Reply To: Re-bolting of “Morning Glory” at Stathams

Hi Richard (and everyone else)

Thanks for your comments.

I agree with you comments on the use of hex bolts with long thread lengths (say 80mm of thread and 20mm of threadless shank). I investigated this option and to date I have not been able to source such bolts – if you know of a source please let me know as this is a cheaper option. However, based on my calcs, I don’t believe 40mm of embedment is sufficient.

On the matter of long-term effects of thermal stress, cyclic load etc on glue in bolts. I agree these are important issues and this is why I an arguing for longer bolts (as opposed to shorter bolts). It may be that 80mm of embedment is not enough over time, or they may require replacing. With a good drill it only takes 1 minute to drill a 90mm deep hole so even doubling the depth is no great burden (and does’nt impact on the visual appearance of the system). Cost is another issue….

Please see below notes that came out of correspondence with others on the issue of fully threaded glue in carrots:

RE: Hex Bolts

I recently contact a couple of chaps over east who were involved an R & D project on bolt testing for RC – they are not aware of any testing on glue in carrots, but there is plenty of published work on threaded bars with different embedment lengths.

I have investigated fully threaded 316 hex bolts. They are available but I dont think the thread at the collars will work with our bolt plates (ie it will catch and apply bending moment to bolt). Also these bolts have only 8mm of effective diameter at the collar, compared to 10mm for a full shank bolts). What we really need is “partially threaded” hex bolts (say 100mm long M10 bolts with 80mm of thread, and 20mm of shank).

Some people roughen the surface of the hex bolt with an angle grinder; in my opinion this is non-ideal and besides potentially creating stress raisers, there will be a differential stiffness in bond which would create uneven load transfer along the axis of the bolt – break glue?

ie fine thread = high stiffness,

widely spaced ground holes = low stiffness

Also, how to create a uniform pattern? Damage to bolt strength due to careless grinding. We should aim to have a repeatable, process that is quality controlled rather than depending on the skill of someone with an angle grinder.

FYI: See load transfer graph for M10 threaded bar with Chemset 801 series.

Summary:

– threaded bar with 80mm of encapsulation: sigmaT = 10KN

– carrot (glue in with 40mm of encap) sigmaT = 2KN

“graph attached”