Sunday, May 27, 2018

High-Strength bolts replaced rivets during the 1960s and 70s

When I described old pin-connected truss bridges, I learned from Brian Solomon's North American Railroad Bridges:
Rivets were replaced with high-strength bolts starting in 1960. Bolts are more expensive to manufacture, but the skill and tools needed to install them and replace them is less sophisticated than that needed by rivets.
Historic Bridges does not include bridges built after 1970s because he defines historic bridges as those built with rivets.

20140514 0004, Lemont Santa Fe Bridge
(Railroads still have old truss bridges because they don't get 80% financing from the Federal Government for new bridges like the state departments of transportation do. For railroads, it is cheaper to maintain a truss bridge than replace it.)

Riveting is labor intensive. You need someone heating them until they are, literally, red hot; someone to catch a rivet and insert it into a hole; and someone on the other side to hammer it down to a round head. More workers are needed if additional people are needed to catch a rivet and toss it to the next person closer to the work site. That is, if a relay team is needed between the forge and the riveters.

In this first video, the receiver literally catches the rivets in his gloves. It looks like both sides use pneumatic hammers. I wonder what synchronizes the hits by the hammers.
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In addition to Rosie the Riveter, Canada chose rivet tossing as an example that a woman can do almost anything a man can do if the country is at war, including tossing and catching hot rivets with a bucket.
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Note that the women in the 1940s were wearing hard hats. Men didn't start wearing them until a few decades later.

This video not only shows more modern riveting tools, it shows a pneumatic wrench that is used on nuts. Note that modern riveters carry the hot rivets rather than toss them.
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This video clearly illustrates that pneumatic hammers are used on both sides. And they also first "tacked" the sheets of steel together using nuts and bolts.
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Obviously, one reason why the rivet needs to be red hot is so that it is malleable enough to be hammered down into a round head. But another reason was so that it would contract when it cooled and provide a compression force, F, to squeeze the plates together. For applications like the boiler being built in the third video, you want the plates squeezed together so tightly that any gap between them is less than the size of a water molecule. That is, you don't want steam escaping from the boiler seams. Furthermore, the rivets are prestressed. It would take a force greater than F to open up a gap between the plates.

I learned recently that another advantage of rivets is that the holes do not need to be exactly the same size or precisely aligned. The diameter of the rivet can be smaller than the hole size because when it is hammered it will mold itself to the width and alignment of the holes. So you not only have good compression strength, you have good slip control.

An issue with rivets is that even though the contraction during cooling creates a compression force, the strength of that force is unknown and varies from rivet to rivet.

In 1934, researchers reported that high-strength bolts (>= 54ksi) could be tightened enough to prevent slip in structural joints.
Based on tests performed at the University of Illinois, Wilson and Thomasin 1938:
The fatigue strength of high-strength bolts appreciably smaller than the holes in the plates was as great as that of well driven rivets if the nuts were screwed up to give a high tension in the bolt.
The Research Council on Riveted and Bolted Structural Joints (RCRBSJ) was formed in 1947. The simplification of bridge maintenance drove the research to replace rivets with bolts. "In the early 1950s, the installation procedures, the slip resistance of joints having different surface treatments, and the behavior of joints under repeated loadings were studied....In 1960, BS 3294 was issued to establish the design procedure and field practice....The first edition of the Guide to Design Criteria for Bolted and Riveted Connections sponsored by the council and published in 1974, provided a valuable summary of connection and connector behavior for designers and specification writers alike."

Three grades of bolts are specified. A307 (>= 60ksi) is made with low carbon steel. A325 (>= 105-120ksi) is "made by heat-treating, quenching, and tempering medium carbon steel." A490 (>= 150-170ksi) is made with "quenched and tempered alloy steel." The higher-strength bolts also have finer threads and are marked [BoltCouncil, p36] to indicate their strength.

[BoltCouncil, pp2-3]

Cyclical loads can cause bolt failure independent of the grade of the bolt. The important aspect of fatigue failure is the range of the stress during a loading cycle. [BoltCouncil, p21]

"North American practice prior to 1985 had been to require that all high-strength bolts be installed so as to provide a high level of preload, regardless of whether it was needed (bolts in a slip-resistant connection or in a connection subject to tension) or not needed (bolts in a bearing-type connection)." The preload should be 70% of the minimum specified tensile strength of the bolt. "To overcome the variability of torque control, early efforts were made to develop a more reliable tightening procedure. The American Association of Railroads (AAR), faced with the problem of tightening bolts in remote areas without power tools, conducted a large number of tests to determine if the turn-of-nut could be used as a means of controlling bolt tension. These tests led to the conclusion that one turn from a finger-tight position produced the desired bolt tension." But "finger tight" was discovered to be highly variable. So now a nut is to be tightened to a "snug fit" with an impact wrench then turned some more according to a table that expresses bolt length in terms of bolt diameter. "Snug fit" is when the wrench starts impacting. [BoltCouncil, pp54-56] Japan uses a "smart electric wrench" that detects the first nonlinearity of current draw of a DC motor. That is, where the bolt transitions from its elastic to its plastic stage. [BoltCouncil, p60]

Holes are typically 1/16 in. larger than the bolt because the friction between the plates caused by the preload tension of turning the nut an appropriate arc past "snug" is enough to prevent slippage. This allows holes to be misaligned up to 1/16 in. Does misalignment reduce the slip strength of a joint? No if the bolts and plates are ductile enough to absorb 1/16 in. of movement. The irony of the high-strength A490 bolts is that they are less ductile. In fact, misalignments will reduce the slip of a joint that is overloaded because some of the bolts are doing more than their fair share of absorbing shear stress. [BoltCouncil, Chapter 11]

A rather obvious advantage of preloading (prestressing) a bolt to 70% of its minimum tensile strength is that it would take a tensile force larger than the preloading before the two plates separated. Another advantage is that it reduces the range of cyclical loading, which reduces the risk of metal fatigue cracks forming, which reduces the risk of metal fatigue failure. If a 10ksi load is applied to a bolt that is preloaded to 100ksi, then the change is 10%. But if it is applied to a bolt with 10ksi preloading, then the change is 100%. Heavy trucks crossing a bridge is a cyclic load on the components of that bridge.


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