Are rockers and rollers mobile support?

[Femme_physics]

Are "rockers" and "rollers" mobile support?

I'm reading about trusses and trying to understand this sentence:

When a bridge or roof trusses extend over large distances, a rocker or roller is commonly used for supporting one end

.

I presume they mean mobile support that only has 1 resistance direction. Am I correct?

 

[Studiot]

Most bridges are supported on bearings, rockers and rollers are two types.

Rollers are exactly that. Solid steel cylinders as in the top sketch.
They can allow substantial horizontal movement of one end of the truss etc, whilst providing the vertical support reactions.

Rockers are used where less movement needs accomodating and are shaped as in the detail.

Now two questions for you to aid understanding.

Firstly why is one end fixed?

Secondly why do we need to allow a free or movement end?

 

[Femme_physics]

Ooh! Oh! I know that! For freedom for expansion or contraction :) Because temp' changes and the loads on it change...with freeom of expansion and contraction it takes more time to wear out because there's less contact at the hinges. I understand what rockers are now. Funny little things...like 1/4 of a circle rollers. Thanks for the sketch...did you make that?

I'm not sure what you mean by one end being fixed. Rollers and rockers are not fixators, they allow for some little basic movement.

 

[Studiot]

2)Yes that's right it's to allow for thermal expansion/contraction.

If you read my post a little more carefully you would see I said on end is fixed and the other is allowed to move horizontally.
I have labelled my sketches as such as well.

So why is there a fixed end?

 

[Femme_physics]

Because if one end wasn't fixed the truss could slide sideways, and there wouldn't be much of a structure if a member can slide away

 

[Studiot]

Yes that's it. To stop it falling off.

go well.

 

[Femme_physics]

Thanks Studiot...you're excellent :)