Attic truss loading and shelves?

[RossAustin]

My trusses are built for 40 psf, that's average material weight distributed over the square footage of the floor as I understand it.

So if I have a free standing shelf that takes up 10 sf, it should be safe to hold 400lbs minis shelf weight, correct?

Second question, if I build shelves that tie into the top and bottom chords how would that affect the capacity if at all?

Attached is a poorly drawn example, blue being free standing and red "built in", black is obviously the attic truss.

 

[PCustoms]

40 PSF is an evenly distributed load on the bottom of the lower chord.

 

[RossAustin]

40 PSF is an evenly distributed load on the bottom of the lower chord.

Ok, so how much weight could a 5'W x 2'D shelf hold? Trusses on 24" center.

 

[PCustoms]

Ok, so how much weight could a 5'W x 2'D shelf hold? Trusses on 24" center.

In theory you need to put a floor down, and that floor can be loaded to 40psf.

You can't just screw a shelf to everything and load it up

 

[RossAustin]

Right, there is a 3/4"TG floor down already. The built in would have a vertical 2x attached to the cross tie on top and a horizontal 2x on bottom, which is screwed through the floor into bottom chords. Then horizontal 2x from the vertical 2x - attached to the vertical truss member, top with plywood of choice.

 

[billconner]

These are attic trusses? the 40 psf is just the attic room area.

 

[RossAustin]

These are attic trusses? the 40 psf is just the attic room area.

Correct, engineer stamped attic trusses, 40 psf is just the "attic" area ~7' x 40' down the center.

 

[firebirdparts]

I would not hesitate to do built in, but it doesn't add any strength in this case. If you really had a hard attachment through the floor, so that the shelf vertical could be in tension, it actually would help a little. I couldn't say how much. That was a good question.

In general, the truss is able to hold up the load if it covered the whole floor. Like, for instance, in your house, one person walking around can easily put down a point load of 300 pounds on the area the size of a shoe. The smaller concentrated loads can be wildly above 40. The 10 square foot shelf is pretty big, so it does start to approach a number like 400 pounds, but the real number is might be double that, for instance.

 

[Beemer]

Playing dumb here but is the rated load for the floor or the roof?
The sketch isn't clear about the construction, atleast to me and I would have suspected total roof load which may be in that general range.

 

[racecougar]

Playing dumb here but is the rated load for the floor or the roof?
The sketch isn't clear about the construction, atleast to me and I would have suspected total roof load which may be in that general range.

It's the bottom chord loading of the designated attic space.

 

[BobnCO]

It’ll hold most things.. don’t store you gold bars up there!

 

[duneslider]

I mean that is roughly the same as the living space in your house, so if you would do this in your bedroom it is probably fine up there too. One thing to note, most attic trusses have a pretty low deflection number which means there is a pretty high amount of deflection that is acceptable. This is usually quite a bit more deflection than your house floor, so as you load that floor know that it will get some sag in it that you wouldn't see in your house. So, maybe be reasonable in what you store up there. Christmas decorations, good. Engine blocks, not good.

 

[Beemer]

It's the bottom chord loading of the designated attic space.

I would hope so but I have never seen roof trusses rated that way so hopefully there is clarity in his documentation.

 

[racecougar]

I would hope so but I have never seen roof trusses rated that way so hopefully there is clarity in his documentation.

Trusses are often stamped with both top chord and bottom chord load ratings. Ask any structural engineer.

 

[Beemer]

Trusses are often stamped with both top chord and bottom chord load ratings. Ask any structural engineer.

Loadings don't get marked on any trusses I've ever seen on built to order commercial project trusses.
That would be good for the OP if his are marked then it's an easy determination.

PS: It is best to avoid little "e" engineers.