Tension rod on double wood beam

[Mackenzie Melo]

Hello everyone,

I have to check the load capacity of a roof structure and I found this wood beam with a tension rod below. I had never seen a structure like this before. (I am attaching a photo of it) Oh, and by the way, is there a name to that specific beam configuration? Another engineer here in the office calls it "Belly Band", but I had no luck in finding it with that name.

Does anyone have any tips on how to calculate what the tension of the rod adds to the moment capacity of the beam so I can check the roof load capacity?

Thanks a lot.

 

[phamENG]

Here's a thread on it from a while back: thread507-273586

There was another one recently - it involved a failure of a roof in Scandinavia...maybe Sweden? I recall that thread had a good discussion on stability concerns with this kind of construction, but I can't find it right now.

 

[KootK]

What you've got there is a post tensioned wood beam. Or a truss, it kinda depends how you look at it. That's most often done as a reinforcement technique but this one looks like original construction to me. It's probably simplest to just analyze the assembly as a truss.

 

[jayrod12]

I agree with Koot. Run it as a truss. It actually looks quite nice from that picture.

 

[SwinnyGG]

Seen some old buildings, but haven't ever seen that arrangement. Intersting.

Got any pictures of the end attachment for that rod? Im curious if that rod actually sees tension or just provides a tiny amount of uplift at mid span from being bent into place.

 

[JoshPlumSE]

This is a "Queen's Post" truss. I've never used one myself, but I've read case studies that use them. My impression is that they are used in the following circumstances:

a) To remove existing unwanted deflection from a beam. This is done by tightening the cables or rods to induce an upward load. Very much like a post tensioned concrete beam, except the post tensioning is on the outside of the member where you can see it.

b) To add more strength or stiffness to an existing beam. Like when you've got a floor vibration issue or you are adding more load to the beam.

Personally, I don't know how to verify the level of pretension in the rods without cutting them. When I took the SE exam 18 years ago, there was a problem where you had to use virtual work to figure out what the tension in the rods where when you knew that the truss deflected x.x inches when they were cut.

 

[dik]

Can you provide a photo of the end condition... it's a neat 'stubby' post.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Tmoose]

http://hyperphysics.phy-astr.gsu.edu/hbase/Waves/string.html

Trolley cars and old Rolls Royces had a similar tension strut thing on the nether side of each frame rail.

http://www.classiccars.co.uk/rolls_royce/silver_ghost/advert/11176#

 

[HouseBoy]

Looks like it was done on purpose. Pretty cool looking.
I've seen a few of those used in old barns.
Always referred to it as a "trussed beam".

Others mentioned the stability thing. I would also add that the tension force can be rather large and anchorage of the rod ends requires a good detail. Generally tru to utilize some kind of end plate (to spread out the load).

 

[dhengr]

Mackenzie Melo:
From the fine quality looks of that queen post/strut/standoff, I suspect it is part of a beam kit of some sort. That kit would include the nuts, bolts, washers, that center span strut, and two end bearing castings which fit over the ends of the two beam members to receive the tensioning nuts, and to tie the two wooden beams together. These end castings are the “good detail” which HouseBoy speaks about. The tension rod would be cut to length and the ends threaded depending upon the beam length.

 

[dold]

Wandering off topic as per usual, but the concourse of the Greenville Spartanburg airport sports some queenposts as well. I know i've been in other airports with more substantial queenposts but I can't recall which ones.

 

[bones206]

I gotta say that's probably in my top 3 favorite structural features to look at and admire. Hope to get a chance to design one of those some day. Dulles airport has some pretty impressive ones.