Pinned & Rolling support in Roof Truss (timber)

[Adam10]

Can anybody explain in simple terms for a truss newbie.

What is the difference between designing a truss with two pinned supports and designing it with one pinned and one rolling?

 

[JAE]

The fixity at both ends can affect the axial loads in the truss members. This is because, as the truss bends down, the bottom chord lengthens (its in tension) and if the far end is fixed laterally (pinned for rotation only) then there will be a resultant lateral force at the pin pushing back against that chord lengthening.

With a rolling support, the lateral reactions are zero from bending.

 

[Lion06]

Agree with JAE. I would also say that a result of the pin-pin truss is that the bottom chord can actually experience compression near the supports. This is a very bad thing for a bottom chord design only for tension. It can also wreak havoc on the webs - again some webs only experience tension in a pin-roller scenario. The pin-pin can put some of those web members that were designed for tension only into compression - again, a bad thing.

I'm not saying that it's wrong to design for a pin-pin support, just be aware of it and detail it out to match your assumed support conditions in the analysis.

 

[BAretired]

If the truss has a horizontal bottom chord and you model it with a pin support at each end, the bottom chord force will turn out to be zero because the chord is not permitted to change length.

BA

 

[Adam10]

Thanks for your responses.

I always design with a pin rolling support. When would you ever design for a pin pin support?

 

[Lion06]

BA-
I think the NET of the axial forces will be zero, but I don't believe it will be zero along the entire length. If you imagine the first diagonal framing into the bottom chord, that will impart an axial load on the bottom chord. It will likely put the portion closest to the pin support in compression and the next segment would likely be in tension.

 

[BAretired]

SEIT,

Upon second thought, I believe you are correct.

Adam10,

I suppose the only time you would design with a pin-pin support is if you had a pin at each end. I don't recall ever seeing that situation in a real truss.

BA

 

[woodman88]

I agree with JAE. I have used the pin-pin bearings mainly for scissor trusses (pitched bottom chords) to calculate the horizontal thrust applied to a beam.

Garth Dreger PE
AZ Phoenix area

 

[BAretired]

For scissors trusses supported on top of walls, I usually consider the supports to be pin and roller, but I agree there is some restraint to lateral deflection from the wall. The problem is, such restraint is usually not very reliable. I guess a strain compatibility analysis could be done, but the pin-roller assumption is conservative.

By the way, have some of the posts in this thread been deleted or did I have one too many beers last night?

BA

 

[paddingtongreen]

I have not ever worked this out, but I wonder what the effect of adding bottom chord bracing has. I see tension in the bracing diagonals and compression in the eaves struts, all due to the vertical loading on the truss. That would make them pin and spring resisted roller supports for the truss!

Michael.
Timing has a lot to do with the outcome of a rain dance.