Design

The Thrill of Conformity

by Don Friedman on March 14, 2019


It’s not often that obscure engineering criteria make it to the op-ed page of the New York Times, but it happened recently with regard to “concrete masonry units” also known as CMU also known as concrete block. “The Joy of Standards” by Andrew Russell and Lee Vinsel is about the way that modern technological system work better because of standards. Their piece does a nice job describing why standards matter; rather than repeat to all here, I suggest simply following the link above.

They discuss the use of CMU as described in TMS 402 – the national masonry code put out by The Masonry Society – and the role of the American National Standards Institute. We deal with TMS when we design and analyze masonry, but ANSI is more focussed on machinery than on structure. It’s worth taking a look at the multiple levels of standards that we rely on and are constrained by.

At the top is are the local building codes, which are laws passed by cities, states, or other local governments. They provide standards for that locality. So, for example, New York has somewhat stricter laws regarding fire ratings than most of the country because (a) it’s logical to do so in a dense city with a lot of high rises and a lot of old buildings with wood framing and (b) we have had horrendous experiences with fires. Just about any part of a local code can vary from one place to another, but most don’t. So this is standardization at the local level.

Below that is the International Building Code, which is a generic US code (despite the name) that serves as the basis for the local codes. A city or state could simply adopt the IBC as is, with zero changes from the generic template, but few do. Even with the local changes, the presence of the IBC as the basis for the local codes provides some national standardization. When I look at the local code in a place I don’t often work – as I recently looked at Virginia’s code – it mostly looks familiar.

Below that are the specialized national-standard codes that are the basis for the IBC. TMS 402 for masonry, AISC 360 for structural steel, ACI 318 for reinforced concrete, ASCE 7 for loads, and so on. (Yes, there is a pattern to those names; yes, it is a quite geeky pattern.)  And that’s just the structural portion of the code: there are similar national standards in other design fields.

At the bottom are the basic standards for materials. Most of these standards in structure come from the American Society for Testing and Materials. So, for example, the type of steel commonly used for structural angles is designated as ASTM A36. The “A” series of ASTM specifications refers to ferrous metals, and some of the standards are quite old. ASTM A6 provides the geometry for most steel shapes, which is the most basic form of standardization imaginable: if I’m going to specify a beam, I have to know that the size is independent of the manufacturer, or else I’d have to specify the manufacturer and not just the size. That’s not speculation, but rather the situation that existed until 1896, when the American Association of Steel Manufacturers created the “American Standard,” which was the first national standard for steel shapes. The illustration above shows some American Standard sections.

And, as Russell and Vinsel discuss, having all of these standards doesn’t really constrain design. Rather, it allows us to concentrate on the interesting and challenging parts of design because we don’t have to spend our time worrying about whether nuts from one factory will fit on bolts from another.

 

 

Concrete, Part 4

March 8, 2019

I’ve said that different materials lend themselves to different forms, and that there is such a thing as a concrete-inflected structural type. That idea leads, unfortunately, to a discussion of the collapse of the Morandi Bridge in Genoa last August. The bridge had a number fo different components (click on the picture above to expand it) […]

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Concrete, Part 3

March 7, 2019

Concrete has become something of a scapegoat for carbon emissions in building, but it’s amazingly difficult to pin down how bad it is, as a material, compared with other options. The problem, simply, is that it is difficult to make apples-to-apples comparisons of efficiency and carbon use across different structural systems. Let’s start with weight. […]

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Unclear Boundaries

January 15, 2019

That beautiful concrete is the undercroft, for want of a better term, at the London Bridge rail station. The station has a top-heavy configuration, with the entrances at street level and the tracks above. This makes sense for a constrained-site urban station, and London Bridge’s operational set-up reminded me of Jamaica Station. The concrete groin […]

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An Old Problem Resurfaces

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Innovations sometimes don’t work well. The Millennium Bridge in London, which was a new twist on suspension bridge design, moved noticeably when it was built and needed a retrofit to reduce the sway. An article on the intersection of structural dynamics, statistics, and the biomechanics of the human gait – Walking Crowds on a Shaky […]

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An Interesting Choice

December 14, 2018

While I’m on the topic of funny-looking subway structures, let’s discuss the mezzanine at the 181st Street Station on the A train. It’s a steel-framed platform hung directly over the tracks. That’s the view from the mezzanine stair above, looking down on the train I had just exited, and here’s the view from the platform: […]

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Food For Thought

July 26, 2018

SCOSS summarizes the full report on Grenfell. Some of the specifics are UK-only, but the general issues apply everywhere. Most architects and engineers are not experts in fire safety, so we follow the codes. But there are always gray areas at the edges (How much flame-spread is acceptable in a pre-manufactured curtain wall? How much […]

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Structural Stability – Lateral Bracing

July 24, 2018

When intervening in existing buildings, floor removals and creating new floor openings for stairs and elevators are a common practice. However, people often underestimate the importance of lateral bracing and the role it plays in the capacity of the vertical supports of the building (walls, columns, etc.). Increasing the distance between the lateral braces of […]

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A Stair Handrail in Wroclaw

July 1, 2018

That is some nice metalwork. Good design is always worth the effort.

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A Serious Bridge For Serious Work

June 30, 2018

Just down the road from the fake covered bridge is a through Pratt truss. Click on the picture to expand it and the bridge is off to the left. It’s not a bridge that people or vehicles can travel over: it carries a large pipe full of sewage from the high-elevation north end of town […]

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