Bolt Load and Joint Load

[kotawsu]

Hello everybody. I have been simulating a bolted joint connection in abaqus. I am trying to understand the load distribution between the joint and the bolt when an external compressive load is applied. I know that there is a formula
Fb= Fp-N*F
where Fb---bolt load
N--load factor
F--external applied load

I understand that when an external load(compressive on the members) is applied the bolt load relaxes than th pretension load and the joint load increases. But when my analysis results is showing me a decrease in both bolt load and the joint load. The shape of the joint is not flat-flat panel. The upper panel is a z-shape beam and the lower panel is a flat panle. I was wondering if this could happen or is there any otehr formula by wich i can calculate that factor(the portion of the external load which relaxes the bolt load). Thanks for your time and patience.
Kota

 

[CoryPad]

Your terminology is somewhat confusing. Take a look at this page and see if it helps:

http://www.boltscience.com/pages/basics7.htm

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[kotawsu]

i am sorry if i am not clear in my explanation. what i meant was when the external load is compressive i understand that the joint load increases and the bolt load relaxes. But i am getting a decrease both in the joint load and the bolt load. for example if 1167 is the pretension load my bolt load and the joint load is showin up as 1103 lbs after the application of external load. Is it possible?

 

[diamondjim]

Depending on the force, you can
have a reduction in clamping load.
Most forces produce a tension force
which increases the load on the
bolts.

 

[kotawsu]

yes, i understand that.my question is what if the external load on the joint is a thermal load and this thermla load is applied only after the pretension step.later u see that the both the bolt load and the joint load decreases.What does it mean?

 

[diamondjim]

If you are saying that only the bolt
sees the thermal load then it is
expanding and thus reducing the clamping
load of the bolt.

 

[kotawsu]

diamond thanks for your time. The thermal load i have applied is on the joint i.e; both the panels. And this thermal load is contracting the plates. In other words the plates are being compressed. So the bolt load should decrease and the joint load should increase. But i see that both the bolt load an dthe joint load is decreasing. This is what is bothering me.I hope I made things clear.
Kota

 

[diamondjim]

Kota,
The problem is that the thermal loads
would normally expand something
and not contract it. Could you possibly
illustrate your application and just
explain how or where the thermal load
is coming from. If you looked at the
url that was provided earlier by CoryPad
the bolt is clamping two members. Would
this represent the two panels being clamped
together?

 

[kotawsu]

Diamondjim,
I am running a sequentially coupled analysis in Abaqus. First i get the temepertaure distribution for the bolted joint assembly and then i perform the mechanical analysis by constraining the model in the right way and apply the thermal load. And you said that the effect of temperature is to expand something. This happens only if the delta T is positive. And delta T can be negative if the temperature of the panel is lesser than the room temperature.
delta T= (t-sink temp)
Contraction= cte* delta T
where cte--- co-efficient of thermal expansion
for example if a particular node in the upper panel has 10 deg F and the sink temp is 7o deg F then the delta T would become -60 deg F and there would be a contraction and not expansion.
So considering this my model is experiencing a compression and hence the bolt load is decreasing. But the joint load is also decreasing which is very weird and I ma trying to figure out why the joint load is also decreasing when it is supposed to follow the reverse trend.Thanks
Kota

 

[Screwman]

I just can't see how thernal contraction in compressive bolted joint could ever lead to an increase in joint loading. Draw up some joint diagrams and work it out graphicly and I think you'll pretty quickly see that it is very improbable that it would occur outside of a formula. I am wondering if you have overconstrained the joint in your model.

 

[yates]

"the thermal load is contracting the plates. In other words the plates are being compressed." ??
Seems to me if joint plates suffer a negative delta-T then contraction would occur, resulting in a lower clamping force. Is clamping force what you mean by 'joint load' ? Since the bolt is holding the joint together, when I was at school, I learnt that force = reaction, consequently both bolt load and joint load would decrease if delta T plate-to-sink is negative.