Structural Steel - discontinuity at base of light pole
Structural Steel - discontinuity at base of light pole
(OP)
I reviewed the structural design of a light pole from another engineer.
The pole is a 8" standard (8.625" OD) pipe with a 6" diameter hole near the base (cable port). While cable ports are necessary and common, I've never seen such a big chunk of steel taken out of the cross section at the point of maximum moment.
Technically, the stresses work. That is, the demand vs. capacity is about 90% after consideration of the reduced section at the port (but ignoring the stress concentration effect). However, I believe the design is just plain bad, and need code ammunition to refuse this nonsense.
Does AISC or any other structural steel building code penalize stresses due to discontinuities? Certainly tension connections get penalized in AISC (chapter B) with shear lag factors - how about main flexural members?
The pole is a 8" standard (8.625" OD) pipe with a 6" diameter hole near the base (cable port). While cable ports are necessary and common, I've never seen such a big chunk of steel taken out of the cross section at the point of maximum moment.
Technically, the stresses work. That is, the demand vs. capacity is about 90% after consideration of the reduced section at the port (but ignoring the stress concentration effect). However, I believe the design is just plain bad, and need code ammunition to refuse this nonsense.
Does AISC or any other structural steel building code penalize stresses due to discontinuities? Certainly tension connections get penalized in AISC (chapter B) with shear lag factors - how about main flexural members?






RE: Structural Steel - discontinuity at base of light pole
2. Lid of opening does contribute some resistance.
in bending.
RE: Structural Steel - discontinuity at base of light pole
RE: Structural Steel - discontinuity at base of light pole
RE: Structural Steel - discontinuity at base of light pole
When in doubt, just take the next small step.
RE: Structural Steel - discontinuity at base of light pole
You are correct that the section looks like a C, and the tips go into compression.
The engineer did a good job of checking combined flexure and compression to define the demand stress at the "tips" of the C in the code check.
We all know that stress flows don't make 90 degree turns so well. AISC doesn't seem to address this (at least not that I can tell).
In my mind, it would be the same as notching the bottom flange of a simply-supported wide flange beam in the middle of the span, then running a stress check and saying that allowable stresses were less than code, and calling it good, all the while ignoring stress concentrations.
Mechanical engineers know better than this. Don't structural engineers?
RE: Structural Steel - discontinuity at base of light pole
Usually you need to stiffen around the opening.
RE: Structural Steel - discontinuity at base of light pole
Yet if wind is an issue you can have clearly expressed that fatigue must be taken into account, and the times wind attains some specific velocity during the life of an structure can be stated. This is not a theoretical situation, signals in roads fail their bolts and plates on wind fatigue.
I also had the opportunity to study this way the old masonry tower of the Basilica del Pilar de Zaragoza (near 100 m tall) and found precisely that in its around 300 year life had already met one of such maxima that had made some of its weakened sections (by included arched openings) fail in flexure. The alternative known source for such an effect, earthquake, the big one at Lisbon, was too far to cause the effect; wind had the strength to do that. And standing formulae I found digging technical literature at my reach showed that on so a long period a peak of solicitation of such strength would have already been met, as surely would, by the standing cracks.
On the other hand I don't think poles need in general thought be fencing off impacts, except if on purpose. Contrarily, I think they should be protected by placement or otherwise.
RE: Structural Steel - discontinuity at base of light pole
RE: Structural Steel - discontinuity at base of light pole
Thanks for the input.
csd72,
You are correct. Ports in poles usually have a stiffening ring around the cut opening. This opening does not. However, even if the opening is stiffened at it's perimeter (such as a round pipe welded in the wall), I think this mainly stiffens the section. I'm not convinced stiffening rings help with longitudinal stress transfer, or significantly reduces stress concentration.
miecz,
Good comment. Yes, the design did take the slender compression element into account. Opening is not that tall, so buckling is not a limit state of the free edge.
RE: Structural Steel - discontinuity at base of light pole
Also, I think a stress concentration factor (of at least 2) should apply to a buckling failure mode. This is just a gut feeling.
RE: Structural Steel - discontinuity at base of light pole
RE: Structural Steel - discontinuity at base of light pole
Your attachment from Minn DOT is useful (though very long).
Regarding wind-induced fatigue, what is your criteria for comparing cycles and stress? For ASCE 7 wind loading (or alternatively EIA/TIE 222) there isn't specific demand/frequency info on the dynamics of low velocity wind loading.
RE: Structural Steel - discontinuity at base of light pole
Gaylord, Gaylord and Stallmeyer in their Steel Structures pages 4 and 5 state how to vary the safety factor for building lives other than those targeted by the code. The spanish code was based in 50 years life-exposure, equivalent to a wind of 161 km/hr. The tower was 289 yr old, so by the formula I had a multiplying factor on the ordinary of around 1.25 times, or for a wind oh 180 km/hr.
Then I considered wind vortex shedding. For the specific situation with akin towers later built at some distance, I found another multiplying (impact, wind vortex shedding) factor of 1.24.
These were factors for the overall design on the structure, not fatigue based. With these factors I examined the behaviour of the building over its 289 years. The building had crack failures consistent with a shearwall behaviour, and effectively FEM analysis at service level showed it having exceeded generously the extant tensile strength of the masonry (around 2.5 times) at worst places.
So in the end was not FATIGUE what I had been considering in this study. But I may have made it in another, if I remind where and the sources will post them.
RE: Structural Steel - discontinuity at base of light pole
RE: Structural Steel - discontinuity at base of light pole
Generally I apply miners rule please read an extract (not as long as last post) from J. Holmes (he is the man is Aust for wind loading). They use this method for roof sheeting, in cyclonic regions.
How you come up with wind loading range that is another issue entirely. I would suggest you review the roof sheeting standard for your region, They probably have a low high low testing arrangement for roof sheeting, this would be best applied as it is easy, then use the fatigue M&n from the steel stanard. however if you want the long hand method i use to check the rain flow given to me by the turbine manufactures, let us know.
When in doubt, just take the next small step.
RE: Structural Steel - discontinuity at base of light pole
Regards
Kosmoooo