IBC 2003: 0.6D vs. 150% factor of safety for stability

[seattlemike]

For wind, I used to design with a 150% factor of safety for the overturning of shear walls per code. And sometimes, to simplify calculations, I applied the 150% overturning stability factor to the 0.9D combination for earthquake as well.

Now that we are using 0.6D under the basic load combinations (IBC 2003), it seems that 150% factor is already included. Is that correct? (yes, I did read 1609.1.3 for the alternate load combinations)

Thanks!

 

[nrguades]

Hi..

I just thought that these informations might help some...

DEAD LOAD FACTORS FOR OVERTURNING FOR WIND AND SEISMIC FORCES:

UBC 1997
DL FACTOR (W) = 0.67 LL FACTOR (S) = 0.9

IBC 2000
DL FACTOR (W) = 0.6 LL FACTOR (S) = 0.6

IBC 2003
DL FACTOR (W) = 0.6 LL FACTOR (S) = 0.6

 

[nrguades]

correction to my thread above... LL shown above is wrong that should be DL..
sorry ....

 

[seattlemike]

Yes, the "0.67" of the 1997 is the reciprocal of the 150% I mentioned before. So it seems the 2/3 is already built in to the basic load combinations.

 

[nrguades]

seattlemike

I guess we have the same interpretation.

From 1609.1.3 (IBC 2003) the 2/3 factor for dead load are for alternate basic load combinations only. Personally, I believed that alternate load combinations are just a carry over from previous editions of building codes ( I might be wrong on this!) and the basic load combinations are considered to be the prepared load combinations...

nrguades

 

[UcfSE]

Some say that the 0.6 on the dead load is to account for the assumed dead load being higher than the actual dead load as-built and that the 1.5 FS should be applied after the dead load is multiplied by 0.6. What do you guys think?

 

[structuresguy]

Well, I don't know about IBC off hand, but in reading FBC requirements it seems to me the intent is 150% on top of the 0.6DL reduction. I can't find anywhere in the FBC code which states this clearly one way or the other, so I am being conservative in my interpretation. This requirement of 150% is clearly stated for South Floride high velocity hurricane zones. But since it is also stated as a basic load combination, I assume that they mean to add the 150% on top of the basic load combinations. Otherwise, they are being redundant and telling you to do the same thing in two different places. And we all know that code officials are never wrong.

 

[nrguades]

Gentleman... try to check this out...

This is taken from Design of Wood Structures - ASD by Donald E Breyer et al...fifth edition based on IBC 2003 pg 2.37

"The design overturing moment (0M) is the difference between the gross OM and 60 percent of the resisting moment (RM). The IBC requires that 0.6 X RM be greater than the OM. In other words, a factor of safety (FS) of 5/3 or 1.67 is required for overturning stability. Notice that in this stability check, an overestimation of dead load tends to be unconservative (normally an overestimation of loading is considered conservative). To obtain the design OM, 60 percent of the RM is subtracted from the gross OM. Up to this point, the DL being used in the calculation of RM did not include the weight of the foundation.
Now, if the design OM is a positive value (i.e., the gross OM is more than 60 percent of the RM), the structure will have to be tied to the foundation. The design OM can be replaced by a couple (T and C). The tension force T must be developed by the connection to the foundation. This tension force is also known as the design uplift force. If the design OM is negative (i.e. , the gross OM is less than or equal to 0.6 x RM), there will be no uplift problem. Should an uplift problem occur, 60 percent of the DL of the foundation plus 60 percent of the DL of the building must be sufficient to counteract the gross OM"

Hope this can clear some confussions on the above topic...

nrguades

 

[SacreBleu]

Reading the above posts, I think the IBC should stand for "Incomprehensible Building Code". (Sorry, couldn't resist)
I was very doubtful that seismic would have as much FS against OT as wind. If you study the load combinations, you will find that the seismic load is multiplied by 0.7, whereas the wind load is not. (1.0 implied). Then, the Dead load resisting moment for either wind or seismic is multiplied by 0.6. However, I could be wrong - I am an old UBC type, and am a bit confused by ther over-use of duelling fudge-factors in the IBC.
Overall, the IBC is more conservative than the UBC. Perhaps the writers of the IBC feel there is more of a chance of engineering error than ever before.

 

[seattlemike]

SacreBleu,

I'm not sure what you are impling by your post: it is clear that we use "W" or "0.7E" and "0.6D" but what are you getting at? You think the 1.5 factor of safety is included already or not?

There is more chance of error because the brainiacs-in-charge wrote too long of a code for anyone to want to read the entire thing: so then we run into problems of knowing part of the code but not all.

Somewhere out there are a bunch of code-writers that could use a good shaking.

 

[SacreBleu]

I think we need to "delete" the 1.5 FS, because the reduction of dead load (0.6) gives us a FS=1.67, as already pointed out by nrguades.

 

[SacreBleu]

postscript:
It just occured to me, 0.7 x 1.67 = 1.17 (actual FS for seismic). The reciprocal of 1.17 is 0.86, which is very close and slightly conservative compared to the simple dead x 0.9 for seismic as in the UBC.
Any comments?

 

[UcfSE]

Seismic loads are given by the code at the ultimate level. If you want to use allowable stress design for seismic design, the loads have to be reduced to a working stress level. That's the purpose of the 0.7 factor on E in allowable stress load combinations.

 

[SacreBleu]

UcfSE,
OK, then the IBC dictates a much larger FS against overturn then the UBC. I still don't get the rationale, because seismic forces vary in direction rapidly.

 

[sme76]

So....if i'm checking a spread footing under a column for overturning for the load case 0.6D+W (IBC 2000), do i take 60% of my foundation and overburden weight in figuring the resisting moment?

 

[seattlemike]

hi sme76,

yes, you would use 60% of the dead load of the footing for your resisting moment vs. the wind overturning (0.6D > W, FS =1.0). single column footings often end up too small for this, and you must bury them several feet to get it to work or attach them to other things via grade beams. good luck!

Mike, PE, SE

 

[sme76]

Thanks seattlemike,

I'm assuming then that the same idea for resistance to uplift would apply.....whatever my uplift reaction from load case 0.6D+W must be less than 60% of foundation and overburden weight. Would you agree?

 

[seattlemike]

yes, i agree.

but really, the wind alone is the upward force, and 60% of all the tributary dead is the holdown or overburden weight. anyway, same thing.

 

[sme76]

thanks for all your input...one last thought. i have been asking about uplift and overturning for cases where the 0.6D+W controls. What about a case such as a pre-engineered metal building where worst case overturning moment on a footing could be from horizontal thrust due to gravity loads on rigid frame? (this happens sometimes for long spans) In this case, I still shoot for a factor of safety of 1.5 against overturning even though IBC doesn't explicitly require this. It seems like the code requires a 1.67 SF for cases involving wind but doesn't say anything about the "metal building" case. Any thoughts? (oh yeah, assume i'm not resisting these horizontal forces through tie rods or hairpins into slab.)

 

[seattlemike]

hmm... well, I would probably want to tie it into the slab since it's going to be there anyway. Yes, the 1.5 FS for sliding was meant for retaining walls and wind. But yes, seemes like it would be appropriate to use: but you may then use 0.9D for the resisting friction force at the conc-soil interface. But you will want to consult the geotech: if you are in areas with liquifaction, you may have to tie to the slab instead of relying on the soil.