0.6 Dead load factor in ASD 13

[urielcdc]

I have been designing anchors for natural stone facades using ASD. And I have conditions where my anchors are subjected to Wind load and dead load, but they generate a moment on opposite directions, then I used ASCE combination: 0.6D+W.

I assume 0.6 factor is to consider that may be a case where not all the dead load is present, but I am thinking that this might be based for many critical structural designs. And I don’t think this is the case for me, since all my natural stone is cut to a 1/16in precision, plus the density of it does not vary more than 2%.

I know is the code, and like a law, it should be followed, but as the laws, the codes factors may not be the right ones for some of the cases.

Do I have an argument trying to use 0.9 factor, instead of 0.6 for the dead load?

Thanks in advance

 

[civilperson]

Applying the building codes to cases not envisioned by the code author is overreaching. Engineering judgement is REQUIRED to be used for cases not fitting the code applications.

 

[JAE]

JKW05 - you beat me to it. vincentpa - I would ask you to cite a code provision where your comment...

The 0.6 is obviously used for overturning and stability analyses. In my opinion, it does not apply to design of components and cladding when you can determine the dead load very accurately

...can be supported. The code requires all load combinations be utilized for all elements, whether main members or components.

The reason for the 0.6 factor is that the codes previously had required a 1.5 factor on sliding or overturning. Under that situation a stone facade piece doesn't have "overturning" so techinically it wouldn't have applied.

The 1.5 factor, the way I understand it, was moved to the load combinations (formalized) since the 1.5 factor was many times missed, or was inconsistently utilized in the body of the code.

The 1.5 factor was more an empircal safety factor that, yes...did take overestimation of dead load into account, but also was used against an overstress condition where an occasional extreme lateral event would occur.

With the stone facade, I'd say pragmatically that the 0.6 factor seems a bit out of place.

Technically it is required by the code.

 

[urielcdc]

Don't die on me JKW05, I am not designing ignoring the codes, but I will surely apply my judgment where the law allows me to.

 

[JKW05]

civilperson,

I again, respectfully, ask that any code section be cited that either says the 0.6 is only required in certain caes, or where the 0.6 is not required in other cases. Where does it say that the load combinations only apply to frame analysis, or column design.....??? What is the basis for the concept that the building code did not "envision" the design of components and cladding?

 

[nutte]

I'm with JKW05 here. I don't even see why the debate has continued on for so long. The building code requires you to check this load combination. End of story.

 

[civilperson]

How does the vertical 0.6 DL apply to a stone facade bearing on a support in combination with a horizontal wind load? The horizontal load will be resisted by the frame and fasteners behind the stone and the vertical load will be resisted by the bearing presures beneath the stone. How can they be combined to the detriment of the structure?

 

[urielcdc]

ASCE 7-95 mention it in 2.4.4
In ASCE 7-98 was incorporated into the combination.
But you are right, they don’t say which cases should it be applied.

Are you telling me that you really believe that my case is overturning and I should applied it in both cases, 98 because it is in the load combinations and in 95 because it would be considered overturning?

 

[urielcdc]

Civilperson
What you are saying is a stone stacked condition. But there are situations where an individual stone anchorage is required, and then both dead load and wind load will be acting on the anchor clip.

 

[civilperson]

Again, in the case where both dead load and wind wind are acting on the anchor clip, the correct load case for design is No. 5 of 2.4.1 of ASCE 7, (D + H + F + (W or 0.7E)). The 0.6D in No. 7 or No. 8 will never be controlling, thus it is useless to apply non critical load cases to the analysis. This makes my point of blind application of code load combinations that DO NOT APPLY.

 

[JKW05]

urielcdc,

I'm still alive

If your question is being addressed to me, my response is that what I "believe" is irrelevant. As nutte says above, "The building code requires you to check this combination. End of story" I absolutely do not agree with everything that is in the code. I work primarily in an area where I cannot find any documented cases of injuries or even building damage due to an earthquake in the past 200 years; but we are stuck with the seismic provisions, and I follow the code for this.

Can you answer my hypothetical lawyer's question above? Would the response be that "I didn't believe the code applied in this case" or "I felt the code was too conservative"? Good luck.

If you truly "believe" that this is wrong, send your recommendation and support data to IBC or ASCE. If they concur, it can be introduced into a future code edition.

 

[JKW05]

civilperson,

All of the load combinations apply, even if the don't control the design. If the DL is 0, then I agree both of the combinations would have the same DL component, and it would be redundant to check (ignoring the H, F, and E for this case). But both combinations are still applicable code combinations. It appears that in uriel's case that the DL does have some impact on his design, and both combinations need to be checked to find the one that controls. If the answer is obvious, I would not suggest cruching through a lot of anaylsis. But it still needs to be considered.

 

[urielcdc]

civilperson
W+0.6D is controlling over W+D, because the windload generates a moment on opposite direction than the moment generated by the deadload

Therefore those moments affect the combinations and it would look like
W+0.6(-D)
That will be more critical than W+D (if the moment generated by WL is greather)
If the moment generated by the Deadload is greather, then D combination alone will be the critical one. But never W+D for my case.

 

[vincentpa]

Thanks JAE. I understood the 0.6 had to do with 1.5 for overturning and overestimation of the dead load. I didn't know it had to anything to do with overstress. I'll take your word for it because it sounds reasonable.

As far as using the 0.6 for this case, I would still not use it. You have to be an engineer and use engineering judgement. This is an absurd combination to apply to designing components. It makes no sense. You can do it but I won't. I don't believe that is the intention of the code writers. I cannot believe that the code writers could be that far out of their minds. Someone write to the committee. I can't. I have two project due next week.

I believe this is just another example of changing codes drastically and not thinking through the consequences.

 

[vincentpa]

JKW05
if your cladding falls off, it will not be because you used 0.6W+D. Your design would have been faulty and there would be 1000 expert witnesses that will attest to that. In other words, 0.6D would be the least of your worries. That is if it ever made it to trial. The question you pose would probably only be asked in an arbitration where your company would be sued in a mass lawsuit usually by lawyers trying to screw everyone they possibly can. Your company would pay to avoid a trial even if it were a product malfunction and not your fault at all. We had to pay (not in our structural department) for a claim which was in no part our fault just to avoid a trial that would have cost three times as much to prove we were not at fault.

You go ahead and use 0.6D+W for designing cladding. Knock yourself out. My cladding won't fail because I use D+W and I will sleep soundly tonight.

 

[JKW05]

vincent,

Your response suggests that the .6D+W will never control in any case for any cladding....ever! I find that to be over-encompassing, especially since it does appear to have a potential impact in the case in the OP. And you also suggest that you already know there is some other fault in "my" design. Please tell me what it is so I can correct it! Sure...in the event of a significant failure there will be a 1000 expert witnesses being paid by both sides. I would rather never get to that point.

Have a great weekend everyone. this has been an interesting thread! I'm just disappointed that there are PE's that feel that ignoring the building codes is "good" judgment.

Respectfully,

JKW

 

[csd72]

Never be arrogant enough to believe that you as an individual knows more than all the commitee members that have worked on this code.

There are reasons we have safety factors.

No point in:

Marking with a micrometer what will be
marked with a crayon and
Cut with a chainsaw!

 

[urielcdc]

JKW05
You never responded to my question completely, will you use the 1.5 factor in ASCE 7-95?

 

[vincentpa]

JKW05,
I was not specifically saying "YOUR" design or anyone elses designs are faulty. Since you posed the hypothetical question, I hypothetically used you as an example. The point I am trying to make is quite simple. The correct answer is either of the answers being debated. The cladding will stay on the building if you use 0.6D or D. Sometimes engineering judgement is required because we are adapting a general code to specific problems, many of which were not meant to be applied to that problem, to come up with a sane and rational design. If you believe that following a code blindly in such an obvious case where the code writers were not clear and specific, you are not wrong. I believe, in this case, it is wise to not follow the code blindly. I am exercizing my rational and in this case sane judgement.

The thing that separates engineers from accountants is that we do and must use our judgement to solve problems and design a cost effective safe structure.

JKW, I was in no way commenting on your engineering ability or designs. I meant no offense. However, if a cladding design fails because 0.6D was used instead of D, there was an error in detailing, construction, manufacturing, or negligence. It won't be from using D instead of 0.6D.

 

[vincentpa]

csd72, please explain who is being arrogant and why.

 

[vincentpa]

I have to add one more post on this subject.

JAE and WillisV have explained the rationale for 0.6D+W. It was a wise move by the code writers but as they are wont to do, they did not explain themselves or their intent. Rarely is there a case to ignore the code. This is one of those rare occasions. It is appropriate, I believe because there is no sane or rational reason to take 0.6D for anything other than overturning and stability.

Now another point:

We are talking about ALLOWABLE STRESS DESIGN!

Many of the engineers posting have master's degrees. I know this from previous threads. We have all or at least most of us have studied ASD and how and why the codes arrive at the allowable stresses used. The safety factors have already been included as an ALLOWABLE stress. There is no need to play with loads to decrease an allowable stress. AISC would have included an allowable stress decrease in the code. Factoring the the dead load in ASD creates a senario that is physically impossible. In no circumstance or situation will something ever weigh 0.6D on this planet. Maybe Mars or Venus but not Earth! The safety factor is in the allowable stress!

We are engineers. We are not scientists or accountants. We apply science to create. We have to use our judgement. That is why we are one of the only professions that has a "standard of care", which does not only apply to "the code".