Bolt checks

[chris456s]

Regarding bolt calcs in EN 1993-1-8
Is the bolt tensile load F_t,Ed equal to total tensile load including preload or is it only applied load external to preload?
The question arises for the following reasons:
1. Notice the tensile resistance check F_t,Ed <= F_t,Rd. Where resistance is K_2*F_ub*A_s/gamma. If Preload is as suggested 0.7F_ub*A_s and K_2 = 0.63 and F_t includes pretension, then the check will always fail under preload alone since 0.63/gamma is less than 0.7.
2. Also notice that the cat C combined slip resistance has a factor equal to preload (F_p,C) minus 0.8F_t,Ed. This make sense if F_t,Ed does not include preload since larger seperating load descreases contact force on the friction plane. But if F_t,Ed includes preload, this factor is double dipping
3. From the first to observations, I would assume that F_t,Ed does not include preload and is exclusively external load. However, that would mean that the tensile checks are not conservative as overly preloading a bolt may exceed tensile capcity which should be checked but isn't explictly. Also F_t,Ed not including preload would mean that the punch shear restistance check F_t,Ed <= B_p,Rd would not be conservative as the total punch shear load would not be accounted for.

Overall there seems to be a condratiction. I see this condiction resolved if F_t,Ed = max(preload ; external load) and the largest allowable preload was reduced for countersunk bolts. However, that is an assumption with no justification other than it resolves the condration (which I could be wrong about) and is not stated in the code.

 

[SSCon]

F_t,Ed is external loading. There's a lot of nonsense on the internet - including this forum - surrounding pre-tensioning bolted joints. See the below document - section 5.1 - for background.

http://www.boltcouncil.org/files/2020RCSCSpecification.pdf

"The shear and tensile strengths of a bolt are not affected by the presence of an initial pretension in the bolt."

 

[chris456s]

Thanks. F_t,Ed being external loading alone makes sense.
"The shear and tensile strengths of a bolt are not affected by the presence of an initial pretension" this is certainly true when bolts are preloaded within their proportional range, as shown by balancing the forces due to preload, contacts and external load.
However tensile capacity, torsional capacity and plate punching shear should still be considered when selecting preload.
Notice, 1993-1-8 calls for preload equal to 70% of ultimate on slip resistance bolts and torquing with a torque wrench has a +/- 25% error. This means preloaded bolts can easily be stressed beyound yeild which certainly effects capacity.
Many beginning mechanics have experienced an over tightened bolt breaking the second it sees even the slightest bit of external load because it is easy to exceed the tensile capacity and/or torsional shear capacity while preloading a bolt. To mitigate this, I alter the tensile check prescribed by EN 1993-1-8 (F_t,Ed <= F_t,Rd) to be max(F_t,Ed ; F_p,C) <= F_t,Rd. This means that when K_2 is less than 0.7 (as when using counter sunk bolts are used) the applied preload should be less than the recommended 70% for slip resistance.
Also, 70% of ultimate (in some cases) can easily exceed the punching resistance of thin plates. I have seen many (omg so many) holes punched though thin plates as a preloaded bolt stresses the plates punching shear plane beyound its limits and is punched though when the plate is very slightly loaded. I see this so much and it is so obvious that the recomended preload is enough to damage plates, that I am shocked that the punching shear check in the code only considers external load and not preload. Thats is why I alter the plate punching shear check prescribed by EN 1993-1-8 (F_t,Ed <= B_p,Rd) to be max(F_t,Ed ; F_p,C) <= B_p,Rd.
Often as engineers we just use the code without considering the real life behaviours, are overly conservative with regard to the consideration presented in codes but not conservative enough with considerations given to practical failure modes not explicitly addressed in the code. All told the selected preload should consider where applicable slip resistance, joint separation resistance, fatigue and leak prevention as well as tensile capacity, torsional capacity and plate punching.