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0.6 Dead load factor in ASD
12

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

RE: 0.6 Dead load factor in ASD

I think the only issue with that is that you would essentially be designing for the actual design wind event and would have no reserve capacity for a larger wind event.

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

If you are not going to use 0.6, why bother with 0.9?

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

When designing a building it is common for engineers to use a coservative estimate of the dead load when designing for gravity loads.  If those dead loads were used to check building overturning the result would be unconservative.

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 0.6 is to give a factor aof safety of 1.5 against uplift.

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

(OP)
Thank you all for your responses.

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

You have a factor of safety built in to bearing pressure, you dont have one for dead weight.

RE: 0.6 Dead load factor in ASD

Urielcdc - you don't give any information on where you are practicing.  Codes in Georgia are given the full force of law since they are referenced by an ordinance that adopts them either fully or as amended by the ordinance.  The best person to answer this question is the reviewer for the authority having jurisdiction.  Right or wrong, you'll have to meet their requirements.

RE: 0.6 Dead load factor in ASD

2

Quote:

I assume 0.6 factor is to consider that may be a case where not all the dead load is present

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

Use your own engineering judgement, as long as state laws don't prohibit it, but don't expect everyone else to agree.

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

3
I am curious; how much difference does using .6 instead of .9 make in the final solution?  If it means, for example, using a 5/16" anchor instead of 1/4", or changing the spacing from 12" to 8", then why fight it.

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

The ASD 0.6 dead load factor is also used in combination with seismic forces. For a stone attachment, this may be more severe than the wind combination.

RE: 0.6 Dead load factor in ASD

I believe that the 0.6 factor is overkill in this instance.  There is no rational to be factoring loads in ASD.  An approproate safety factor is provided by the allowable stresses.  This is a veiled attempt by the code committee to promote load factor design by eliminating an advantage of using ASD.

RE: 0.6 Dead load factor in ASD

I would stick with the 0.6.  But can you please tell me how the dead load and wind counteract each other?  Can the wind not blow in either direction?

RE: 0.6 Dead load factor in ASD

(OP)
StructuralEIT
This would be my piece:

http://files.engineering.com/getfile.aspx?folder=ee818e96-61f5-4f74-a352-f1aa6acf08a9&file=anchor.dxf

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

urielcdc,

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

2
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.  You are designing a support for a cut piece of stone that you should be able to determine the unit weight very accurately from the manufacturer. If you are truly concerned, ask the manufacturer what the range of unit weights are. Use the highest unit weight. If it makes you feel comfortable, put a small safety factor of 0.9.  I wouldn't do that because the variability in dead load and loading in general is taken into account in the allowable stress. The allowable stress used for steel design takes into account material varaiability AND loading variability.  You are penalizing yourself twice.

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

The facade should only experience the negative wind pressure, the positive pressure can be transmitted to the structure behind the facade.  If good connections are made are small intervals, then even the negative pressure can be transmitted to the structure.

RE: 0.6 Dead load factor in ASD

Can anyone who is suggesting that the 0.6 is not required please cite any section of IBC, ASCE-7, or any other current code that states such.  We all use engineering judgment every day, but I do not believe ignoring the building codes is appropriate.

RE: 0.6 Dead load factor in ASD

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.

RE: 0.6 Dead load factor in ASD

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

Quote:

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.

RE: 0.6 Dead load factor in ASD

(OP)
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.

RE: 0.6 Dead load factor in ASD

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?

RE: 0.6 Dead load factor in ASD

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.

RE: 0.6 Dead load factor in ASD

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?

RE: 0.6 Dead load factor in ASD

(OP)
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?

RE: 0.6 Dead load factor in ASD

(OP)
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.

RE: 0.6 Dead load factor in ASD

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.

RE: 0.6 Dead load factor in ASD

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.

RE: 0.6 Dead load factor in ASD

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.

RE: 0.6 Dead load factor in ASD

(OP)
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.

RE: 0.6 Dead load factor in ASD

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.

RE: 0.6 Dead load factor in ASD

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.

RE: 0.6 Dead load factor in ASD

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

RE: 0.6 Dead load factor in ASD

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!

RE: 0.6 Dead load factor in ASD

(OP)
JKW05
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

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.

RE: 0.6 Dead load factor in ASD

csd72, please explain who is being arrogant and why.

RE: 0.6 Dead load factor in ASD

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".

RE: 0.6 Dead load factor in ASD

Can you justify using the alternate ASD load combinations?  They do not appear to include the 0.6 factor.

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

BINGO! PMR06.  I think this is the ultimate answer to this dilema.  There is no 0.6D + W combo in the alternative ASD combinations.

RE: 0.6 Dead load factor in ASD

ASCE 7-02 load combinations for ASD:

5. D + W
7. 0.6D + W

Use both!

RE: 0.6 Dead load factor in ASD

Using Section 1605.3.2 – Alternative basic load combinations of the 2006 IBC gains you very little.  Included in the paragraph above the alternative equations is the sentence:  "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."  You are almost in the same place as 0.6D + W.  However, if you are in a jurisdiction that still uses the 2000 IBC you are in luck as that sentence does not appear in that version of the code.  I don't know about the IBC 2003.

RE: 0.6 Dead load factor in ASD

vincentpa,

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

SSStrPREng, I was looking at IBC 2003, which does not have that 2/3 dead load verbiage.

RE: 0.6 Dead load factor in ASD

JAE & PMR06,
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

Whether you're applying 0.6 to the dead loads, or ω=1.3 to the wind loads, either way, you are factoring your loads.  Factored loads are a feature of Load Factor Design or Load Factor Resistance Design.  It's really amazing that the code committee managed to slip these Load Factors into allowable stress design.  Look at the mess it has caused.  

RE: 0.6 Dead load factor in ASD

The IBC 2003 and 2006 do not agree with ASCE 7. ASCE 7 states that you can take an increase in allowable stress if the material permits it for certain load cases.  The commentary of ASCE 7 explains that you cannot use allowable stress increases with combined loadings that decrease the loading such as D + 0.75S + 0.75W.  But you can use allowable stress increases for loadings like D+W and 0.6D+W.  Therefore, 0.6D+W would really be a wash because the stress would be allowed to increase for wind loading. ASCE 7 does not contain "alternate load combos". I don't know where and why IBC gets them and uses them. The IBC does not have a commentary. I really wish the IBC had a separate commentary.  It would help to explain a lot of things.

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

urielcdc: Sorry for the delay in response, I have been out of the office.  Unless you are working in a location that has adopted a code that refers to the older ASCE requirements, it doesn't really matter what ASCE 7-95 says, other than the historical evolution of the code requirements.  I don't have ASCE 7-95 with me here, but will try to locate it tonight.

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 don't see where there is anything to discuss. The code says that one of the load cases to consider is 0.6D+W. In my opinion this doesn't make sense in many situations but it is the governing code. How can you possibly justify ignoring the code requirements in the situations where you don't like the results but follow the code when you like the results.

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

In response to vincentpa's question regarding "I don't know where and why IBC gets them and uses them" in reference to the "alternate" load combinations, these "alternate" load combinations of the IBC are holdovers from the UBC (same for the "alternate" live load reduction method) and are esentially there as a holdover for designers used to working with them.  

RE: 0.6 Dead load factor in ASD

Urielcdc:  I did find my old ASCE standards here.

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

I don't have the IBC commentary and when you look at the ASCE 7-05 Commentary it suggests the opposite (I cannot believe that I have to pay extra for the commentary to IBC.  It's like pay bathrooms that you have to pay $1 to go in and $2 to get out).  You can use the 0.6D+W and take an allowable stress increase in ASCE 7-05.  The allowable stress increase, in effect, negates the 0.6 LOAD FACTOR for designing members for strength, which to me makes more sense.  

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

IBC is the code that PA has adopted by way of the UCC. It refers to the ASCE 7 standard where the code committees felt it was appropriate and/or redundant. ASCE 7 is not a code in itself. Section 1605 of the International Building code specifically details the load combinations to be used with no provision for allowing the load combinations or commentary from the ASCE 7standard.  Keep in mind that the IBC Commentary is also not a part of the code.  But the commentary does explain the code's intent that the load combinations do apply to all elements.

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

The stress increase only applies to wood for the typical materials we use, because wood shows a strength increase that is related to load duration.  This is not the case for steel or concrete when we're considering static or equivalent static loads.  Where is the stress increase shown in the ASCE 7-05?

RE: 0.6 Dead load factor in ASD

After the load combinations and in the commentary. Yes steel does have a "strength increase" with the rate of loading. That is the reason that there was a 1.33 increase in allowable stress for wind and seismic.

RE: 0.6 Dead load factor in ASD

Would you mind pointing that out please?  I'm not reading anything in the commentary that suggests that the stress increase is based on material behavior due to load duration instead of being based on the low probability that more than one variable load will be at its maximum  value at the same time.

RE: 0.6 Dead load factor in ASD

The 1/3 stress increase for steel for wind and seismic has a long and "mysterious" past, with several dissenting opinions of what is was really intended to be for.  The most popular of which is for the decreased probability of multiple transient loads occuring at the same time as UcfSE said.  

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

UcfSE,
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

if 0.6D-W controls your design and you want to use D-W in that combination, couldn't you multiply D X 1.667 and use that in all you other combinations. if the 0.6D-W still controlled with you amplified dead loads, you'll be good to go, still within the code requirements, and be able to sleep soundly tonight.
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

(OP)
Grizzman
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

2006 IBC Section 1605.3.1.1 allows a stress increase only for wood!  There is no ambiguity in the verbage of that section either.  And there is nothing in the building code that lets ASCE 7 trump the code requirements.

RE: 0.6 Dead load factor in ASD

vincentpa,

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

ASCE 7 comentary clearly (sort of) explains the .6D+W. It is based on counteracting wind load. If you have lateral load, use it, if not, it is not asppropriate and should not be checked.  It is like any other load combination. If it is germaine you check it, if not you don't. If all we did was blindly follow codes with out understanding what we are doing, why would we need to be PE's?

RE: 0.6 Dead load factor in ASD

It's been shown that steel yield and tensile strength, and modulus of elasticity I believe, are affected by strain rate.  Salmon and Johnson have a good blurb - but only a blurb - about it also.  

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

If we don't follow the building codes, or we can pick and choose which sections we want to use, then why even have them?  

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

Regarding the building codes, one thing to keep in mind is that they are minimum standards.  I hear a lot of talk about making a judgement to design something for less load then required by the code.  However, I only remember a few rare occasions where people were talking about exceeding the code requirements.  

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.

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