Stud wall and trusses

[Bob49]

Hello,

Sort of a theoretical question regarding roofs constructed out of trusses rather than rafters and internal stud walls.

If the header plate of the stud wall is fixed to the trusses and the bottom plate fixed to the floor then does that mean that some of the weight of the roof is being transferred to the floor via the stud wall?

If you think of a situation where the stud wall runs parallel to the floor joists and does not sit exactly on top of the joists does that mean the flooring is taking some weight of the roof?

The reason I am asking is that I have the above situation and the flooring is squeaking when walked upon near the stud wall. Probably riding up and down on a nail in the bottom plate of the stud wall.

I can fix this by using a plastic shim or wedge or something under the stud wall where it is squeaking. It's only moving by a couple of mm, but enough to squeak and be annoying.

But could this cause problems?

Thanks,

Bob

 

[tomfe]

Maybe the floor is keeping the roof up!

You'll have no problems in wedgeing the stud wall, if you have access you might want to screw down into the floor joists as well.

 

[Bob49]

Thanks for the reply.

I get so worried about this kind of thing. I'm even worried that maybe wedging the floor could cause the chord part of the truss to crack in extreme temperature conditions!

My mind is a nightmare.

 

[^woody^]

Trusses self-support. A stud wall won't take any load from a truss, but movement due to loading of the truss can cause the situation you describe. Screw stuff down or add extra timber noggins to reduce movement.

 

[Bob49]

Thanks Woody, I understand that trusses self-support. But imagine if I got a length of timber 1 inch larger than the height of my room. In the center of the room I knocked this timber in under a truss and on top of a joist. Surely now there is weight of the roof on this timber? Assuming the truss or joist doesn't break.

 

[^woody^]

There is a very very slight loading by virtue of the contact, but still the bottom chord of the truss does not deflect so much as to put load to the wall, rather load still goes to the end bearings of the truss. All that happend is you take some of the deflection out of the truss.

 

[Bob49]

I see thanks, so by the same logic then stud walls must have a very very slight loading, if constructed very tightly.

I find it all very interesting, I should have been a structural engineering.

 

[^woody^]

I see thanks, so by the same logic then stud walls must have a very very slight loading, if constructed very tightly.

I find it all very interesting, I should have been a structural engineering.

We see it regularly when non-structural stud walls are removed and then people complain about extensive cracking to the ceiling. All that has happened is that the stud wall has taken the deflection out of the joists, and then the joists deflect a little but are adequate for the span. In practical terms, there is still a cracked ceiling that needs to be replaced, but there can be some comfort that the floor is not going to actually fall down.

 

[John D v2.0]

Yes anything that deflects imposes a load on anything fixed hard under it. When we had the full width doors at the back, the deflection on the 5m beam was potentially quite a few millimetres, so the top of the doors had to be fitted on brackets and compressible foam to avoid issues if we have a foot of snow.
For a stud wall it is probably fine but it would make some interesting load paths if you really needed to understand them in full.

 

[^woody^]

Yes anything that deflects imposes a load on anything fixed hard under it. When we had the full width doors at the back, the deflection on the 5m beam was potentially quite a few millimetres, so the top of the doors had to be fitted on brackets and compressible foam to avoid issues if we have a foot of snow.
For a stud wall it is probably fine but it would make some interesting load paths if you really needed to understand them in full.

There is deflection and bending.

Deflection is primarily concerned with finishes and is the initial loading (new build) or post install loading (retro fit) - beams should deflect and then stop once installed.

That foot of snow at some future random time should not be causing the beam to deflect as it should have been allowed for, and designed out with a beam that does not alter under future loading - that's more like bending which should not be occuring either.

 

[John D v2.0]

That foot of snow at some future random time should not be causing the beam to deflect as it should have been allowed for, and designed out with a beam that does not alter under future loading - that's more like bending which should not be occuring either.

I'm happy to accept there's a terminology difference between the two, but i am not happy to believe that adding load to a beam that is in place wouldn't cause further movement of the middle of the beam.
Look at someone standing on a diving board for a start.

 

[^woody^]

but i am not happy to believe that adding load to a beam that is in place wouldn't cause further movement of the middle of the beam.
Look at someone standing on a diving board for a start.

It might do, but it shouldn't as a competent person will design it out.

Only an idiot will design a beam that causes the lounge ceiling to crack once the shirts are hung in the wardrobe above. Likewise for your snow.

 

[John D v2.0]

It might do, but it shouldn't as a competent person will design it out.

Only an idiot will design a beam that causes the lounge ceiling to crack once the shirts are hung in the wardrobe above. Likewise for your snow.

I think you are mixing up the cause and the effect.
Yes the beam will deflect under load, and yes a competent person will design the system so that the deflection won't cause any problems.
That was my point, in our case the doors had to be attached with cranked straps rather than direct fixed due to the deflection calculated by the se being too much for the doors (when the next ice age comes)
If a beam deflects and no one notices, then there's no problem. If a beam deflects and the ceiling cracks and your door jams you'll complain.

 

[tony1851]

Deflection of a beam or joists is not a structural issue - it's a seviceablity issue - in other words, it's a case of how much deflection you are prepared to tolerate.

In domestic timber floors, deflection is usually limited to span x 0.003 - this is to ensure plaster ceilings don't crack, or that the floor doesn't bounce too much when walking across it. For steel beams, the design code limits only live load deflection (usually to span/360) - it leaves dead load deflection to be agreed between client and SE.

(There was a thread on here a few years ago about a client who had a wide-span opening for bifold doors, with a steel beam above supporting part of a flat roof, and a parapet wall. The SE apparently only allowed for live load (eg snow) deflection, and forgot to add the dead load deflection of the structure above the beam, with the result that the beam bent excessively and the bifolds wouldn't fit. Don't know what happened, but presumably the parapet wall would have had to be taken down, rebuilt, and a deeper beam installed = ££££s).