In trusses, does the number of screws on a joint matter?

  • #1
Femme_physics
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Main Question or Discussion Point

we're gonna start trusses soon (statics) and I wanna know if I should pay attention to several screws on a joint...for instance, I attached a question and in the drawing I see sometimes 2 and sometimes 3 screws on a joint...is it relevant, or should I treat them as though they were one screw?
 

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  • #2
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In the real world of complex structural design, it likely does matter,,,just as the diameter of the bolts matters.....but if this is an introductory undergradate class you'll likely ignore such details....besides, unless you are given some theoretical tools for dealing with different connections you'd be hard pressed to figure it out on your own.

Around 1958 I built a house with my Dad...split rings were a new design to strengthen the trusses...an a innovation. With a single 1/2" bolt and a circular ring drilled party in each mating surface around the center bolt, even at 12 feet or so long, two men could not move that joint!!! Yet without that split ring, we could easily turn the two truss pieces.
 
  • #3
Femme_physics
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I see. I sorta suspected it to be the case--thanks. But I just thought of something-- since there's more than 1 screw, doesn't it mean that the joint can't rotate?
 
  • #4
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In the old days, when rivets were used, they would to some extent fill the hole in their making, and the joint would be tight. This effect was also aided by the cooling shrinkage of the hot rivet. So a two- or three-rivet joint would be approaching full moment-fixity, if properly made. the trouble was that you could not inspect it to see that. Demolition of old riveted bridges has shown that in some cases, contractors cheated, sometimes just glueing in a rivet head on both sides. However, these days we use clearance bolts or welding. This means that the hole has to be larger than the bolt (same as rivets had to be) but the tightening of the bolt does not fill the gap. So, initially, if friction is overcome, the joint will rotate a bit until the clearances are taken up, and then some indeterminate moment fixity will ensue. Another practical point is that steel fixers prefer two fasteners because they use the pointed end of their podger spanner to capture the members and align them before inserting the bolt in the second hole. Removing the podger spanner enables the second bolt to be inserted. If the truss is loaded only at the joints, then there is insignificant bending anyway. However, this is fairly unusual, and secondary moments are the reality. They really are small when the length to thickness ratio of the member is say more than 10, but can be significant if less than 5. Long answer but interesting commentary, though I say it myself.
 
  • #5
Femme_physics
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Actually, this was a very interesting commentary, pongo, keep it up.

From what I gather-- even 1 screw on trusses joints means complete fixation?
 
  • #6
AlephZero
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Actually, this was a very interesting commentary, pongo, keep it up.

From what I gather-- even 1 screw on trusses joints means complete fixation?
I think you are trying to read too much into this. The first step in analysing a truss is to find the loads in the members. The basic point of a truss structure is that if the joints are pinned, the structure is statically determinate so you can calculate the loads independent of the deflections.

Once you have the loads, you can then select the size (area) of the members and figure out how many bolts or rivets you need to connect them, so the bolts don't fail in shear, etc.

What pongo38 says is true to some extent, but in a sensibly designed truss the members themselves will not be stiff enough in bending to carry a significant amount of bending moment. You make the members of a truss structure so they look like rods, not like I-beams that are meant to carry loads in bending.

If you are just starting to learn about trusses, then assume all the joints are either pinned or on rollers. IMO the picture you attached is just somebody trying to make it look more realistic than a diagram with lines representing the members, and I wouldn't try to read anything more into it.
 
  • #7
Femme_physics
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I see, Aleph. I thought it was thrown in there to add another element, but I won't read too much into it as you said.
 

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