Net Uplift on Steel Joist 1

[strucguy]

When specifying net uplift pressure on steel joist, do any of you indicate design approach (say ASD) or the exact load combination (say 0.6D+W) used to arrive at that value. With SJI promoting both LRFD and ASD for the design of steel joists, isn't it misleading for the joist designer to not have this info. Any thoughts on this??

 

[WillisV]

Section 5.11 of Joist Spec (K series in this instance) requires you to indicate design methodology. So you would say Net Uplift = 15psf (ASD) or something similar.

If you have anything more than typical uniform gravity and wind uplift loading, I would recommend creating a table with each load case separately defined and letting the joist manufacturer combine the loads.

 

[TLHS]

This isn't just an issue in the US where you're using both methods. For a lot of contractor/specialty engineered items there can be confusion between specified and factored loads. Personally, I think it's important to explicitly state the type of load every time you mention it (not just hidden in the notes) and also, where necessary, provide service *and* factored loads or a way to convert between them. That way the specialty engineer can properly apply loads for strength and servicability as necessary and they don't have to guess or over factor.

In Canada it's most confusing when dealing with geotechs or foundation work. That part of the industry is slowly shifting towards working with factored loads, so depending on the company they'll default to different things.

This is really a situation where a few words can save everyone a world of headaches.

 

[TDIengineer]

Yes, make it clear it is an ALLOWABLE load. And I agree, wind loading should be provided as an allowable or service load rather than strength. (old school)

 

[msquared48]

The more I use factored loads, the more I like un-factored.

To me, there are just too many places to make a mistake with factored loads, and they are constantly changing.

Mike McCann
MMC Engineering

 

[strucguy]

Thanks for the input guys. I have decided to modify my steel joist notes to read as below.

"Net uplift pressures on roof joists as indicated in the drawings is based on 0.6D+W load combination"

 

[XR250]

I agree with Mike. I still do all of my design in ASD except concrete.
Serviceability controls most of my designs anyway so why should I do the extra work?

 

[haynewp]

AISC's ASD method is more of a simplified version of LRFD now anyway, it's not like it used to be in the 9th edition manual ("Allowable Strength Design" vs "Allowable Stress Design").

 

[Ron]

ASD forever! It actually makes sense.

Strucguy...yes, let 'em know your premise.

 

[steellion]

IBC 2012/ASCE 7-10 changed the load combinations because wind is now based on ultimate strength. So the combination would be 0.6D-0.6W under that code. It's a always a good idea to provide notes if it is factored or unfactored for delegated designs. We have a note for net uplift for joists with the load combination indicating how we arrived at that number.

 

[Archie264]

Whoa, this is a big development for me...I didn't know I was allowed to publicly admit that I preferred ASD, especially since the debate has been officially "resolved" in the other direction for many years now. Whoo-hoo! I feel like a free man! No more (well, fewer, anyway) silly load combinations to skinny the design down to a gnats whisker! Especially since, as Excel noted, deflection often controls anyway.

I'll still be prepared to submit calcs in LRFD to demonstrate that I am "competent" but for my own use it's back to ASD. Except for concrete, again, as Excel noted.

 

[TLHS]

The LRFD factors don't bring your design down a gnat's whisker. The average factor of safety generally stays about the same. For systems with small dead loads and heavy live loads, you'll actually have a more expensive structure with *more* material using LRFD.

 

[Archie264]

Alright fine, but they're a pain to use all in pursuit of the (in my opinion misguided) idea that you can know all dead and live loads to within a gnat's whisker over the life of the structure. Others might feel differently, of course.