Simple Supports

Anyone can design a building that stands up. But it takes a structural engineer to design one that BARELY stands up.

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The San Francisco Earthquake and Reinforced Concrete

Concrete has been used as a material for thousands of years. But reinforced concrete – concrete with steel embedded in it – is a much more recent invention. It didn’t start to be used until the mid 1800’s (it’s first use is usually traced to some reinforced garden tubs built in France), and it was years after that before people started using it effectively.

One of the first “modern” systems of reinforced concrete was the Ransome system, invented by Ernest Ransome in 1884. This system was distinguished by using twisted steel bars to improve their bond with the concrete.

ransome

Engineers are sort of skeptical of new technology by nature (and by incentive), and reinforced concrete (including the Ransome system) wasn’t any different. Up through the end of the 19th century it remained unpopular to use as a building material, being used for foundations but not much else. Most building codes didn’t even recognize it.

One of the few buildings made of reinforced concrete during this period was a museum on the Stanford campus, built in 1891 using the Ransome system. Reinforced concrete was chosen for it’s speed – it could be put up much quicker than a traditional masonry building. It was later enlarged with wings on either side, but these were built of conventional masonry construction and built to match the appearance of the original building.

stanford

In 1906, a 7.8 magnitude earthquake struck the San Francisco bay area. Thousands of buildings were damaged or destroyed by the shaking and the subsequent fires. The wings of the Stanford museum, built out of masonry, were reduced to rubble. But the original reinforced concrete structure suffered no damage at all.

Unreinforced masonry (masonry with no steel embedded in it) is perhaps the worst possible material to use if you want your building to survive an earthquake. As the building shakes back and forth, parts of it are put in tension which normally only see compression. Masonry is exceptionally weak in tension, but reinforced concrete, with it’s steel skeleton, is far more resilient. Engineers inspecting the Stanford Museum, built using the Ransome system, were impressed with how little damage it suffered.

Buildings are designed to survive worst-case loading that, in all likelihood, they’ll never see. Because of this, engineering practice tends to proceed one disaster at a time. The success of the Stanford Museum (and other reinforced concrete buildings in the bay area) helped popularize reinforced concrete as a building material. And modern steel reinforcing is specifically designed and shaped to grip the concrete, like Ransome’s bars were.

Why Roman Concrete Outlasts Ours

Recently, there’s been a flurry of news surrounding a new paper which examined the mineral structure of concrete samples taken from a 2000 year-old Roman breakwater. The articles range from measuredly pointing out it’s carbon efficiency, to extolling it’s  near-mystical properties. The fact that these structures are still intact after millennia, while ours often decay to the point of uselessness after less than 50 years, obviously raises some questions. Namely, was Roman concrete better than ours? Why does ours fail so quickly?

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New Southern Pine Design Values

On June 1st, new design values for southern pine lumber came into effect. These results are based on full-scale testing of various lumber sizes, and supersede the interim results that went into effect last year, which only affected 2″-4″ sized lumber. The kick in the teeth is that the new values show a sizable decrease in capacity for compression, bending, and tension, with reductions ranging from 10-30%. More information can be found at the SPIB site.

Allowable design values vs. actual strength at failure.

Allowable design values vs. actual strength at failure.

The changes are the result of the large-scale destructive testing of thousands of pieces of southern pine lumber. Wood is a highly variable material, and so requires a large number of samples to reliably establish safe design values. This sort of testing first began in the late 1800’s, and is conducted every so often by lumber testing organizations. Testing standards have changed over the years, but currently must follow ASTM D 1990. Testing organizations must be certified by the American Lumber Standards Committee. There are currently seven organizations, which are responsible for various regions and wood varieties. Southern pine lumber is covered by the Southern Pine Inspection Bureau.

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Using The Tools You Have

MacGyver: World’s Greatest Engineer

Engineering is a game of optimization under constraints. Problems are never just “design a beam that can span a hundred feet“, but “design a beam that can span a hundred feet, is made of concrete, weighs less than 40 tons, and is less than five feet tall.”  Or, more likely, “design a beam that can span a hundred feet as cheaply as possible”. Problems with only one requirement are easy to solve – it’s the ones with multiple, sometimes conflicting requirements that require clever solutions.

One of the most important of these requirements is “…and design it using only these tools“. This isn’t something that shows up in the design contract, but it’s a necessary reality. The tools humans have invented so far, be they wrenches or word-processors, are a limited subset of what’s theoretically possible to accomplish. And the tools any given engineer will have available are a limited subset of that. Much like MacGyver, we can’t solve engineering problems any way we’d like. We have to use whatever junk happens to be lying around.

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An Introduction to Graphic Statics

As I’m so fond of mentioning, engineering design required the use of a number of creative methods before the invention of calculators and computers. Some of the most important and widespread of these were graphic methods of analysis. Graphic methods essentially translate problems of algebra into geometric representations, allowing solutions to be reached using geometric construction (ie: drawing pictures) instead of tedious and error-prone arithmetic.

Unfortunately, these methods are slowly being forgotten. It’s extremely rare to ever see them used, outside of a select few occasionally taught in structural analysis courses. But understanding how, and more importantly why, they work unquestionably makes for a better engineer.

To remedy this, this post will lay out some of the basics of graphic statics. If there’s interest, more posts on more advanced methods will follow.

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Structural Engineering Resources on the Net

[Edit: I’ve added this as it’s own page, which I’m frequently updating, so check there for the most recent version]

Here’s a list of engineering resources around the net I’ve found useful. Feel free to suggest any additions in the comments.

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Fracture-Critical Bridges

The Golden Gate – a fracture-critical bridge

The story of the I-5 collapse continues to evolve. Currently the focus seems to be on the fact that I-5 (not to mention I-35W) was classified as “fracture-critical”, and the danger it and other fracture critical bridges pose. As there’s a great deal of misinformation being spread about the nature of fracture-critical bridges, an explanation of what it actually means is in order.

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The I-5 Bridge Collapse

The I-5 Bridge in Washington

Right on the heels of my last post about structural collapse, we get news that the I-5 bridge in Washington has collapsed.

Information is still limited, but based on the facts available, it looks as if it was caused by a truck carrying a heavy piece of mining machinery striking a cross member. The 50 year old bridge wasn’t classified as “structurally deficient“, but it was “functionally obsolete”, indicating it probably had less clearance height than a modern bridge.

First off, despite what some news outlets are reporting, there are no indications that the bridge was “unsafe”. Functionally obsolete means things like lane width, approach curvature, clearance, and other nonstructural aspects aren’t up to modern bridge code standards. However, none of this has anything to do with the load carrying capacity of the bridge, or the status of the structural members.

So is this just another example of why we need more infrastructure spending? I’m not so sure.

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Collapse and Connections

I-W35 Bridge Collapse

Consider the failure of the I-35W Bridge from several years ago:

During the wreckage recovery, investigators discovered that gusset plates at eight different joint locations in the main center span were fractured. The Board, with assistance from the FHWA, conducted a thorough review of the design of the bridge, with an emphasis on the design of the gusset plates. This review discovered that the original design process of the I-35W bridge led to a serious error in sizing some of the gusset plates in the main truss…On November 13, 2008, the NTSB released the findings of its investigation. The primary cause was the under-sized gusset plates, at 0.5 inches (13 mm) thick.

Or the infamous Hyatt Regency walkway collapse:

…someone looking at the original details of the connection must have said he had a better idea or an easier way to hang one skywalk beneath the other…But no matter how much more convenient to assemble, the new rod configuration effectively doubled the push of the washer on the box beam supporting the upper walkway’s floor, and this made the already under-designed skywalks barely able to support their own weight.

Or the partial collapse of the Centergy parking deck in Atlanta:

First, the connection holding the fourth floor spandrel beam to the column broke. This caused the beam to slide away from the column. The beam moved away from the garage far enough that the T-beams composing the main floor of the parking deck were bearing on a very thin edge on the ledge of the beam. The concrete of the ledge spalled and as the t-beam fell it pushed the spandrel off of the structure. The fourth floor of the deck fell on top of the floor beneath. The weight of the falling floor overwhelmed the capacity of the floor beneath, and initiated a progressive collapse of the entire bay beneath.

Unless they’ve been catastrophically damaged, structural failure in the US tends to occur at the connections between members.

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Accuracy vs. Simplicity in Engineering

Once you move beyond basic statically determinate cases, engineering problems rapidly become very difficult to solve (leading to some creative solutions in the days before computers). An exact solution often requires the aid of either expensive software packages or extensive calculus training. However, while it might be difficult to, say, calculate the exact bending moment in a beam, it’s often easier to put an upper bound on it’s value. And sometimes, a reasonable upper bound is all you really need.

Engineering, in all it's glory.

Engineering, in all it’s glory.

Here’s a real life example I faced recently. A single-story building has a room dedicated to file storage. In this case, the files are stored in large shelves that can be moved along tracks mounted to the floor. I had to design the concrete slab to support the weight of the files.

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