Through Bolt Truss 1

[MIStructE_IRE]

Hi,

Perhaps someone can tell me if I’m going crazy here.. The bottom joint in a truss is shown below where I have 100kN opposing tension forces. If I balance the forces I get 100kN.

The fabricator is proposing a through bolt connection which I believe needs to be designed for 200kN tension as the bolt is being pulled from both sides.

Its the classic tug of war! Is the tension force double at the joint or am i going crazy..?

Thanks!

 

[Ron]

You are correct.

 

[MIStructE_IRE]

Thanks Ron,

I thought so, but started to doubt myself once I was questioned.

 

[sbisteel]

Respectfully disagree.

 

[rb1957]

I agree ... the bolts react 100kN ... you have a load and reaction.

another day in paradise, or is paradise one day closer ?

 

[bootlegend]

Isn't the bolt just a tension member being pulled at each end?

 

[MIStructE_IRE]

I drew out the same sketch bootlegend and I get 100kN also. I think you’re right actually.

I’m picturing a guy holding two ropes with a horse pulling each one! Is the force in his arm any greater than if he was holding only one rope and the other was stabilising him to an anchor? No.. So i’m back thinking the force is 100kN in the bolts.. Doh!

 

[kostast88]

It's 100. Simple experiment:

Imagine you are holding a rubber band upside down.
The weight is a downward force= the mass of the band x g=9.81 m/s2.
Then fix it on a wall, on top of a pin.
Then remove the pin - let the band fall down. Why did this happen?


This whole time you were applying an upwards force = W, So the band was always in tension from both sides. The internal force was W all the time. You know this because when you put in on the pin, it didn't change shape from when you were holding it. If it was in 2-sided tension it would have to deform more, because it's rubber.

Tension is constant in every part of a bolt. That only changes for very long bolts (because there is moment generated inside the bolt - see pin connections in EC3 EN 1993-1-8 - Connections).

 

[MIStructE_IRE]

I agree guys. Thanks! Friday morning...

 

[BAretired]

The tension in the bottom chord is 100kN (not 200kN). I would prefer to butt the chord members together, rather than using long bolts as shown in the sketch. The vertical member has no force, so a nominal connection to the bottom chord is all that is required. The butt joint could be offset from the vertical if desired.

BA

 

[human909]

The tension in the bottom chord is 100kN (not 200kN). I would prefer to butt the chord members together, rather than using long bolts as shown in the sketch. The vertical member has no force, so a nominal connection to the bottom chord is all that is required. The butt joint could be offset from the vertical if desired.

Yep, agreeed. This joint doesn't excite me. Though we don't have the full picture so who knows.

 

[SWComposites]

You need to put a solid block into the center member in order to get good preload on the bolts.

 

[dik]

It's tricky to get the nut in place and tighten it up... might have to use locktite red... can you use a couple of end plates with a bolt on either side?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

a double wedge so you can tighten it up?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[MIStructE_IRE]

Thanks all, PS - that’s just a clear image to demonstrate the question i had. Its not representative of the actual connection.