Intermediate Stiffeners in Plate Girders 1

[normm]

In the UK the intermediate stiffeners in plate girders are welded to the top flange and web of girders but not connected to the bottom flange. A gap is left between the bottom flange and the end of stiffener; as the sketch shows. This details is followed no matter if the deck is at the top or bottom flange of the girder.

I have asked a few colleagues of mine but have only been told that this is a better detail for fatigue considerations.

I cannot satisfy myself how this detail prevents the stress reversal in weld between the web and the bottom flange if that is the aim. How does the gap help? Why this is better for fatigue?

I will appreciate your views if this detail is followed in most countries and if somebody can get me a better or more detailed explanation.

 

[KootK]

How does the gap help? Why this is better for fatigue?

It turns out that it doesn't help. You're instincts are sound. See section 2.4.2.1 of this document: Link. There are, of course, some other practical reasons to cut the stiffener short.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sponton]

as transcribed from Salmon et al:


"Intermediate stiffereners are provided to assist te web; to stiffen and create nodal lines...

On the tension fflange, the effects of stress concentration increase the fatigue or brittle fracture possibilities, i.e. welding in no way helps to the tension flange. since the work of Basler has shown that welding of stiffeeners to the tension flange is unnecessary for proper functioning of stiffeners, AISC-G2.2 permits stopping stiffeners "short of the tension flange provided bearing is not needed to transmit a concetrated load or reaction" the weld by which the stiffener is attached to the web "shall be terminated to closer than 2 times the web thickness nor more than 6 times the web thickness from the near toe of the web-to-flange weld". " Structural Steel design, salmon et al. p555

 

[sponton]

Here's Basler's paper

http://preserve.lehigh.edu/cgi/view...xt=engr-civil-environmental-fritz-lab-reports

 

[SlideRuleEra]

KootK, great Federal Highway Administration (FHWA) link, page 17.

The idea of not connecting intermediate web stiffeners to the tension flange originated over a century ago when riveted plate girders were developed. Actually, stiffeners were not connected to either the tension or the compression flange - it was not practical to do this with rivets. The point load bearing stiffeners, e.g. at the ends of the girder, had to be machined to obtain a close, tight fit between flanges. The intermediate stiffeners were sheared for a close fit, but were neither machined nor did they necessarily bear on either flange. What made this concept workable was the longitudinal angles, top and bottom of the web, that allowed the web / flanges rivet connections. These angles braced the web at the right location. See this image:

When welded plate girders came into use, welding the stiffeners to the flanges was relatively easy and braced the stiffener. Keeping the intermediate stiffeners a little short was a way to save money, and was not thought to cause problems. Distortion-induced fatigue cracking that is the problem was not fully understood until the 1980's.

It took a long time to realize the technical difference between riveted and welded structural steel (at least the 1940's through the 1980's). Lehigh University was, and is, a leader in that area. Basler's 1961 paper was one step in finding the truth... but more has been learned since then.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[normm]

Excellent Responses from all of you.
Many thanks.