Monopole Collapse 1

[ISEAG]

I work primarily in the communications structures arena designing monopole structures. Requests from jurisdictions have begun to come in for fall zones and fall zone requirements. A focal point "buzz" phrase has emerged requesting or requiring "self-collapsing" monopoles.

Poles generally fail at base welds, bolt/foundation failure or simply by buckling at critical sections. I have been on testing programs where we performed full scale failure testing of poles and we never dropped one to the ground...it is virtually impossible to produce enough sustained load (without winches or a dozer) to plastically deform a pole to the point of dropping it.

Does anyone here have any thoughts or experience on this? I can understand designing poles to have fusible points but it takes really small sections that are atypical or not available to force failure at a defined location to identify a fall zone or cause self collapse.

I know letters are written kind of explaining away this issue for the jurisdictions but, as one guy told me, he just writes a letter stating the pole will fail in a self contained mode and will not fall outside a legnth of the top pole section.

THanks in advance for any input.

 

[Ron]

I have testified before county and city commissions regarding this issue. Mostly my analysis and testimony was related to guyed structures, but similar would apply to monopoles. It is not practicable to design monopoles to have "break points". The best you can do is decide that failure will occur at the base and that the full length of the monopole will fall over. That gives a fall radius equal to the height of the monopole. That's not unreasonable and should be one of the site design criteria.

For guyed structures we could design break points and cause them to "drop down" within a reasonable radius. That's achievable with guyed structures, but not necessarily so with monopoles. I don't like designing failure points into any structure...it's just not in our nature to design for pre-determined failure...although our designs will fail if subjected to overload...that's just the nature of efficient design.

 

[BAretired]

The only way I can think of is to construct the pole in two sections, the upper one much weaker than the lower one so that failure takes place at the junction of the two. This would mean oversizing the lower section so that it cannot fail under expected loads.

If the overall height is 9 units and the lower section is 4 units, collapse could be contained within a radius of 3 units, or 1/3 of the original height.

BA

 

[dik]

If I understand this correctly, some jurisdictions restrict construction in the radius of the height of a monopole?

This seems silly... be a shame to prevent construction within 300' of a 30 storey building... or similar construction...

Dik

 

[MikeHalloran]

Buildings never fall over dead like a rigid body, right?

http://www.eng-tips.com/viewthread.cfm?qid=248426&page=9

Mike Halloran
Pembroke Pines, FL, USA

 

[mimgq2]

Doesn't the TIA-222-G standard have any design guildlines for failure? Such as a fall zone? Does it not have any details on how the pole should connect to the base plate that can be used as guild? or specifically how the pole should be desinged to fall that you can use?

 

[mimgq2]

Cell Towers fall all the time...From fatigue or fire.

http://cms.firehouse.com/web/online...ellular-Tower-Crushes-Chiefs-Vehicle/46$21901

http://www.wusa9.com/news/columnist/blogs/2009/01/must-see-video-cell-phone-tower-burns.html

Where are the standards?

 

[mimgq2]

Sorry Tread broke

"

http://cms.firehouse.com/web/online...llular-Tower-Crushes-Chiefs-Vehicle/46$21901"

 

[ISEAG]

Thank you all for the input. I too am a bit curious/frustraed about the issue because some jurisdictions ARE trying to require a "defined" fall zone. I have not been able to get them to answer my qiestion as to what they want but, at the same time they want me to state/certify that the pole won't fall outside a certain radius, typically over a property line or onto a building near by. I refuse to state that a pole won't fall anywhere less than its height for fear of future risk claims. I have worked up designs with 2 or more pole sections with the top section holding all carriers, oversizing the lower sections to have at least 15% to 20% more capacity to insure top section failure. Problem is, the pole mfg and the carriers don't like it because of cost. I am not sure what PJF, Valmont, Ft Worth Tower, or Sabre are doing about this but their designs haven't seem to change in the jurisdictions in question. My designs fall within the same realm as theirs.

Thanks again for your input. As I learn more I will post here just to educate us all.