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

 

[Grizzman]

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.

 

[urielcdc]

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.

 

[JKW05]

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.

 

[JAE]

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.

 

[DRC1]

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?

 

[UcfSE]

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.

 

[JKW05]

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.

 

[RARWOOD]

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.