[TowerTalk] Epoxied bolts

[kr7x@comcast.net]

Mark / Michael:

Please see my post on this subject yesterday. 

I am involved in this type of thing daily.

L shaped or J-bolts are not stronger than straight bolts both being properly designed and installed. The two limiting factors are the inherent strength of the bolt in tension and the strength of the concrete area that has to be developed to resist a concrete cone shaped pull out failure. In theory you design the installation such that the strength of the concrete is just a bit more than the strength of the bolt. This way the bolt fails in tension which is a less sudden type of failure due to the ability of the bolt to fail inelastically, i.e., deform. Concrete does not fail in this manner. It has virtually no inelastic deformation, it is a sudden failure.

As I stated earlier J-bolts, L shaped bolts actually can fail sooner than their tensile strength due to crushing of the concrete at their crook or bend area. 

Post-installed chemical (epoxy) and mechanical anchors are also design to behave the same as cast in place anchors. The idea is to develop the tensile strength of the anchor before you fail the concrete. The limiting factors in both CIP(cast in place) and Post Installed is the size and shape of the concrete area they are installed in. Imagaine a block of concrete versus a large floor slab. The distance to the free edge of the concrete area and depth of the concrete versus the depth of embedment of the anchor, etc.

There is no limitation to the use of epoxy chemical anchors to tensile loading if one neglects the creep effect. Most anchors for amateur towers are not in constant tension. Look at the base of a free standing crank-up, the anchors only see tension, if at all, when there is a lateral force caused by wind or seismic events. Then only the anchors on the upwind side of the force direction may see tension load and only for a short duration, not for a constant duration.

Some local building codes do not allow post installed chemical anchors for overhead use (ceiling installation where the load is always in tension) and in some  anchor situations due to the creep effect and fire resistance. They require that a special type of mechanical post installed anchors be used (undercut style).

Google, "Hilti", "American Concrete Institute", and other sites for a great deal more information on concrete anchoring both CIP and post-installed.

Regards
Lonberg Design Group, Ltd.

H. Lonberg, P.E., S.E / KR7X
President
-------------- Original message -------------- 
From: "AA6DX - Mark" <aa6dx at arrl.net> 

> Michael ..So far, your conclusion does not mesh with what I have read at 
> various sites, and in the past on this reflector..... But then, the 
> companies are selling their glue .... 
> but but but ... to engineer types, so... must be truthful? 
> Hmmm ... watching this thread, as I have a back porch that is going to be a 
> base for a crank-up --- Mark, AA6DX 
> ----- Original Message ----- 
> From: "Thin Air Communications" 
> To: 
> Sent: Thursday, February 02, 2006 9:45 AM 
> Subject: Re: [TowerTalk] Epoxied bolts 
> 
> 
> I am not an engineer nor do I pay one on TV. But the use of epoxied bolts 
> from my experience drafting structural plans is limited to objects in sheer 
> and never in tension. Meaning that yes, the epoxied bolt is no where near as 
> strong and solid as a "J" or "L" or caged bolt in tension. The whole 
> reasoning behind a "J" or "L" is so the bolt has something to grab on to. It 
> is possible to break the epoxy connection and lift the bolt from the hole. I 
> would refrain from epoxying bolts used in tension but would also rely on an 
> engineer to evaluate the application. 
> 
> Michael Wood 
> KE7ENC 
> 
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