A Caution to Consider for All 2

[msquared48]

Just analyzed another tower, 245 foot lattice on the top of an 1800 foot high ridge where, in 2011, an out of state engineer had apparently not done his topographical homework, and misapplied the definition of the Crest Height as well as the Topographic Category, if even he looked at Google Maps. He was not even aware of the local northeasters that can come down from the Frasier Valley, the critical direction for the tower.

He used a crest height of 0 and a Topo Cat of 1 where it should have been a Topo Cat 3 for a ridge, and a crest height of at least 1500 feet according to the ASCE 7-10 definition per section 26.8.1. His analysis was originally marginal at 102%, but mine with the new equipment and correct topo fails at 246%, way more than just the additional equipment (+20% to his original analysis).

Now I have to explain this to the telecom client. Man I live this job!

This is not the first time I have seen this. We really need to be careful with these wind factors, particularly with critical communication equipment, and such terms as "crest Height" needs to better defined in ASCE7 with examples, specific, real world examples, not just textbook where it is all left to interpretation and chance.

I'll step down now.

Mike McCann, PE, SE (WA)

 

[TehMightyEngineer]

First thing my boss taught me was make sure your loads are right. Everything else will likely be minor by comparison to screwing up your design load calculations.

Maine EIT, Civil/Structural.

 

[CELinOttawa]

Agreed!

Strength
Stability
Stiffness

All analysis is meaningless if you do not get what goes on the top of the */Phi correct.

 

[TehMightyEngineer]

Plus, it's one of the few things that can invalidate an entire design. So even if you catch your error it can seriously delay a project.

Maine EIT, Civil/Structural.

 

[JedClampett]

I know nothing about tower design. And because of that, before I took one on I would make sure to pick someone's brain who does these. It seems like it wouldn't take long to realize that this Crest Height is a pretty big deal.
There's no harm in admitting you need help with something. Or turning down a project because it's in a unfamiliar (either geographical or type of design) area.

 

[chicopee]

I guess one should visit and know the territory before dealing with towers.

 

[racookpe1978]

246% load?

Obviously, that is maximum expected (but a realistic load) force by ionterpreting the "Rules" of the "Game of Towers" out on the frontier. It "might not" occur each Tuesday afternoon in May, but probably every fourth November when a storm front comes through, right?

Why do you think the original felt a 102% load was reasonable? Did he "read the book" and decide that storms will never be higher than what the book decided was the maximum?

 

[hokie66]

racookpe,

2.5 times from basically flat ground to an 1800 ft high ridge sounds about right for the wind load difference. I think the original "engineer" did not "read the book".

 

[msquared48]

Obviously, the Ohio "EIT" has not experiences a Northeaster like I have. And Hurricane Ridge is only 30 miles or so to the west. Didn't get its name for no reason. Being local has its benefits.

Mike McCann, PE, SE (WA)

 

[BUGGAR]

"Now I have to explain this to the telecom client".

Let me do this! I will even pay my own airfare to have the joy of doing this. Oh please!

 

[Ron]

Mike...have seen this numerous times....even in the flatness of Florida. Happens on all styles....lattice, guyed and monopoles! Add new equipment to tower that didn't make it the first time around! Always fun...Always a challenge to explain to money driven client! Good luck.

 

[dhengr]

BUGGAR:
No need for airfare. Just wait for them to call about the status of the project. Then put the computer generated recorded answering message on...., “you are currently #398 in line, please hold, we pretend to appreciate your business.” Then you turn the nasty loud music on, interrupted just often enough by verbal adverts, so they can’t get any other work done while they are waiting and listening for an unhelpful human voice. Alternatively, tell them you will have the engineer call them, the window for that call is tomorrow from 8:00a to 8:00p, and there will be an extra charge if the responsible party is not there the take the service call.

As for Mike’s OP issue.... anyone can do engineering these days, all they need is a computer program. No real knowledge of the subject is required, engineering experience and judgement are much too expensive. They can be from any place, and e-mail it to you. No need for them to learn their trade (profession doesn’t really apply here) under an experienced engineer who guides them and looks over their work. Their output in design and final products is just a commodity, and their work is just a job, certainly not a profession endeavor that they take pride in. Then there is a reasonable likelihood that their customer just reused an old design, intended for a different region. The towers were both 245' tall and had about the same junk hanging on them, why pay for engineering all over again. Those engineerss are just way to pushy, they ask to many questions, they are always so damn conservative and they are way to expensive. No wonder we can’t afford more beautiful towers dotting our landscape.

 

[msquared48]

Unfortunately I think the tendency in the Telecom industry for structural engineers is to not have the results be drastically different than a previous analysis by another engineer. This creates problems and dangerous situations when certain considerations have been ignored. Your personal liability and reputation are at stake as opposed to having a major client. Some people fall into this trap. You have to be absolutely sure of your input and let the chips fall where they may. I had a similar problem just recently on another tower I posted about. Problem there was that although the code allowed a certain condition, I disagreed with it. Judgement call.

Mike McCann, PE, SE (WA)

 

[Ron]

Go Mike!

 

[msquared48]

I went... 'nuff said.









Mike McCann, PE, SE (WA)