upside down truss

[James M. Troth]

Anyone ever see this? I am recommending evaluation by an engineer.

[rgrizzle]

what??? I take it that they made a flat roof out of this. An engineer is a good call. they are not being used for their intended use.

[Brian E. Kelly]

http://www.sbcindustry.com/images/tr...igurations.swf

I do not see that truss here.

[James M. Troth]

Originally Posted by bkelly2

http://www.sbcindustry.com/images/tr...igurations.swf

I do not see that truss here.

Nice link.

Yep they made a flat roof trhat had some dips.

[mdavis5]

Thats a first for me. It would look nice sheetrocked

[James M. Troth]

Originally Posted by mdavis5

Thats a first for me. It would look nice sheetrocked

Sure would, anything to add extra weight to it.

[David P. Valley]

Good call.

I don't see how this sort of truss installation can properly support a flat roof. The webs are not resting on a solid chord. It appears that the solid chord is on top, which does nothing in this location.

Do you have a bigger picture?

[jrivera]

Originally Posted by dvalley

Good call.

I Do you have a bigger picture?

How about a picture of the outside too.

Mic

[pdickerson]

I would need a better picture of it before I decided there was something wrong. The structure looks OK to me. There are lots of different types of truss systems that are used in commercial buildings. How do the ends bear on the walls?

[Richard A. Hetzel]

The truss doesn't care which end is up. It's deepest where the moment is greatest, and shallow where the moment is least, just like if the same truss were inverted. Occasionally you'll see railroad bridges which are similar, with the trusses hanging from the top chord. I don't see a problem with this roof.

[tneyedli1]

Originally Posted by jtroth

Anyone ever see this? I am recommending evaluation by an engineer.

Good catch!
Most likely the wind blew the plans upside down and the construction crew installed it that way.
T.Neyedli
www.alphahomeinspections.ca

[Marc D. Shunk]

Both of these links show the use of inverted trusses:

http://www.ufpi.com/product/rooftrusses/types.htm
http://www.northlandtrusssystems.com/products.html

I see them sometimes in agricultural buildings to get a higher ceiling along one side to store combines and tractors and such.

[David P. Valley]

This is a flat roof truss...


If you notice any style truss, they have a solid chord at the lower portion, even if it's inverted. If you look real close at the picture that James posted, it appears to have split cords at the bottom. This doesn't do anything as far as supporting the weight of the roof. Any excessive weight added to the pictured truss would just make the webs push the chords at the seams. Just picture yourself standing on top of this truss and jumping up and down at the top center. The lower cords (at the seams) will open right up.

The truss doesn't care which end is up. It's deepest where the moment is greatest, and shallow where the moment is least, just like if the same truss were inverted. Occasionally you'll see railroad bridges which are similar, with the trusses hanging from the top chord. I don't see a problem with this roof.

8/26/07 12:53 AM

I have to disagree. Some railroad bridges do have upside-down trusses, but they are welded and made of iron. That's a whole different subject.

IMO...The pictured truss is improper.

[John Nosworthy]

Seems to me this looks improper installation of the existing designed truss. The portion that would normally be in compression is really in tension! Add a blizzard worth of snow and ice and I believe this roof will fail right down the middle. Just my 2 cents.

[Richard A. Hetzel]

In a truss, depending on how connections are made, the bottom chord does not have to be continuous, nor does any other member, The truss shown would have equal load-carrying capacity whether it were inverted or conventional. The point is that it is deepest where the moment is greatest, and shallow where the moment is least. That would be true no matter which way it were erected.

Yes, compression and tension mebers are reversed, but it functions exactly the same way upside down or right-side up. It just needs to be designed for this specific application, as all trusses do.