I've only done one tower base, but I agree with Bob and Steve.
I hired a contractor to do the base for my UST MA-770MDP 72' tubular tower.
As I recall, the spec called for a 3'x3'x6' hole. Despite using his smallest
scoop, the guy on the excavator, who was quite experienced and a real artist
with the machine (he had done a lot of work on our house remodeling and
driveway overhaul) was not able to dig a perfectly square hole. It ended up
sort of pyramid shape. The guy dug the hole too deep -- I think it was 9' --
and had to refill the bottom with a couple of feet of gravel. I don't recall
the exact measurements, but I think the top is something like 4'x4' or
4'x5'and the bottom is even wider -- perhaps as much as 5'x6'. And the hole
is at least 7' deep. It should have taken 2-3 of yards of concrete to do the
job, but they told me it took most of a truckload -- 9 yards, I think. I
don't know if they were trying to churn the fee or if that was what it took,
but as I'd made a fixed-price deal with the contractor on the base he had to
eat the difference. He complained, but I held him to it -- after all, they
missed the spec by a mile. Besides, his company made plenty on the house and
driveway!
Anyhow, there's no way that giant pyramid of concrete is going anywhere. The
cable on the MA-770MDP will fail long before the pier moves.
I also agree with whoever said these pad-pier base designs are rediculous. I
looked at getting an AN-Wireless tower, which is otherwise a nice
cost-effective solution for medium loads, and couldn't get past the base
design. They seem to sell a lot of towers to amateurs, so they really should
rethink the base. BTW, both the fixed Bertha and Big Bertha sold by Array
Solutions use a square block type base. Those monsters have *huge* windload
capacity. I don't have specs on the base, but judging from some pictures
I've seen the fixed Bertha base is on the order of 6'x'6'x6'. Although I've
stood on the base for NT1Y's Big Bertha (140' rotating pole that's something
like 3 feet in diameter at the base) and *that's* a real chunk of concrete.
73, Dick WC1M
> -----Original Message-----
> From: Bob Smith [mailto:na6t@na6t.com]
> Sent: Tuesday, July 12, 2005 5:42 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Pad and Pier Foundations
>
>
> I kinda have to side with Steve on This one,, I've just
> completed (in the past 10 months) installing two of the
> Trylon 96' towers. But, When I had the holes dug my backhoe
> operator was able to dig the hole and put the bottom undercut
> into the hole with the machine
> and bucket that he brought to the job. Yes the undercut was a
> little bigger (about 1') but that's what the concrete was
> for, filling the hole. A building inspector will probably
> NEVER condemn you for using MORE concrete that the plan calls for,
>
> When a Building Inspector check the Trylon plan he just wants
> to see the plans, the hole, the undercut, the re-bar and
> UNDISTURBED soil. Oh, one other thing in California, the
> Crush test for 3000 # concrete
>
> I think the solution to the 'hole problem' is one of using
> the tools and skills 'that you brought to the table' ,
>
> IE: hire the proper people and equipment to do the correct JOB,
>
> Also as to the OSHA rules, I don't think a small company
> digging a hole falls under OSHA regs, don't you have to have
> at least 10-15 employees before OSHA 'raises it's ugly head?
>
>
> nuf said, back to my work at hand
>
> Bob Smith
> NA6T
>
>
> Robert Smith Consulting
> "Wireless Installations -- Government, Businesses & ISP's"
> F.C.C. Licensed-Commercial & Amateur Services A.R.S NA6T ARRL
> Life Member 1-530-591-2230 (cell in
> Colusa-Yuba-Sutter-Glenn-Butte Counties) 1-707-964-4931
> w/answering machine
> Fort Bragg, California 95437
>
> "On The Air-Conditioned Mendocino Coast, In REAL Northern
> California" No trees were destroyed in the sending of this
> message. However, a large number of electrons were terribly
> inconvenienced.
>
>
>
> ------------------------------
>
> Message: 4
> Date: Mon, 11 Jul 2005 11:18:42 EDT
> From: K7LXC@aol.com
> Subject: Re: [TowerTalk] Pad and Pier Foundations
> To: towertalk@contesting.com, nv8a@att.net
> Message-ID: <210.477678b.3003e7d2@aol.com>
> Content-Type: text/plain; charset="US-ASCII"
>
> In a message dated 7/10/2005 10:39:28 PM Pacific Daylight
> Time, towertalk-request@contesting.com writes:
>
> > The ability to dig an "undercut" hole no doubt depends on
> the soil. I gave a local concrete contractor the AN Wireless
> foundation plan, and he said there was no way to dig such a
> hole. He said the only way to do a pad and pier foundation
> around here would be to dig the hole the size of the pad,
> construct a form the size of the pier, then backfill.
>
> A couple of comments about the Trylon base design.
> First, IMO it's a stupid design. I'm sure there's a valid
> engineering reason to have the bottom belled-out but it's not
> only dangerous to be in a hole that deep but it's also
> illegal per OSHA rules to be that deep in a hole without
> being shored up. This was designed by an engineer in an
> air-conditioned office that never has to install one of them.
> In many cases the soil won't allow it anyway.
>
> Second, the design calls for 7 yards of concrete. Make
> the hole bigger (usually happens when you use a backhoe
> anyway), put in a little more
> concrete,
> and you're good to go. The Trylon design is one-size-fits-all
> so there's some engineering overhead already built in.
>
> Cheers,
> Steve K7LXC
> TOWER TECH -
> A professional tower erector, not an engineer
> Cell: 206-890-4188
>
>
> -
>
>
>
>
_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
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