CHBC · California Historical Building Code
What seismic forces and R-values apply and what caps/exceptions exist?
Under the CHBC you generally use the regular‑code R for the system (or a rational R if none exists), but seismic forces can be reduced to 0.75× the regular‑code forces and are further limited by absolute base‑shear caps of 0.30W for Risk Category I/II and 0.40W for Risk Category III/IV; URM buildings must follow CEBC Appendix A1 as referenced in the CHBC.
Last reviewed: July 6, 2026
What the code requires — plain English
The CHBC requires that seismic evaluation use the same R-values used in the regular code for comparable lateral‑force‑resisting systems (or a rationally assigned R when none exists) and that the resulting strength‑level seismic forces be reduced in prescribed ways. The controlling rule is § 8-706.1 of the CHBC.
Use the regular‑code R for the system (or a rationally assigned R) — but the CHBC lets you limit forces to 0.75 times regular‑code seismic forces and applies explicit base shear caps by Risk Category.
Requirements in detail
Primary rule
- Use the R‑values tabulated in the regular code for similar lateral‑force‑resisting systems, considering the existing structural detailing. If there is no tabulated R‑value, assign one rationally based on detailing. See § 8-706.1.
Mandatory reductions, caps and special numeric rules
- 0.75 cap on seismic forces: Seismic forces used for evaluation need not exceed 0.75 times the seismic forces prescribed by the regular code. (This is an explicit allowance in § 8-706.1.)
- Near‑fault increase exclusion: For Risk Category I, II or III structures, near‑fault increases in ground motion (MCER 0.2‑sec > 150%) need not be considered when the building fundamental period in the direction under consideration is ≤ 0.5 s. See § 8-706.1, Exception 2.
- Base shear caps by Risk Category (absolute maximum seismic base shear allowed under CHBC exceptions):
- For Risk Category I or II: seismic base shear need not exceed 0.30 W (30% of W). See § 8-706.1, Exception 3.
- For Risk Category III or IV: seismic base shear need not exceed 0.40 W (40% of W). See § 8-706.1, Exception 4.
- New lateral system R‑value use: When a strengthening project adds a new lateral system, the R‑value of the new system may be used provided the new system resists at least 75% of the building’s base shear regardless of relative rigidity. See § 8-706.1.1.
- Unreinforced masonry (URM): Evaluation and seismic improvement of URM bearing‑wall buildings must comply with the California Existing Building Code (CEBC), Appendix A1 (2013) as modified by the CHBC — see § 8-706.1.2. That CEBC appendix contains the URM analysis procedures (including R = 1.5 for ordinary plain masonry shear walls as used there).
- Deviations from detailing: Any deviations from regular‑code detailing must be evaluated for stability and ability to maintain load capacity at expected inelastic deformations — see § 8-706.1.3.
Quick reference table (decision dimensions and values)
| Decision item | Value / threshold | Code Reference |
|---|---|---|
| Source of R‑value for evaluation | Use regular‑code R‑values for similar systems; assign rational R if none exist | § 8-706.1 |
| Max fraction of regular code seismic forces allowed | 0.75 × (regular‑code seismic forces) | § 8-706.1, Exception 1 |
| New lateral system may set R | New system R allowed if it resists ≥ 75% of base shear | § 8-706.1.1 |
| URM buildings analysis | Follow CEBC Appendix A1 (2013) as modified by CHBC | § 8-706.1.2 ; CEBC Appendix A sources |
| Near‑fault increase exclusion | May omit near‑fault increase when fundamental period ≤ 0.5 s (Risk Cat I–III) | § 8-706.1, Exception 2 |
| Base shear cap — Risk Category I or II | 0.30 W (30% of W) maximum | § 8-706.1, Exception 3 |
| Base shear cap — Risk Category III or IV | 0.40 W (40% of W) maximum | § 8-706.1, Exception 4 |
| Evaluate detailing deviations | Must evaluate for inelastic stability and reserve strength | § 8-706.1.3 |
Exceptions & special cases
- The CHBC explicitly permits reducing the design seismic force to 0.75× the regular‑code value (not a discretionary reduction beyond that) — § 8-706.1, Exception 1.
- Near‑fault spectral amplification may be ignored for Risk Category I–III when the building period is ≤ 0.5 s (Exception 2). Apply this only directionally (the direction under consideration). § 8-706.1, Exception 2.
- Absolute base shear caps: regardless of the computed reduced force, final base shear need not exceed 0.30W (Risk Cat I/II) or 0.40W (Risk Cat III/IV). These caps can control for low‑R systems such as URM. § 8-706.1, Exceptions 3–4.
- URM buildings: must be evaluated per the CEBC Appendix A1 (2013). The CEBC contains special procedures, allowable R‑values, h/t limits, wall anchorage rules and pier shear computations that the CHBC points you to — see § 8-706.1.2 and the CEBC Appendix A references.
- New lateral system rule: when adding a new lateral system as part of strengthening, you may apply the new system’s R only if that new system will resist at least 75% of the building’s base shear. See § 8-706.1.1.
Common mistakes
- Treating the CHBC as identical to the regular code and ignoring the 0.75 allowable reduction and the 0.30W / 0.40W caps — these caps are explicit exceptions in § 8-706.1.
- Using the new lateral system’s R‑value when it does not resist ≥ 75% of base shear (violates § 8-706.1.1).
- Forgetting that URM must follow CEBC Appendix A1 procedures referenced in § 8-706.1.2, including special assumptions and allowable strengths (not the regular code’s R for many URM walls).
- Applying the near‑fault exclusion without checking the fundamental period ≤ 0.5 s requirement or Risk Category limits (see § 8-706.1, Exception 2).
- Failing to evaluate deviations from regular detailing for inelastic performance per § 8-706.1.3.
Worked example (concrete numbers)
Scenario: historic unreinforced masonry (URM) bearing‑wall building, Risk Category II, total seismic weight W = 500 kips, short‑period design spectral acceleration S_DS = 0.8 g, CEBC/ASCE R for ordinary plain masonry shear walls R = 1.5 (CEBC uses R = 1.5 for that URM category).
- Compute the regular‑code base shear (V_reg) using the usual form V = (S_DS × W) / R (CEBC Appendix A provides the analysis basis). For our numbers: V_reg = (0.8 × 500) / 1.5 = 266.7 kips.
- Apply the CHBC 0.75 reduction (maximum allowed): 0.75 × V_reg = 0.75 × 266.7 = 200.0 kips. (This is the maximum CHBC permits by Exception 1 of § 8-706.1.)
- Apply the base shear cap for Risk Category II: 0.30 W = 0.30 × 500 = 150 kips (Exception 3 of § 8-706.1).
Final allowable seismic base shear under CHBC = the minimum of {V_reg, 0.75 V_reg, base shear cap} = 150 kips (the 0.30 W cap controls in this example).
Note: if the same building were assigned a different Risk Category or had a higher R (e.g., moment‑resisting frame), the controlling term might be 0.75×V_reg or V_reg rather than the W cap — always evaluate all three limits.
Related provisions (CHBC and referenced CEBC)
- § 8-706.1 — Seismic forces, R‑values and caps.
- § 8-706.1.1 — Use of new lateral system R when it resists ≥ 75% of base shear.
- § 8-706.1.2 — URM buildings: follow CEBC Appendix A1 (2013) as modified by CHBC.
- § 8-706.1.3 — Evaluate deviations from detailing for inelastic stability.
- CEBC Appendix A (A1) — Procedures, equations and details for seismic retrofit of existing buildings and URM; includes base shear formulas and URM analysis methods referenced by the CHBC.
Code references
Grounded in the retrieved California Historical Building Code — click a citation to read the verbatim passage:
CHBC § 8-706 High relevance — show source text
SECTION 8-706 — LATERAL LOAD REGULATIONS
8-706.1 Seismic forces. Strength-level seismic forces used to evaluate the structure for resistance to seismic loads shall be based on the R -values tabulated in the regular code for similar lateral-force-resisting systems including consideration of the structural detailing of the members where such R -values exist. Where such R -values do not exist, an appropriate R -value shall be rationally assigned considering the structural detailing of the members.
Exceptions:
- The forces need not exceed 0.75 times the seismic forces prescribed by the regular code requirements.
- For Risk Category I, II or III structures, near-fault increases in ground motion (maximum considered earthquake ground motion of 0.2 second spectral response greater than 150 percent at 5 percent damping) need not be considered when the fundamental period of the building is 0.5 seconds in the direction under consideration.
- For Risk Category I or II structures, the seismic base shear need not exceed 0.30W.
- For Risk Category III or IV structures, the seismic base shear need not exceed 0.40W.
8-706.1.1 When a building is to be strengthened with the addition of a new lateral force resisting system, the R -value of the new system can be used when the new lateral force resisting system resists at least 75 percent of the building’s base shear regardless of its relative rigidity.
8-706.1.2 Evaluation and seismic improvement of unreinforced masonry bearing wall buildings shall comply with the California Existing Building Code (CEBC), Appendix Chapter A1 2013 Edition, and as modified by the CHBC.
Exceptions:
- Alternative standards may be used on a case-by-case basis when approved by the authority having jurisdiction. It shall be permitted to exceed the strength limitation of 100 psi in Section A108.2 of the CEBC when test data and building configuration supports higher values subject to the approval of the authority having jurisdiction.
- CEBC Section A102.2 shall not apply to Qualified Historical Buildings in Risk Category III buildings and other structures whose primary occupancies are public assembly with an occupancy load greater than 300.
8-706.1.3 All deviations from the detailing provisions of the lateral-force-resisting systems shall be evaluated for stability and the ability to maintain load-carrying capacity at the expected inelastic deformations.
8-706.2 Existing building performance. The seismic resistance may be based upon the ultimate capacity of the structure to perform, giving due consideration to ductility and reserve strength of the lateral-force-resisting system and materials while maintaining a reasonable factor of safety. Broad judgment may be exercised regarding the strength and performance of materials not recognized by regular code requirements. (See Chapter 8-8, Archaic Materials and Methods of Construction.)
8-706.2.1 All structural materials or members that do not comply with detailing and proportioning requirements of the regular code shall be evaluated for potential seismic performance and the consequence of non-compliance. All members that would be reasonably expected to fail and lead to collapse or life threatening injury when subjected to seismic demands shall be judged unacceptable, and appropriate structural strengthening shall be developed.
8-706.3 Load path. A complete and continuous load path, including connections, from every part or portion of the structure to the ground shall be provided for the required forces. It shall be verified that the structure is adequately tied together to perform as a unit when subjected to earthquake forces.
CHBC § 8-705.1 High relevance — show source text
8-705.1 Gravity loads. The capacity of the structure to resist gravity loads shall be evaluated and the structure strengthened as necessary. The evaluation shall include all parts of the load path. Where no distress is evident, and a complete load path is present, the structure may be assumed adequate by having withstood the test of time if anticipated dead and live loads will not exceed those historically present.
8-705.2 Wind and seismic loads. The ability of the structure to resist wind and seismic loads shall be evaluated. Wind loads shall be considered when appropriate, but need not exceed 75 percent of the wind loads prescribed by the regular code. The evaluation shall be based on the requirements of Section 8-706.
8.705.2.1 Any unsafe conditions in the lateral-load-resisting system shall be corrected, or alternative resistance shall be provided. When strengthening is required, additional resistance shall be provided to meet the minimum requirements of the CHBC. The strengthening measures shall be selected with the intent of meeting the performance objectives set forth in Section 8-701.2. The evaluation of structural members and structural systems for seismic loads shall consider the inelastic performance of structural members and their ability to maintain load-carrying capacity during the seismic loadings prescribed by the regular code.
8.705.2.2 The architect or engineer shall consider additional measures with minimal loss of, and impact to, historical materials which will reduce damage and needed repairs in future earthquakes to better preserve the historical structure in perpetuity. These additional measures shall be presented to the owner for consideration as part of the rehabilitation or restoration.
2025 CALIFORNIA HISTORICAL BUILDING CODE 13
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
STRUCTURAL REGULATIONS
SECTION 8-706 — LATERAL LOAD REGULATIONS
8-706.1 Seismic forces. Strength-level seismic forces used to evaluate the structure for resistance to seismic loads shall be based on the R -values tabulated in the regular code for similar lateral-force-resisting systems including consideration of the structural detailing of the members where such R -values exist. Where such R -values do not exist, an appropriate R -value shall be rationally assigned considering the structural detailing of the members.
Exceptions:
- The forces need not exceed 0.75 times the seismic forces prescribed by the regular code requirements.
- For Risk Category I, II or III structures, near-fault increases in ground motion (maximum considered earthquake ground motion of 0.2 second spectral response greater than 150 percent at 5 percent damping) need not be considered when the fundamental period of the building is 0.5 seconds in the direction under consideration.
- For Risk Category I or II structures, the seismic base shear need not exceed 0.30W.
- For Risk Category III or IV structures, the seismic base shear need not exceed 0.40W.
8-706.1.1 When a building is to be strengthened with the addition of a new lateral force resisting system, the R -value of the new system can be used when the new lateral force resisting system resists at least 75 percent of the building’s base shear regardless of its relative rigidity.
8-706.1.2 Evaluation and seismic improvement of unreinforced masonry bearing wall buildings shall comply with the California Existing Building Code (CEBC), Appendix Chapter A1 2013 Edition, and as modified by the CHBC.
Exceptions:
CHBC § 8-706.1.3 High relevance — show source text
Exceptions:
- Alternative standards may be used on a case-by-case basis when approved by the authority having jurisdiction. It shall be permitted to exceed the strength limitation of 100 psi in Section A108.2 of the CEBC when test data and building configuration supports higher values subject to the approval of the authority having jurisdiction.
- CEBC Section A102.2 shall not apply to Qualified Historical Buildings in Risk Category III buildings and other structures whose primary occupancies are public assembly with an occupancy load greater than 300.
8-706.1.3 All deviations from the detailing provisions of the lateral-force-resisting systems shall be evaluated for stability and the ability to maintain load-carrying capacity at the expected inelastic deformations.
8-706.2 Existing building performance. The seismic resistance may be based upon the ultimate capacity of the structure to perform, giving due consideration to ductility and reserve strength of the lateral-force-resisting system and materials while maintaining a reasonable factor of safety. Broad judgment may be exercised regarding the strength and performance of materials not recognized by regular code requirements. (See Chapter 8-8, Archaic Materials and Methods of Construction.)
8-706.2.1 All structural materials or members that do not comply with detailing and proportioning requirements of the regular code shall be evaluated for potential seismic performance and the consequence of non-compliance. All members that would be reasonably expected to fail and lead to collapse or life threatening injury when subjected to seismic demands shall be judged unacceptable, and appropriate structural strengthening shall be developed.
8-706.3 Load path. A complete and continuous load path, including connections, from every part or portion of the structure to the ground shall be provided for the required forces. It shall be verified that the structure is adequately tied together to perform as a unit when subjected to earthquake forces.
8-706.4 Parapets. Parapets and exterior decoration shall be investigated for conformance with regular code requirements for anchorage and ability to resist prescribed seismic forces.
An exception to regular code requirements shall be permitted for those parapets and decorations which are judged not to be a hazard to life safety.
8-706.5 Nonstructural features. Nonstructural features of historical structure, such as exterior veneer, cornices and decorations, which might fall and create a life safety hazard in an earthquake, shall be evaluated. Their ability to resist seismic forces shall be verified, or the feature shall be strengthened with improved anchorage when appropriate.
8-706.5.1 Partitions and ceilings of corridors and stairways serving an occupant load of 30 or more shall be investigated to determine their ability to remain in place when the building is subjected to earthquake forces.
8-706.5.2 Seismic forces used to evaluate and improve nonstructural components and their anchorage, where required, shall comply with ASCE 41 or need not exceed 0.75 times the seismic forces prescribed by the requirements of the regular code.
14 2025 CALIFORNIA HISTORICAL BUILDING CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
8-8 ARCHAIC MATERIALS AND METHODS OF CONSTRUCTION
SECTION 8-801 — PURPOSE, INTENT AND SCOPE
California Historical Building Code High relevance — show source text
|100 psf|1 hr
23 min|||7|1, 2|11/3| |F/C-4-RC-9|4″|4″ deep (4370 psi);1/4″ reinforcement bars
at 6″ pitch with3/4″ cover;1/4″ main rein-
forcement bars at 4″ pitch perpendicular
with1/2″ cover; 13′1″ span restrained.|150 psf|2 hrs|||7|1, 3|2| |F/C-4-RC-10|4″|4″ thick (5140 psi) deck;1/4″ reinforce-
ment bars at 71/2″ pitch with7/8″ cover;3/8″
main reinforcement bars at 33/4″ pitch
perpendicular with1/2″ cover; 13′1″ span
restrained.|140 psf|1 hr
16 min|||7|1, 5|11/4| |F/C-4-RC-11|4″|4″ thick (4000 psi) concrete deck;
3″ × 11/2″ × 4 lbs R.S.J.; 2′6″ C.R.S.; flush
with top surface; 4″ × 6″ x 13 SWG mesh
reinforcement 1″ from bottom of slab; 6′6″
span restrained.|150 psf|2 hrs|||7|1, 3|2| |F/C-4-RC-12|4″|4″ deep (2380 psi) concrete deck;
3″ × 11/2″ × 4 lbs R.S.J.; 2′6″ C.R.S.; flush
with top surface; 4″ × 6″ x 13 SWG mesh
reinforcement 1″ from bottom surface;
6′6″ span restrained.|150 psf|1 hr
3 min|||7|1, 2|1| |F/C-4-RC-13|41/2″|41/2″ thick (5200 psi) deck;1/4″ reinforce-
ment bars at 71/4″ pitch with7/8″ cover;3/8″
main reinforcement bars at 33/4″ pitch
perpendicular with1/2″ cover; 13′1″ span
restrained.|140 psf|2 hrs|||7|1, 3|2| |F/C-4-RC-14|41/2″|41/2″ deep (2525 psi) concrete deck;1/4″
reinforcement bars at 71/2″ pitch with7/8″
cover;3/8″ main reinforcement bars at
33/8″ pitch perpendicular with1/2″ cover;
13′1″ span restrained.|150 psf|42 min|||7|1, 5|2/3| |F/C-4-RC-15|41/2″|41/2″ deep (4830 psi) concrete deck;
11/2″ × No.CHBC § 903.2.6. Medium relevance — show source text
For new Group I-4 occupancies, see Exceptions 2 and 3 of Section 903.2.6.
h. New Group R occupancies are required to be protected by an automatic sprinkler system in accordance with Section 903.2.8.
i. See Sections 407.1.1 and 408.1.2 for specific exceptions to construction type, allowable building areas and allowable heights.
j. Restraint shall not be permitted in any building except in Group I-3 occupancies constructed for such use (see Section 408.1.2).
k. Nonambulatory persons shall be limited to the first 2 stories.
l. Nonambulatory persons shall be limited to the first 5 stories.
m.Nonambulatory elderly clients are not permitted in buildings of these types of construction. See Sections 435.3.3 and 435.3.4.
n. In other than Group A, E, H, I, L and R occupancies, high-rise buildings, and other applications listed in Section 1.11 regulated by the Office of the State Fire Marshal, the S increases
for height and stories in Tables 504.3 and 504.4 are permitted in addition to the S area increase in accordance with Table 506.2.
o. For Group R-2 buildings of Type VA construction equipped throughout with an approved automatic sprinkler system in accordance with Section 903.3.1.1, S area increase is permit-
ted in addition to the height and story increase provided the height shall not exceed 60 feet and 4 stories.
p. See Section 436.1 for additional regulations for child-care centers and adult day care.|UL = Unlimited; NP = Not Permitted; NS = Buildings not equipped throughout with an automatic sprinkler system; S = Buildings equipped throughout with an automatic sprinkler
system installed in accordance with Section 903.3.1.1; S13R = Buildings equipped throughout with an automatic sprinkler system installed in accordance with Section 903.3.1.2;
S13D = Buildings equipped throughout with an automatic sprinkler system installed in accordance with Section 903.3.1.3.
a. See Chapters 4 and 5 for specific exceptions to the allowable height in this chapter.
b. See Section 903.2 for the minimum thresholds for protection by an automatic sprinkler system for specific occupancies.
c. New Group H occupancies are required to be protected by an automatic sprinkler system in accordance with Section 903.2.5.
d. The NS value is only for use in evaluation of existing building height in accordance with the_California Existing Building Code_.
e. New Group I-3 occupancies are required to be protected by an automatic sprinkler system in accordance with Section 903.2.6.
f. New and existing Group I-2 occupancies are required to be protected by an automatic sprinkler system in accordance with Section 903.2.6 and Section 1103.5 of the_California_
Fire Code.
g. For new Group I-4 occupancies, see Exceptions 2 and 3 of Section 903.2.6.
h. New Group R occupancies are required to be protected by an automatic sprinkler system in accordance with Section 903.2.8.
i. See Sections 407.1.1 and 408.1.2 for specific exceptions to construction type, allowable building areas and allowable heights.
_j.CHBC § 8-706.3 Medium relevance — show source text
8-706.3 Load path. A complete and continuous load path, including connections, from every part or portion of the structure to the ground shall be provided for the required forces. It shall be verified that the structure is adequately tied together to perform as a unit when subjected to earthquake forces.
8-706.4 Parapets. Parapets and exterior decoration shall be investigated for conformance with regular code requirements for anchorage and ability to resist prescribed seismic forces.
An exception to regular code requirements shall be permitted for those parapets and decorations which are judged not to be a hazard to life safety.
8-706.5 Nonstructural features. Nonstructural features of historical structure, such as exterior veneer, cornices and decorations, which might fall and create a life safety hazard in an earthquake, shall be evaluated. Their ability to resist seismic forces shall be verified, or the feature shall be strengthened with improved anchorage when appropriate.
8-706.5.1 Partitions and ceilings of corridors and stairways serving an occupant load of 30 or more shall be investigated to determine their ability to remain in place when the building is subjected to earthquake forces.
8-706.5.2 Seismic forces used to evaluate and improve nonstructural components and their anchorage, where required, shall comply with ASCE 41 or need not exceed 0.75 times the seismic forces prescribed by the requirements of the regular code.
14 2025 CALIFORNIA HISTORICAL BUILDING CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
8-8 ARCHAIC MATERIALS AND METHODS OF CONSTRUCTION
SECTION 8-801 — PURPOSE, INTENT AND SCOPE
8-801.1 Purpose. The purpose of the CHBC is to provide regulations for the use of historical methods and materials of construction that are at variance with regular code requirements or are not otherwise codified, in buildings or structures designated as qualified historical buildings or properties. The CHBC require enforcing agencies to accept any reasonably equivalent alternatives to the regular code when dealing with qualified historical buildings or properties.
8-801.2 Intent. It is the intent of the CHBC to provide for the use of historical methods and materials of construction that are at variance with specific code requirements or are not otherwise codified.
8-801.3 Scope. Any construction type or material that is, or was, part of the historical fabric of a structure is covered by this chapter. Archaic materials and methods of construction present in a historical structure may remain or be reinstalled or be installed with new materials of the same class to match existing conditions.
SECTION 8-802 — GENERAL ENGINEERING APPROACHES
Strength values for archaic materials shall be assigned based upon similar conventional codified materials, or on tests as hereinafter indicated. The archaic materials and methods of construction shall be thoroughly investigated for their details of construction in accordance with Section 8-703. Testing shall be performed when applicable to evaluate existing conditions. The architect or structural engineer in responsible charge of the project shall assign allowable stresses or strength levels to archaic materials. Such assigned strength values shall not be greater than those provided for in the following sections without adequate testing, and shall be subject to the concurrence of the enforcing agency.
SECTION 8-803 — NONSTRUCTURAL ARCHAIC MATERIALS
California Historical Building Code Medium relevance — show source text
000|32,500|37,500|77,500|32,500|42,500|41,500|77,500| |Sheathing|24/16|27,000|35,000|40,500|83,500|35,000|45,500|44,500|83,500| |Sheathing|32/16|27,000|35,000|40,500|83,500|35,000|45,500|44,500|83,500| |Sheathing|40/20|28,500|37,000|43,000|88,500|37,000|48,000|47,500|88,500| |Sheathing|48/24|31,000|40,500|46,500|96,000|40,500|52,500|51,000|96,000| |Single
Floor|16 o.c.|27,000|35,000|40,500|83,500|35,000|45,500|44,500|83,500| |Single
Floor|20 o.c.|28,000|36,500|42,000|87,000|36,500|47,500|46,000|87,000| |Single
Floor|24 o.c.|30,000|39,000|45,000|93,000|39,000|50,500|49,500|93,000| |Single
Floor|32 o.c.|36,000|47,000|54,000|110,000|47,000|61,000|59,500|110,000| |Single
Floor|48 o.c.|50,500|65,500|76,000|155,000|65,500|85,000|83,500|155,000|Col1 Col2 Structural Sheathing Col4 Col5 Structural I Col7 Col8 Thickness
(in.)A-A, A-C Marine All Other
GradesA-A, A-C Marine All Other
GradesSanded
Plywood1/4 24,000 31,000 24,000 31,000 31,000 31,000 Sanded
Plywood11/32 25,500 33,000 25,500 33,000 33,000 33,000 Sanded
Plywood3/8 26,000 34,000 26,000 34,000 34,000 34,000 Sanded
Plywood15/32 38,000 49,500 38,000 49,500 49,500 49,500 Sanded
Plywood1/2 38,500 50,000 38,500 50,000 50,000 50,000 Sanded
Plywood19/32 49,000 63,500 49,000 63,500 63,500 63,500 Sanded
Plywood5/8 49,500 64,500 49,500 64,500 64,500 64,500 Sanded
Plywood—
continued23/32 50,500 65,500 50, CHBC § 3.1. Medium relevance — show source text
Gypsum wallboard, blocked edges|Per side: 600 lbs per foot for seismic shear
550 lbs per foot for seismic shear
200 lbs per foot for seismic shear
400 lbs per foot for seismic shear| |3. Existing footings, wood framing, structural steel and reinforcing steel
3.1. Plain concrete footings
3.2. Douglas fir wood
3.3. Reinforcing steel
3.4. Structural steel|f ′c = 1,500 psi (10.34 MPa) unless otherwise shown by
tests3
Allowable stress same as D.F. No. 13
_ft _= 40,000 lbs per square inch (124.1 N/mm2) maximum
ft= 33,000 lbs per square inch (137.9 N/mm2) maximum| |1. Material must be sound and in good condition.
2. Shear values of these materials may be combined, except the total combined value shall not exceed 900 pounds per foot (13,140 N/m).
3. Stresses given may be increased for combinations of loads as specified in the regular code.|1. Material must be sound and in good condition.
2. Shear values of these materials may be combined, except the total combined value shall not exceed 900 pounds per foot (13,140 N/m).
3. Stresses given may be increased for combinations of loads as specified in the regular code.|2025 CALIFORNIA HISTORICAL BUILDING CODE 17
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
ARCHAIC MATERIALS AND METHODS OF CONSTRUCTION
TABLE 8-8B—STRENGTH VALUES OF NEW MATERIALS USED IN CONNECTION WITH EXISTING CONSTRUCTION Col2 NEW MATERIALS OR CONFIGURATIONS OF MATERIALS STRENGTH LEVEL CAPACITY1 1. Horizontal diaphragms2
1.1. 15/32 inch minimum plywood sheathing fastened directly over existing straight
sheathing with edges of plywood located on center of individual sheathing
boards and fastened with minimum #8x 11/4 inch wood screws or nails with
helical threads 0.13 inch min. diameter and 11/4 inch min. length at 4 inch
centers all panel edges and 12 inch centers each way in field
1.2. Same plywood and attachments as 1.1 fastened directly over existing diago-
nal sheathing
1.3. 3/8 inch plywood sheathing fastened directly over existing straight or diagonal
sheathing with ends and edges on centers of individual sheathing boards and
fastened with #6 wood screws or nails with helical threads 0.13 inch min.
diameter and 11/4 inch min. length at 6 inch centers tall panel edges and 12
inch centers each way in field1,500 lbs per foot
1,800 lbs per foot
900 lbs per foot2. Shear walls:
Plywood sheathing applied directly over wood studs. No value shall be given to
plywood applied over existing plaster or wood sheathing100 percent of the value specified in the regular code for
shear walls3. Crosswalls: (special procedure only)
3.1. Plywood sheathing applied directly over wood studs.CHBC § 25.4 Medium relevance — show source text
Values are not cumulative with other horizontal diaphragm values and are for short-term wind or seismic loading. Values shall be reduced 25 percent for normal loading.
b. Values shall be reduced 50 percent in Seismic Design Categories D, E and F.
c. 11/4-inch, No. 6 Type S or W screws are permitted to be substituted for the listed nails.|For SI: 1 inch = 25.4 mm, 1 pound per foot = 14.59 N/m.
a. Values are not cumulative with other horizontal diaphragm values and are for short-term wind or seismic loading. Values shall be reduced 25 percent for normal loading.
b. Values shall be reduced 50 percent in Seismic Design Categories D, E and F.
c. 11/4-inch, No. 6 Type S or W screws are permitted to be substituted for the listed nails.|2508.6.1 Diaphragm proportions. The maximum allowable diaphragm proportions shall be 1 [1] / 2 :1 between shear resisting elements. Rotation or cantilever conditions shall not be permitted.
2508.6.2 Installation. Gypsum panel products used in a horizontal diaphragm ceiling shall be installed perpendicular to ceiling framing members. End joints of adjacent courses of gypsum board shall not occur on the same joist.
2508.6.3 Blocking of perimeter edges. Perimeter edges shall be blocked using a wood member not less than 2-inch by 6-inch (51 mm by 152 mm) nominal dimension. Blocking material shall be installed flat over the top plate of the wall to provide a nailing surface not less than 2 inches (51 mm) in width for the attachment of the gypsum panel product.
2508.6.4 Fasteners. Fasteners used for the attachment of gypsum panel products to a horizontal diaphragm ceiling shall be as defined in Table 2508.6. Fasteners shall be spaced not more than 7 inches (178 mm) on center at all supports, including perimeter blocking, and not more than [3] / 8 inch (9.5 mm) from the edges and ends of the gypsum panel product.
2508.6.5 Lateral force restrictions. Gypsum panel products shall not be used in diaphragm ceilings to resist lateral forces imposed by masonry or concrete construction.
2508.6.6 Diaphragm ceiling connection to partitions. [DSA-SS & DSA-SS/CC and OSHPD 1, 1R, 2, 4 & 5] Gypsum board shall not be used in diaphragm ceilings to resist lateral forces imposed by partitions. Connection of diaphragm ceiling to the vertical lateral force resisting elements shall be designed and detailed to transfer lateral forces.
Exception: [OSHPD 2] Single-story Type V skilled nursing or intermediate care facilities utilizing wood-frame or light-steel-frame construction.
SECTION 2509—SHOWERS AND WATER CLOSETS
2509.1 Wet areas. Showers and public toilet walls shall conform to Section 1210.2.
CHBC § 104.1 Medium relevance — show source text
[A] 104.1 General. The building official is hereby authorized and directed to enforce the provisions of this code.
[A] 104.2 Determination of compliance. The building official shall have the authority to determine compliance with this code, to render interpretations of this code and to adopt policies and procedures in order to clarify the application of its provisions. Such interpretations, policies and procedures:
- Shall be in compliance with the intent and purpose of this code.
- Shall not have the effect of waiving requirements specifically provided for in this code.
[A] 104.2.1 Listed compliance. Where this code or a referenced standard requires equipment, materials, products or services to be listed and a listing standard is specified, the listing shall be based on the specified standard. Where a listing standard is not specified, the listing shall be based on an approved listing criteria. Listings shall be germane to the provision requiring the listing. Installation shall be in accordance with the listing and the manufacturer’s instructions, and where required to verify compliance, the listing standard and manufacturer’s instructions shall be made available to the building official.
[A] 104.2.2 Technical assistance. To determine compliance with this code, the building official is authorized to determine compliance with this code, to require the owner or owner’s authorized agent to provide a technical opinion and report.
[A] 104.2.2.1 Cost. A technical opinion and report shall be provided without charge to the jurisdiction.
[A] 104.2.2.2 Preparer qualifications. The technical opinion and report shall be prepared by a qualified engineer, specialist, laboratory or specialty organization acceptable to the building official. The building official is authorized to require design submittals to be prepared by, and bear the stamp of, a registered design professional.
[A] 104.2.2.3 Content. The technical opinion and report shall analyze the properties of the design, operation or use of the building or premises and the facilities and appurtenances situated thereon to identify and propose necessary recommendations.
[A] 104.2.2.4 Tests. Where there is insufficient evidence of compliance with the provisions of this code, the building official shall have the authority to require tests as evidence of compliance. Test methods shall be as specified in this code or by other recognized test standards. In the absence of recognized test standards, the building official shall approve the testing procedures. Such tests shall be performed by a party acceptable to the building official.
[A] 104.2.3 Alternative materials, design and methods of construction and equipment. The provisions of this code are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved.
Exception: Performance-based alternative materials, designs or methods of construction and equipment complying with the International Code Council Performance Code . This exception shall not apply to alternative structural materials or to alternative structural designs. [DSA-SS, DSA-SS/CC] The International Code Council Performance Code is not permitted by DSA.
[DSA-SS, DSA-SS/CC & OSHPD 1, 1R, 2, 4 & 5] Alternative system shall satisfy ASCE 7 Section 1.3, unless more restrictive require- ments are established by this code for an equivalent system.
[DSA-SS, DSA-SS/CC] Alternative systems shall also satisfy the California Administrative Code, Section 4-304.
CHBC § 0.75 Medium relevance — show source text
[BS] A108.4 Masonry tension. Unreinforced masonry shall be assumed to have no tensile capacity.
[BS] A108.5 Wall tension anchors. The tension strength of wall anchors shall be the average of the tension test values for anchors having the same wall thickness and framing orientation.
[BS] A108.6 Foundations. For existing foundations, new total dead loads are permitted to be increased over the existing dead load by 25 percent. New total dead load plus live load plus seismic forces may be increased over the existing dead load plus live load by 50 percent. Higher values may be justified only in conjunction with a geotechnical investigation.
SECTION A109—ANALYSIS AND DESIGN PROCEDURE
[BS] A109.1 General. The elements of buildings hereby required to be analyzed are specified in Table A102.1.
[BS] A109.2 Selection of procedure. Buildings with rigid diaphragms shall be analyzed by the general procedure of Section A110. Buildings with flexible diaphragms shall be analyzed by the general procedure or, where applicable, are permitted to be analyzed by the special procedure of Section A111.
SECTION A110—GENERAL PROCEDURE
[BS] A110.1 Minimum design lateral forces. Buildings shall be analyzed to resist minimum lateral forces assumed to act nonconcurrently in the direction of each of the main axes of the structure in accordance with the following:
Equation A1-5
V = 0.75----------------------- S DS W - R
APPENDIX A-10 2025 CALIFORNIA EXISTING BUILDING CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
APPENDIX A—GUIDELINES FOR THE SEISMIC RETROFIT OF EXISTING BUILDINGS
[BS] A110.2 Seismic forces on elements of structures. Parts and portions of a structure not covered in Section A110.3 shall be analyzed and designed per the current building code, using force levels defined in Section A110.1.
Exceptions:
- Unreinforced masonry walls for which height-to thickness ratios do not exceed ratios set forth in Table A110.2 need not be analyzed for out-of-plane loading. Unreinforced masonry walls that exceed the allowable h/t ratios of Table A110.2 shall be braced according to Section A113.5.
- Parapets complying with Section A113.6 need not be analyzed for out-of-plane loading.
- Where walls are to be anchored to flexible floor and roof diaphragms, the anchorage shall be in accordance with Section A113.1.
CHBC § 2.2 Medium relevance — show source text
APPENDIX A-14 2025 CALIFORNIA EXISTING BUILDING CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
APPENDIX A—GUIDELINES FOR THE SEISMIC RETROFIT OF EXISTING BUILDINGS
[BS] FIGURE A112.2.2—ANALYSIS OF URM WALL IN-PLANE SHEAR FORCES
PIER IS OVERSTRESSED IN SHEAR
V a = Allowable shear strength of a pier.
V p = Shear force assigned to a pier on the basis of a relative shear rigidity analysis.
RETURN
V r
V WX
∑ V r
= Rocking shear capacity of pier.
= Total shear force resisted by the wall.
= Rocking shear capacity of all piers in the wall.
[BS] A112.2.3 Shear walls without openings. Shear walls without openings shall be analyzed the same as for walls with openings, except that V r shall be calculated as follows: Equation A1-23 V r = 0.9( P D + 0.5 P w ) D / H
[BS] A112.3 Plywood-sheathed shear walls. Plywood-sheathed shear walls may be used to resist lateral forces for URM buildings with flexible diaphragms analyzed according to provisions of Section A111. Plywood-sheathed shear walls shall not be used to share lateral forces with other materials along the same line of resistance.
[BS] A112.4 Combinations of vertical elements.
[BS] A112.4.1 Seismic force distribution. Seismic forces shall be distributed among the vertical-resisting elements in proportion to their relative rigidities, except that moment-resisting frames shall comply with Section A112.4.2.
[BS] A112.4.2 Moment-resisting frames. Moment resisting frames shall not be used with an unreinforced masonry wall in a single line of resistance unless the wall has piers that have adequate shear capacity to sustain rocking in accordance with Section A112.2.2. The frames shall be designed in accordance with the building code to resist 100 percent of the seismic forces tributary to that line of resistance, as determined from Section A111.2. The story drift ratio shall be limited to 0.0075.
SECTION A113—DETAILED BUILDING SYSTEM DESIGN REQUIREMENTS
[BS] A113.1 Wall anchorage.
[BS] A113.1.1 Anchor locations. Unreinforced masonry walls shall be anchored at the roof and floor levels as required in Section A110.2. Ceilings of plaster or similar materials, where not attached directly to roof or floor framing and where abutting masonry walls, shall either be anchored to the walls at a maximum spacing of 6 feet (1829 mm) or be removed.
[BS] A113.1.2 Anchor requirements. Anchors shall consist of bolts installed through the wall as specified in Table A108.1(2), or an approved equivalent at a maximum anchor spacing of 6 feet (1829 mm). Wall anchors shall be secured to the framing members parallel or perpendicular to the wall to develop the required forces.
2025 CALIFORNIA EXISTING BUILDING CODE APPENDIX A-15
Frequently asked questions
Can I always use 0.75× the regular code forces for a CHBC project?
Yes — § 8-706.1, Exception 1 permits using seismic forces up to 0.75 times the regular‑code forces. However, you also must respect the absolute base shear caps (0.30W / 0.40W) and other applicable CHBC/CEBC requirements.
When may I apply the new lateral system’s R‑value?
You may apply the new system’s R‑value only when the new lateral system will resist at least 75% of the building’s base shear. See § 8-706.1.1.
How does CHBC treat URM buildings?
URM bearing‑wall buildings are evaluated under the CEBC Appendix A1 (2013) procedures as referenced in § 8-706.1.2; that appendix prescribes specific R, h/t limits, anchorage, pier shear checks and more.
Does CHBC change how near‑fault spectral amplification is handled?
Yes — for Risk Category I, II or III you may omit near‑fault increases when the building’s fundamental period in the direction under consideration is ≤ 0.5 s (see § 8-706.1, Exception 2).
If my computed base shear is higher than 0.30W (Risk Cat II), which governs?
For Risk Category I or II, the CHBC allows the seismic base shear to be no greater than 0.30W (Exception 3 of § 8-706.1), so that cap would govern if it is lower than your reduced/calculated base shear.
More in California Historical Building Code
Ask about the CHBC
Get cited, plain-English answers on the California Historical Building Code for your project — any code section, any scenario.
Start Free Trial