California's Current and Former Structural Ordinances

In the previous blog post, we discussed that notable earthquakes often lead to changes in the upcoming edition in the building code, or at the least spur research to that effect. What about the buildings that were approved under prior building codes, but based on what we know now, may prove to be unsafe? Cities and states have the ability to pass acts and ordinances, which can mandate the retrofit of such structures. 

We saw the first instance of this with the Field Act of 1933, in which all public-school buildings were required to be upgraded to be earthquake compliant. We also saw it in the Alfred E. Alquist Extension in 1994, which mandated that all hospitals must be earthquake-code compliant by 2030-work for which is still ongoing. 

Currently, the most urgent and widespread ordinances are for buildings with soft stories. These ordinances are in place in several cities throughout California, but we'll specifically discuss those of San Francisco and Los Angeles since those are the two largest municipalities. 

First, what defines a building with a 'soft story'? A soft story is defined to be a story in which the structural stiffness is calculated to be less than 70% of the stiffness of the story above or less than 80% of the average story stiffness of the three stories above. This configuration has been shown to suffer large ground floor displacements, leading to structural damage and even collapse during a seismic event. 

The 1971 San Fernando Earthquake and the 1994 Northridge Earthquake both caused significant damage to buildings with soft stories in the Los Angeles area, and the 1989 Loma Prieta Earthquake caused similar damage in the San Francisco Bay Area. 

In San Francisco, the seismic ordinance applies to wood-frame buildings with three or more stories, which were permitted for construction before January 1, 1978. The ordinance was rolled out in tiers, beginning with Tier 1, which included buildings used for education, assembly, or daycare. Tier 2 consisted of buildings with 15 units or more; Tier 4 consisted of buildings with ground-floor commercial use, or buildings located in a liquefaction zone; and Tier 3 consisted of buildings not falling into one of the other tiers. About 5,000 of the buildings in the city of San Francisco were subject to this ordinance. The tiers were used to stagger the mandatory completion dates with all seismic retrofits required to be completed by this year, 2020. 

The mandatory ordinance in Los Angeles encompasses a much larger amount of buildings-nearly 13,500 (compared to San Francisco's 5,000). The ordinance applies to wood-frame buildings permitted for construction before January 1, 1978, however, this ordinance also includes buildings with two stories. The Tiers are also slightly different, including that in the Los Angeles framework, they are called Priority levels. Priority 1 is for buildings with 16 units or more; Priority 2 is for buildings with three or more stories (less than 16 units); and Priority 3 is for buildings not included in Priority 1 or 2, which would mean two-story buildings with fewer than 16 units. The Ordinance states that from time of notice, the owner has two years to submit either proof of prior retrofit, or plans to retrofit and demolish; three and a half years to obtain a permit to start either construction or demolition; and seven years to complete construction. Following this framework, retrofit completion dates in order of Priority are 2022, 2023, and 2024. 

While there is still a lot of work to do, we can have confidence in the fact that progress is underway and the cities in California will be much more resilient to earthquakes in the coming years. 

About the Author: Erin E. Kelly

Ms. Kelly is an experienced structural engineer with a focus on seismic risk. She has extensive experience in structural failure investigations, seismic structural design, and seismic risk assessments. Through the School of P.E., she has taught a 32-hour course for the California Seismic P.E. Exam, authored several blog posts, and contributed to other review products. She has a Bachelor of Science in Civil Engineering from Johns Hopkins University and a Masters of Engineering in Structural Engineering from Lehigh University.

Lessons from the ASCE 41 Basic Checklist

As you may be aware, there is a document used in assessing the seismic performance of existing structures entitled, ASCE 41 - Seismic Evaluation and Retrofit of Existing Buildings. Among other topics, this guide offers instruction for basic assessment of seismic vulnerabilities through "checklists" on general building configurations as well as one specific checklist for each building type. The checklist on configurations is used for all building types and is referred to as the "Basic Checklist." 

A quick scan through the Basic Checklist will highlight several of the concepts that you'll need to be comfortable with as you prepare for the CA Seismic P.E. exam, so I thought it would be helpful to run through some of them here. 

Consider a building you know well as you go through this list. Maybe it's a building you designed, or maybe it's the one you're sitting in right now.

Here are some of the criteria:

Load Path: The structure shall contain a complete, well-defined load path, including structural elements and connections, that serves to transfer the inertial forces associated with the mass of all elements of the building to the foundation. 

 As we covered previously, lateral loads are applied to the exterior walls of the building, then transferred to the diaphragm, then to the vertical elements of the LFRS, then to the foundations. Each of these elements needs to be sufficiently connected (i.e. dowels in concrete construction, bolts/welds in steel construction). Most buildings will pass this one. 

Adjacent Buildings: The clear distance between the building being evaluated and any adjacent building is greater than 4% of the height of the shorter building. 

 We cover this concept specifically in the course as we discuss both drift and separation. Pounding can cause considerable damage, particularly if the adjacent buildings are not the same height or do not have the same floor-to-floor heights. If the shorter building is displaced toward the taller building, and the contact point is between floor heights, the contact could occur at the midpoint of a column and cause catastrophic damage. If the buildings are the same height and experience contact during a seismic event, the damage will be less significant but could cause damage at the roof/wall connection. 

Weak Story, Soft Story, Vertical Irregularities, Geometry, Mass, Torsion:

 You should recognize each of these as some of the Horizontal and Vertical irregularities from ASCE 7. These irregularities were only codified in the 1994 Uniform Building Code (UBC), so buildings designed and constructed prior to the adaptation of the 1994 UBC are more likely to have these irregularities. Also notable is that weak story and soft story are listed here separately. While they are the result of similar configurations, "weak story" relates to strength and "soft story" relates to stiffness. 

Liquefaction, Slope Failure, Surface Fault Rupture: 

 These all relate to the soil below the structure. However, we now know that the soil conditions can be a large factor in how the buildings will behave during a seismic event. Liquefaction relates to a type of soil in which the cohesion between the soil particles is likely to decrease to a point of instability when saturated. This can occur during an earthquake, as was seen specifically in the 2011 Christchurch Earthquake. Slope Failure relates to landslide hazard, which is relatively common after an earthquake. Surface Fault Rupture relates to the proximity to the closest known fault. If a structure is located very close to a fault, the building could be damaged by the surface rupture during an earthquake. Based on recent legislation, structures shall not be built immediately on top of or within 50 feet of these fault lines. 

If you have time to review the building-type-specific checklists, you'll notice some of the improvements that we discussed in the Building Code Blog Post. These checklists are designed to show how much of the modern seismic detailing can be found in these existing structures, and I think they double as a great study tool for this exam.

About the Author: Erin E. Kelly

Ms. Kelly is an experienced structural engineer with a focus on seismic risk. She has extensive experience in structural failure investigations, seismic structural design, and seismic risk assessments. Through the School of P.E., she has taught a 32-hour course for the California Seismic P.E. Exam, authored several blog posts, and contributed to other review products. She has a Bachelor of Science in Civil Engineering from Johns Hopkins University and a Masters of Engineering in Structural Engineering from Lehigh University.

How is ASCE 7 Organized for Seismic Engineering

Besides signing up for this course, obtaining a copy of and getting comfortable with the latest version of ASCE 7 is the best thing you can do for yourself to prepare for the California Seismic P.E. exam. 

This test is fast paced, so the last thing you want to spend time doing is flipping through the code to find the information you need. 

Here's a quick overview of some important sections to get you started: 

Chapter 11 

Chapter 11 is used on the exam to determine the ground acceleration parameters, site class, and seismic design category. Section 11.4 includes formulas and tables to determine the site class, and Section 11.6 contains tables to determine the Seismic Design Category. As we discuss in the course, Seismic Design Category influences so much in a building's design including, but not limited to, permitted lateral systems, maximum building heights, lateral analysis procedures, restrictions on irregularities, and seismic detailing requirements. 

Chapter 12 

Chapter 12 contains the seismic design requirements for building structures, so for most of the exam, you'll be using this chapter. This blog post is by no means comprehensive, but I'll highlight a few of the important features. 

In my opinion, one of the most important tables in the whole code is Table 12.2-1. This should always be your starting point on the exam. It outlines each type of lateral force-resisting system, its corresponding seismic parameters for ductility, overstrength, and deflection amplification, and provides guidance on the applicability or appropriate building height limit in each seismic design category. This table is useful as a personal teaching tool or point of reference and is also a great place to start on any exam question. 

Table 12-3.1 describes each of the horizontal and vertical irregularities that are considered by the code. A building with any of these irregularities will require additional analysis or the consideration of additional seismic load, so it's important to review these definitions and commit them to memory. 

Section 12.8 outlines all the parameters needed for the Equivalent Lateral Force Procedure, from the seismic response coefficient, to base shear, to period, and deflection. If the building is permitted to be analyzed by this procedure (and for the purpose of the test, 99% of structures will be) this is where all your calculations should begin. 

Section 12.12 contains limits for allowable story drift. This is an easy place to pick up some points. Table 12.12-1 includes limits based on type of structure and risk category, but if you have a moment frame structure in Seismic Design Category D through F, be sure to consider section 12.12.1.1. 

The final section in Chapter 12 worth including here is Section 12.14-the "simplified alternative structural design criteria." There are many factors that will determine if you can use this section, and typically as far as the test is concerned, they will ask you to use this section if required. It's a simple way to determine the base shear, etc. for a building if it meets all the qualifications. 

Chapter 13 

Finally, Chapter 13, or more specifically, Tables 13.5-1 and 13.6-1 provide the a_p, R_p, and Ω_o values for nonstructural components. These will be used to determine their anchorage forces. 

As I said, this is in no means a full guide to ASCE 7, but if you are able to tab/bookmark/highlight these sections and get familiar with them, you'll save a lot of time on the test and, let's face it, we could all use some more time for this exam.

About the Author: Erin E. Kelly

Ms. Kelly is an experienced structural engineer with a focus on seismic risk. She has extensive experience in structural failure investigations, seismic structural design, and seismic risk assessments. Through the School of P.E., she has taught a 32-hour course for the California Seismic P.E. Exam, authored several blog posts, and contributed to other review products. She has a Bachelor of Science in Civil Engineering from Johns Hopkins University and a Masters of Engineering in Structural Engineering from Lehigh University.

Discussion on Building Frame vs. Bearing Wall Systems

As you may have already recognized, the load path for lateral loads differs greatly from that of gravity loads, and can, in some cases, be completely independent. 

Lateral loads are assumed to be applied initially to the faade of the structure or the exterior walls. The loads applied half-a-floor height above and below a given floor level are then assumed to be transferred to that level, which acts as a horizontal diaphragm. The loads are then transferred from the diaphragm to the vertical elements of the lateral force-resisting system either by tributary area or rigidity, based on the type of floor system. These vertical elements are what we typically refer to the whole system as the moment frames, shear walls, braced frame, etc. The lateral loads are finally resolved into the foundation at the ground level. 

In a case where the gravity system consists of slabs and beams supported by interior columns, which carry the loads to the foundation, the gravity and lateral load paths are essentially independent. In other cases, however, such as in conventional wood-frame structures or concrete tilt-up structures, the load paths overlap as the walls act both as lateral and gravity load-resisting elements. 

We identify these two types of load paths in the Code as Building Frame and Bearing Wall. Building Frames structures contain a separate load path for gravity and lateral loads. The Bearing Wall structures involve elements that act simultaneously as gravity and lateral load-resisting elements. 

Building frame systems are preferred for several reasons, but one major advantage is that they allow for the stiffness of the structure to the maintained through a limited number of elements. 

To understand the importance of this, we must consider one of the serviceability functions of the lateral load-resisting system, which is to limit the deflections in the structure. Deflections need to be limited in order to avoid structural damage, plastic deformations, increased load effects due to the P-Delta effect, and, importantly, occupant comfort. 

If the entire structure were to be designed to achieve this purpose, the building would need to be extremely stiff, heavy, and expensive. Instead, if we separate the two systems, and make just the lateral system sufficiently stiff, a considerable amount of both labor and materials can be saved. 

In the book Why Buildings Stand Up, Mario Salvadori explores this concept as it applied to steel-framed skyscrapers. Salvadori explains, "One must be aware that in steel construction, rigid or moment connections are costly. They require specialized manpower and dangerous work at great heights. Their cost may represent 10% of the entire cost of the structure. But, if the inner core were stiff enough, one could forsake the rigid connections between the beams and columns of the exterior frames and use much cheaper connections, which allow beams and columns to rotate with respect to each other, as if they were hinged. Such hinged, or shear, connections could not be used without a core since the frame would collapse like a house of cards, but they are economical and practical if the core stands up rigidly and the outer hinged frame leans on it. The separation of the two structural functions is now complete." 

You can see through this quote that the separation of the gravity and lateral systems has allowed us to build bigger, taller, and stronger structures while being economical and practical in our design choices. This division is now codified in seismic engineering and plays a key role in how lateral systems are selected and evaluated.

About the Author: Erin E. Kelly

Ms. Kelly is an experienced structural engineer with a focus on seismic risk. She has extensive experience in structural failure investigations, seismic structural design, and seismic risk assessments. Through the School of P.E., she has taught a 32-hour course for the California Seismic P.E. Exam, authored several blog posts, and contributed to other review products. She has a Bachelor of Science in Civil Engineering from Johns Hopkins University and a Masters of Engineering in Structural Engineering from Lehigh University.

How Hard is the California Seismic P.E.?

Before I answer this question, I feel that I should give you a quick summary of my background: I'm originally from the East Coast, meaning that I never took a course in seismic engineering at any point in my education. I dove in headfirst when I moved to California and decided to take this exam. I remember being told "it's easy!" by so many people, only to fail the first time I took the exam. The second time around, I studied hard and was able to pass. 

Personally, I found the exam to be challenging, but not tricky. With the right preparation and a good understanding of the principles of seismic engineering, this exam is very doable. To me, the level of difficulty comes down to several factors, which I'll get into here: 

Table of Contents

1. The exam is fast

2. It matters that the engineers who pass this exam are proficient in seismic engineering

3. Not everyone learned seismic engineering in college

4. Do you really want to do this again?

 

1. The exam is fast

We're talking 55 problems in 2.5 hours. Gone are the days of 40 questions in 4 hours (assuming you've passed the national P.E. exam). This breaks down to just over 2 minutes per problem. There will be some definition questions, but don't rely on these to give you extra time on the calculation-based questions. My experience was that there are a lot of calculations to be done during the exam, so make sure you're prepared to tackle these with the necessary speed. 

2. It matters that the engineers who pass this exam are proficient in seismic engineering

Understanding the code requirements and engineering philosophy behind seismic engineering could not be more important if you plan to practice engineering in California or any other seismically active region. As structural engineers, we design for a "life safety" criteria, which is literally another way of saying our work could be the difference between life and death for ourselves and our fellow citizens. That's just to say, if this exam seems hard, it's for good reason. I have a great respect for this exam and the way it pushes the engineers of California to be true experts in what we do. 

3. Not everyone learned seismic engineering in college

I firmly believe that this exam will be "hard" or "easy" based on where you went to college-literally the geographical WHERE. I went through both undergrad and grad school without taking a single course on seismic engineering, so this exam challenged me to turn everything I know sideways and learn new material. While I was doing that, my California native colleagues were saying to me, "The Seismic P.E. is easy! It's just everything we learned in college!" If you're not from California, that's OK. See this as an opportunity to get up to speed. 

4. Do you really want to do this again? 

This question is how I've approached each of these licensing exams-The F.E., the national P.E., and both California state exams. You're going to put your life on hold (to some extent) to prepare for these exams. It's so worth it to apply yourself now, pass, and never look back. There is nothing wrong with over studying! 

Bottom line: this exam is hard. It's designed to be hard so that we, and the rest of the population, can have confidence in the engineers designing our structures. But you made it this far! If you're reading this, you likely have one or more degrees in engineering and achieving that was a challenge too. I have no doubt you can rise to this challenge and succeed.

About the Author: Erin E. Kelly

Ms. Kelly is an experienced structural engineer with a focus on seismic risk. She has extensive experience in structural failure investigations, seismic structural design, and seismic risk assessments. Through the School of P.E., she has taught a 32-hour course for the California Seismic P.E. Exam, authored several blog posts, and contributed to other review products. She has a Bachelor of Science in Civil Engineering from Johns Hopkins University and a Masters of Engineering in Structural Engineering from Lehigh University.