[TowerTalk] windload rating of triex LM470e tower to be independently verified
Tower2sell@aol.com
Tower2sell@aol.com
Wed, 27 Sep 2000 14:33:30 EDT
Bob,
I have to dissagree with some of the things you mentioned. The wet sealing of catalog sheets from the manufatcurer is still done. The catalog sheets are used with a sealed, cover page that describes the site and antennas for which it intended and other items and disclaimers.
The "normal soil" is up to the consumer to verify that the site has normal soil, and most of the time they don't. Normal soil was developed for bidding proposes so someone can compare quotes from different tower companies without having to have a soil boring. Too many people have elected to not to have their foundation redesigned with more appropriate soil parameters. The problem with the presumtive values in the UBC build code is that they are very conservative. The EIA-222 specifies a minimum factor of safety of 2.0 and most geotects use a 3 for bearing. The code factor of safety is considerable, partly because it assumes a constant loading like a building that could be controlled by settlement. A tower on a calm day have very small foundation loads and the foundations are only loaded to the max at the extreme wind loading (rarely). This means that settlement is not normmaly an issue and a higer bearing can be used. Many geotech allow a 1/3 increase in bearing capacity due t!
o the temporary wind load conditions.
The problem is not the lack of engineering staff, it is the price of the engineering certification package can exceed the cost of the steel. This is a simple supply and demand situation for small towers. The tower companies can get thier prices for the commercial customers who consider them resonable considering the price of thier large towers. It is about the same engineering work to design a large tower as a small one. My father was an engineer and designed sewer lines. He taught me that the difference between a large job and a small job was the distance between the lines representing the pipe. He charged a percentage of the construction costs. Towers are similar, except the engineering is more of a flat rate fee.
The EIA-222 has been incorporated into most building codes with only a few exceptions. If you look at the UBC for example, it states that the EIA-222 is an accepted national standard that can be used. The EIA-222 rev F has eliminated the Class A, B, C... (The proposed revison G may have something similar) The main problem it that many plan checkers are not familar with the EIA-222.
Tower2sell@aol.com
In a message dated Tue, 26 Sep 2000 5:13:59 PM Eastern Daylight Time, Bob Otto <N8NGA@one.net> writes:
<<
Hello Al,
Just wanted to share a couple of thoughts with you. I have no idea
whether the data you have is correct or not, but I do have quite a bit
of general experience with building depts., and tower companies. Most
tower companies will not provide what is called a "wet signature" any
more. They will give you construction "guidelines", and analysis
figures. There are two reasons for this. First, each building
department has different requirements, and wants analysis calculations
done different ways. Tower companies, rather than tailor each set of
figures to a specific local, publish one set and requests you get a
local engineer that knows the local requirements to determine
applicability to the local building dept. requirements. Additionally,
there is a uniform code that was adopted some years ago that
represented a more stringent building requirement, and most companies
had to re-calculate their towers strength ratings. Many of these
companies do not have engineers on staff, so it was done once, and the
results became the basis for each data package delivered. Tower
companies with no substantial engineering staff, cannot afford to
develop a specific analysis for each local as it comes up.
The second reason tower companies are reluctant to provide the "wet
signature" is liability. Tower companies cannot be there to assess
soil conditions, the amount of trees (affecting class A or B or C,
etc.), and have no control over what happens after the tower is
delivered. Unfortunately, in this letigious society we live in,
should anything happen, the tower manufacturer is sued. By placing
the responsibility on the local engineer, they avoid suits.
I hope you find the analysis you got is correct, and that all is well
with your tower setup.
73's from.......
Bob Otto
N8NGA@one.net
Cincinnati, Ohio
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Tuesday, September 26, 2000, 2:22:23 PM, you wrote:
AW> Background:
AW> On Aug 3 TexasRF on this post asked " ...if anyone had received a copy of
AW> engineering calcs supporting the claim of ...24sqft/70mph for LM470E?".
AW> On Aug30 FirstCall on this post wrote "The web pages and ads are 100%
AW> correct in the stress analysis figures. Of those wanting a certified copy of
AW> stress analysis it can be ordered from FirstCall directly." The posting
AW> also contained information about the re-start of the business of making
AW> "Tri-Ex" towers with the LM470 now called the SkyTower470
AW> My Experience:
AW> On July 27 I received an Analysis package, three months after placing the
AW> order and also requesting analysis for the countys 80mph requirement. The
AW> Analysis was dated May 2, 2000!!!!!!
AW> Yesterday, after an hours long session with the county engineer and his
AW> supervisor reviewing the analysis I was informed that I would have to have
AW> the tower and foundation analysis reviewed and certified by a Washington
AW> State Professional Engineer. I believe they were uncomfortable with the
AW> analysis (copy including stamped by a California State registered
AW> professional engineer) because of the following:
AW> 1. The county engineers were informed several times that the California
AW> engineer would call to discuss the analysis but not call was received.
AW> 2. The Foundation Installation stated that "The tower footing design
AW> shown is based on normal soil criteria...." and "Normal soil is defined as
AW> cohesive soil with an allowable vertical bearing load capacity of 4000
AW> pounds per square foot". The analysis contains a table listing various
AW> soils. The listing for 4000 pounds is defined as massive crystalline
AW> bedrock !!!!!!!!
AW> 3. The county engineer asked several times who crossed out 70 and hand
AW> wrote 80 for the wind speed and who crossed out 24 and hand wrote 15 for the
AW> maximum antenna wind area.
AW> 4. The county engineer was also uncomfortable with the diagrams and
AW> anaylsis of the rebar in the footing but I am not knowledgable to describe
AW> this concern.
AW> Summary:
AW> In a week or two I will report on the findings of the Washington State
AW> Professional Engineer. Hopefully, it will substantiate the manufacturer and
AW> FirstCalls rating claim; Further, that the new company, PARAGON MFG.,LLC
AW> with help from FirstCall will be providing amateurs with a line of towers
AW> and service at competitive prices.
AW> --
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AW> Submissions: towertalk@contesting.com
AW> Administrative requests: towertalk-REQUEST@contesting.com
AW> Problems: owner-towertalk@contesting.com
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>>
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