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RE: Steel-Detail: cut threads vs rolled threads

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>In regards to threaded rods ... , what
>is the difference between having cut
>threads or rolled threads?  Is there
>any difference in strength or performance?

Below is an excerpt from the answer I wrote on this subject in the October issue of AISC's Modern Steel Construction magazine. It is not oriented toward cases involving fatigue. When fatigue is a consideration, Chris Wright's summary of differences will be helpful.

Charlie

*******************
[When comparing rolled and cut threads ....] Unless an upset rod is used, the strength values of the rods are based on the root area, which is identical for both methods of threading. 

However, there will be a difference in elongation under load as follows. In general, the rod elongation D for a load P on a length L is:

Δ=PL/AE

For a 2"-diameter rod with cut threads, which has a minimum 1.96" body diameter per ASTM F1554:

Δ = 1.04PL/(πE) 

For a 2"-diameter rod with rolled threads, which has a minimum 1.84" body diameter per ASTM F1554:


Δ = 1.18PL/(πE)

Thus, although there is identical strength, there is a decrease in axial stiffness when rods with rolled threads are used, versus rods with cut threads-a 13.5% decrease in this particular case. Therefore, it is a design consideration which type of threading is to be used. If strength controls the design of the rods, I don't think there is an issue. If drift controls the design, there might be an issue. This is an aspect that I think should be considered by the engineer of record (EOR) in the design of the rods.

With this in mind, the designer should specify in the contract documents either a design based upon rolled threads or that cut threads be provided. If cut threads are specified, the fabricator should provide the rods with cut threads, even though a slightly larger-diameter rod with rolled threads might be acceptable to the Engineer as an alternate.

If the type of threading is unspecified, ASTM F1554 Section 6.2 allows that the manufacturer can provide either. If the rods are of another specification, such as ASTM A307, there are similar provisions in the ANSI/ASME B1.1 threading specification that reference them.

Charlie Carter, S.E., P.E.
American Institute of Steel Construction Chicago


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