Round Bar as joist web

[Italo01]

Hello,

I've never used or seen round bars used as joist webs in my country but i've seen them used in some SJI catalogs and in some threads of this forum. For me, a round bar is a very bad element for compression due to it's low radius of gyration.

I'd like to knoy from you guys what are the advantages of using round bars as joist webs, instead of channels or double angles from you guys who use it. Is it something to due with speed of fabrication?

Thanks in advance.

 

[271828]

They're only used in fairly small joists, so the compressive axial load in the bar is presumably low.

I've always assumed these are used because the joist is easier and cheaper to fabricate with them than with angles.

 

[phamENG]

For modern usage, they really only show up in K-series joists (if you're following SJI here in the US) which, as 271828 said, are small. They can get up over 2' deep, so some of those bars are fairly long, but they are very lightly loaded.

If you need long spans with higher loading, LH and DLH joists typically use angles as web members.

 

[Italo01]

Thanks 271828 and phamENG.

 

[Once20036]

Most of the rod web joists that I`ve seen using a single pieces of rod, bent several times to create multiple web members. Sometimes the rod diameter is constant over the full length of the joists. Othertimes, it's spliced at the fifth or seventh top chord panel point and a smaller rod is used in the center. Design is typically governed by compression forces, and the tension members are lightly utilized.

I think that the advantage here (in light load situations, as mentioned above) is that they`re very easy to fit together in the shop. There's no dealing with different angles for each individual web, everything is preset by the bends.

 

[KootK]

The primary advantage is the form that the webbing takes with respect to fabrication efficiency. Solid round webs are usually just one continuous piece bent into the shape of the truss configuration. To bump up the "awesome" even more, they'll be bent to maintain a constant panel point spacing and any non-modularity in the span length relative to that spacing will be dealt with locally at the ends. That's called making money.

If you reinforce a round with a single angle, it will have axial capacity for days. This is typically not done during fabrication because it messes with truss stacking for shipping purposes.

 

[Italo01]

Thank you Guys.

I imagined that had to do with easy of fabrication but is good hearing for someone who has experience.

Kootk, in this image that you showed, both web and chords are made of Round Bars?
I hadn't seen it yet.

 

[phamENG]

KootK's post is of an old and antiquated joist style. Rounds for chords was rare, and isn't done at all now. It's all double angle chords with round bar webs for K-series. In the past, some companies used light gauge hat channels for chords. Lots of things have been attempted. A lot of them failed - either commercially or physically.

 

[KootK]

In my neck of the woods, the bend round web system is often paired up with heavy gauge, cold formed hat chord sections. Slick to fabricate. Not a ton of fun to reinforce.

 

[Brad805]

Efficient design depends on pounds of steel. Any large fabricator that is tooled up with all the equipment, and design tools will not use round bar much anymore for the reasons you cite. Many large fabricators order their steel in rolls and custom bend exactly what they want. That reduces waste a ton, and gives them endless flexibility. Below is a recent design calc sheet from one. They spank the other competitors using their level of automation. Old school fabricators or small shops will use round bar due to ease of fabrication with low cost equipment. I could setup a die in my own shop to bend out webs using round bar.

 

[Italo01]

Thank you guys. I've understand it.