[TowerTalk] Small free-standing to Bigger free-standing

[Lonberg, Hank]

David:

It is perfectly possible to do what you ask. I have done this very thing
many times when my clients insist on putting 6 pounds in a pound sack. The
trick is in the connecting the new concrete to the existing. What I do is to
drill and epoxy rebar dowels into the existing concrete to act as shear
connectors between the new and old concrete. Typically they are at 1'-0"
centers and project 12 inches out from the existing and are imbedded per
manufacture's recommendation for size of bar. This insures that the new
foundation acts as a single unit and develops the larger size strength. If
you don't do this then under the worst design case, i.e., fully loaded with
lateral wind load the foundation would act as two separate pieces and only
have the strength of the new one with the old one acting like the soil.

I recommend you find a local structural engineer to design up the new
foundation.

73

Hank Lonberg / KR7X  P.E.


	-----Original Message-----
	From:	David Jones [SMTP:djones@hwcn.org]
	Sent:	Tuesday, March 28, 2000 4:10 PM
	To:	TowerTalk
	Subject:	[TowerTalk] Small free-standing to Bigger
free-standing


	If one was thinking about moving from a relatively small 48' HD
	free-standing tower (concrete base roughly 4x4x4' - top at ground
level) to
	a much larger free-standing (80-96' HD Trylon) with a much wider
lowest
	section?............
	Could you build up-and-around the current base to install your
lowest
	section?
	For example:
	1) Dig out another foot (or more, because you certainly couldn't get
	underneath again) all the way around and build another rebar cage to
go
	around it.
	2) Then build up the concrete another six inches or so above the
current
	grade to gain at least some additional height/volume.
	3) Plant your bottom stubs in the new concrete around the old?
	Thoughts?
	David Jones  VE3STT
	e-mail: ve3stt@rac.ca


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