Simplified method per ASCE7-05 Section 12.14.8.1

[teda]

Here is the interesting code text. In section 12.14.8.1, Fa is permitted to be taken as 1.0 for rock sites, 1.4 for soil sites, or determined in accordance with section 11.4.3. But in section 11.4.3, it referred back to 12.14.8.1 where simplified design procedure of section 12.14 is used. God! It is a loop, isn’t? For high seismic areas, this will be a big difference for design seismic base shear using different Fa.

Example:

A simple single story wood house in Los Angeles, CA.

Ss=1.5 without any soil report (sure it is not site class E and F).

We can assume site class D per section 11.4.2

Fa will be 1.0 per Table 11.4-1.

If using section 12.14.8.1, Fa=1.4. This is 40% base shear increase.

What is your opinion? Thanks.

 

[kevo_55]

teda,

Yes, you'll generally get higher seismic forces when using the simplified procedure.

Basically the more complicated the seismic approach that you take, the smaller the seismic forces.

In order of difficulty the seismic procedures are: Seismic Response History procedure, Modal Response procedure, equivalent laterial force procedure, and the simplified procedure. In some cases, any of these can be used. In other cases, not all of them can be used. Of course, you'll need a PHD to do a the seismic response history procedure.

Just my 2 cents.

 

[teda]

Kevo_55, thanks for the text but I think you did not understand my question. My question is how to get correct Fa. Could you read again?

 

[kevo_55]

I do understand your question.

You have the option of choosing an Fa of 1.4 or determining it per table 11.4-1.

The good thing with the simplified procedure is that if your Ss is 0.2, you can use an Fa of 1.4 (if you have a soil type site) and not 1.6 as seen in table 11.4-1 (assume site class D). Of course, you'll get an overall higher seismic load in the end.

Does this help?

 

[teda]

What confusing me is how to decide correct Fa. Code does not clearly specify that, it is a loop between section 12.14.8.1 and 11.4.3. For Ss=1.5, site class D, you will see two different results and can not say which one is correct. You can not just say use the conservative one. That is a big difference.

Personally I think Fa=1.4 is NOT necessary, this is not in IBC1613.5.3. Also it is NOT consistant with IBC2003 and IBC2000.

 

[kevo_55]

Teda,

I agree. The thing is with the simplified method, both ways in finding your Fa are correct. Consistent or not, it's still in there.

 

[teda]

When a plan checker putting shot gun on my head and force me to use Fa=1.4 or redesign the whole thing to use Equivalent method, it is really bad.

 

[MOOK]

There are many things in the code like this case. This is not the only thing you may get confused from the code. The code is trying to generalize everything which makes it more confusing.

For example, in ACI code there are requirements for Flexura Members and then there are requirements for beams and requirements for walls. Are not beams and walls considered flexural members.

Actually, when I talk with my co-workers I find all of them overwhelmed by the ambiguity and contraditons in all codes and I believe this is the case for most of Structural Engineers.

We really need a revolution in the codes. All codes (with no exception) needs to be rewritten in more clear and stringet way.

 

[McEngr]

teda,

Plan checkers can be a pain the butt, especially in the area which you live. LV, Nev is known to be tough. CA is also known to be tough. I suppose that you know that you cannot use the simplified procedure except for the table in which you give it, right? You and kevo had a discussion on this, which is a good one, but you also know that the simplified procedure will only make your lateral design more conservative I hope. If a plans checker is giving you that kind of feedback, it's probably because you did something incorrectly and it's not allowed in the table. For instance, you can't use simplified for light-framed wood construction for 4 story structures.

 

[teda]

McEngr, I will never use Simplified method for 4 story building for sure. I believe what I did is correct and just for some reason, this special plan checker can not agree with me on Fa=1.0. There is no problem for other plan checkers. I hope code can resolve this issue in the incoming ASCE7 new edition or supplement.

 

[kevo_55]

teda,

I don't think that the simplified procedure is permitted for buildings over 3 stories.

Simply doing the ELF procedure is good enough for smaller buildings.