Truss Strength

[jrepp44]

Typical prefabricated roof trusses (2 x 6). Spacing is 29" on center in the shop area. If I temporarily place 3/4" plywood on top of the bottom chord would it be safe to walk on?

 

[Deere Dude]

I stand on my 2x4 trusses all the time in the house, and on my brothers pole shed 2x4 trusses 24" on center and 30 foot long. I never heard of 29" on center. That is odd.

Those are spaced close enough to where walking won't be a problem unless you are really a beefer. You will like the shed.

 

[bcp]

I would do it.

Fasten a couple of 2x4's to the plywood bottom, about 2 inches or 32 inches apart. When you are standing on it and exerting force, you don't want the plywood to slide off.

Bruce

 

[/pine]

For just "walking around"...running wires etc it will be fine...but unless specified when the trusses were built...the bottom chords are not designed to carry any live loads...over time, even a minor load on a single unfortified truss member will cause it to sag...

 

[EddieWalker]

Yes, it will be fine,but for the effort involved in getting plywood up there, I would just use long 2x6's.

 

[bigtiller]

A 2x6 is what my 54x75 building has, right down the center of the building. My trusses are 7.5 feet on center.

 

[LD1]

Yes, it will be fine,but for the effort involved in getting plywood up there, I would just use long 2x6's.

Agreed.

 

[/pine]

I'm surprised the truss engineering doesn't specify at least a "rat run" along the bottom chords...

 

[hddnis]

For just "walking around"...running wires etc it will be fine...but unless specified when the trusses were built...the bottom chords are not designed to carry any live loads...over time, even a minor load on a single unfortified truss member will cause it to sag...

That must be why so many trusses specify that they don't reach rated load capacity unless you hang thousands of pounds of drywall from the bottom of them.

 

[Raspy]

Of course that truss is strong enough to hold the weight of a person.

If there was a failure it would be because the bottom chord was not connected to the diagonals strong enough. I've seen those little plates they press on to hold it all together, come off. I agree with Eddie that long 2X6s would be better. That 29" thing is interesting.

 

[/pine]

That must be why so many trusses specify that they don't reach rated load capacity unless you hang thousands of pounds of drywall from the bottom of them.

The weight of a ceiling etc., etc...is considered "dead load" NOT live load...and dead load is figured into the size of the gusset plates...Also with a sheetrock etc. ceiling the weight is distributed quite evenly across the (generally 2') span where applicable...with the spacing cited by the OP...there is no way to realistically distribute loads to more than a single truss...

 

[hddnis]

The weight of a ceiling etc., etc...is considered "dead load" NOT live load...and dead load is figured into the size of the gusset plates...Also with a sheetrock etc. ceiling the weight is distributed quite evenly across the (generally 2') span where applicable...with the spacing cited by the OP...there is no way to realistically distribute loads to more than a single truss...

You said:

...over time, even a minor load on a single unfortified truss member will cause it to sag...

Usual is 10psf dead load on the bottom chord. That is over twice what is needed to carry 5/8" standard weight drywall and full insulation. Hardly a minor load and trusses are designed to not sag under that load.

Heavier you load a truss the less likely the bottom chord will sag, it is a tension member and wants to be tensioned.

 

[/pine]

You said:

You said:

Quote Originally Posted by /pine View Post
...over time, even a minor load on a single unfortified truss member will cause it to sag...
Usual is 10psf dead load on the bottom chord. That is over twice what is needed to carry 5/8" standard weight drywall and full insulation. Hardly a minor load and trusses are designed to not sag under that load.

Heavier you load a truss the less likely the bottom chord will sag, it is a tension member and wants to be tensioned.

What I said was: "...unless specified when the trusses were built...the bottom chords are not designed to carry any live loads..."

As I said before it's all about the size of the gusset plates...except on dinky little trusses you rarely see single member bottom chords...it's all about the plates that hold the members together...

S.F. dead load factors are generally calculated for an equally distributed area...with usually no less than 2' centers...

 

[hddnis]

I'm not talking about live loads in any way shape or form. I'm addressing that you said "minor" loads cause a truss to sag, they don't.

Gusset plates have nothing to do with it, they are part of the design and therefore part of the rating for the truss.

I'm also not saying anything about the spacing, that again has nothing to do with the rating since it is part of the truss package.

 

[/pine]

I'm not talking about live loads in any way shape or form. I'm addressing that you said "minor" loads cause a truss to sag, they don't.

Gusset plates have nothing to do with it, they are part of the design and therefore part of the rating for the truss.

I'm also not saying anything about the spacing, that again has nothing to do with the rating since it is part of the truss package.

For the sake of your pathetically ignorant argument you continue to misrepresent what I have stated...Here is what I said: "...over time, even a minor load on a single unfortified truss member will cause it to sag..." this was in the context of live loads...Doh!

Another factor is trusses have a camber built into the bottoms to allow for natural and imposed forces (loads)...the weight of the truss itself will cause it to sag "over time"...!

 

[Raspy]

For the sake of your pathetically ignorant argument

:laughing::laughing::laughing:

 

[hddnis]

For the sake of your pathetically ignorant argument you continue to misrepresent what I have stated...Here is what I said: "...over time, even a minor load on a single unfortified truss member will cause it to sag..." this was in the context of live loads...Doh!

Another factor is trusses have a camber built into the bottoms to allow for natural and imposed forces (loads)...the weight of the truss itself will cause it to sag "over time"...!

Exactly how much time do you think he's going to spend sitting on his trusses? Your point is absurd. You will not deform a truss using it to hold the weight of a worker while constructing the building, not a wood truss anyway, which is what the OP shows in his picture. If you think that your "live load" scenario is going to deform a truss then you have no experience building and no engineering background, your claim is baseless in the extreme.

Come off it already, you are simply wrong, you made a point that was silly, just admit it and move on.

 

[Egon]

You said:



Usual is 10psf dead load on the bottom chord. That is over twice what is needed to carry 5/8" standard weight drywall and full insulation. Hardly a minor load and trusses are designed to not sag under that load.

Heavier you load a truss the less likely the bottom chord will sag, it is a tension member and wants to be tensioned.

:thumbsup:

You mean it holds the walls together and upright with proper bracing!

Always liked figuring out the stresses in truss members. ( log table, slide rule days ) might even have gotten one right on a test!

 

[hddnis]

:thumbsup:

You mean it holds the walls together and upright with proper bracing!

Always liked figuring out the stresses in truss members. ( log table, slide rule days ) might even have gotten one right on a test!

Yes, they do that very well.:thumbsup:

 

[hawkeye08]

I remember when we had our shop built, the salesman wanted to sell us an uplift on the pricing to include some bottom loading in case we wanted it. I declined.. wish I had opt'd for it on a couple of trusses to let me use them to lift things into lofts.