List:
This has been an interesting discussion but it has not been quite accurate and
could be misleading to someone who wanted to actually install elevated guys
that were sized according to "Newton's Laws" and the basics of vector
mechanics.
An "old wives" tale needs to be dismissed. The weight of the foundation is not
the important factor in resisting the lateral force and moment generated by the
horizontal component of the angled guy forces at the top of the elevated guy
post. It is the depth and the width of the foundation and the insitu
soils' load bearing resistance.
For the sake of argument lets assume that you have 2 guys attached to the top
of a 6 foot high elevated guy post. The vector summation of the maximum guy
forces (tensions) compute to 3000 lbs at a 45 degree angle up from the
horizontal (chosen for simplicity). This equates to a horizonal force component
of 2121 lbs and 2121 lbs vertical (upwards). This is the design loading. The
resulting bending moment at the ground is (6 x 2121) is 12,726 ft-lbs.
The resulting uplift (vertical up) force at the ground is 2121 lbs, and the
resulting shear force (horizontal) at the ground is 2121 lbs. This combination
of loads is what the foundation and soil have to resist.
First off, lets size the vertical post for allowable stress and minimum weight
per foot. Assume that the post shape will be embedded a minimum of 4 feet into
the foundation. This is assuming that the foundation will be a post type of
foundation not a mat or large raft type. The post is then to be 10 feet in
total length.
The major loading on the post is bending and therefore bending stress will
determine the size of the section to be utilized. Maximum allowable bending
stress will be assumed to be 0.60 Fy (per AISC 14th Ed. Steel Construction
Manual). Lets look at 4 types of sections: wide flange beams (W), rectangular
hollow structural sections (HSS), round hollow structural sections (HSS), and
schedule 40 pipe.
W (wide flange beam) Fy= 36,000 psi (ASTM A36)
HSS (rectangular) Fy= 46,000 psi (ASTM A500 gr B)
HSS (round) Fy= 42,000 psi (ASTM A500 gr B)
Sch 40 Pipe Fy= 35,000 psi (ASTM A53 gr B)
The formula for stress due to bending is f= M / S where f is the stress (psi),
M is the moment (in-lbs), and S is the section modulus of the section (in*3)
Fortunately the 14th Edition of the AISC has tables for sections and their
maximum moments for the above alloys. Checking for maximum moment capacity the
results are:
W6x12 12 lbs/ft 10 feet 120 lbs. (wide flange)
HSS 5x5x1/4 15.62 lbs/ft 10 feet 156.2 lbs (square structural
tube)
HSS 5.5x0.258 14.46 lbs/ft 10 feet 144.6 lbs (round structural tube)
6" Sch 40 pipe 19.0 lbs/ft 10 feet 190 lbs (standard pipe)
The wide flange beam W6x12 seems to be the winner in the minimum weight for
load supported contest. This is exactly why W's are used extensively in
building construction for beams where bending stresses determine size.
Note that in the industry there are no I beams or H beams the structural shapes
are W (wide flange beams), M (misc. beams), S (s shapes), C (channels), MC
(misc. channels). L (angles) and WT (T shapes cut from W's), HSS (hollow
structural sections), and Pipe. There are indeed HP shapes which are steel
piling.
The discussion about concrete filled pipe and tubes was interesting but note
that the concrete does little for the bending strength. It however contributes
greatly to the compressive strength of a column (loaded vertically down) and to
provide additional margin of safety for fire protection.
I however specify, if my client wishes to use hollow sections for the guy post,
that they be concrete filled for corrosion protection of the inside of the tube
for minimal cost.
So now we have a W6X12 for the post, now to determine the foundation.
Fortunately for post type foundations in gereralized soils, the IBC
(International Building Code) Chapter 18 has a table (Table 1806.2) that
classifies soils types with presumptive load bearing values if one does not
have a soils report available for the specific site. If there is not a
geotechnical report available the best bet is to use Class 5 as the soil at the
site. For this example, use class 5 which has an allowable bearing capacity of
1500 psf and a lateral bearing pressure of 100 psf/ft. This lateral bearing
capacity increases in depth per foot of depth up to 15x the table value and
also increases due to the fact that the elevated guy post is an isolated
foundation. The increase is 2x the table lateral value. This causes the 100 psf
to be 200 psf.
The IBC also provides an iterative formula for post type foundations to
calculated the required embedment for a given width of foundation and assumed
depth. which is equation 18-1.
I will not bore you with the details of the calculation but with the above
assumed loadings of 2121 lbs horizontal and 12,726 ft-lbs of moment at the
ground surface I have calculated the depth of a 3'-0" square concrete
foundation to be 5.5'. The elevate guy post (W6X12) is embedded 4'-0" into this
concrete foundation. This is similar to putting concrete around the hole
base of a fence post to stiffen it for lateral loads.
What about the vertical component you say, the weight of the foundation itself
is 3x3x5.5x150= 7425 lbs which is 3.5 times as much (3.5 safety factor) as the
vertical upward load so that is taken care of. You see the weight of the
concrete did play some role in this system!
I have used this design proceedure for over 40 years and have not had a failure
of the post or foundation if all are installed as specificed.
Your situation may be different and there are other methods to anchor single
and mulitiple guys so use this as a guide to give you an idea of what you are
dealing with.
Lonberg Design Group, Ltd.
Hank Lonberg, P.E., S.E. / KR7X
President
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: kip@kdream.com
Cc: "towertalk" <towertalk@contesting.com>
Sent: Tuesday, November 1, 2011 6:39:44 AM
Subject: Re: [TowerTalk] PIPE ANCHORS
On 10/31/11 9:13 PM, Kipton Moravec wrote:
> On Mon, 2011-10-31 at 19:34 -0700, Jim Lux wrote:
>> On 10/31/11 2:02 PM, Dorn Hetzel wrote:
>>> But of course, filling the pipe or square tube with concrete probably
>>> does help since bending would have to compress the concrete
>>> fill and concrete *seriously* resists compression. Especially if the
>>> interior of the tube is rough enough for the concrete to bond
>>> to it well (or some bolts are run through or such).
>>>
>>
>>
>> I'm not sure about that. I looked through a bunch of references, and
>> nobody seems to have measured the strength with/without concrete.
> http://www.ctsee.org.tw/pdf/200310/ee0401-05.pdf
>
Excellent.. A first glance is interesting. They're talking structural
columns, mostly, and I can see easily how filling the column greatly
reduces local buckling. I'll have to read some more about loading in
bending (it's in there, but I haven't had my first cup of coffee yet..)
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