Wrought Iron Bridge Tie Bars 2

[MOB1]

I am examining wrought iron road bridge components. The tie rods (joining top and bottom timber chords) have been upset forged at the threaded end to increase the thread root diameter above the shank diameter. Was this standard practice 120 years ago? Was it for fatigue resistance? I assume this would have been empirically worked out as little was known about fatigue in those days.
The bridge authority must replace some of the rods (due to corrosion) and are considering rolled threads and other measures to cater for heavier and numerically greater loads.
On trying to come up with numerical values for fatigue improvement for this design of fastener and for fasteners with radiused guide surfaces, rolled threads, etc., the lack of design information for fatigue resistance becomes obvious.
Can anyone help with some references?

 

[dik]

It was not uncommon to use a greater diameter at threaded areas. Use of threads, themselves, was less common.

Regarding fatique, one of the texts I have... "A Practical Treatise on Bridge Construction, being a Text Book..." by T. Claxton Fidler, M.Inst.C.E., 1887... debates if failure by fatigue is possible and that it might be some type of dynamic response to a moving load...

Dik

 

[unclesyd]

Interesting, but not much substance.
Here are several references on bridge construction using wrought iron and some discussion on fatigue

Hearing:

http://www.icac.nsw.gov.au/files/html/GRENDALPVT00338.htm#top

Paper on Structurals

http://www.istructe.org.uk/thestructuralengineer/HC/Abstract.asp?PID=1856

History of fatigue:

http://courses.washington.edu/mengr541/ramulu/541/notes/notes1.pdf

 

[metengr]

Talk to a reputable licensed, structural engineer.

 

[MOB1]

To metengr,
I have spoken to Structural Engineers about this issue but the Standards and Codes of Practice (South African, Australian - do not know about US) do not address design of fasteners to prevent fatigue failure. There are a few very old references on the subject.

 

[SlideRuleEra]

What you are describing is called an "upset rod". In the past they have been used with steel sheet piling installations, and are still used from time to time. As you have assumed, the intent of the upset is to keep the threaded end root diameter stresses similar to that of the unthreaded length. Here is a link to a current supplier

http://www.clevelandcityforge.com/products/images/threaded_upset.pdf

and a some general information

http://www.aisc.org/MSCTemplate.cfm...tomSource/Faq/SteelInterchange.cfm&FaqID=2269

http://www.SlideRuleEra.net