Beam reactions on plans 1

[JStructsteel]

When putting beam reactions, the engineer specified the total is DL+LL (ASD). Would that be used thru the shop drawings and connection design, or would you try and break it down to be able to LRFD the connections?

 

[CDLD]

We usually specify in the notes if the loads are ASD or LRFD, and show the factored (combined) loads on the drawing.
We break the loads down by type if it is for OWSJ roofs.

 

[JStructsteel]

CDLD, they have the breakdown of the loads, so I could figure the DL reactions. Thanks

 

[Eng16080]

It’s not possible to know the magnitude of the LRFD load combo 1.2D+1.6L based on the ASD combo D+L.

So, ideally the fabricator should be given LRFD values if that methodology will be used for the connection design. Lacking that, if I was the fabricator or connection designer, I might multiply the ASD value by 1.6 to get a conservative value.

I know that some folks might say to multiply by 1.4 or 1.5 to convert from ASD to LRFD, but I don’t agree with this. It’s possible for example to have a floor with a low dead load, like a 10 psf wood framed floor, and a high live load of 100 psf like for an assembly occupancy. This would exceed the values from using the 1.4 or 1.5 multipliers.

 

[JStructsteel]

Eng16080, I suppose that ASD could just be used thru the connection design process, and not worry about it.

 

[phamENG]

I suppose that ASD could just be used thru the connection design process, and not worry about it.

This. If you have the design loads in ASD, just do an ASD design. Nice and simple.

 

[271828]

If they gave ASD reactions, they probably want or expect the connections to be designed using ASD. There's rarely a compelling reason to care, so I'd just go with ASD.

 

[Celt83]

In my experience ASD reactions have been traditionally given under the assumption that the detailing outfits are grabbing the green book and pumping out the connections. These days it's more likely that the reactions are being dropped into RISA Connection, RAM Connection, or some other design package, in these instances since all the load factors under gravity are 1.0 for ASD the detailers can simply input the value given as a DL and get an accurate connection design if instead you provide an LRFD reaction then the load factors differ from 1.0 so either provide the component reactions so the detailer can do the proper load combos or the detailer has to make an educated guess about the load ratios to get the software inputs correct.

 

[Eng16080]

Certainly, connection design per ASD is most common. Maybe once or twice on a project has that not been the case for me.

With the increasing reliance on software, doing at least a minimal spot check for sanity of the fabrication drawings is a good idea, at least to ensure that the correct design methodology (ASD or LRFD) is being used. I imagine it’s easy for an inexperienced user of SDS or Tekla or whatever to click the wrong button.

 

[lexpatrie]

I would not provide LRFD connection designs when the final ASD reactions are provided, at least not without an RFI to the EOR that accepts it.

 

[haynewp]

I agree that you risk submittal rejection if using LRFD from ASD provided loads and LRFD was not mentioned as acceptable in the construction documents.

 

[lexpatrie]

Why not set up all the standard stuff as dual format ASD/LRFD?

 

[cliff234]

The AISC 303 “Code of Standard Practice” requires the EOR to state on the drawings whether the loads are service level or factored. Likewise, the CoSP requires the EOR to indicate on the drawings whether the connections are to be designed using ASD or LRFD. Converting the reactions from ASD to LRFD (or vis versa) by the connection designer with a simple conversion factor is contrary to the CoSP.

 

[phamENG]

whether the loads are service level or factored

This nomenclature really bugs me and I think we need to an overhaul to better represent the way loads are calculated in current codes. For instance: what is service level wind vs. factored wind? Service level wind (the wind you use for serviceability checks) isn't even codified, though the general recommendation is 0.42xW for Risk II buildings. That's only in a couple of load combinations used for ASD. And besides, 0.42 is a factor. So by the definition of the word factored, service level wind load is a factored wind load. Strength level wind load, however, has a factor of 1.

So while I understand what service level and factored are intended here, the actual loads are moving away from fitting these definitions and confusion can result. It's another 'it's what we've always done/said so we'll just keep doing it' even though the language and the reality aren't matching up anymore.

For me, unfactored would be the individual loads as determined by the code (L, D, W, S, E, etc.). Factored would a combination of loads, either ASD or LRFD which would be clearly specified.

Rant complete. Sorry.

 

[haynewp]

Why not set up all the standard stuff as dual format ASD/LRFD?

Someone pointed out to me a long time ago that AISC's intent by the below is to use either ASD or LRFD throughout the design of the building and not to mix them. He was on an AISC committee at the time, but his comment was not provided as an official AISC interpretation.

AISC 360 spec:

B3. DESIGN BASIS

...."Design for strength shall be performed according to the provisions for load and resistance
factor design (LRFD) or to the provisions for allowable strength design (ASD)."

 

[Celt83]

In my opinion if you want the connections designed for LRFD then you need to provide the individual load components (D, L, S, etc.) so that the appropriate combinations are being considered in the connection design since the load factors in LRFD deviate from 1.0.

 

[DrZoidberWoop]

If the EOR has spec'd ASD loads, design the connections using ASD. The only time I've had mixed loads on a job is when the EOR provides LRFD loads and the erector provides ASD loads for the crane tie-in/supports. Then you design for whichever connection requires the more substantial connection and verify the loads for the other loads.

 

[lexpatrie]

The AISC design examples run them side by side, if that's the question.

Or are you meaning that the "or" in that AISC spec prohibits presenting/designing for both calculation methods? They show both methods side-by-side in the AISC design examples. I realize that conventionally through a building the steel is either all ASD or all LRFD, my point being if you calc it both ways it works both ways and is thus acceptably safe regardless of the design methodology used by the design engineer, barring the pathological case where one particular load case in LRFD is really funky. They are supposed to match at L/D=1.5 as I recall.

 

[haynewp]

I meant the "or" part of the spec, which isn't relevant if running ASD and LRFD side by side is what you meant and not mixing.

 

[lexpatrie]

Overall context here is connection design (unclear from post title), so when you do those you're not in control of the original design methodology, but if you develop them in tandem it should help for the rare LRFD job that comes along.