Hi, Chuck;
You are correct on both points.
The moment arm I mentioned is correct for the righting moment created
by the weight of the pier and structure on it. The moment arm for the
re-bar is the distance from the re-bar to the opposite edge of the pier,
assuming the footing is rigid and the pier will not fail between the
re-bar and the edge.
Safety margins are an essential part of good engineering. They are
typically chosen based on standards, past experience, knowledge of the
variables in load and strength, consequences of failure, and the
economic and performance costs of strengthening the item being designed.
I don't know the values recommended for tower structures. Using yield
strength instead oft ultimate strength may not be sufficient in some
cases.. In designing the re-bar cage for my base, I concluded that the
re-bar was superfluous as far as structural integrity of the base was
concerned, so I didn't research the matter. That is not the case for
those using re-bar to tie a pier to a base; the re-bar is essential to
safety and correct safety margin is critical. What safety margin would
a P.E. use in this case?
A builder I respect observed that the cost of re-bar was such a minor
part of the total cost of a project I asked about that he recommended
more re-bar than my worst case calculations indicated is required. In
that case, the cost of more re-bar was trivial, so stop calculating and
put in more steel. That is not always the case, though, and one must be
smart enough to make a good decision. Life is full of compromises, a
secret to long life is to choose one's compromises correctly!
73 de Red, WOØW
Chuck Lewis wrote:
>Red,
>Thanks for supporting my post!
>I do challenge part of yours, however: shouldn't the allowable moment be the
>yield strength times the distance from the rebar to the OPPOSITE SIDE of the
>small pier? And what about safety factors?
>
>Cheers,
>Chuck, N4NM
>
>----- Original Message -----
>From: "Red" <RedHaines@centurytel.net>
>To: "Towertalk" <towertalk@contesting.com>
>Sent: Wednesday, December 04, 2002 12:40 PM
>Subject: Re: [Towertalk] Pouring base of tower
>
>
>
>
>>Chuck is correct regarding the direction of stresses, and that the pier
>>must be fastened to the base or it will overturn on top of the base.
>> However, it is not difficult to achieve that with the re-bar,
>>especially if the re-bar is anchored in the concrete with J bends at
>>each end. Calculate the overturning moment at the junction (overturning
>>moment at the tower base minus the righting moment of the pier) and use
>>that difference to calculate the tension that may appear in the re-bar.
>> Do not place the re-bar too close to the outer surface of the pier; 3"
>>absolute minimum, 6" insures against variables. The moment that each
>>re-bar can support is the yield (not ultimate) strength of the re-bar
>>times its distance from the center of the pier.
>>
>>Putting the J bend at each end of the re-bar is easily accomplished with
>>a 'rod buster', a tool available at contractor supply firms that both
>>cuts the re-bar and bends it. I recently priced one at about $200,
>>manually operated. No local rental agencies offered one.
>>
>>I bought the re-bar from a local ready mix supplier, already cut to my
>>specifications. I didn't put in the J bends, as I used a single pour
>>and had welded cross members on each of the vertical members, so each
>>vertical member had its full length engaged with the concrete and had
>>several welded cross members giving it additional grip on the concrete.
>> The tower manufacturer didn't specify any re-bar and the tower base
>>legs have welded lugs to grip the concrete. I added the re-bar
>>primarily to make a UFER cage; any additional strength was a plus.
>>
>>73 de WOØW
>>
>>Chuck Lewis wrote:
>>
>>
>>
>>>Wait a minute!!!
>>>This application is NOT like a footing! I hope someone with some
>>>
>>>
>credentials
>
>
>>>will chime in here and keep us from doing some dumb stuff! Footings do
>>>
>>>
>not
>
>
>>>have to withstand overturning moments, and your freestanding tower base
>>>does. The tower base does more than simply support the weight of the
>>>
>>>
>tower,
>
>
>>>and the tower is not held vertical simply because of the weight of the
>>>
>>>
>base.
>
>
>>>When the wind tries to tip the tower, the small 'pier' is pulled up and
>>>
>>>
>away
>
>
>>>from the large 'pier' on the windward side. The large 'pier' is hooked
>>
>>
>under
>
>
>>>several feet of (hopefully) undisturbed soil, and the two chunks of
>>>
>>>
>concrete
>
>
>>>are being pulled apart. Now there might indeed be an epoxy or similar
>>>compound or a rebar configuration which will give the two pours the
>>>
>>>
>tensile
>
>
>>>strength of a single pour, but you need to ask the question relative to a
>>>tower base, NOT a foundation/footing, and the answer needs to come from
>>>
>>>
>the
>
>
>>>right source. Your concrete contractor no doubt knows a great deal about
>>>foundations and footings, but I'll bet he knows diddley-squat about TOWER
>>>BASES. Neither do I, for that matter, but let's not make the mistake of
>>>thinking that if it looks just like a foundation/footing it can be
>>>
>>>
>treated
>
>
>>>as a foundation/footing. These are two distinct applications despite the
>>>fact that they LOOK identical. Tower bases have a unique set of
>>>
>>>
>requirements
>
>
>>>which include a combination of compressive, tensile and shear loads, but
>>>
>>>
>a
>
>
>>>foundation/footing is mostly compressive. Didn't the manufacturer specify
>>>
>>>
>a
>
>
>>>SINGLE pour? Why?
>>>
>>>Please check with someone who knows about TOWER BASES!
>>>
>>>Chuck, N4NM
>>>(hoping someone who knows about TOWER BASES will tell me I'm full of
>>>
>>>
>cr*p)
>
>
>>>----- Original Message -----
>>>From: "Phil - KB9CRY" <kb9cry@attbi.com>
>>>To: "TowerTalk" <towertalk@contesting.com>
>>>Sent: Tuesday, December 03, 2002 9:30 PM
>>>Subject: Re: [Towertalk] Pouring base of tower
>>>
>>>
>>>
>>>
>>>
>>>
>>>>Well I finally talked to my industrial concrete contractor and described
>>>>the original question. He told me that the pier on a larger pier is
>>>>just like a footing/foundation wall design and as long as the rebar ties
>>>>the two pours together, one does not normally use an epoxy bonding agent
>>>>between the two. If one if worried about water seeping in the cold
>>>>joint and attacking the rebar, then normally epoxy coated rebar is
>>>>specified. The bonding agent would certainly work but it is overkill.
>>>>So I stand corrected and now slightly more educated. Hope this helps
>>>>others too. Phil KB9CRY
>>>>
>>>>
>>>>
>>>>
>>>>Guy Olinger, K2AV wrote:
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>>I think you're a little off on this one, Steve.
>>>>>
>>>>>In construction of all concrete framed buildings, pours separated by
>>>>>weeks and having rebar in common are all over the place, and have to
>>>>>be. They occur at the boundary of floors and columns.
>>>>>
>>>>>The floor above cannot be poured until the one below has hardened to
>>>>>the strength to carry the weight of the one above.
>>>>>
>>>>>There are simple, common, well-documented steps that are always taken
>>>>>at pour boundaries by professionals, no more mysterious than the
>>>>>vibrator used to eliminate voids in the pour.
>>>>>
>>>>>That it's not bandied about much on Towertalk merely reflects that
>>>>>double pours are commonly not needed for amateur towers, not that
>>>>>there's anything wrong with them.
>>>>>
>>>>>The base for my Trylon was designed by a fellow that does building
>>>>>foundations. It was done in two pours so that we could stand the base
>>>>>and bottom section on a firm surface, shim it to vertical and tie the
>>>>>base directly into rebar aligned to the desired orientation (one face
>>>>>edge on to 45 degrees).
>>>>>
>>>>>When the rest of the base was poured, the bottom section was securely
>>>>>positioned by angled rebar across the square and fastened to the base
>>>>>legs, no possibility of movement, and an automatic UFER ground.
>>>>>
>>>>>The layers were joined by a generous coating of some nasty epoxy stuff
>>>>>put down the morning of the second pour.
>>>>>
>>>>>The base itself was an interesting 11 cubic yard over-engineered
>>>>>design that forces the base to be lifted out of the ground and kick
>>>>>sideways into undisturbed hardpan in order to be overturned. The tower
>>>>>could be ripped off the base by a tornado, but the base is never going
>>>>>anywhere. If I move, the base will be marketed as a pad for an
>>>>>incinerator or large outdoor barbeque.
>>>>>
>>>>>When I asked the guy about the double pour and seepage, he just looked
>>>>>at me like I was crazy, and my contractor son-in-law gave me one of
>>>>>those shut-up-before-you-embarrass-me looks.
>>>>>
>>>>>Just because there's a wrong way to do something doesn't mean it
>>>>>shouldn't be used.
>>>>>
>>>>>73, Guy.
>>>>>
>>>>>----- Original Message -----
>>>>>From: <K7LXC@aol.com>
>>>>>To: <wz7i@arrl.net>; <towertalk@contesting.com>
>>>>>Sent: Sunday, December 01, 2002 11:04 AM
>>>>>Subject: Re: [Towertalk] Pouring base of tower
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>In a message dated 11/30/02 4:51:50 PM Pacific Standard Time,
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>wz7i@arrl.net
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>writes:
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>>I am planning the installation of a self-supporting tower. My
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>contractor
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>>asked me a question to which I don't know the answer. Is there
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>any reason
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>>why he cannot pour the base in two pours as long as they are tied
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>together
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>>by the rebar cage? The plans call for the foundation to be 9 x 9
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>at the
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>>base and then 5 x 5 at the surface. He would prefer to pour the
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>18" thick
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>>9 x 9 foot section before the 5 x 5 foot section. Any reason why
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>not?
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>> It should be in one pour so that the base is totally bonded
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>together. If
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>the bottom pour has 'gone off' and hardened, you wind up with two
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>separate
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>layers.
>>>>>>
>>>>>> Sometimes there are two pours if you have to have two trucks
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>worth of
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>concrete but they are poured consecutively with little or no time
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>delay.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>> Are you looking at something like a pedestal base where it's not
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>a cubic
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>hole? Did you take the manufacturer's specs or have a PE design it?
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>Sometimes
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>manufacturer's specs are pretty impractical (i.e. Trylon's undercut
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>base, US
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>Tower's deep skinny holes) so it'd probably be easier if you could
>>>>>>reconfigure it for simplicity's sake. The problem is that the bases
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>are
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>designed by engineers sitting in an office somewhere and they never
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>have had
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>to install one of their (stupid) designs.
>>>>>>
>>>>>> What kind of contractor wouldn't know the answer to your
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>question?
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>Cheers,
>>>>>>Steve K7LXC
>>>>>>TOWER TECH
>>>>>>_______________________________________________
>>>>>>AN Wireless Self Supporting Towers at discounted prices,
>>>>>>See http://www.mscomputer.com
>>>>>>
>>>>>>Wireless Weather Stations now $349.95. Call Toll Free,
>>>>>>888-333-9041 for additional information.
>>>>>>_______________________________________________
>>>>>>Towertalk mailing list
>>>>>>Towertalk@contesting.com
>>>>>>http://lists.contesting.com/mailman/listinfo/towertalk
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>_______________________________________________
>>>>>AN Wireless Self Supporting Towers at discounted prices,
>>>>>See http://www.mscomputer.com
>>>>>
>>>>>Wireless Weather Stations now $349.95. Call Toll Free,
>>>>>888-333-9041 for additional information.
>>>>>_______________________________________________
>>>>>Towertalk mailing list
>>>>>Towertalk@contesting.com
>>>>>http://lists.contesting.com/mailman/listinfo/towertalk
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
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>>>>multipart/alternative
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>>>>---
>>>>_______________________________________________
>>>>AN Wireless Self Supporting Towers at discounted prices,
>>>>See http://www.mscomputer.com
>>>>
>>>>Wireless Weather Stations now $349.95. Call Toll Free,
>>>>888-333-9041 for additional information.
>>>>_______________________________________________
>>>>Towertalk mailing list
>>>>Towertalk@contesting.com
>>>>http://lists.contesting.com/mailman/listinfo/towertalk
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>>_______________________________________________
>>>AN Wireless Self Supporting Towers at discounted prices,
>>>See http://www.mscomputer.com
>>>
>>>Wireless Weather Stations now $349.95. Call Toll Free,
>>>888-333-9041 for additional information.
>>>_______________________________________________
>>>Towertalk mailing list
>>>Towertalk@contesting.com
>>>http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>>>
>>>
>>>
>>>
>>>
>>
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>> text/html
>>---
>>_______________________________________________
>>AN Wireless Self Supporting Towers at discounted prices,
>>See http://www.mscomputer.com
>>
>>Wireless Weather Stations now $349.95. Call Toll Free,
>>888-333-9041 for additional information.
>>_______________________________________________
>>Towertalk mailing list
>>Towertalk@contesting.com
>>http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>>
>>
>
>
>
>
>
>
>
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