Eurocode EC3 Strength Check of the Preloaded Bolt

[Vitkacy1989]

Hi Folks,

In reference to the figure below, excerpt from EN-1993-1-8:2005:
A - In clause 3.6.1 (2) Gamma M7 is used
B - In clause for Design Slip resistance 3.9.1 (2) there is no safety factor used.

Question: if I am checking Tension of the bolt (not a Design Slip), should I use Gamma M7 as per figure A?

 

[JBlack68]

is Gamma M7 not the preload factor for high strength bolt - see Table 2.1?

 

[Vitkacy1989]

Yes it is, recommended value is 1.1.
When one would use this factor, it would decrease load in the bolt, thus tensile strength utilization ratio of the bolt will be smaller than if this factor would be equal to 1.00.
Maybe this factor 1.1 accounts for preload loose due to embedding? It is not stated in the code. What do you think?

 

[klaus.]

it depends on how the preload is applied

 

[Vitkacy1989]

@klaus. could you please explain how do you use this gamma M7 factor? thanks!

 

[klaus.]

you can read here page 15 ( in german only)

 

[Vitkacy1989]

Thanks for reference. This however does not answer my doubts.
Load which sees bolt is one of these two:

(A) Load in Bolt = Preload + Part of the External Load
or
(B) Load in Bolt = Preload/gamma M7 + Part of the External Load

if I would check this bolt against tensile strength, I would rather use (A) as it is larger than (B).
So, bolt's tensile strength must be greater than (A).

Would you agree?

 

[klaus.]

careful ...the load in the bolt is NOT the sum of external tension + preload !!!

The preload is used to transfer friction load....

the gamma_M7 reduces the preload for friction design...not for tension design

depending which methode for pretensioning you are using the Gamm_m7 is 1.1 or 1.0

usually it is 1.0 so equation (3.7) is the one to use

 

[Vitkacy1989]

Thanks for contribution!
Well, I would like to discuss tension case for preloaded bolts, not slip resistance ...

As you noted, the load in the bolt is not the sum of external tension and for this very reason I previously named it "Part of the External Load" to underline this fact.

Therefore, would you use (A) or (B) for tension check?

 

[klaus.]

for tension check.... neither......external tension only

 

[Vitkacy1989]

Be aware that preloaded bolt does not see all the external load as it is partially transferred via compressed plates. What is stressing the bolt to its limits it is the preload itself and therefore I think you should take care about that in your approach as well.

If you would use external load only, this is not what physically you have got in the bolt in real world. You may be on the unsafe side.

 

[klaus.]

well...how much of the preload (prestress) would you use for tension check of bolt ????
10%...50%....100%. ????

 

[Vitkacy1989]

0,7*fub*As/SF with Safety Factor depending on the method of tightening used (so as you already pointed out, 1.0 or 1.1, other for non civil engineering cases).
We assume the loss of the preload is taken into account within SF.

 

[klaus.]

Well this is wrong....
How can you apply external tension load if the design capacity of the bolt is already reached just by preload ???

 

[Vitkacy1989]

Using preload value 0,7*fub*As/1.1 = 0.64*fub*As
Tensile strength is 0.9*fub*As/1.25 = 0.72*fub*As

We get (0.72-0.64)*fub*As = 0.08*fub*As "space" for the part of external load
In common preloaded tensioned connection approx. 90% of the external load is transferred via plates, while the rest 10% is going through the bolt,
therefore tension value in bolt equals 0,7*fub*As/1.1+10% of External Load and it is smaller than Tensile Strength and that is the answer how one can apply external tension. Simply, there is still space for it in bolt's capacity.

What do you think?

 

[klaus.]

If you apply external tension to a connection which has pestressed bolts ...
the extenal tension will reduce the pressure which was build up by the pretensioned bolts
The tension forse in the bolts stay approx the same ( = prestress)

Once the external tension reaches the value of prestress then the pressure will be zero (=gap) and tension load is taken by bolts

so in the ultimate state the pretension is "gone" and the bolt tension force equals the external tension fore

==> bolt design is done for external tension force only

Prestressing bolts also does not incluence the load capacity of the connection
prestressing reduces deformation (rotations) and can be used for friction shears transfer

 

[Vitkacy1989]

No, that is not how it works.
Please refer to VDI2230 or this plot of what is happening with the preloaded bolt during loading: Link
This concept explains real world behavior.
This diagram explains also itself that bolt sees additional tension due to external load applied BEFORE external tension reach the value of prestress.
This diagram is the main benefit of using bolt preloading.
And that was the reason I started this thread.

Please reconcile and let me know your thoughts.

 

[klaus.]

well yes...there is a littel bit influence (maybe 10 %) ...but we structural engineers like to keep things easy

Do it the way you think it is correct as long as it is on the save side it is OK