Structural Steel - discontinuity at base of light pole 2

[ATSE]

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?

 

[dgkhan]

1. stress concentration. I have noticed edges around openings are thickened.
2. Lid of opening does contribute some resistance.
in bending.

 

[asixth]

It may be a failure mechanism. If there are no safety barriers preventing a car hitting the light pole there will be a slip base, so the pole will fail when hit by a car and prevent the car from being wrapping around the post.

 

[miecz]

I imagine that the reduced section looks like a C. So I'm curious. Can the tips of the C go into compression? The section then would tend to buckle. Were you able to take that into account?

 

[rowingengineer]

I know light poles and cantilever wind towers are different for analysis but also similar. In wind tower design we have large doors in the side of the towers, to analysis this we look at fatigue fairly heavily with rain flow analysis of the wind loads and then fatigue of the steel tower. Generally, the walls are thickened as per dfkhan post with some cool welding details. Therefore, if you wanted to get them on fatigue i found an interesting article that make help.

When in doubt, just take the next small step.

 

[ATSE]

miecz,
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?

 

[csd72]

blodgetts books go into this exact situation.

Usually you need to stiffen around the opening.

 

[ishvaaag]

You may find difficult to extract a stress concentration out of a structural code on a standard shape (well, not so standard after cutting it). In any case you may argue this just on technical considerations. A look on the Galambos text that has something that I remember on holes on tubes and pipes and their rigidizers may help.

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.

 

[miecz]

If I take a sheet of paper, curl it into a C shape and then try to bend it so that the tips go into compression, the tips buckle radially from the center of the C. Granted, the unsupported length of the hole is short, but if the plate is thin, it may buckle long before it gets to yield. Did the designer adjust the allowable compressive stress for buckling?

 

[ATSE]

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.

 

[miecz]

Curious. What did they use for an effective length?

Also, I think a stress concentration factor (of at least 2) should apply to a buckling failure mode. This is just a gut feeling.

 

[bdSE]

Is it possible that the opening cover is attached with the intent of becoming part of the structure? That would certainly not the conservative approach we like to see in design, but perhaps that was the design philosophy. Take a look at the screws / bolts used to attach the plate cover, maybe this is where your additional strength will come from. A reinforced ring with a cover plate providing a micro "shear wall" might just be the trick needed to make the pole work at the base under full design bending.

 

[ATSE]

rowingengineer,
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.

 

[ishvaaag]

Have located the factors I used to evaluate the wind exposure of the tower.

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.

 

[ishvaaag]

More properly, in the second paragraph of the previous post the referred formula expresses the variation of forces on wind exposure, rather than of the safety factor.

 

[rowingengineer]

ATSE,
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.

 

[KOSMOOOO]

By my point of view, in static you can ignore stress concentration factor. But in cycling loading eg. fatigue is mandatory

Regards

Kosmoooo