or seven years…

We’ve already been able to show off the city a bit at dinner last night, where some early arriving European scholars got to see some of the Loop’s best on a really glorious evening.  Seeing people see Sullivan for the first time is kind of priceless.

My bit of welcoming involves explaining the city in terms of its infrastructure–how exchange has really been the one constant in Chicago’s history.  This involves looking at grain elevators (via William Cronon, whose chapter on these long-lost monuments in Nature’s Metropolis remains one of the best reads in construction, economic, or urban history I’ve ever seen), at railroads, canals, expressways, etc.  And at how these things intersect.  So, skyscrapers above railyards (see the Prudential, above) seem like classic cases where the city’s commercial, civic, and infrastructural aims come together to create something new.

The back and forth between the activity of building, buildings themselves, and the activities that buildings then go on to support (think of Wacker Drive after the de-industrialization of the river shoreline and re-construction as the city’s greatest business address) calls to mind that Dewey quote that students of mine are overly familiar with…seeing “construction” as both a noun and a verb requires a certain blurred vision, but if Dewey was right then it’s precisely that blurriness where things get most interesting.  So, some thoughts on that today, too, to kick things off.

A good few days ahead, highlights to follow…

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