0.6 Dead load factor in ASD
0.6 Dead load factor in ASD
(OP)
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
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






RE: 0.6 Dead load factor in ASD
On another note, the codes aren't "laws" per se. They are meant as a guide. Experience can teach you when, where and why exceptions can be made.
Lets assume you are designing something. All your assumptions during the design process tend to be a bit conservative. If when you check the capacity of your member you are at say 98% capacity, I think no reasonable engineer could fault you for saying it was ok.
RE: 0.6 Dead load factor in ASD
I think in this case the code should be followed. Besides, if it ever fell off, you could be blamed because you didn't follow the code, whether or not that in itself was the cause of the problem.
RE: 0.6 Dead load factor in ASD
In the past this was addressed sometimes by designing for a resisting moment equal to 1.5 times the overturning moment. This accounted for the over estimation of dead load and provided for an additional factor of safety. Using 60 percent of the dead load would produce similair results.
I don't know if this is the justification given for the 0.6 load combination in ASCE 7 or not. However there is an ASCE forum on this web site that you could also post your question if you have not already done that.
RE: 0.6 Dead load factor in ASD
The 0.9 factor should never be used with ASD, it a a LRFD factor.
You must remember that all design methods and weights are approximate (particularly for stone) and there needs to be a factor to allow for this.
RE: 0.6 Dead load factor in ASD
Although I am using natural stone I have pretty much control over it (dimensions and weights), then I don’t see why I should be using so much conservative design with 0.6 factor.
Let’s take for example, in an event where there is very low wind load, or none at all, which would happen most of the time, there will be no overturning, and the dead load will be the only one that will be supported by my anchors, then I will use only the next combination:
100%D
Then why should I be conservative in the event of a wind load, and not with the dead load acting alone?
If I were talking about LRFD then my dead load acting alone would have a factor also, but not for this case ASD.
Should that means that the 0.6 factor is most because I have no control over the wind load, more than that I have no control over dead load? If so, then why when both loads are applied and creating a moment in the same direction, why the combination for this case it is only:
D+W (notice than there is no factor for the wind load)
What that tells me is that the 0.6 factor is due to the unknowing of the dead load accurately, and returning to my point, what if I have total control on my dead load design? Can I substitute the 0.6 for 0.9?
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
Your situation falls into the situations discussed in the last paragraph of this post.
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
Well...sort of. But the 0.6 factor is primarily a safety factor to account for potential extreme variations both in dead load and on the actual wind load applied. Despite the fact that the factor is placed on the dead load, it is there to provide essentially a 1.5 factor against overturning, extreme unbalanced winds, etc.
You are right to think you have complete control over the dead loads - but I also feel I have very good understandings of structural floor dead loads.
Its all about the level of safety, not the uncertainty over the variation of dead loads.
RE: 0.6 Dead load factor in ASD
I can see some logic in your approach but definitely would not use 0.9. Perhaps I would think about using closer to 0.75 but I would probably just use 0.6.
RE: 0.6 Dead load factor in ASD
If the design "appears" to be excessive, I would try to legitimately fine-tune the wind load before altering the "generally accepted", code-specified, load combinations.
Who wants a 500 pound piece of stone falling on a by-passer?
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
This would be my piece:
http:
The dead load and ex is always generating a moment on the clip
When the negative pressure wind load is acting, it generates (with ey) a moment in the opposite direction that the moment that is generated by the dead load.
When we have positive pressure, the wind load does not generate any moment, because the load would be transmitted by the clip directly to the backup (CMU, concrete, etc)
SteveGregory
I consider seismic loads also, and it would be the same example as this one, just changing seismic load instead of wind load, nevertheless, for most of the project that I have worked on, wind load is more critical than seismic loads.
JKW05
For the exposed conditions to weather, we have to use typically S.S. and that is getting more expensive over time. Plus it gets really more expensive when changing the longitude of the clips or the thickness, especially when we have huge projects were we need thousands of clips.
You are telling me to fine tune the wind load, but I have to design for the worst condition that could happen in the building, it is very difficult to control installation process if we have different size of clips, and the installers guys could put a small clip on a required big clip zone. And that is what could really worry me about a stone falling on a by-passer.
Thank you all for your responses, this has been very constructive to be my first post.
Please let me know if anyone else of you think that my case is not a sliding or overturning condition as already explained by WillisV
RE: 0.6 Dead load factor in ASD
I don't know how you have figured your wind loads. But if you are designing all of the facade for the end zone pressure for conservatism or simplicity, I am simply suggesting that a more efficient design would be to design just the panels in the end zones with the higher pressures. The exposure class and variables for building height could also be reviewed. Maybe you've already done that. But that is what I meant by "fine-tuning".
Of course there is always a trade-off of design time vs. the cost of the final design that is being constructed. Not to mention keeping details consistent for ease of construction. But if the argument is that material cost is forcing a more "precise" design, and material cost is more important than the design time and complexity, I would elect to use every reduction permitted by the code, but I would stick with the code combinations, rather than making up my own.
Unfortunately, we are in a very litigious society (at least here in the U.S.).
Lawyer: Why didn't you use the 0.6D, which would have prevented by clients husband from being killed?
Engineer: (please fill in the blank).
Maybe I'm paranoid, but if there isn't a good response to the question above, I will stick with the code.
RE: 0.6 Dead load factor in ASD
The 0.6 is a safety factor on the overestimation of dead load for individual member design. The overestimation of dead load for individual member design makes overturning analysis of buildings (where the dead load has a more chance to be estimated inaccurately) unconservative. I don't agree with JAE in that the effect of wind variation is included in the 0.6 factor. JAE, do you have or know of a paper discussing the reason for the 0.6 factor? It is not discussed in the commentary. The effect of wind variation is also accounted for in the components and cladding wind loads, which you should be using not the MWFRS loads. I have run into this problem also. I cannot justify taking what amounts to a huge reduction/penalty for something that can be accurately calculated.
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
...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.
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
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?
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
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?
RE: 0.6 Dead load factor in ASD
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.
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
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.
RE: 0.6 Dead load factor in ASD
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.
RE: 0.6 Dead load factor in ASD
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.
RE: 0.6 Dead load factor in ASD
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.
RE: 0.6 Dead load factor in ASD
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.
RE: 0.6 Dead load factor in ASD
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
RE: 0.6 Dead load factor in ASD
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!
RE: 0.6 Dead load factor in ASD
You never responded to my question completely, will you use the 1.5 factor in ASCE 7-95?
RE: 0.6 Dead load factor in ASD
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.
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
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".
RE: 0.6 Dead load factor in ASD
The w coefficient is confusing though, it appears to be 1.3 if simplified method is used (IBC 1609.6), but what is "Section 6" of ASCE 7? Do they mean chapter 6?
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
5. D + W
7. 0.6D + W
Use both!
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
Arrogant was probably too strong a word, but many of the people on these code board are there because they are experts in that field. If you go against the code, you better be real certain that you are aware of all the reasons it has been written that way.
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
In IBC 2003, the alternate load combos specify to multiply the wind load by a factor labeled Omega equal to 1.3. It also states that an allowable stress increase is permitted. This section applies to if IBC section 1609.6 or "Section 6" ASCE 7 is used to calculate wind loads. It appears that "Section 6" of ASCE 7 was a typo and "Chapter 6 should've been used because IBC 2006 changed it to Chapter 6 ASCE 7. I think that IBC 1609.6 was a misprint because IBC 2006 doesn't mention it. Anyway, in this instance, if using a load factor of 1.3 and an allowable stress increase of 1.33, it would be a wash or better.
However in IBC 2006, section 1605.3.2 states "For load combinations that include the counteracting effects of dead and wind loads, only two-thirds of the minimum dead load likely to be in place during a design wind event shall be used." The only thing about this is that the factor 2/3 is not included in the load combinations.
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
Now, the IBC has adopted ASCE 7 so where does that leave us? I really hate it that the IBC repeats information from ASCE 7. IBC 2003 is a mess with this. IBC 2006 is much better but still needs work. Thankfully, PA has adopted IBC 2006 and ASCE 7-05. They are much better than 2003 and 7-02.
RE: 0.6 Dead load factor in ASD
Vincentpa: I agree IBC and ASCE has really made things difficult by not cooperating with each other to provide a single standard. I also agree that 2006 got better, and my understanding is that the trend of repeating info from the ASCE standard is being phased out future editions of the IBC.
I find the difference in wording between IBC and ASCE about stress increase interesting: IBC limits it specifically to wood design (1605.3.1.1), while ASCE is quite vague about it, leaving it "to be justified" (2.4.1). ASCE 7 also does not present the option for the "alternative basic load combinations"
There has been some suggestion that the building code is not a law (re: the 1st response to the OP), but "just a guide". A totally disagree with this. The use of the International Building Code was adopted as a law, at least in PA, by the Uniform Construction Code Statute and its amended acts. I will leave it to the legal minds as to the difference between the code being a law, and the requirement that specific codes be used is the law. But either way, it appears to me that not following the code is inconsistent with the state law. I would imagine other states or municipilaties have adopted the use of the code in some similar legal manner.
I also agree that, as engineers, we have to use our judgment. But it is also our responsibility to conform to the applicable building code(s). I cite the NSPE Code of Ethics for Engineers: Article II, paragraph 1b: "Engineers shall approve only those engineering documents that are in conformity with applicable standards." ASCE 7 is an applicable standard per IBC. I realize this is not "a law", but I believe it is worth considering as responsible professional.
Finally, the IBC actually does have a commentary that can be purchased separately. I quote:
"Previous editions of the model codes specified that the overturning moment and sliding due to wind load could not exceed two-thirds of the dead load stabilizing moment; however it was not typically applied to all elements in the building. In the code this limitation on dead load is accomplished through the load combinations. The applicable combination is 0.6D+W+H. This load combination limits the dead load resisting wind loads to 60 percent but it apples to all elements." (Re: 2006 IBC Code and Commentary, Volume II, page 16-18) Based on the commentary, I can't see how one could argue that the intent to use it for the design of cladding was "forgotten".
I realize many different municipalities have adopted different codes, and my reflections are based on the 2006 IBC and ASCE 7-05, which are the current codes in PA. I guess the point I have been trying to make here is that yes, we have to use our technical knowledge and judgment in our designs, but we also need to comply with the building code. Period. Without a doubt, the way the codes are written it is often difficult to interpret them. But I see no ambiguity in the load combinations required by IBC, no matter how much one disagrees with it. I will step off my soap box for one final time.
Regards,
JKW
RE: 0.6 Dead load factor in ASD
I live in south florida and this comes into play regularly. Interior footings on one story steel framed builds, uplift on joists, overturning on shearwalls... It is my understanding that the intent is to provide a safety factor of 1.5 (1.67 actually) against dead load resistance. While this makes sense to me for something like a gravity retaining wall it makes no sense at all when you use piles to resist uplift because the pile capacity already has a signficant safety factor.
It seems to me that the older that I get the more ridiculous that the codes become. But in this case, in my opinion, you had better follow the code.
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
With respect to ASCE 7-95. If the piece of stone façade was simply bearing on the foundation (or a shelf angle), with no anchors, the way 2.4.4 of ASCE is written, I would say yes, the 1.5 safety factor (2/3 DL) would apply. This would obviously require a very thick, impractical panel. But 2.4.4 also says "…unless the building or structure is anchored…" From your sketch, I'm assuming that the anchor may be located near the top of a panel (?) to resist the potential pull-out from a negative pressure. I would say in this case, the standard would allow you to just use D+W. As engineer-of-record, one could use a DL reduction, .9, .75, .6, or whatever (s)he may feel is appropriate to provide what (s)he judges to be an appropriate safety factor, as long as the D+L also worked.
ASCE 7-98 simply says the load combination shall be applied to produce the "most unfavorable" effect in the building, foundation, or structural member, and the 0.6D + W combination is introduced. In this case, I see no ambiguity; the 0.6D+W is a combination that must be considered to comply with the standard, as it is with the current IBC.
There have been several comments in this thread about "blindly" following the code. Complying with a building code is not the same as "blindly following" it. Your OP was a good question. But my post just above provides support documentation that cladding design was not "obviously" overlooked by the code committee(s). I agree with csd72's remark that we, individually, "should not believe we know more than all the committee members." Codes evolve over time, largely due to previous failures. I cannot say exactly why the committee chose to start specifying the 0.6DL combination. My guess is that somewhere during review of failures from an earthquake or hurricane, they determined that that combination may have prevented failures. The suggestion that the code writer's "obviously" did not intend that the 0.6D apply to your case is obviously not that obvious. Otherwise this thread would have died long ago, and the IBC Commentary would not clearly state "it applies to all elements."
(Sorry, I'll get off my soap box again.)
RE: 0.6 Dead load factor in ASD
As for not following the code; this is the only instance (facade anchorage) that I have chosen to "interpret" the code in another way in my career. It appears that IBC and ASCE conflict in this instance. So I choose to use the code that makes more sense to me. I am in PA so the UCC adopted the IBC 2006 which adopted the ASCE 7-05. There are too many hands in the cookie jar and this is what you get, conflicting codes and threads with 50+ responses. There are many instances where the code is crazy but I still follow it. One is a ringwall foundation for a tank controlled by seismic in Pittsburgh!!!! Nothing will be perfect but as the owner of this thread put it, SS anchorage is expensive and can have a big impact on the bottom line. Little things like this need to be explained in greater detail if they are not going to spark debate and controversy.
RE: 0.6 Dead load factor in ASD
In addition, Section 102 of the IBC says "Where, in any specific case, different sections of this code specify different materials, methods of construction or other requirements, the most restrictive shall govern. So even if you use the ASCE Commentary as rationale for some other load combination, you must still use the most restrictive to comply with the code.
Finally, I would propose that the building codes exist exactly to deter the use of the almight $ as an excuse to skimp on the design. Is it appropriate for my competitor across the street to decide for himself that the seismic provisions are "just rediculous" and permit him to omit reinforcing from his masonry to save the owner $$$$$???? If your answer is yes, then you should go to the legislature and convince them repeal the adoption of the building code.
As a professional, you don't get to pick and choose which codes, or which sections of a code you wish to follow.
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
See this thread:
thread176-168330: 1/3 Stress Increase History
And this link:
www.aisc.org/stressincrease
RE: 0.6 Dead load factor in ASD
if you can read that in many text books, including Steel Structures by Gaylord and Gaylord, which is one that I have handy. With an increased rate of loading, steel has a higher yield point. Loading like wind and seismic fall into and "increased rate of loading" category. Therefore, material behavior for steel, indicates that you would be able to use an increase in allowable stresses for load combinations D+W and 0.6D+W.
RE: 0.6 Dead load factor in ASD
Yeah, I know that's circumventing the intent of the code, but that's what the OP was looking for in the first place.
RE: 0.6 Dead load factor in ASD
If I do that, the combitation of D (acting alone) will be the one to control. And surely would end up over designing the anchorage.
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
I think the Northridge Earthquake showed many that an increased load rate (i.e. the WHAM of energy impacted into moment resisting frames) actually resulted in brittle fractures much lower than anticipated in steel structures.
Now the yield might have been higher, but there was little if any inelastic response and I wouldn't use a 1.33 factor in that condition.
RE: 0.6 Dead load factor in ASD
RE: 0.6 Dead load factor in ASD
I was asking about where in the code or commentary, or anywhere, is it mentioned that we used the stress increase due increased yield at high strain rate. Blast design I can agree that you could adjust your steel mechanical properties. How do you know your earthquake or wind loading are sudden enough to produce the effect required?
If I understand your post, the authors state that increase rate of load increases steel properties, but how do you know that wind and seismic loads increase the strain rate enough? I suspect they don't and that's why we don't allow the increase any longer. It's also worth noting that the AISC has removed the stress increase for steel with the 2001 supplement to the green book.
RE: 0.6 Dead load factor in ASD
Again, I am not suggesting anyone "blindly" follow the code. But out of 65 posts, no one has provided any section from any current code that says the 0.6D+W combination is not appropriate. The only arguments have, essentially, been that "I know better than the code committees", and the reference to the ASCE commentary, which is helpful in decribing the evolution of the combination. But even in ASCE 7, there is no section (or commentary, for that matter) that clearly precludes the use of this combination for the OP's case. The IBC is very clear, both in the body of the code and in the commentary, that it is to be used.
I know I have been beating a dead horse. And while my posts sound like I am a big fan of the IBC, let me say I do not like the direction the codes have gone. I think they have become far to over-reaching, complicated, poorly-written, and often conflicting with other standards. But, where adopted, they are the construction standard that professionals are obligated to design by. The code committee and, in PA, the state legislature by adopting the UCC, have decided to regulate our "engineering judgment" by including this particular combination. We can feel offended by it, and we can disagree with it, but until the legislature changes it, or the code committee revises it, it is the building code. If in our judgment the code is not sufficient to provide a safe design, we can improve the design, as long is it also complies with the code.
If you have a project in a municipality that has not adopted a building code, than I agree with most of what vincentPA (and others) have said. The rationale and engineering judgment that has been presented seems rational to me. But it does not trump complying a building code that has been legally adopted.
RE: 0.6 Dead load factor in ASD
For example some states establish snow load zones. I know at least one designer who uses the established snow load even in areas where there are lake effects. The argument was that the state requires X p.s.f. snow, so considering lake effect was not required.