When to use Joist Girders vs Wide Flange

Joist girders are typically used for long spans that require a heavy section I-shape. The economy comes because the joist girder is lighter (less steel) and cheaper to build than to buy a heavy I shape. Too really get some economy use as deep a joist girder as possible. That is the trade off in some cases, the joist girder is deeper than the I shape.

I will use rolled beams when:

1) there is a possibility that the girder will need to be reinforced for added loads in the future. (Have you ever tried to reinforce a joist girder?)
2) Short schedule and quick delivery is important.
3) Where minimizing the depth of the girder is important.
4) Varying magitude of loads.
5) Other special situations.

Although joist girders can be designed for varying loads, pass thru ductwork, end moments, etc., the place that they really shine are equal loads and equal panels with no depth restriction.

Query... a joist girder is simply a truss member replacing a girder? Not familiar with the terminology.

Dik

Both the joist and joist girder are truss. The joist support the roof, itself is supported by the joist girders at the ends.

dik, the steel joist manufacturers here in the US also provide a product called a "joist girder" which is really just a big joist, but is designed with vertical web members in a set rhythm to correspond with the supported joists.

They are usually quite deep and have deeper seats at the ends. They can be designed to also take end moments from a wind frame mechanism at the columns.

Thanks... dik

JAE brings up another reason to use a joist girder instead of a wide flange--they work better as part of a moment (rigid) frame, because they are deeper.

DaveAtkins