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

 

[PMR06]

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?

 

[JAE]

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

 

[SteveGregory]

ASCE 7-02 load combinations for ASD:

5. D + W
7. 0.6D + W

Use both!

 

[SSStrPREng]

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.

 

[csd72]

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.

 

[PMR06]

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

 

[vincentpa]

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.

 

[miecz]

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.

 

[vincentpa]

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.

 

[JKW05]

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

 

[ron9876]

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.

 

[WillisV]

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.

 

[JKW05]

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.)

 

[vincentpa]

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.

 

[JKW05]

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.

 

[UcfSE]

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?

 

[vincentpa]

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.

 

[UcfSE]

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.

 

[WillisV]

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

And this link:

http://www.aisc.org/stressincrease

 

[vincentpa]

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.