Since I've been practicing structural engineering in California, I've kind of taken my experience with seismic design for granted.
Not only it’s something that we do nearly every single day, it’s also required in order to get the PE license (CA has an additional 4-hour seismic portion on top of the regular NCEES 8-hour exam for the PE exam).
I understand that a lot of people in “non-earthquake areas” are not used to seismic design; since it will be on the SE exam, my goal is to help you as much as possible by covering a number of important topics.
If you practice structural engineering in states that rarely deal with earthquakes or if you are fairly new to the world of lateral design, I hope you find these articles helpful in your preparation for the SE exam.
In this post, I will start with the explanation of “ASCE 7–05 Chapter 11: Seismic Design Criteria”.
Note that this chapter is very closely related to Section 1613 of IBC 2009. Parts of them are pretty much identical. To me, ASCE 7–05 is slightly easier to read so once you get the hang of it, finding and understanding the related sections in IBC should be cake.
Description – Variables of “Seismic Design Criteria”
Every lateral design problem usually starts with the variables described in Chapter 11.
It’s important to understand what these variables are and how to obtain/calculate them so that you can start on the right track.
This chapter of ASCE 7 is the very basic of determining the required seismic demand. So if you get this part wrong, the rest of the design is essentially incorrect.
Now, since this part is so important, what is the best way to understand them?
My recommendation: Think of “reading the code” like “solving a problem”. There are “givens” and there are “things to be determined”. That’s how I am going to break down these variables.
“Given” (4 Variables)
By ASCE 7–05 definition, this is the “mapped MCE, 5 percent damped, spectral response acceleration parameter at short periods as defined in Section 11.4.1”. (IBC 2009 defines short periods as 0.2 seconds).
If you have time, you can dig in to different seismic related books and figure out what this is all about. But as far as the exam goes, this number is usually given.
If not given, you’ll have to look at Figure 1613.5(1) to 1613.5(14) of the IBC. It’s good know what they look like although I’ve never had to use them.
One thing you need to pay attention to when looking at the figures is the title. If it mentions “0.2 sec” then you are looking at the SS. Otherwise, it’ll say “1.0 sec” which is the S1 value that I’ll talk about next.
If you need to determine this number outside of the exam settings (like in your actual actual job), USGS has a handy tool that helps you determine the values for you. You just have to enter site latitude and longitude which you can obtain from Google Maps by right clicking on a location and select “what’s here”.
S1 is pretty similar to SS except that it’s for longer period.
By ASCE 7–05 definition, this is the “mapped MCE, 5 percent damped, spectral response acceleration parameter at a period of 1 s as defined in Section 11.4.1”.
Everything I said above for SS applies here also so I am not going to repeat it.
3. Site Class
I've came up with three possibilities of how this can be given in a problem.
Possibility 1: Given Directly
On the exam, the problem may just flat out tell you what the site class is.
In actual practice, most of time, this is determined by the soil engineer.
Possibility 2: Given Indirectly
The exam might also give you bunch of variables like “soil shear wave velocity” or “standard penetration resistance”. In that case, you just need to look at IBC 2009 Table 1613.5.2 and match the numbers to the correct class.
Occasionally, you may need to do some calculations if the problem gives you properties of different soil layers. I am not going to go into details in this post but basically you’ll have read and understand IBC 2009 Section 1613.5.5 and 16126.96.36.199 which has all the formulas and explanations.
Possibility 3: Unknown
Sometimes the problem might not give you any clues at all regarding site class.
If that’s the case, you can assume it’s site class D per section 1613.5.2 which states: “When the soil properties are not known in sufficient detail to determine the site class, Site Class D shall be used…”.
Note that if this occurs in the afternoon portion (written part), make sure you cite the code section!
4. Occupancy Category
This is usually pretty straightforward.
The exam problem will usually give you some kind of building usage description; you will have to match it to the description listed In IBC 2009 Table 1604.5.
Note that Occupancy Category II is for “Buildings and other structures except those listed in Occupancy Category I, III, and IV”; therefore, you should familiarize yourself with this table so that you have a good idea what kind of buildings might not belong to I, III, and IV.
“Determine” (6 Variables)
1. Importance Factor, I (I call it “IE”, “E” for earthquake)
Once you know what the building “Occupancy Category” is, use ASCE 7–05 Table 11.5–1 to obtain the importance factor.
Side note: Personally I like to call it “IE” in my calcs just to distinguish it from the importance factor for wind load (which I call “IW“).
2. Short-period Site Coefficient, Fa
This is determined based on Site Class and SS using ASCE 7–05 Table 11.4–1.
However, note that the numbers given in the table for SS are in increment of 0.25. So what happens if you have a SS somewhere in between (e.g. SS = 0.85)?
In that case, you’ll have to interpolate, which can be tedious.
To save time, I created a little handy chart for SS up to a few decimal points. That way I only need to look at the chart to find the corresponding Fa rather than doing the cumbersome calculation myself.
Click Here to get the timesaving Chart
3. Long-period Site Coefficient, Fv
Pretty much the same as Fa except it’s based on S1 using ASCE 7–05 Table 11.4–2. The tabulation for different SS is also included in the above file.
Once you you have Fa, you can calculate SMS = FaSs
From SMS, you can then determine SDS = (2/3) SMS
Similarly, SM1 = FvS1
And SD1 = (2/3) SM1
6. Seismic Design Category, SDC
Now that you have determined SDS and SD1, you can determine the Seismic Design Category by looking at Table 11.6–1 and Table 11.6–2 whichever governs (i.e. SDC “D” governs over SDC “A”).
Note that this only applies if S1 < 0.75 per ASCE 7–05 Section 11.6.
If S1 is greater than or equal to 0.75, you basically has two possible outcomes depending on what your occupancy category (OC) is. If your OC is I, II, or III then the SDC is “E”; if your OC is IV, then you have SDC “F”.
Other Topics (3 items)
So far we have discussed the most important parts of Chapter 11. There are a few other things in this chapter that I didn't mention because I don’t encounter them too often. However, these are things that you will come across from time-to-time. I’ll just briefly go over them so that you are aware of their existence.
Design for SDC A (ASCE 7–05 Section 11.7)
I've never had to use this section myself either in real practice or in the exam. However, since the exam can cover pretty much anything they want (as long it's referenced in the spec), it’s still important to at least have some idea of what this is.
I won't go over it here since I believe it will be easier for you to just read the section. If you are familiar with seismic design concepts for higher SDCs, this section will be a light read. If not, I suggest that you come back to read this section in the ASCE7 after you have completed your study – it will make more sense.
Design Response Spectrum (ASCE 7–05 Section 11.4.5)
Sometimes you may be asked to calculate the “design spectral response acceleration, Sa” which is based on a couple of variables that you have already determined above (i.e. SD1 and SDS) with the exception of TL and T. All the other terms mentioned in this section is derived from these 4 variables which you can see in this chart below:
- “TL” is the long-period transition period which is determined from looking at figures 22–15 to 22–20.
- “T” is the fundamental period of the structure. It is kind of like the variable “X” in an X-Y chart whereas the “Sa” is the “Y”.
In other words, the spectrum chart describes: “What is the Sa for a building with fundamental period T in a location that has certain SD1, SDS, and TL?”
General, Definition, and Notation (ASCE 7–05 Section 11.1, 11.2, and 11.3)
Finally, it is prudent to look at the “General” section and be aware of the different “exceptions”. You may or may not run into them but having a mental image of what they are will help you if they come up during the exam.
Also, “occasionally,” you’ll run into terms that you are not exactly sure what they mean. One of the first places to look at will be the “definition and notation sections”. I would just skim through them very quickly so that you have an idea of what's there.
Recap & Thank You!
To recap, these are the things in ASCE 7–05 Chapter 11 that you should know by now:
- Site Class
- Occupancy Category
“To be determined” variables:
- Importance Factor, I (IE)
- Seismic Design Category (SDC)
- Design for SDC A Response spectrum
- General, Definition, and Notations
That’s it for now. If you find this helpful (or not), please let me know in the comments below. Thank you for reading!
(PS: Ian has created a handy flowchart that is related to this section. I will post them soon – stay tuned!)