Specifying a moment of inertia for an open web steel joist or truss

[Tim_B]

I work for an open we steel joist manufacturer. Occasionally we see EORs specify a minimum required moment of inertia. There are a number of reasons I can imagine them specifying this but I wanted to get a more comprehensive list from this community. If you are a specifying engineer, why might you specify a minimum moment of inertia for an open web steel joist?

Thanks in advance!

Thanks!

Tim_B

 

[SlideRuleEra]

To control bending stress and/or deflection.

1. The designer still has freedom to select joist depth details (for example, shallow joist with heavy members or deep joist with smaller members).

2. Owner gets a design that exceed the minimum required to meet code. Good way for the Owner to specify reserve capacity in the structure to allow for future changes.

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[MotorCity]

One of the things we run into is that many analysis software are not capable of modeling joists. Engineers will often pick a wide flange and run the model accordingly. When it comes time to translate the beam to a joist on the drawing, using an equivalent moment of inertia is a convenient way to do this. It comes into play more for lateral analysis, diaphragms and that sort of stuff. Not really for gravity loads since the joist tables tell you the load that will produce a given deflection. Oh, and we like to mess with the joist engineers

 

[dik]

Best to just spec a load and deflection limitation and often a joist depth... and let the fabricator go at it... Typically spec a joist depth equal to [0.7xspan(ft)] in inches... a 30' span would use 22" (0.7x30')in inches deep joists... as a quick rule of thumb...

Dik

 

[KootK]

For me this is usually about structural vibration of floors. I can't really control the vibration characteristics of a floor panel without exerting some control over the infill floor joists.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.