Some Comments On the UBC 97

[drasticxxxx]

Hello guys
First,
For strength load Combination, the load combinations are clear and the code require to multiply all seismic combination with 1.1, but the confusing part, if you search the internet and books, you will find tons of examples of situation where they use ubc97 but with unknown combinations (at least to me, like 1,4D+1.7L(I don’t know where is the reference, and it looks like I am missing something

Second
For Checking Drift, scaling should not considered, but my main question is under which case or combination should the drift be checked, the code is note clear in this but it seems to me it is under strength combinations, however others saying use allowable combinations, so what do you think guys?
Third,
Is it accepted if we assume all reinforced concrete diaphragms as rigid in analysis (I think this will give reasonable answers in most cases)

Last and Most important point,
I use Etabs for scaling between static and dynamics, and this fine with me ,however another question jump to my mind, in dual system the moment resisting frame should contribute with 25% of shear resistance of structure and shear walls with the rest, and base on that R value is given in table,
So, is this mean if I check static method and found 40 % for example instead of 25%,that I need to scale the static first (maybe by interpolation between dual system and pure moment resisting system), then after this I run dynamic analysis and scaling according to new modified static analysis(They will be two scaling process???)

 

[sponton]

1) explain what you mean by unknown combinations? UBC 1612.2.1 has the basic combinations which are pretty much the same as ASCE or IBC, well what it seems to me is that you haven't really opened the code and read it, specially the seismic provisions where it tells you the cases you have to test it with (for example full load in x direction with 30% of the force applied in the orthogonal direction and the iteration between these combinations)

2)All combinations involving lateral forces. You'd check it against the limit, usually drifting is limited to 0.02 of the story height (though this depends on the material). These are serviceability, it has nothing to do with allowable stress or strength requirements. Also drifts come into place when you're doing the column analysis, in the direct design method [i've seen people not even bothering to check whether method is applicable]

3) Depends on the thickness of the slab [rigidity], ASCE i think has a method of numerically testing whether your diaphragm is rigid or not so it has more of a technical feel if you don't feel comfortable with assumptions.

4) why complicate yourself, I'd just check whether the rest can take the 15% difference that the frames are taking. If you're planning on calibrating the system to exactly take a percentage you're going to have a bad time.

 

[JStephen]

I think the 1.4D + 1.7L was in the older ACI-318's, not sure when it changed. It's 1.2 and 1.6 now.

 

[drasticxxxx]

First of All Thank you

1)Regarding this point it is true I didn't read IBC, but I fully read UBC97, and I found many books refer to load combination but instead of using 1.2 D+1.6 L as UBc 97 said , it says 1.4 D +1.7 L, so I don't know why?
Please don't judge people and say they don't open, I read this section very well and still confused about this part?

2)So if it is for serviceability, could you please mention load case and load combination fro checking??

3)UBC has statement too ,put if not seen anyone in practice checking, that why I assume it most likely rigid as it make scene.

4)It is not of making life harder, it is matter of implementing the code and see why people ignoring this(maybe they are right but they should have reasons)


Thank you again for your help

 

[drasticxxxx]

UP guys

 

[mike20793]

1. See JStephen's post. Load combinations have been recalibrated over time.

2. You use the seismic load cases. The process is defined within the UBC.

3. People do check this provision, but only when they have to. You can assume rigid or flexible if you meet certain conditions. If you do not meet those conditions, then you either need to use a semi-rigid diaphragm or check whether you can idealize the diaphragm as flexible or rigid based on the procedure in the UBC.

4. I agree with sponton.

Try reading over ASCE 7 and it's commentary. I think this will help with your issues.