Appendix A — GUIDELINES FOR THE SEISMIC RETROFIT OF EXISTING BUILDINGS
A108 Design Strengths — 2025 California Existing Building Code
California Existing Building Code (Title 24, Part 10) · 2025 edition · updated 2026-10-19 · California
CBSC: official editions, supplements and errata
Italicized text is a California amendment to the model code, as printed in the official publication.
Cite as: 2025 California Existing Building Code (Title 24, Part 10) § A108 · Effective January 1, 2026 · Official source: Building Standards Commission
A108.1 Strength values.¶
[BS] A108.1 Strength values.
Strength values for existing materials are given in Table A108.1(1) and for new materials in Table A108.1(2).
The strength reduction factor, φ, shall be taken equal to 1.0.
The use of materials not specified herein shall be based on substantiating research data or engineering judgment, as approved by the code official.
[BS] TABLE A108.1(1)—STRENGTH VALUES FOR EXISTING MATERIALS
| EXISTING MATERIALS OR CONFIGURATION OF MATERIALSa | EXISTING MATERIALS OR CONFIGURATION OF MATERIALSa | STRENGTH VALUES |
|---|---|---|
| EXISTING MATERIALS OR CONFIGURATION OF MATERIALSa | EXISTING MATERIALS OR CONFIGURATION OF MATERIALSa | × 14.594 for N/ |
| Horizontal diaphragms | Roofs with straight sheathing and roofing applied directly to the sheathing. |
300 lbs. per ft. for seismic shear |
| Horizontal diaphragms | Roofs with diagonal sheathing and roofing applied directly to the sheathing. |
750 lbs. per ft. for seismic shear |
| Horizontal diaphragms | Floors with straight tongue-and-groove sheathing. | 300 lbs. per ft. for seismic shear |
| Horizontal diaphragms | Floors with straight sheathing and finished wood flooring with board edges offset or perpendicular. |
1,500 lbs. per ft. for seismic shear |
| Horizontal diaphragms | Floors with diagonal sheathing and finished wood flooring. | 1,800 lbs. per ft. for seismic shear |
| Horizontal diaphragms | Metal deck welded with minimal welding.c | 1,800 lbs. per ft. for seismic shear |
| Horizontal diaphragms | Metal deck welded for seismic resistance.d | 3,000 lbs. per ft. for seismic shear |
| Crosswallsb | Plaster on wood or metal lath. | 600 lbs. per ft. for seismic shear |
| Crosswallsb | Plaster on gypsum lath. | 550 lbs. per ft. for seismic shear |
| Crosswallsb | Gypsum wallboard, unblocked edges. | 200 lbs. per ft. for seismic shear |
| Crosswallsb | Gypsum wallboard, blocked edges. | 400 lbs. per ft. for seismic shear |
| Existing footing, wood framing, structural steel, reinforcing steel |
Plain concrete footings. | f′c = 1,500 psi unless otherwise shown by tests |
| Existing footing, wood framing, structural steel, reinforcing steel |
Douglas fir wood. | Same as D.F. No. 1 |
| Existing footing, wood framing, structural steel, reinforcing steel |
Reinforcing steel. | Fy = 40,000 psi maximum |
| Existing footing, wood framing, structural steel, reinforcing steel—continued |
Structural steel. | Fy = 33,000 psi maximum |
| For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2, 1 pound = 4.4 N, 1 pound per square inch = 6894.75 N/ a. Material must be sound and in good condition. b. Shear values of these materials may be combined, except the total combined value should not exceed 900 pounds per foot. c. Minimum 22-gage steel deck with welds to supports satisfying the standards of the Steel Deck Institute. d. Minimum 22-gage steel deck with3/4-inch diameter plug welds at an average spacing not exceeding 8 inches and with sidelap welds appropriate for the deck span. |
[BS] TABLE A108.1(2)—STRENGTH VALUES OF NEW MATERIALS USED IN CONJUNCTION WITH EXISTING CONSTRUCTION
| NEW MATERIALS OR CONFIGURATION OF MATERIALS | NEW MATERIALS OR CONFIGURATION OF MATERIALS | STRENGTH VALUES |
|---|---|---|
| Horizontal diaphragms |
Plywood sheathing applied directly over existing straight sheathing with ends of plywood sheets bearing on joists or rafters and edges of plywood located on center of individual sheathing boards. |
675 lbs. per ft. |
| Crosswalls | Plywood sheathing applied directly over wood studs; no value should be given to plywood applied over existing plaster or wood sheathing. |
1.2 times the value specified in the current building code. |
| Crosswalls | Drywall or plaster applied directly over wood studs. | The value specified in the current building code. |
| Crosswalls | Drywall or plaster applied to sheathing over existing wood studs. | 50 percent of the value specified in the current building code. |
| Tension anchorsf | Anchors extending entirely through unreinforced masonry wall secured with bearing plates on far side of a wall 30 square inches of area.b, c |
5,400 lbs. per anchor for three-wythe minimum walls. 2,700 lbs. for two-wythe walls. |
| Shear boltse, f | Anchors embedded not less than 8 inches into unreinforced masonry walls; anchors should be centered in 21/2-inch-diameter holes with dry-pack or nonshrink grout around the circumference of the anchor. |
The value for plain masonry specified for solid masonry TMS 402; and no value larger than those given for3/4-inch bolts should be used. |
| Combined tension and shear anchorsf |
Through-anchors—anchors meeting the requirements for shear and for tension anchors.b, c |
Tension—same as for tension anchors. Shear—same as for shear anchors. |
| Combined tension and shear anchorsf |
Embedded anchors—anchors extending to the exterior face of the wall with a 21/2-inch round plate under the head and drilled at an angle of 221/2 degrees to the horizontal; installed as specified for shear anchors.a, b, c |
Tension—3,600 lbs. per anchor. Shear—same as for shear anchors. |
[BS] TABLE A108.1(2)—STRENGTH VALUES OF NEW MATERIALS USED IN CONJUNCTION WITH EXISTING CONSTRUCTION—continued
| NEW MATERIALS OR CONFIGURATION OF MATERIALS | NEW MATERIALS OR CONFIGURATION OF MATERIALS | STRENGTH VALUES |
|---|---|---|
| Infilled walls | Reinforced masonry infilled openings in existing unreinforced masonry walls; provide keys or dowels to match reinforcing. |
Same as values specified for unreinforced masonry walls. |
| Reinforced masonryd | Masonry piers and walls reinforced per the current building code. | The value specified in the current building code for strength design. |
| Reinforced concreted | Concrete footings, walls and piers reinforced as specified in the current building code. |
The value specified in the current building code for strength design. |
| For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2, 1 pound = 4.4 N, 1 degree = 0.017 rad, 1 pound per foot = 14.43 N/ a. Embedded anchors to be tested as specified in Section A107.4. b. Anchors shall be1/2 inch minimum in diameter. c. Drilling for anchors shall be done with an electric rotary drill; impact tools should not be used for drilling holes or tightening anchors and shear bolt nuts. d. Load factors or capacity reduction factors shall not be used. e. Other bolt sizes, values and installation methods may be used, provided that a testing program is conducted in accordance with Section A107.5.3. The strength value shall be determined by multiplying the calculated allowable value, determined in accordance with Section A107.5.3, by 3.0, and the usable value shall be limited to not greater than 1.5 times the value given in the table. Bolt spacing shall not exceed 6 feet on center and shall be not less than 12 inches on center. f. An alternative adhesive anchor bolt system is permitted to be used providing: its properties and installation conform to an ICC Evaluation Service Report; and the report states that the system’s use is in unreinforced masonry as an acceptable alternative to Sections A107.4 and A113.1 or TMS 402, Section 2.1.4. The report’s allowable values shall be multiplied by a factor of three to obtain strength values and the strength reduction factor,φ, shall be taken equal to 1.0. |
A108.2 Masonry shear strength.¶
[BS] A108.2 Masonry shear strength. The unreinforced masonry shear strength, v mL, shall be determined for each masonry class from one of the following equations:
- When testing is performed in accordance with Section A106.2.3.1, the unreinforced masonry shear strength, v m, shall be determined by Equation A1-3.
Equation A1-3
v mL
= ----------------------------------------------0.75 0.75 v tL + P ---- A Dn - - 1.5
The mortar shear strength values, v tL, shall be determined in accordance with Section A106.2.3.6. 2. When alternate testing is performed in accordance with Section A106.2.3.3, unreinforced masonry shear, v mL, shall be determined by Equation A1-4.
Equation A1-4
v mL
0.75 f + P ---- D -
= ---------------------------------- sp A n - 1.5
A108.3 Masonry compression.¶
[BS] A108.3 Masonry compression. Where any increase in wall dead plus live load compression stress occurs, the maximum compression stress in unreinforced masonry, Q G /A n, shall not exceed 300 pounds per square inch (2070 kPa).
A108.4 Masonry tension.¶
[BS] A108.4 Masonry tension. Unreinforced masonry shall be assumed to have no tensile capacity.
A108.5 Wall tension anchors.¶
[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.
A108.6 Foundations.¶
[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.
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