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Re: Welding of Crane Runways

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> There is a requirement in the Canadian code for welding that forbids 
intermittent
> welds in dynamically loaded structures except in compression
> zones and then those welds must not be spaced any further apart than
> 6" or 12 times the thickness of the thinner part, whichever is less. 

Thses are fatigue and weld quality requirements. Intermittent welds 
involve a large number of starts and stops. The ends of a stitch weld are 
usually malformed, there's a great tendency towards embrittlement because 
of sudden cooling and the ends tend to acquire defects such as inclusions 
and undercut. Moreover the load transfer into a fillet is non-uniform: 
The ends are always very highly stressed and the sorter the stitch the 
worse the situation. The reason for the provision is to avoid fatigue 
failure. You may have low nominal stress, but the local stresses will be 
many times higher, especially if there's a built-in sharp corner due to a 
rough weld contour.

In your particular case the stitch welds on either side of the 
instantaneous wheel location will also be subject to uplift, like a beam 
on an elastic foundation which also aggravates susceptibility to fatigue 
failure.

The economy issues associated with stitch welding seem obvious, but the 
real price is that of poor quality. You should really require that the 
ends of those welds be mag particle or penetrant examined and all defects 
removed.

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw


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