consensus recommendations for “soft story” retrofit · 2018-04-02 · consensus recommendations...

2017 SEAOC CONVENTION PROCEEDINGS

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Consensus Recommendations for “Soft Story” Retrofit

David Bonowitz, S.E. and Daniel Zepeda, S.E.1

Abstract1 Retrofit programs for “soft story” woodframe buildings are now underway around California. Most recently, smaller jurisdictions are implementing their own programs following the lead of Berkeley and San Francisco in the north and the City of Los Angeles in the south. However, some of the new programs might be missing the lessons learned by those pioneering programs, and practice is diverging. For five key topics, this paper discusses the limitations of applicable codes and guidelines, including California Existing Building Code Appendix Chapter A4, lessons learned by practitioners on the leading programs, and findings by SEAONC and SEAOSC committees. The paper reviews the different approaches taken by the two largest programs, San Francisco and Los Angeles, as a step toward establishing consensus among SEAOC committees, SEAOC members, and local building officials. Background The collapse-prone nature of woodframe multi-unit residential buildings with soft stories, weak stories, or open front wall lines was observed as early as the 1971 San Fernando earthquake. Well-publicized failures in Loma Prieta (1989) and Northridge (1994) confirmed the risk and motivated voluntary retrofits in the Bay Area and in greater Los Angeles. After Northridge, SEAOSC members worked with the Los Angeles Department of Building and Safety (LADBS) to develop retrofit provisions that would evolve into both Division 93 of the Los Angeles Building Code and Appendix Chapter A4 of the current International Existing Building Code (IEBC) and California Existing Building Code (CEBC). A decade later, the city of Berkeley used the IEBC provisions in the state’s first mandatory mitigation program, requiring evaluations but not retrofits. In 2007, the city of Fremont would enforce California’s first “soft story” retrofit program, mandating improvements for about two dozen buildings.2 In 1 Both authors are members, chairs, and/or past-chairs of various SEA Existing Buildings Committees, and both consult to multiple California jurisdictions. This paper does not necessarily represent the position of any particular EBC, SEA, or jurisdiction. Nor does it necessarily represent the position of either author. Rather, it is intended as an informed presentation of certain issues as a basis for further discussion by SEAOC members, committees, and other stakeholders. 2 As engineers know, the term “soft story” refers to a type of structural irregularity in new construction or to a calculable deficiency in an existing

2013, after voluntary incentives proved ineffective, and nearly a quarter century after Loma Prieta, San Francisco approved the state’s first sizable mandatory retrofit program. Other cities would follow, as shown in Table 1. In particular, the City of Los Angeles program, approved in 2015, will be the largest single-jurisdiction mandatory seismic retrofit program ever in the United States. The Los Angeles and San Francisco programs together will result in over 18,000 retrofits, more than all of California’s post-1986 unreinforced masonry programs combined. The San Francisco and Los Angeles programs Table 2 (provided at the end of the paper due to its length) compares the scopes and criteria of the San Francisco and Los Angeles programs. Both programs allow the use of ASCE 41 (ASCE, 2014) and FEMA P-807 (ATC, 2012) as alternatives to CEBC Appendix Chapter A4 (cited as “Chapter A4” below for brevity). With SEAONC or SEAOSC input, both programs have also applied certain lessons of those more specialized documents to modify the code-based provisions of Chapter A4. Today, a large majority of the retrofits in the S.F. and L.A. programs are being designed with these modified or abbreviated versions of Chapter A4, so this paper focuses on the code-based Chapter A4 approach. In this paper, we consider five issues on which the S.F. and L.A. programs differ. On the key conceptual points, however, the two programs are the same, and the SEAONC and SEAOSC Existing Buildings Committees concur:

building. It is not limited to buildings of any specific structural system, structural material, occupancy, size, height, or age. Advocacy efforts, however, have led to an understanding among building owners, contractors, and policy makers that “soft story” refers only to collapse-prone woodframe multi-unit residential buildings built generally before 1978. The title of this paper, and the text to this point, puts the term in quotes to acknowledge this not-quite-correct usage. The balance of the paper will use the shorthand term, without quotes, to mean the type of woodframe residential buildings covered by typical ordinances. The paper also uses the term SWOF, an acronym covering soft story, weak story, and open front deficiencies, as defined in IEBC or CEBC Appendix Chapter A4.


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• As a class, pre-1978 multi-unit woodframe buildings frequently have collapse-prone deficiencies, such as soft or weak stories, related to or exacerbated by open wall lines and torsional irregularity.

• Appropriate mitigation programs should be

determined by local political processes with technical input from organizations like SEAONC, SEAOSC, and SEAOC. Housing stock vulnerabilities in both Los Angeles and San Francisco appear to justify their mandatory retrofit programs.

• The retrofit objective used in current programs –

Life Safety with reduced force levels, with allowances for practicality – is appropriate. Life Safety is a performance level defined in ASCE 41. Collapse Prevention, also defined in ASCE 41, might be appropriate as well, especially for the weakest or most collapse-prone existing conditions.3

3 Chapter A4 and FEMA P-807 do not state performance objectives in ASCE 41 terms, and each might deliver performance different from strict Life Safety. The CEBC, however, draws a policy equivalence between Chapter A4 and ASCE 41 Life Safety with reduced loads, which is why Life Safety has been adopted as the performance level for use with that standard.

• Allowances for practicality include acceptance of lesser deficiencies above the critical SWOF story. Thus, a first-story-only and structural-only scope is appropriate for mandatory retrofits, even though such a scope might not meet the full intent of Life Safety Building Performance as defined in ASCE 41.

• For typical buildings, various steel and wood retrofit

systems, alone or in combination, are suitable. Stiffer systems, such as concrete or masonry shear walls or steel braced frames, are also suitable within the limits of the selected analysis and design criteria.

• Mandatory retrofit programs warrant special

consideration in terms of design and construction quality assurance.

Table 1. California Cities and Their Soft Story Mitigation Programs, as of July 2017

City Current Program Type (year implemented)

Approximate Number of Buildings

Program Status

Berkeley Mandatory evaluation (2005) Mandatory retrofit (2014)

270 All retrofits due to be complete by the end of 2018.

Fremont Mandatory retrofit (2007) 22 Complete in 2012. Alameda Mandatory evaluation (2009) 100 Complete in 2012. Oakland Mandatory screening (2009)

Subsidized voluntary retrofit (2017) 1400 Screening complete in 2011. Subsidy program with FEMA,

covering about 100 buildings, ongoing. Mandatory retrofit ordinance in development.

San Francisco Mandatory retrofit (2013) 4900 Ongoing with phased deadlines. About 4000 retrofits expected to be complete by mid-2019, balance by late 2020.

Mountain View None 100 Inventory and ordinance development in progress. Palo Alto None 300 Inventory and ordinance development in progress. Los Angeles Mandatory retrofit (2015) 13,500 Ongoing with staggered deadlines. All retrofits expected to be

complete by 2024. Santa Monica Mandatory retrofit (2017) 1,600 6-year plan expected to begin end of 2017, with all retrofits

complete by end of 2024. West Hollywood

Mandatory retrofit (2017) 800 5-year plan expected to begin in April 2018, with all retrofits complete by 2023.

Beverly Hills None 300 Inventory and ordinance development in progress. References: Berkeley (2017); Fremont (2007; ABAG, 2016); Alameda (2017); Oakland (2009, 2017); San Francisco (2013); Mountain View (2016); Palo Alto (Schotanus et al., 2017); Los Angeles (2017); Santa Monica (2017A; 2017B); West Hollywood (2017); Beverly Hills (2017).


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• In general, the intent and limits of any retrofit should

be communicated clearly from the engineer to the owner/client. In a prescriptive mandatory program, the intent might be only to comply with the legal requirements, with no stated performance objective. Even this is useful for owner/clients to understand.

While there is consensus on these broad points, the programs around California vary in their details. Many of the differences are small relative to the overall goals. For example, different jurisdictions target different groups of buildings by age, stories, or number of units. They set different schedules and deadlines to suit their building inventories, owners, and staff capacities. These are largely policy decisions. Other differences between programs are technical, and some are significant, reflecting a divergence in code interpretation and engineering practice. The following discussion addresses five distinct topics. Others are also worthy of review, but this is a start, intended as a step toward establishing consensus among SEAOC committees, SEAOC members, and local building officials. 1. Should Chapter A4 be used to identify, screen, or evaluate existing conditions? Chapter A4 offers prescriptive retrofit provisions based on the code for new construction. The base shear is reduced with a 0.75 factor, and the retrofit scope is limited to the critical story. Otherwise, Chapter A4 relies on CBC/IBC/ASCE 7 provisions for allowed materials and systems, acceptability criteria, and design parameters including R and Cd. Are these code-based criteria appropriate for assessing the potential SWOF deficiencies and other non-conforming conditions in existing woodframe buildings? Specifically, are they appropriate for creating a list of buildings subject to an ordinance, or for demonstrating that a given building should be exempt from retrofit? L.A. approach: The 13,500 buildings subject to the L.A. ordinance were identified by LADBS staff from existing public records and from visual assessment of open front conditions. The process did not involve any analysis or any calculation of strength or stiffness, so it did not rely on Chapter A4 or related criteria. However, the program does rely on a modification of the Chapter A4 definitions (discussed in topic 2 below) to set each building’s retrofit scope (SEAOSC, 2016A). In concept, the program allows the same modified definitions to be used to show that a listed building is already sufficient, and therefore exempt from retrofit. S.F. approach: As in Los Angeles, the San Francisco list was compiled without building-specific calculations. Unlike L.A.,

the S.F. process did not involve city staff judgments. Instead, a list of over 6600 buildings was generated from Housing Department data on building age and number of units (ESIP, 2014). A 1-year screening phase was then used to confirm and correct the list, leaving about 4900 buildings subject to the retrofit mandate (SFDBI, 2017B). The screening relied on a judgment-based definition of a new term – target story – that lets design professionals identify vulnerable woodframe structures without calculations (SFDBI, 2017A). A building screened into the program can comply by evaluation (that is, without retrofit), but San Francisco does not allow Chapter A4 to be used as evaluation criteria. Thus, Chapter A4 does not figure in either the screening or the evaluation of buildings subject to the ordinance. Discussion: There are two aspects to this question. The first involves the use of Chapter A4 definitions of “soft wall line,” “weak wall line,” and “open-front wall line.” The second involves the use of material properties and design parameters, including R, for analysis of obsolete conditions. Two of the Chapter A4 definitions are essentially the same as the definitions of “soft story” and “weak story” used by ASCE 41 for Tier 1 checklist evaluation and by ASCE 7 to define vertical irregularities in new construction. They compare the strength or stiffness of the critical story to that of the story above, defining a soft condition if the stiffness ratio is less than 70 percent and a weak one if the strength ratio is under 80 percent. As such, identifying either of these conditions requires a knowledge of the wall layout, construction, and detailing of two whole stories, as well as some basic calculations. These prescriptive rules are suitable for catching irregularities during design and for flagging potential deficiencies to be confirmed or ruled out by more careful calculation. But they are not feasible for compiling a list of properties subject to an ordinance. In both San Francisco and Los Angeles, lists of owners to receive ordinance notifications had to be developed without full knowledge of any specific building. Even in smaller jurisdictions, it is not feasible – and probably not a good idea in liability terms either – to classify a building as having a calculated soft story or weak story deficiency based only on public records or sidewalk surveys. Whether these two definitions are adequate for exempting a building from retrofit is an important question too. The SEAOSC Design Guide (2016A) takes just this approach, suggesting that the retrofit scope can be reduced, possibly to nothing, if each perimeter wall line is as long in the first story as it is in the second. This logic is flawed, however, if it misses the contributions of interior walls or partitions, or if it overlooks a significant torsional deficiency (see topic 2 below


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for a more thorough discussion of L.A.’s line-by-line approach). For this reason, the S.F. program does not allow exemption by the calculated soft and weak rules alone, but does allow evaluation with ASCE 41, which considers torsion, diaphragms, load path, a basic stress check, and other issues. The third Chapter A4 definition, “open-front wall line,” requires no calculation, relying instead on an understanding of wall configurations that require diaphragm rotation. Thus, an open wall line can often be identified by a quick visual assessment. The characteristic appearance of an open wall line, comprising a series of shop windows, garage doors, or open parking stalls separated by pipe columns, has become the basis for many a sidewalk, windshield, or even online image survey performed as an early step in developing a mitigation program. These surveys can miss buildings where the open wall line is obscured from view, however, as is known to have happened in Berkeley’s pioneering program (Bonowitz and Rabinovici, 2013). Reliance on the look of an open wall line can also miss collapse-prone conditions caused by open ground floors with weak or soft – but not open – perimeter walls. The best approach for compiling a list of buildings subject to an ordinance is probably to cast a wide net, as San Francisco did, and let each owner (with the help of a design professional) make the case for exemption based on building-specific conditions or thorough evaluation. While this has the political disadvantage of making buildings “guilty until proven innocent,” it worked reliably in San Francisco (ESIP, 2014), and owners were happy to pay a small fee to confirm their status early in the program. The second aspect of using Chapter A4 for evaluation involves the existing building’s obsolete or non-conforming conditions and whether they are properly addressed by code provisions for new construction. What is the R value for horizontal board siding? What is the best way to calculate the drift of a stud wall sheathed with stucco on one side and 1920s plaster on the other? If I count on the strength and stiffness of the non-conforming perimeter walls, should I also model the interior partitions? ASCE 41’s predecessors were first developed precisely because the code for new construction could not handle these questions well. Even if good engineers find alternative sources to inform a code-based analysis, Chapter A4 itself does not help them, so inconsistency between engineers is almost certain (Bonowitz and Rabinovici, 2013). Strawman recommendation: Chapter A4 definitions can be useful for identifying potential deficiencies as a first step toward more thorough evaluation (with ASCE 41 or similar criteria) or toward retrofit scoping. They are not appropriate

as a way to identify or select buildings for a mitigation program, however, because they rely on calculation that is impractical in a program’s development stage. Chapter A4 criteria should not be used for analytical evaluations of existing non-conforming buildings because they rely on assumptions and structural attributes that apply only in new or retrofit construction. 2. Can deficiencies be identified and mitigated line-by-line, or is a “whole story” approach necessary? Past collapses of SWOF buildings are associated with characteristic open-front conditions. If the open front is the critical flaw, perhaps adding strength and stiffness to just that wall line would be an adequate retrofit, especially if doing so mitigates the diaphragm rotation that an open-front wall line allows. Similarly, if only one or two wall lines in the critical first story are soft or weak compared with the same wall lines above, is it sufficient to retrofit just those lines? Or does an adequate retrofit require consideration of overall structural redundancy, torsion control, and strengthening of weak and non-conforming stud walls? L.A. approach: The Los Angeles ordinance allows a line-by-line retrofit, re-defining the key terms from Chapter A4. For example, here is the definition of “weak wall line” from Chapter A4, followed by the revised definition from Los Angeles (2015). Note the shift from story strength to wall line strength:

WEAK WALL LINE. A wall line in a story where the story strength is less than 80 percent of the story above in the direction under consideration. WEAK WALL LINE is a wall line at the ground floor where the wall strength is less than 80 percent of the wall above in the direction under consideration.

To implement the revised definitions, LADBS (2016) requires, “Wall lines along the parking or similar open space shall be evaluated to determine the soft, weak, or open front wall lines.” SEAOSC (2016A, Sec 1.2) advises consideration of wall lines beyond those at the parking area but still understands the ordinance scope as “limited to wall lines around the perimeter of the building.” Thus, the L.A. ordinance does not – and is perhaps not meant to – fully mitigate a soft or weak story deficiency. Its intent is to reduce risk by strengthening suspect perimeter wall lines whose inadequacy is thought to contribute to or aggravate a soft or weak story deficiency.


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Other Southern California cities, including Santa Monica (2017A) and West Hollywood (2017), have taken the same line-by-line approach as Los Angeles. S.F. approach: By contrast, San Francisco implements Chapter A4 as the CEBC and its predecessor documents intend, requiring consideration of the whole target story. Other Northern California cities, including Fremont (2007), Berkeley (2017), and ABAG (2016), have taken the same full story approach as San Francisco. Discussion: With Northern and Southern California cities taking different approaches, this topic points to a significant divergence in practice across the state. There is no question that the line approach, particularly if it results in less retrofit construction, represents a lower implied performance objective than the story approach (SEAOSC, 2016A, Sec 2.2; Burton et al., 2016).4 The SEAOSC guide to the L.A. ordinance acknowledges as much, as noted above. But even the story approach of Chapter A4, while backed by consensus development, represents a limited objective. For practicality, Chapter A4 does not address structural deficiencies above the target story, and it does not address nonstructural deficiencies at all. Rather, the question is whether the lower performance delivered by the line approach is too low to be worthwhile. A related question is whether the line approach leads to an incomplete understanding of the retrofit’s intent or effectiveness by owners, policy-makers, or even engineers. As the SEAOSC guide notes, “the goal [of the line approach] is to strengthen the portion of the building that has performed poorly in past earthquakes.” That is, the developers of the L.A. ordinance intentionally called for a limited retrofit scope with the intent of mitigating the building’s worst deficiency and keeping costs to a minimum by leaving the rest alone. That’s a sensible philosophy. But can a collapse-prone deficiency be ascribed to a single wall line, ignoring interactions with the rest of the structure? And can a one-line retrofit reliably address the risk posed by these non-conforming buildings? Recent studies (and engineering common sense, critically applied) suggest the answer is often no. A focus on individual wall lines can lead to errors, leading to ineffective retrofits, tagging acceptable structures as deficient, and missing the big picture that good engineering tools are meant to illuminate. 4 In both cases, the performance objective is “implied” because CEBC Appendix Chapter A4 does not state an objective in performance-based terms.

Consider the example building used by the SEAOSC guide to illustrate the L.A. ordinance requirements, shown in Figure 1. By the L.A. ordinance (and as described by SEAOSC), this is a vulnerable building in both directions. In the short direction, line B is a problem because it’s open. In the long direction, line 1 is deficient because in the first story it has less than 80 percent of the sheathed length of line 1 in the story above. But just looking at the plan, this is not an obviously bad structure at all; it has two substantial lines of resistance in both directions, each located at or near the perimeter for torsion control.5 True, if the sheathing is non-conforming (stucco, plaster, board siding), then the existing walls might not pass a stress check, but that has nothing to do with the openness of line B or the “weak” first story of line 1. In fact, focusing on the look of line B or the length of line 1 misses any absolute (not relative) weakness on lines A and 2. It’s also true that because of the tuck-under parking area, the second floor’s center of mass will be eccentric to the first story’s center of stiffness, but that mild irregularity is hardly the collapse-prone condition a mandatory mitigation program is looking for.

Figure 1. First story plan of tuck-under building, after

SEAOSC (2016A) Fig. 1.4-2 By focusing on the open line B, the L.A. ordinance’s line approach also misses the real problem with this building, which is not the lack of structure on line B but the potential overstress on line A.8. If existing line A.8 (and line A at the other end) has decent wood sheathing, as many 1960s buildings do, there’s a good chance the structure is fine in the short direction, even with the tuck-under parking. If it’s sheathed only with stucco, and is perforated with doors and

5 Some engineers have speculated that the prescriptive nature of the L.A. program and heavy marketing by contractors have discouraged owners from hiring engineers to actually study their existing buildings and avoid retrofitting already acceptable structures. The market effects of soft story programs are important and interesting but beyond the scope of this paper.

B A A.8 A.3

1.2 1

2

Upper story perimeter walls

First story perimeter walls Open parking

area

Pipe column


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windows, there’s a good chance this critical line will be over-stressed – but the line approach doesn’t help the engineer find the deficiency on line A.8, as it looks only at line B. Finally, by focusing on line B, the line approach misses the chance to locate the retrofit on line A.8 instead. Strengthening line A.8 would take advantage of the existing wall framing and footing and would avoid the far greater cost and disruption of installing a new steel frame and foundation on line B. Figure 2 presents a similar building. Without a substantial force resisting element on line A.8, the torsional deficiency and weakness in the short direction are more obvious. With a line approach, the L.A. ordinance’s prescription for this building is to strengthen and stiffen only line B – not to balance the existing structure and reduce torsion, but to carry the tributary 75% base shear required of new construction. The retrofitted structure, newly stiff on one end but untouched on the other sides, could easily end up less balanced and subject to more torsion. That makes existing line A (old stucco with openings?) even more vulnerable – but line A is outside the ordinance scope.

Figure 2. First story plan, short side open

Figures 1 and 2 thus show how a line approach can over-strengthen and over-stiffen one side of a building. At worst, this can aggravate an existing deficiency in the un-retrofitted wall lines. At best, this approach might only reverse the torsional collapse mode by shifting the critical wall line from one end of the building to the other, with only a small reduction in the overall probability of collapse.6

6 Unbalanced strengthening can be made worse by two other aspects of the L.A. provisions. A redundancy factor greater than one and an R factor of only 3.5 are both likely from a line approach, and both, artificially, will lead to even stronger and stiffer walls or frames along the one or two retrofitted lines. R factors are discussed further in the next section.

The unintended consequences of a line approach are also demonstrated by Figure 3, which shows a building with the same plan dimensions as Figures 1 and 2, but with the long side open instead. The line approach would again target only the open side, in this case along line 2, possibly leading to an unbalanced structure after retrofit. The issue with a structure like this is not the long direction strength; the solid wall at the back of the parking area on line 1 is often adequate. Rather, the problem is the torsion resulting from the open front, resisted by the short direction walls on lines A and B. The line approach discourages the engineer from thinking about these walls or understanding their role in the overall performance. If the open line 2 is retrofitted carefully, that can eliminate much of the torsion. But the short direction walls are still important. A study of typical Berkeley buildings found that with a long narrow plan, despite the open long side, the short direction is actually critical, not because it’s open but because it simply has less wall (Bonowitz and Rabinovici, 2013).

Figure 3. First story plan, long side open

Burton et al. (2016) made the same finding with nonlinear response history analysis of a similar SEAOSC example building. In the existing building, the long direction with the open front already has acceptable drifts of 2.5% but is forced to retrofit anyway. Meanwhile, the short direction without an open wall line is prone to higher drifts (3.6%) and accounts for 61 percent of collapses in bi-directional loading. Yet that critical direction would be left untouched by the L.A. ordinance. After retrofit, the median collapse acceleration is increased a mere 14 percent. Why? Because the short direction is not retrofitted at all, so after retrofit it accounts for 93 percent of the collapses. The open-front retrofit removed the visually obvious irregularity in one direction, but the next weakest link, ignored by the L.A. ordinance, remains and is not much better.

B A A.8 A.3

1.2 1

2

Non-conforming partition

B A A.8 A.3

1.2 1

2

Non-conforming partition


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This was one of the findings from the work that led to FEMA P-807, confirmed by the Berkeley study: “The strength of the first story walls is far more significant to a building’s performance than either its observable openness or the relative strength of its first and second stories” (Bonowitz and Rabinovici, 2013). A line approach to SWOF mitigation makes it too easy to miss this lesson. The line approach is an invention of the current Los Angeles program not found in other codes or standards. Its purpose, well intended, is to avoid the construction-phase disruption to occupied ground floor units that would result from Chapter A4’s story approach.7 But these buildings with ground floor units are also the ones in which a first story collapse kills tenants. If the line approach does not measurably improve performance, the reduced cost and disruption might not be worth the marginal benefit. Certainly, a line approach can improve a range of deficient buildings, and in many cases the results of a line approach and a story approach will be similar. Where the non-open wall lines already have wood sheathing and few openings, the ill effects of a line approach can probably be avoided. But in other cases, including the SEAOSC example buildings discussed here, the line approach can obscure the critical issues and lead to ineffective or unintended consequences. Strawman recommendation: In the short term, the City of Los Angeles should revise its program criteria to use a story approach like that of Chapter A4 as the default, and allow a line approach only in cases where it can be shown to produce equivalent results – essentially the opposite of the policy now in place. Because the line approach is written into the definitions in the L.A. ordinance itself, this revision will probably require new or trailing legislation, but that should not be an obstacle, as technical revisions to the ordinance have already been made (Los Angeles, 2016). Anticipating adjustments as the L.A. program continues over the next seven years, the city should commission more studies of actual buildings to identify the cases where a line approach provides acceptable risk reduction and is cost-beneficial. 7 Typical Los Angeles buildings (like some smaller Berkeley buildings) are more likely to include ground floor units than typical San Francisco apartment buildings. The San Francisco building stock, however, probably includes more mixed-use buildings with ground floor commercial space. Related to this is the question of whether the primary purpose of a citywide SWOF mitigation program is to save lives or to preserve housing. This is an essential policy question, but one beyond the scope of this paper.

Other jurisdictions that allow a line approach based on the L.A. precedent should make similar changes. At the same time, San Francisco and other cities that allow only a story approach should participate in the recommended studies and remain open to reduced retrofit scopes. 3. What are the appropriate R-factors to use for new retrofit elements installed into existing structures? As code-based criteria, Chapter A4 and the L.A. provisions both require the selection of retrofit elements from a list of systems defined for new construction; each system has its own prescribed design parameters, including the response modification coefficient R. The code for new construction also references rules from ASCE 7 regarding modifications and limits on R values when different system types are combined within a single structure. Two of these ASCE 7 rules are of special interest to the design of code-based retrofits of woodframe SWOF buildings: • Section 12.2.3.1 requires that for any vertical

combination of systems, the lowest stories must use the smallest of the various R values. This means that for a first-story-only retrofit, the R value for the retrofit system is limited by the value that would be assigned to the non-conforming woodframe walls in the upper stories. Thus, even a new steel moment frame (R = 3.5 or higher) or wood structural panel (R = 6.5) would be limited to an R value of just 2.

• Section 12.2.3.3 requires that for any combination of

systems providing resistance in the same direction within the same story, the smallest of the various R values must be used for all systems. An exception is made for 2-story buildings with light frame construction and flexible diaphragms assigned to Risk Category II. This rule is especially relevant where steel frame elements are used in combination with wood structural panel shear walls.

In both cases, the code’s reasonable conservatism for new construction has the effect of making the retrofit more expensive and possibly stronger or stiffer than the optimal design. Should these rules for new construction be applied as written, or should they be adjusted or waived when applied to retrofits of non-conforming woodframe structures? L.A. approach: Los Angeles addressed the first issue, vertical combinations, with its 2016 legislative revision, allowing that R need not be taken less than 3.5, except for cantilever column systems (Los Angeles, 2016). This allows the engineer to use an ordinary steel moment frame (R = 3.5) without having to increase the design forces by imposing R =


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2. If the engineer prefers an intermediate (R = 4.5) or special steel moment frame (R = 8), however, the design will still have to use R = 3.5.8 The second issue, horizontal combinations within a story, is largely moot in Los Angeles because of its focus on individual wall lines (see topic 2 above). Even if multiple wall lines are found deficient, each different system would still be held to the limits of the vertical combination rule, so both steel and wood systems would have to use R = 3.5, no matter how ductile. In essence, then, the entire L.A. program is premised on retrofits designed prescriptively with R = 3.5. S.F. approach: By adopting Chapter A4, San Francisco addresses the first issue with an allowance already in Section A403.3. This section waives the ASCE 7 restriction as long as the retrofit eliminates certain vertical irregularities. This allows wood structural panel shear walls and steel moment frames (of any type) to be designed using the R values prescribed for new construction. On the second issue, based on work by SEAONC Existing Buildings Committee volunteers, San Francisco has amended its Administrative Bulletin 107 to allow the design of separate retrofit systems with separate R values. The revised bulletin adds an exception to Section A403.3, as follows:

For retrofit systems involving different seismic force-resisting systems in the same direction within the same story, resisting elements are permitted to be designed using the least value of R for the different structural systems found in each independent line of resistance if the following conditions are met: (1) Risk Category I or II building, (2) four stories or less above grade plane, and (3) the seismic force- resisting systems are composed of WSP shear walls, steel moment-resisting frames, steel cantilever columns, and steel braced frames. Values for Cd and Ω0 shall be consistent with the R value used. (SFDBI, 2017A)

Discussion: R values for vertical combinations were identified as a retrofit issue when California adopted the 8 Engineers recognize that an IMF or SMF will provide more ductility than an OMF. Many are nevertheless using OMF systems for these retrofits because the ordinary system has fewer limits on member size and detailing, improving constructability. Also, even if steel SMF systems can be designed for smaller forces, their sizing is often controlled by drift limits, so the frame ends up similar to an OMF anyway, and any cost saving in the steel is not realized.

International Building Code (referencing ASCE 7-05) as its model code for the 2007 CBC. Earlier versions of what would become IEBC Chapter A4 had been written for use in California in coordination with the 1997 Uniform Building Code; the UBC had offered an R value for wood framing of 4.5, not 2. The SEAOC-written (and now obsolete) commentary to the 2006 IEBC called for consistent use of the IBC values (R = 2) but suggested that the effects would be mitigated by the need to design new steel systems for drift. For the 2012 IEBC, SEAOC and NCSEA proposed the revision that is in IEBC and CEBC Section A403.3 today. The change was based on judgment, not on a technical proof of equivalence or sufficiency. The reason statement for the change read, in part:

[T]he question is how to allow a more practical design (and R value) suited to the risk reduction retrofit of existing buildings, where new systems are typically used to supplement old ones, as opposed to new construction, where all new systems must and should comply completely with the building code. The right solution should acknowledge that no single R value can be stipulated for the design of strengthening elements, that Chapter A4 is intended as a prescriptive and somewhat simplified alternative to ASCE 41, and that the principal goal of retrofitting SWOF buildings is to eliminate their SWOF deficiency” (ICC, 2009, proposal EB60-09/10).

Thus, Chapter A4 and the L.A. ordinance take different approaches to a common issue, but both solutions are based primarily on judgment and practicality. Considering horizontal combinations within a single story, the AB 107 exception developed from discussions within the SEAONC EBC about proper retrofit design for a common San Francisco situation. In many buildings, an effective retrofit scheme involves adding wood structural panel sheathing over existing stud walls, and installing steel frames or cantilever columns along an open-front wall line so as not to block parking bays or storefront windows. But by the ASCE 7 rule, the ductile wood shear walls in the rear half of the building would have to be designed with the conservative R value of the ordinary steel elements along the open front line. EBC members noted that the ASCE 7 rule allows an exception for new 2-story buildings. It seemed that the same exception might apply fairly to first-story-only retrofits of existing collapse-prone buildings, using the same consensus judgment applied to the case of vertical combinations years earlier. Plus, neither ASCE 41 nor FEMA P-807 (both of


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which are also allowed by San Francisco’s program) imposes a similar restriction when different systems are used together. The argument against this suggestion is that ASCE 41 and FEMA P-807 use unreduced loads and displacement-based acceptability criteria. If an engineer using Chapter A4 wants to size steel and wood elements separately, she is free to use either of those alternative documents. But if code-based criteria are going to be used and are preferred for their familiarity, they should be applied without major allowances for convenience. To resolve the issue, SEAONC volunteers studied the relative performance of combined systems designed with and without the ASCE 7 rule. They found that combining systems, each with its own R value, has no detrimental effect on collapse probability (Maison et al., 2017). Strawman recommendation: If Los Angeles and other cities that use a line approach to scope retrofits (see topic 2 above) retain that approach, their use of R = 3.5 for all retrofit elements might be an equally appropriate simplification. Cities that use (or switch to) a story approach should use the provision of Chapter A4 regarding vertical combinations and should consider San Francisco’s allowance regarding different parallel systems within a story. 4. How should engineers address potential over-strengthening of the first story? In a collapse-prone SWOF building, lateral deformations concentrate in the critical first story. The studies that led to FEMA P-807 demonstrated that as the first story is strengthened and stiffened, upper stories can become critical instead, as illustrated in Figure 4. In the existing building, the first story is prone to collapse-inducing drifts while the upper stories feel practically no deformation. As stiffness is added to the critical first story, its drift is reduced to an acceptable level (4% in FEMA P-807, assuming a ductile retrofit system), while drifts in the upper stories increase slightly. With an optimal or “best performance” retrofit, the first story drift is reduced as much as possible while keeping the upper story drifts below their critical level (1.25% in FEMA P-807, assuming brittle or non-conforming existing materials). If the first story retrofit is made even stiffer, the second story will lose strength as its drift limit is exceeded. This is a concern if the retrofit is so overly-strong as to induce a collapse in the brittle second story (a possibility confirmed by Buckalew et al., 2015 and Burton et al., 2016). It can even be a concern if the upper story has enough damage to require move-out during repairs, since the point of many retrofit

programs is to preserve occupiable housing during post-earthquake recovery. For these reasons, FEMA P-807 limits the strength of the retrofitted first story to about 1.3 times the strength of the second (ATC, 2012, Section 6.2.2). In a building with robust upper story walls, and given a nominal safety-based performance objective, the limit might have little effect. In weaker (usually older) buildings, or with a more aggressive objective, the limit effectively rules out a first-story-only retrofit if FEMA P-807 is used. But what about retrofits designed with Chapter A4, which sets no limit on the first story strength and requires no check of the upper stories at all? Should the retrofit strength be capped to prevent damage or collapse in an upper story? Or is some overdesign of the first story retrofit acceptable, given the project or program objective?

Figure 4. Story drift ratios in a building with constant upper story strength and varying first story strength,

from FEMA P-807 Figure 2-6 L.A. approach: With its code-based criteria, the Los Angeles provisions set no limit on the strength of the retrofitted first story.9

9 Los Angeles does allow the use of FEMA P-807. Presumably, the first story strength limit would apply when that alternative is taken. Application of the alternative criteria is beyond the scope of this paper.


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As the LADBS guidelines were being developed, SEAOSC recommended an optional cap on the required first story strength (see SEAOSC, 2016A, Section 1.8). An explicitly stated option would at least encourage engineers to check for an overly-strong retrofit. LADBS staff declined the SEAOSC recommendation, however, on grounds that it would involve investigation and documentation of the second story walls, as well as tracking of future upper story alterations. The corresponding criteria and procedures, it was thought, would make enforcement infeasible; without proper enforcement, engineers might be encouraged to game the provision to improperly reduce the scope of retrofit. S.F. approach: San Francisco attempts to apply the lesson of FEMA P-807 even to retrofits based on Chapter A4. Administrative Bulletin 107 adds the following sentence to Section A403.3:

Despite any other requirement of Section A403.3 or A403.4, the total expected strength of retrofit elements added to any target story need not exceed 1.3 times the expected strength of the story immediately above, as long as the retrofit elements are located symmetrically about the center of mass of the story above or so as to minimize torsion in the target story.

For calculation of story strength, the bulletin’s commentary references its own provisions implementing FEMA P-807. To account for future alterations that might affect elements contributing to the second story strength, San Francisco added a one-sentence provision to SFEBC Section 403 for Alterations (then SFBC Section 3404):

403.13 Mandatory Seismic Retrofit. Submittal documents shall include plans indicating locations and construction of existing, new and modified building elements used to comply with [the mandatory soft story provisions].

It is not known how many engineers are applying the optional cap to their San Francisco projects. Discussion: Despite the LADBS concern over implementation and enforcement feasibility, neither SEAONC nor SEAOSC opposes the idea of capping the strength of the retrofitted first story. On the contrary, some engineers see a strength limit as essential. Most, however, view it as useful, perhaps advisable, but not necessary. The argument to set a maximum allowable first story strength is the same as that made in FEMA P-807: Over-strengthening the first story drives damage into the occupied upper stories, leading possibly to post-earthquake disruption and repair

costs and, in the worst cases, structural damage or even story collapse. From this perspective, it is better to have the damage in the ductile retrofit elements within the first story than in the existing brittle elements in the story above. The argument for an optional cap recognizes the FEMA P-807 finding that strengthening beyond an optimal point does not increase overall structural capacity. Designing to that optimal point with a code-based approach like Chapter A4, however, is difficult at best. Requiring such a precise design would be misguided. Still, if over-strengthening is wasteful, it makes sense at least to allow engineers and owners to save the cost of excess construction. If capping is optional, as in San Francisco, or not even considered, as in Los Angeles, is that unconservative? Buckalew et al. (2015) studied two 1920s era San Francisco buildings with non-conforming construction in the upper stories. In one case, a Chapter A4 retrofit did not exceed 1.3 times the strength of the upper story, so a cap would not have applied. In the other case, a 3-story midblock building, the uncapped Chapter A4 retrofit reduced the probability of collapse (POC) from nearly 100 percent in an extreme event (SS = 1.50g) to 46 percent. With the same model rigged to prevent second-story collapse, the Chapter A4 retrofit would have cut the POC even further, to 19 percent. The difference is due to the potential for upper story collapse, which controlled 15 of 22 response history analyses. Meanwhile, a FEMA P-807 retrofit of the same building, representative of a capped design, reduced the POC in the extreme event to only about 70 percent. So the choice is between: • 70 percent POC with damage confined to a ductile first

story (FEMA P-807, capped), and • 46 percent POC with damage likely in the brittle upper

story (Chapter A4, uncapped). An argument can be made that the uncapped Chapter A4 retrofit is the safer choice. The capped retrofit is not bad, however. In smaller, more likely events, both designs deliver low probabilities of collapse, and the capped design keeps any structural damage in the ductile retrofit elements. Even with an over-strengthened first story, structural framing damage or collapse in an upper story would not be expected except in the very weakest and least-conforming buildings, or in an extreme earthquake that typical retrofit criteria do not consider. The prospect of costly or disruptive sheathing or cosmetic damage in an upper story is real, but it must be weighed against the cost of something other than a code-based first-story-only retrofit. That is, if a Chapter A4 retrofit is considered too stiff and strong, the alternative is to do a (far more costly) full-building retrofit or to accept a capped design that might provide lower performance.


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The political and logistical feasibility of a mandatory program for thousands of buildings relies on the selection of practical retrofit criteria tailored to obvious and severe deficiencies – such as collapse-prone SWOF conditions. Indeed, practicality is the driving idea behind Los Angeles’ line-by-line approach, San Francisco’s longer compliance period for commercial occupancies, and the whole Chapter A4 notion of a first-story-only retrofit to begin with. San Francisco’s implementation of FEMA P-807 even relaxes the performance objective in certain cases to avoid requiring work above the target story (SFDBI, 2017A, Part B). For the most collapse-prone buildings, where the strength limit would be most likely to apply, studies to support San Francisco’s FEMA P-807 criteria showed that even a practical, capped design would cut the probability of excessive drift from more than 90 percent to about 50 percent in a design event (Bonowitz, 2013). Thus, as a policy matter, if over-strengthening is deemed undesirable, then capping the first story strength seems preferable to requiring a full-height retrofit, at least for mandatory programs. Strawman recommendation: Los Angeles should modify its guidelines to allow at least an optional “need not exceed” cap on the strength of the first story retrofit. Ideally, this change would accompany the shift from a line-by-line approach to a story approach, as discussed above, as the benefit of a strength cap along a single line has not been studied. FEMA P-807 imposes a first story strength cap by default. For a Chapter A4 retrofit, Los Angeles could take the simple approach adopted by San Francisco. San Francisco’s optional cap, appropriate to 3- and 4-story buildings, reflects the drift-based criteria of FEMA P-807. Another approach, described by SEAOSC (2016A, Section 1.8), would have the first story retrofit elements sized to give a consistent demand-capacity ratio from story to story. Thus, a weaker second story would set a lower cap on the first story retrofit strength while also considering the variation of shear demand over the height of the building. (However, this approach does not account for differences in system ductility and reliability between the existing materials in the upper story and the retrofit system in the first story.) Both approaches require investigation of the second story that Chapter A4 by itself does not. More study of capped and uncapped retrofits of typical Los Angeles and San Francisco buildings would benefit all stakeholders.

5. In a citywide mandatory program, are additional quality assurance measures needed? Existing buildings pose design and construction quality issues different from those raised by new construction. Mandatory retrofit programs are different still. In a mandatory program, no matter how carefully developed by policy-makers and consultants, some owners will be reluctant to comply. Forced by an ordinance to spend money, they will understandably seek the least-cost path to compliance, opening the door to cut-rate engineers and contractors. This can create market pressures on the rest of the profession, discouraging even normal quality control measures. Even owners willing to pay for good work might find, depending on the local market, that qualified professionals are unavailable to meet program deadlines. Further, many owners of residential buildings are inexperienced with heavy construction. While most will have dealt with contractors on architectural renovations or small repairs, many are unsure of how to find and hire a structural engineer. Some owners also have the false impression that it is the job of city plan checkers and inspectors to protect them from unprofessional work. For these reasons, the mandatory retrofit market is different from the normal market for design and construction services. From observations of the San Francisco and Los Angeles programs, it seems that a substantial portion of the owner group either does not know how to recognize (or worse, does not care about) reliable, high quality design. Given these challenges, should mandatory soft story retrofit programs include special procedures or code provisions to assure the quality of work by building department staff, engineers, contractors, and special inspectors? L.A. approach: In general, the Los Angeles program is relying on its existing administrative and technical code provisions for quality assurance. As one of the largest building departments in the country, LADBS also has a thorough set of customized submittal forms and approval checklists. In addition, the department has created a special unit to handle the thousands of additional retrofits now mandated over the next seven years. Though the program is still in its first year, this is expected to provide consistency in plan review and field inspection. Even so, and even with the suddenly huge demand for professional services, some engineers are already recognizing the shift in the market described above. Whether through inexperience or to gain a competitive advantage, engineers are bidding soft story projects without up-front investigation, a practice expected to cause problems during construction.


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S.F. approach: San Francisco’s Department of Building Inspection (SFDBI) is also relying on its existing procedures and provisions. The department dedicated a number of plan check engineers and one manager to its soft story program but has not yet created a special unit for field inspectors. Neither the California Building Code nor the San Francisco amendments would necessarily require structural observation for these soft story retrofits, but Section 1704.6.1 does allow the building official to require structural observation by discretion. For its soft story program, the SFDBI program manager has committed to require structural observation for every project (Chun, 2017). As of mid-August 2017, construction is already essentially complete on about 1400 projects. Incidents of unacceptable design and construction errors, even in completed and inspected projects, have come to light. They appear to be rare, but without an independent, randomized quality assurance review, the rate remains unknown. Prompted by testimony at a public meeting of the city’s Building Inspection Commission, SFDBI staff performed 29 follow-up inspections and found one case that required revisions (Sabatini, 2017). The program is approaching a critical deadline on September 15, when owners of 3500 buildings (in the third of four Tiers) are due to submit plans and permit applications. As of mid-August about 1300 remain to be submitted in the final month (SFDBI, 2017B), posing a challenge for the owners, their engineers, and SFDBI. Discussion: To be sure, most of the mandated work being done in San Francisco is acceptable, with no more quality concerns than on any voluntary retrofit or new construction. (In Los Angeles, the construction phase is still in its early days.) Yet “most” is hardly good enough. Without a concerted quality assurance program involving follow-up inspections and design reviews, neither SFDBI, nor SEAONC, nor the city’s owners and tenants can say for sure that the program is yielding consistent high quality work. Meanwhile, examples of unacceptable work have been found in the design, construction, inspection, and regulation (Bonowitz, 2016). On a typical soft story retrofit project, any of five key players can directly affect the quality of design or construction: the engineer, the contractor, the special inspector, the plan checker, and the field inspector. A sixth player, the owner – whether experienced, informed, and motivated, or not – is indirectly involved, as he or she chooses the first three. As noted above, the mandatory retrofit market has opened opportunities for cut-rate engineers, some of whom bid the

design without making a site visit. In some cases, the resulting plans simply do not match the building, leading to obvious problems down the line. The same problem has surfaced in Los Angeles, where engineers report that in order to be competitive, they must forego any budget for site visits during design. Engineers can also offer a lower fee if they omit structural observation, which the building code might not require. Unsophisticated owners do not know to ask for this and might not understand when comparing bids that they are looking at different scopes of work. The contractor, handed those deficient plans, must either ad lib a fix or delay the project by engaging the engineer to make a proper (and code-required) design revision. Absent strong enforcement, all incentives are toward the former. Wouldn’t the special inspector, required by the building code to observe the installation of hold-downs or shear panel nailing, catch a mistaken set of plans? Actually, no. The special inspector is there to observe the work being done, not to ensure that the shear walls are of proper length or the hold-downs are in the right spots. What about the plan checker? First, large mandatory programs are made feasible in part by promising over-the-counter reviews. Every plan checker has discretion to reject a set of plans or calculations, but if applications are backing up, throughput can become a priority. Second, retrofit plans often quite reasonably call for assumed conditions to be “verified in field.” The plan checker must rely to some degree on the contractor and the field inspector to do that verification and to conscientiously file a revision if the assumption proves false. The regulatory process relies on the engineer’s final affidavit of compliance, the certificates of special inspection, and the field inspectors’ signatures on a job card. Bottom line, the plan checker does not go to the field. As for the field inspector, even the best ones will tell you that they never have time to look at everything. In retrofit, that matters, since the existing conditions on one wall line can vary substantially from those elsewhere in the same building. Besides, the ultimate responsibility to build to the plans lies with the contractor, and checking every detail is not the inspector’s job. So with five players, there are at least four gaps in which quality can be lost. The nature of a mandatory program, especially with an unmotivated or uninformed owner, tends to widen each of them. If quality is lacking, engineers are no more responsible than their inspector, contractor, and building official colleagues. But as leading advocates for retrofit programs, engineers and associations like SEAONC, SEAOSC, and SEAOC do have a


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responsibility to narrow those gaps where we can. The success of the citywide programs we endorse is as important as the success of the individual projects we design. Strawman recommendation: Cities should recognize the special challenges posed by mandatory retrofit programs and should supplement their normal regulatory procedures accordingly. At a minimum, program staffing by the building department (or other lead agency within the jurisdiction) should allow for random follow-up inspections and design review to quantify and ensure program consistency. Plan checkers and field inspectors should receive training specific to the retrofit program. Aside from the engineering criteria, all of the regulations needed to ensure quality design and construction are already in the local building code; they need only be identified and prioritized in the context of mandatory seismic retrofit. Department managers should develop program-specific job aids (forms, checklists, templates, cheat sheets, etc.) to improve efficiency and consistency. For large programs, building departments should consider designating certain staff members to specialized units.

Structural observation in accordance with the local building code should be required on every soft story retrofit project, regardless of building size, occupancy, risk category, seismic design category, or performance objective. In addition, building officials should consider supplementing the structural observation provisions with requirements for design phase site visits and condition assessment so that relevant information is available to the plan checker. Finally, building departments and professional associations, including SEAONC, SEAOSC, and SEAOC, should work with property owner groups to educate them about how to identify, assess, and hire qualified engineers and contractors.

Table 2. A Comparison of the San Francisco and Los Angeles Mandatory Soft Story Retrofit Programs

Topic San Francisco Los Angeles Effective date September 15, 2013 November 22, 2015 Code section San Francisco Existing Building Code Chapter

4D, “Mandatory Earthquake Retrofit of Wood-Frame Buildings.”

Los Angeles Municipal Code, Chapter IX, Article I Los Angeles (2015) replaces Division 93, “Mandatory Earthquake Hazard Reduction in Existing Wood-Frame Buildings with Soft, Weak or Open-Front Walls” Los Angeles (2016) further modifies Division 93 to adjust time limits and clarify engineering criteria

Bulletins “Application of Engineering Criteria in SFEBC Chapter 4D,” also known as AB-107 (SFDBI, 2017A). Plus various Information Sheets.

“Structural Design Guidelines” (LADBS, 2016)

Subject buildings About 4900 About 13,500 Age Pre-1978 Pre-1978 Height / Massing 3 or more stories above grade

2 or more stories above a basement or “underfloor area” that extends above grade

1 or more stories above a subject “ground floor” (See Structural Attribute row) “Cripple walls exceeding four feet in height” count as a story.

Occupancy and Units

5 or more dwelling units, regardless of other uses or occupancies “Dwelling unit” is defined to include almost any residential unit in a building of R-1 or R-2 occupancy, including hotels and motels.

4 or more residential units if building is all Residential. Any number of units if building has non-Residential use (presumably Business, Mercantile, or Assembly, not Storage or Utility)

Structural system “Type V (wood-frame) construction” “Wood-frame construction, or wood-frame portions thereof.” LADBS (2016) requires special consideration of mixed systems with existing concrete or masonry walls.


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Topic San Francisco Los Angeles Structural attribute SFDBI (2017A): Any building with a “target

story in which the existing SFRS relies on wood-frame elements.” SFDBI (2017A) defines target story as “Any story above grade plane, or any basement or underfloor space that extends above grade at any point, in which the wall layout or plan configuration is substantially different from the wall layout or plan configuration in the story above,” except for top stories and stories below a pitched-roof attic.

“Ground floor portion of the structure contains parking or other similar open floor space that causes soft, weak, or open-front wall lines.” As written, the ordinance applies to any building with one of the three types of defined wall lines, but it is being enforced only for buildings with “open floor space.” Uses IEBC Chapter A4 definition of Open-Front Wall Line Uses modified version of IEBC Chapter A4 definitions of Soft Wall Line and Weak Wall Line.

Notable exemptions Recently retrofitted buildings in two categories: 1. Retrofits triggered by other work within the prior 15 years, and 2. Retrofits completed under the city’s previous voluntary program using provisions in Administrative Bulletin 94.

SEAOSC (2016A): Cripple walls exempt, regardless of type or extent of sheathing, even if calculations would show them as weak or soft.

Retrofit scope Structure only, from the diaphragm above any wood-frame target story to the foundation, including load path elements.

Structure only, from the diaphragm above any deficient wall line to the foundation, including load path elements. LADBS (2016): “Wall lines along the parking or similar open space shall be evaluated to determine the soft, weak, or open-front wall lines.” SEAOSC (2016A): Evaluate all perimeter wall lines for soft, weak, or open-front conditions.

Potential triggered requirements

All existing triggers apply, including those regarding disabled access in buildings with commercial use and planning review for exterior alterations and historic buildings.

None identified, but work is likely to trigger disabled access improvements for buildings with public accommodations.

Waived requirements

None. “[E]xisting electrical, plumbing, mechanical or fire-safety systems [need not] be altered to comply with existing code unless they constitute a hazard to life or property.”

Compliance schedule

Tier I: Buildings with A, E, or certain residential occupancy for vulnerable tenants. Tier II: Buildings with 15 or more dwelling units Tier III: All others Tier IV: Buildings with B or M (commercial) occupancy in the target story, and buildings in mapped liquefaction zones All buildings received notices in 2013. All were required to complete Screening in 1 year. Permit application and work completion deadlines then varied for each successive Tier, with the final tier, Tier IV, having permit applications due in September 2018 and a completion deadline of September 2020.

Priority I: 16 or more dwelling units Priority II: 3 or more stories Priority III: All others Notices staggered by building department. Each building has 2 years from order to produce evaluation or retrofit design, 3.5 years from order to obtain permits, 7 years from order to complete construction or demolition.


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Topic San Francisco Los Angeles Screening criteria The four criteria: Pre-1978, 3 or more stories

(or 2 over an “underfloor area”), 5 or more dwelling units, and the presence of a wood-frame target story. The screening process was used to assign buildings to compliance tiers.

None, but owner has 60 days from order to appeal subject status

Evaluation criteria Same as the retrofit criteria, except that IEBC Chapter A4 is not allowed for evaluation.

Same as the retrofit criteria.

Retrofit criteria: “Code based”

2015 IEBC Appendix A4. Additional requirements (SFDBI, 2017A): • A building investigation and report are

required to confirm analysis and design assumptions.

• For buildings located in Site Class E, Fa must be taken as 1.3.

Additional allowances (SFDBI, 2017A): • “[T]he total expected strength of retrofit

elements added to any target story need not exceed 1.3 times the expected strength of the story immediately above, as long as the retrofit elements are located symmetrically about the center of mass of the story above or so as to minimize torsion in the target story.”

• R factors may be assigned line-by-line based on findings and guidance provided by SEAONC (Maison et al., 2017)

• Except for straight lumber-sheathed diaphragms, the diaphragm need only be checked locally for its capacity to transfer forces to new or strengthened vertical elements of the SFRS.

Ordinance gives abbreviated criteria based on ASCE 7. LADBS (2016) gives details. Similarities to 2015 IEBC Chapter A4: • Reduced design forces (75% of new construction) • R need not be less than 3.5 except for cantilever

column systems (sim A403.3) • Story drift limit (2.5%) But LADBS criteria differ from IEBC regarding: • Definition of Soft Wall Line and Weak Wall Line. • LADBS (2016) replaces A403.8 diaphragm check

with 3:1 ratio limit and requirement to design cantilevered diaphragms for shear transfer.

• Non-conforming materials (A403.9.1) • Hold-down connectors (A403.9.2) Additional requirements: • LADBS (2016): Specific weights “shall be

considered”. • LADBS (2016): Redundancy factor set to 1.3 by

default, to be considered line by line. • LADBS (2016): 2.0% drift limit if “soft story”

irregularity after retrofit is not explicitly checked. • As for new construction for 3+ story buildings with

re-entrant corners, discontinuous diaphragm, out-of-plane offset, non-parallel systems. (No mention of torsional irregularity)

• Div 94 for hillside buildings (sim IEBC A403.2) Retrofit criteria: Alternative

ASCE 41-13, with a performance objective of Structural Life Safety with a BSE-1E hazard, and: • Scope limited as above • SFDBI (2017A): “Retrofit strength need

not exceed 1.3 times the strength of the story above.”

FEMA P-807, with a performance objective of Onset of Strength Loss with a maximum Probability of Exceedance of 30 percent given a demand of 0.5SMS. The maximum POE may be taken as high as 50 percent for the weakest buildings if the target story is used only for parking, storage, or utility.

ASCE 41-13. LADBS (2016) sets a performance objective of Structural Life Safety with a BSE-1E hazard, and: • Scope limited to the SWOF deficiencies • “Retrofit strength need not exceed 1.3 times the

strength of the story above.” FEMA P-807. LADBS (2016) sets a performance objective of Onset of Strength Loss with a maximum Probability of Exceedance of 20 percent given a demand of 0.5SMS.


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Topic San Francisco Los Angeles Steel system design criteria

No limits on system types, except as imposed by the selected criteria. Additional allowances: • Limits on R factors waived, so that each

SFRS element may be designed with its own R value, based on SEAONC guidance (Maison et al., 2017)

• Strong column-weak beam requirements waived for columns that carry no gravity load.

LADBS (2016): • Limits on use of OMF, IMF (see Allowed retrofit

systems) • Requires bracing of tops of moment frame columns • Allows special cantilever columns, but requires

amplifies seismic load with overstrength factor

Allowed retrofit systems

Any, except as prohibited by the selected criteria.

Not allowed: • Concrete walls • Masonry walls • Steel braced frames • Steel IMF in SDC E and steel OMF in SDC D or E,

unless building is less than 35 ft tall and of limited weight.

SEAOSC (2016A) suggests reason is based on stiffness incompatibility. LADBS (2016): Existing concrete or masonry walls require full-story analysis with reduced ASCE 7, or alternative criteria.

Design quality assurance

Site investigation report required. Affidavit of compliance required.

Requires affidavit of compliance on plans.

Construction quality assurance

No special requirements. DBI has begun requiring structural observation based on code official discretion.

LADBS (2016) requires note on plans that “size and spacing” of all existing load path is “To be verified in field during construction.”

References ABAG, 2016. Soft Story Retrofit Program Development [ABAG Publication #P16001EQK]. Association of Bay Area Governments, March. Alameda, City of, 2017. “Seismic Retrofit.” alamedaca.gov/community-development/building/seismic-retrofit ASCE, 2014. Seismic Evaluation and Retrofit of Existing Buildings [ASCE/SEI 41-13]. American Society of Civil Engineers. ATC, 2012. Seismic Evaluation and Retrofit of Multi-Unit Wood-Frame Buildings With Weak First Stories [FEMA P-807]. Federal Emergency Management Agency, May. Berkeley, City of, 2017. “Soft Story Program – Regulations for Potentially Hazardous Buildings Containing Soft, Weak or Open Front Stories.” www.cityofberkeley.info/softstory/ Beverly Hills, City of, Community Development Department, 2017. “Proposed Seismic Retrofit Program for Existing

Wood-Frame Soft-Story Buildings.” www.beverlyhills.org/business/ constructionlanduse/siesmicretrofit/softstoryprogram/ Bonowitz, D., 2013. “San Francisco’s ‘Soft Story’ Program: Screening & Evaluation Forms.” Unpublished slides for a SEAONC Mini-Seminar, September 9. Bonowitz, D., 2016. “SFBC Chapter 34B ‘Soft Story’ Program: Quality Assurance Issues.” Unpublished presentation to the SEAONC Existing Buildings Committee, August 29. Bonowitz, D. and Rabinovici, S., 2013. Soft Story Risk Reduction: Lessons from the Berkeley Data. Earthquake Engineering Research Institute, January. Buckalew, J., Maison, B., McDonald, B., Schotanus, M., and McCormick, D., 2015. “Case Studies of Soft-Story Retrofits Using Different Design Guidelines.” A SEAONC Special Projects Initiative Report, December. www.seaonc.org/ sites/default/files/article/2013_spi_final_report.pdf


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Burton, H., Rad, A., and Buckalew, J., 2016. “A Comparative Assessment of the Collapse Performance of Soft, Weak or Open-Front Wall Woodframe Buildings Retrofitted using Alternative Procedures,” in 2016 SEAOC Convention Proceedings. Structural Engineers Association of California. Chun, R., 2017. Personal correspondence with the author, August 8. ESIP, 2014. “Mandatory Soft Story Program: Preliminary Returns.” Earthquake Safety Implementation Program, October 10. Fremont, City of, 2007. “Earthquake Retrofit Standards and Requirements for Soft-Story Residential Buildings” [Chapter 15.75 of the Fremont Municipal Code]. ICC, 2009. Proposed Changes to the 2009 International Codes. International Code Council, August. LADBS, 2016. “Structural Design Guidelines” [Document No. P/BC 2014-137], Revised. City of Los Angeles Department of Building and Safety, September 30. Los Angeles, City of, 2015. “Ordinance 183893.” Los Angeles, City of, 2016. “Ordinance 184081.” Los Angeles, City of, 2017. “Soft-Story Retrofit Program.” www.ladbs.org/services/core-services/plan-check-permit/plan-check-permit-special-assistance/mandatory-retrofit-programs/soft-story-retrofit-program Maison, B., Palmer, K., Matteson, T., and Olson, B., 2017. “ASCE 7 R-Values in Soft-Story Building Retrofits.” Proceedings of the 2017 SEAOC Convention. Mountain View, City of, 2016. “Soft Story Study – Scope of Work.” Community Development Department, unpublished. Oakland, City of, 2009. “Soft-Story Seismic Screening Program FAQs.” www2.oaklandnet.com/Government/ o/PBN/OurOrganization/BuildingServices/o/Permits/DOWD008964 Oakland, City of, 2017. “Safer Housing for Oakland: ‘Soft Story’ Rental Property Seismic Retrofit Grant Program. www2.oaklandnet.com/government/o/hcd/s/HousingRepairRehabPrograms/OAK059370 Sabatini, J., 2017. “SF hoes to avoid code enforcement ‘nightmare’ over soft-story retrofit mandate.” San Francisco Examiner, August 1.

San Francisco, City and County of, 2013. “Ordinance 66-13: Building Code – Mandatory Seismic Retrofit Program – Wood-Frame Buildings; Optional Evaluation Form Fee.” [Code language adopted by ordinance is now found in San Francisco Existing Building Code Chapter 4D.] Santa Monica, City of, 2017A. “Ordinance 2537: An Ordinance of the City Council of the City of Santa Monica Amending Articles IV and VIII of the Santa Monica Municipal Code by Updating Seismic Retrofit Standards and Tenant Protections Laws.” Santa Monica, City of, 2017B. “Seismic Retrofit Program.” www.smgov.net/Departments/PCD/Programs/Seismic-Retrofit/ Schotanus, M., Lizundia, B., Hoyt, G., Seligson, H., and Rabinovici, S, 2017. “Identifying and Mitigating Structural Seismic Vulnerabilities at the Community Level.” Proceedings of the 2017 SEAOC Convention. SEAOSC, 2016A. Design Guide Vol. 2: City of Los Angeles Mandatory Earthquake Hazard Reduction in Existing Wood-Frame Buildings with Soft, Weak or Open-Front Walls (SWOF). Structural Engineers Association of Southern California. SEAOSC, 2016B. “Los Angeles Soft, Weak, or Open-Front Wall Line Ordinance Retrofit Example.” Presented by members of the SEAOSC EBC Task Group at 2016 Strengthening Our Cities Seismic Summit. SFDBI, 2017A. “Application of Engineering Criteria in SFEBC Chapter 4D” [Administrative Bulletin 107]. San Francisco Department of Building Inspection, June 27 (Supersedes prior editions dated Nov 12, 2013; Nov 10, 2014; Dec 16, 2015; and May 9, 2016). SFDBI, 2017B. “Soft Story Properties List.” sfdbi.org/soft-story-properties-list West Hollywood, City of, 2017. “WeHo Seismic Retrofit Program.” www.weho.org/city-hall/city-departments-divisions/community-development/building-and-safety/seismic-retrofit

consensus recommendations for “soft story” retrofit · 2018-04-02 · consensus recommendations...

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