Concurrent Loading "&" Standard Load Combinations 4

[BSVBD]

Please see attached...

A steel joist manufacturer is asking, "To confirm that all the loading in loading diagram for 32LHSP-63 is concurrent and that standard load combinations apply"

If loading is concurrent, then all loads in diagram could occur, 100%, "concurrently" or simultaneously (at the same time).

If standard load combinations apply, then a 0.75 reduction factor can apply to "L" and "S" in one combination. In another combination, either "L" or "S" can be omitted altogether.

I don't see one ASD (is there anything else?) load combination that requires both "L" and "S", 100%, concurrently.

(Sorry for being so conservative, but, isn't this part of our unwritten law? - A "What if?" scenario "COULD" happen; And when it does, I don't consider ASCE load combinations holding any weight in the courtrooom. MY bottom line is, for safety sake, I don't WANT a failure to occur.)

Therefore, isn't the manufacturer inquiry a contradiction?

Any suggestions on how to reply to this inquiry "To confirm..."?

 

[JAE]

They probably should have asked, "Are all joists to be designed for all the applicable code load combinations with the loads indicated or are some loads NOT concurrent with each other?"

There might be portions of a specific load case that don't occur simultaneously.

For example:
Attic live loads are usually not concurrent with roof live loads.

Snow drift loads should be added to actual flat roof snow loads and not the minimum flat roof snow loads.

Check out Eng-Tips Forum's Policies here:
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[KootK]

A steel joist manufacturer is asking, "To confirm that all the loading in loading diagram for 32LHSP-63 is concurrent and that standard load combinations apply"

I don't believe this to be a nonsensical ask. As JAE pointed out, there are some situations where it is quite reasonable to assume non-concurrency of loads. Not everything fits neatly into the stock ASCE load categories and it's not uncommon for engineers to apply some extra-code judgment in deciding what needs to be applied concurrently and what does not.

As an engineer with some experience doing component design, I can tell you that many a well intentioned engineer has neutered the cost effectiveness of a pre-engineered component by being excessively conservative. These things are built around "typical". When I first left WI and returned to Canada, I was annoyed that we don't have KCS joists here. So I invented my own by writing the SJI requirements into my joist spec. The joist guys freaked out and the contract came back 3X what it should have been. In that case, it was my good intentions that went awry.

A "What it?" scenario "COULD" happen; And when it does, I don't consider ASCE load combinations holding any weight in the courtrooom. MY bottom line is, for safety sake, I don't WANT a failure to occur.

I disagree strongly with this sentiment. Everything that we do as structural engineers has a stochastic aspect to it. Our snow loads are statistically derived. Our material factors are statistically derived. Our load factors are statistically derived. To turn your back on the load combination factors, which are also statistically derived, strikes me as arbitrary and logically inconsistent. Of course, that said, I would never deny any engineer their right to design to loads in excess of the code requirements where they feel that to be necessary.


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.

 

[BSVBD]

JAE & Koot... Thank you!

Is it unreasonably over-conservative to insist the following:

Standard “ASD” basic load combinations CAN apply ONLY if all loads are 100% concurrent. In other words “L” and “S” may NOT be omitted from any basic load combination, and, “L” and “S” may NOT be reduced to 0.75 in any basic ASD load combination.

Can we insist / specify the employment of ASD only rather than LRFD? Perhaps this a question for another thread?

 

[BSVBD]

Koot... I respect your disagreement... BUT... in all sincerity, has your stochastic probability distribution ever been exceeded?

I admit, much to the detriment of higher cost and probable inefficiency, I tend to lean toward "What if?" quite frequently.

I also admit, I had to look up the definition of stochastic.

Thank you!

 

[KootK]

Standard “ASD” basic load combinations CAN apply ONLY if all loads are 100% concurrent. In other words “L” and “S” may NOT be omitted from any basic load combination, and, “L” and “S” may NOT be reduced to 0.75 in any basic ASD load combination.

I'm finding this bit baffling. The ASCE ASD provisions do include cases where one or the other of snow or live is omitted. They also include cases where snow and live are multiplied by 0.75. Is it your intention to create your own, more conservative load cases for this project?

Can we insist / specify the employment of ASD only rather than LRFD?

I don't see why not.

in all sincerity, has your stochastic probability distribution ever been exceeded?

I couldn't say. I've had the good fortune to not be involved in any failures that were blamed on excessive loading. Of course, over a long enough time horizon, my stochastically derived loads will eventually be exceeded. That's just the nature of the beast. There's no such thing as "safe". Only "safe enough" in the statistical sense and as dictated by society's tolerance/perception of risk.

How have you handled seismic loads over the years? I'd think that the load factor of 1.0 there must be driving you nuts. And the omission of live loads. We statistics lovers are able to be at peace with that kind of thing.




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.

 

[Once20036]

Like the folks above, I think it's a reasonable question to ask.
Seems to me like there are two different questions posed by the manufacturer..

1) Are all loads applied concurrently?
I believe that this has nothing to do with load combinations and everything to do with the 4 different "load diagrams" present on your diagram... I think it's a little confusing...
You have top chord dead load listed in three different places. Are these meant to be additive? Or are they taken one at a time?
Similarly, the snow load is listed in three different places. Are these additive? 0.14 KLF is listed in two different places. Maybe a plan view would clarify this, but it strikes me as odd. Also, one of the "load diagrams" shows a drift that flattens out and another shows a uniform snow load. Is the uniform load Pf, to be combined with the drift or Pm, treated independently. If it is Pf, where is Pm? (Assuming ASCE 7-10)
I think they're asking if they combine each of these 4 diagrams into one master diagram, or if each represents one load scenario that is treated independently.

2) Do standard load combinations apply?
This seems like an after thought to me, based on their confusion about concurrency.

I absolutely believe that we can insist on an ASD or LRFD design. I recently selected a standard joist that worked fine with an LRFD design, but would need to be two sizes larger using an ASD design. If they were free to arbitrarily choose which methodology to use, I could find myself in trouble.

To clarify, I believe that you're talking about Live Load (L) and not roof Live Load (Lr), correct?
LRFD allows you to utilize 50% of the live load in many situations in which both snow and live occur (ASCE 7-10 2.3.2 exception 1).
ASD takes 75% L + 75% S (ASCE 7-10 2.4.1 combo 4)

To me, the interesting question is whether or not we can insist a joist manufacturer NOT utilize that exception (LRFD) or insist that they not utilize that 75% factor.
I would be inclined to think that we could, as long as the requirement is clearly outlined in the drawings at the time that the project is bid.

 

[BSVBD]

Thank you all for your credible comments and further inquiries.

I must leave the office and will return in the morning with the intent to reply.

Thank you and have a wonderful day!

 

[Teguci]

I guess I'm a bit puzzled by the loading detail:
- God bless you for including bottom chord dead load! I would assume that load applies to all of the joists
- I get the typical top chord dead plus snow condition, but what happens to the dead load when you have the snow drift loading? Are these 2 different snow loading conditions or did you subdivide the snow loads?
- How can you have snow and dead load on only 22 ft and no loading on the rest of the joist?
- If you want all the live loads and snow loads included together, then why didn't you add them together? I can't think of a condition where the snow loads would start and stop like that if they are all combined.
- What's the load break down for the point loads (dead, live, etc...)?

If I were the joist mfg. I'd have to assume that the deads, lives and snows are all added together under 1 load condition and then apply the load combos. If I won the bid, I'd then ask for clarification.

 

[sticksandtriangles]

Teguci,

I noticed you said "God bless you for including the bottom chord dead load". Is this something I should be doing on bar joist jobs? Does it affect their design significantly?

Since we are on the topic of joist load diagrams, I have recently been given direction by higher ups that simply showing RTU locations and unit weights on our framing drawings is sufficient enough for joist designer. Is this acceptable or is there some code mandated SJI requirement that special joist loading diagrams need to be provided? I am still relatively new to structural engineering and have not received shop drawings yet on these joists. Here's to hoping that the joist manufacturer gets it right!

Thanks,
S&T

 

[wannabeSE]

Sticksandtriangle,
Probably need a note saying the joists need to be designed for the additional loads shown on plan. Modern Steel had an article in 2011 about specifying joist

https://www.aisc.org/globalassets/modern-steel/archives/2011/02/2011v02_2011v02_more_practical.pdf

 

[sticksandtriangles]

Thanks for the link wannabeSE!

 

[Teguci]

sticksandtriangles - I currently do a lot of design involving hanging loads from existing superstructures. The official response from OWSJ manufacturers is that you can't hang more than 100 lbs concentrically off the bottom chord without a tall ladder and a micrometer to find out what flavor of joist has been installed (or install a mechanical connection within 4" of the panel point - looks good on a drawing but don't leave your desk to see what actually got installed).

2 things to add further perspective:
1. Fire protection pipes under NFPA 13 require a support to be designed for a "non-concurrent" 250 lb point live load plus it's dead load. Any fire protection pipe hanging from the bottom chord of an OWSJ must be - reinforced to the upper panel point, mechanically connected concentrically at the panel point (use magic as needed), or specifically analyzed given the bottom chord properties.
2. Looked at a data center job where the owner was installing hot dipped galvanized OWSJs which were required to support a suspended structural grid on the order of 20 psf or so. If the Owner had included a bottom chord loading, a considerable cost for reinforcing the joists (remove galvanizing and welding angles at each connection point)could have been avoided.

So, yes, God bless engineers that specify bottom chord loading - when appropriate.

 

[sticksandtriangles]

It is also interesting to note that from the article on page four under option 2 states: "This option (option 2) works well for joists with concentrated or snow drift loads with known magnitudes and locations."

For concentrated loads, this seems to make sense, but I guess I do not understand how I would specify snow drift loads to the joist manufacturer without a specific loading diagram. Does simply showing a roof snow loading diagram like the one below give the joist manufacturer enough information work from?

 

[BSVBD]

For what it's worth, please see attached (partial) Roof Framing Plan...

Is it your intention to create your own, more conservative load cases for this project?

Yes… That WAS my original intention. However, through the effectiveness of Eng-Tips advice, I am leaning toward being “at peace” with the already established ASCE combinations. Admittedly, I still struggle with “What if?” BUT… I do NOT lose sleep!

Once20036…
- I itemize my loads when I make diagrams, so that, in the future, I can more easily account for how I derived the total(s).
- (3) top chord DL’s, from the top, are: 1. Penthouse Floor. 2. Penthouse Roof. 3. Warehouse Roof.
- (3) uniform SL’s are from the same. They are Pf. Wisconsin – (Still) ASCE 7-05.
- Partial and trap SL are drift from north side of 21’-3” long Penthouse and east side respectively.
- All loads are concurrent.
- I am specifying that standard loads apply.
- The LL applies to the Penthouse Floor which is intended to be used for storage.
- I will be specifying ASD (or LRFD) on the documents, in the future; something I have yet to practice.

Teguci…
- Bottom chord DL applies to all of the joists.
- (In another part of the building I have bottom chord LL for a walk-on ceiling.)
- The snow loading on the 22’ is from the Penthouse Roof.
- This “32LHSP-63” Joist supports the long wall of the Penthouse.
- I didn’t break down the Point Loads since, for whatever reason, SJI typically does not ask for a breakdown of point loads when designing girders. I realize we are not discussing girders here, but, that was my apparent logic. Perhaps a change is due in the future.
- I agree… assume the DL, LL and SL are concurrent and then apply the load combos.

sticksandtriangles… Although it DOES consume more time, I like the load diagrams for the historical inevitable necessity to clarify and reanalyze, if necessary, in the future… especially for complicated situations.

Teguci... Why a "non-concurrent" 250 lb point live load? I apply a minimum 3 PSF BCDL to account for such piping. How do you (or they) derive a 250 LB point LL?

Thank you all for your input!

 

[Teguci]

NFPA 13 identifies the 250 lb point live load as non-concurrent. You could call it an accidental load. It represents a person accidentally putting their weight (up to 250 lbs) on a fire protection pipe. If I was specifying an OWSJ for a roof over a sprinklered space, I'd be tempted to add a detail calling for a bottom chord section capable of supporting a 300 lb point load within 12" of the panel point and then just list all of the dead load without worrying whether its top or bottom chord.

 

[BSVBD]

Teguci... (2) OWSJ vending competitors have both advised me, when considering "relatively" small point loads, such as 0.25K, it is far more efficient to specify the following:

"JOIST MANUFACTURER TO DESIGN TOP (OR BOTTOM) CHORD SO THAT POINT LOADS SHOWN (either on framing plan or load diagram) CAN BE PLACED ANYWHERE THROUGHOUT LENGTH OF JOIST."

than to install additional bottom to top chord reinforcing afterward in the field. (see attached...)

This allows HVAC installers, and others, much flexibility in the placement of RTU's and other items.