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

 

[frv]

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.

 

[UcfSE]

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.

 

[RARWOOD]

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.

 

[csd72]

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.

 

[urielcdc]

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?

 

[csd72]

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

 

[WillisV]

See my first post in thread507-202887

Your situation falls into the situations discussed in the last paragraph of this post.

 

[SSStrPREng]

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.

 

[JAE]

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.

 

[apsix]

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.

 

[JKW05]

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?

 

[SteveGregory]

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.

 

[miecz]

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.

 

[Lion06]

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?

 

[urielcdc]

StructuralEIT
This would be my piece:

http://files.engineering.com/getfil...6-61f5-4f74-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

 

[JKW05]

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.

 

[vincentpa]

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.

 

[civilperson]

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.

 

[JKW05]

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.