[TowerTalk] new AN-Wireless tower, floundaion question
ersmar at comcast.net
ersmar at comcast.net
Fri Jun 30 17:10:29 EDT 2006
JC, Jeff et ux:
Without stirring up some who don't like math (it has happened to me
before on this subject on this reflector), be careful that you don't exceed
the vertical bearing pressure of the soil by dumping concrete into the hole.
In other words, too much concrete in the hole spread over too small an area
(81 sqft in JC's case) could cause the foundation to sink into the dirt.
Do the math before you deviate from the manufacturer's specs -OR-
Do what the manufacturer says.
Gene Smar AD3F
----- Original Message -----
From: "Jeff Goldman" <k3dua at erols.com>
To: "TowerTalk" <towertalk at contesting.com>; "JC Smith"
<jc-smith at comcast.net>
Sent: Friday, June 30, 2006 4:44 PM
Subject: Re: [TowerTalk] new AN-Wireless tower, floundaion question
> I guess the item I would look at is the total cost/effort. Yes, the
> pad-pier uses less concrete and rebar, a cost savings. But is the
> difference worth the time, and extra effort of 2 pours and renting the
> tamper and backfilling. It is much simpler to use there 9 by 9 by 5
> full foundation. One pour, no back fill, no tamper rental. Yes it
> takes more rebar and concrete, but its a lot easier in my book.
> Jeff, K3DUA
> On Jun 30, 2006, at 9:55 AM, ersmar at comcast.net wrote:
>> It's nearly impossible to undercut safely the full size of the
>> pad for this type of foundation. When I was working in the electric
>> utility industry as an engineer, we used these foundations all the
>> time for our substation structures. The crews dug out the full size
>> hole for the pad (in your case, that would be a hole 9 feet by 9 feet
>> by five feet deep.) They set the rebars for both the pad and pier,
>> the poured the concrete for the pad. Next day they set the forms for
>> the pier on top of the pad and poured the pier. ANd I believe there
>> is a material that is applied to the concrete to allow this second
>> pour to adhere to the first. Anyone help here?
>> The next step is the trickiest: backfilling the excavation after
>> removing the pier's form material. In your case you ought to back
>> fill a foot deep around the pier, then spend a bit of time with a
>> tamper banging the soil in compaction. You can rent a motorized
>> tamper for this. Then add another foot and tamp it, etc until the
>> entire excavation is filled and the earth put back in near-undisturbed
>> My Trylon tower's foundation is similar in construction except
>> that the undercut is a slanted cut only a foot wider than the main
>> hole. I was able to use a spade, with proper shoring of course, to
>> loosen the earth and haul it out with a backhoe. But yours is a bit
>> too wide to safely support the overlying earth.
>> I know - a PITA, but it has to be done that way so you don't
>> injure anyone should the hole collapse.
>> Good luck, and remember - On towers as in driving a car, there
>> are no such things as accidents.
>> 73 de
>> Gene Smar AD3F
>> -------------- Original message ----------------------
>> From: "JC Smith" <jc-smith at comcast.net>
>>> Hello All,
>>> As mentioned previously I am stick-building (like a giant erector
>>> set) a new
>>> AN-Wireless HD-80. I will be posting photos at:
>>> if anyone cares to watch it go up and the antenna stack go on it. We
>>> started digging the foundation yesterday. It's a pad and pier
>>> (like an inverted tee) with a 9'x9'x1.5' pad and a 5'x5'x3.5' pier.
>>> Has anyone on here ever dug one of these by digging the 5'x5' hole
>>> first and
>>> then undercutting the bottom to create the pad? If so, I'd sure
>>> hearing how you did the digging for the undercut. We got the 5'x5'
>>> hole no
>>> problem but our experiments (so far) with the undercutting haven't
>>> been very
>>> successful. We are in clay soil, fortunately with no rocks. If
>>> don't go better today we will probably dig out the entire 9'x9' hole,
>>> the pad, form the pier and backfill around it.
>>> Suggestions appreciated.
>>> 73 - JC, K0HPS
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>>> TowerTalk at contesting.com
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