My 589HDX base attachments are 6 structural solid rods and only 36" into
the concrete. It seems to me that bottom half of the Rohn 6' section
may only be there as part of the concrete rebar, it seems unlikely tower
forces are transferred more than a few feet into the concrete. Since
Rohn wants the legs to drain, perhaps they could be plugged at the above
ground connection or replaced with a base you fabricate from solid steel
rod and use a rebar cage for the rest. I was told by a structural
engineer that the rebar cage is primarily designed to keep the concrete
as a solid structure - no cracking.
If the leg draining problem is solved, I don't think being into the
water table with the concrete is a significant concern. Many
foundations here in eastern WA are into a water table, mine at the tower
varies from surface to 4' down, winter to summer. The HDX 9' deep base
holes needed lots of pumping. After gas pumping we pumped with
electric sump pumps to about 3" of water and then pumped in the
concrete, which will displace the water and harden properly. The soil
is hard clay below the 18" surface mixed forest soils. When water
saturated, it turns into pudding when disturbed.
I agree that a soils engineer is advised for the sand, I think the
cyclic loading on a tower can cause movement over time. Solid bar stock
welded into a cage can replace the tube legs and solve the draining
problem. A structural engineer can do the design for that.
Grant KZ1W
On 3/19/2012 6:11 AM, David Robbins wrote:
> you want a local engineer who understands how to build towers in that soil to
> design you a base and anchors. that is well outside the 'normal' stuff that
> is documented in the rohn standard drawings. not to mention the threat of
> hurricane force winds.
>
> Mar 19, 2012 08:37:02 AM, llibsch@bellsouth.net wrote:
>
> Group -
>
> Our club has bought a tower for use at our new club shack on
> the edge of the 'glades. The 'glades is fresh water this far north and
> west. The water table at the site is 30 inches below ground level. The
> soil is mixed gravel, sand, and earth fill for about 2 1/2 feet, then
> wet sand below 30 inches. Our tower is an old Rohn with a three legged
> tilt base to which the tower attaches. The Rohn plans for a concrete
> tower base suggest the bottom of the 6 foot, open, galvanized steel legs
> of the tilt base be placed into 9 in of (dry) sand and the concrete base
> poured around the legs above the sand. It seems likely that the the tilt
> base legs below the concrete base will always be in water, and that
> water will stand inside the legs to the level of the water table. Much
> of the concrete base will stand in wet sand below the water table. As
> concrete is water permeable and drys slowly, it is likely that the
> outside of the tower base legs will also be wet from the top of the
> concrete base down. We seek an understanding of the special problems
> this situation presents to the safe construction of a tower base and
> solutions to those problems. So far, we recognize three likely problems:
>
> 1) Corrosion issues with the steel legs both inside and below
> the concrete base. What can we do to minimize this problem?
>
> 2) It seems likely wet sand is a less firm base than dry
> soil. What changes should we make to the size and /or the shape of the
> concrete base from the Rohn plans?
>
> 3) Any hole deeper than 30 inches fills quickly with water.
> Will we be able to dig a hole 7 feet deep and 5 feet by 5 feet wide? If
> we manage to dig an acceptable hole, it will fill with water.
> Can we pour concrete into a water filled hole and have it set
> up properly?
>
>
>
> Larry, K4KGG
>
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