[TowerTalk] windload rating of triex LM470e tower to be indep endently ver...

[Lonberg, Hank]

Al:

I understand that the tower mfg specs say normal soil. However the building
guys will not accept that typically unless you have a report or can show
that it is indeed normal soil of 4000 psf allowable bearing pressure.

Actually I've run across soils,  a gravel, that can withstand 6000 psf and
which was not bedrock. It is just that, not given any other info, the
building officials will use the table.

The allowable soil values effect the size of the concrete foundation not the
tower structure. Remember the tower structure loads travel down to the
foundation. The foundation is sized for the tower loads and the allowable
soils values. 

The tower mfgr designs to EIA/TIA specifications but the county will use the
UBC. Each code defines the "normal" soil differently and each has a
different audience and application. It is well that the mfgr shows this
assumption for normal soil as it helps define the parameters of the
foundation design.

The amount of concrete (cubic yards) is not a good measure of a foundation's
strength. It depends on the geometric shape of the foundation in plan and
section to a larger extent. It also depends if it is a spread footing type
or a pier type foundation. Each transfers the load a little differently to
the ground.

If you could get the county to define which soil class in table 18-I-A you
should use that would help your engineer. If they don't then the worst case
soil is used for design in lieu of better information.

Hank Lonberg  P.E. / KR7X



	-----Original Message-----
	From:	Al Williams [SMTP:alwilliams@olywa.net]
	Sent:	Tuesday, September 26, 2000 1:40 PM
	To:	Lonberg, Hank; K7LXC@aol.com; towertalk@contesting.com
	Subject:	Re: [TowerTalk] windload rating of triex LM470e
tower to be independently ver...


	I seem to be missing something-but this my first tower permit
experience.
	It wasnt the county that specified the 4000 lb rating.  Rather it
was the tower mfr indirectly specifying 4000lb by stating that the
foundation design (I presume that means the mfrs rebar, welding to erection
plates, and amount/strength of the concrete) requires normal soil and then
goes on to state that "normal soil" is 4000lb.
	Not being experienced, I would read this as either faulty
documentation or his design is not adequate for "typical" soil i.e. not
massive crystalline bedrock.
	BTW, the design calls for only 3 ½ cu yards of cement for a
wind/antenna of 70mph/24 sq ft whereas the comparable US tower HDX572 calls
for 5 cu yards for 70mph/18 sq ft.
	k7puc
	I really appreciate the responses.

	k7puc

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