Masonry strength f′m

The specified compressive strength of masonry, f′m, verified by the unit strength method of TMS 602-22 Article 1.4 B.2: from the strength of the units and the mortar type, for concrete masonry (Table 2) and clay masonry (Table 1). The calculation also checks the bed joints, the grout and the range of f′m the Code allows, and the spacing of control or expansion joints against criteria you set. US and metric units.

Edition. TMS 602-22 is not on hand. Tables 1 and 2 were read in TMS 602-16 (pp. S-16 and S-18); TMS's overview of the 2022 changes lists no change to the unit strength method. The f′m limits were read in TMS 402-16 9.1.9.1; TMS says the 2022 edition moves them to 4.3 so they hold for every design method. Steps that rest on the 2016 text say "to confirm against the code text".

Inputs of the f′m calculation #

• Masonry: concrete masonry units (ASTM C90) or clay units (ASTM C62, C216, C652).
• Unit strength: the net area compressive strength of the units, from their tests or the supplier.
• Mortar: Type M or S, or Type N (ASTM C270).
• Units less than 4 in. nominal height: concrete masonry only; Table 2's footnote takes 85 percent of the tabulated f′m.
• Bed joint thickness: the method needs 5/8 in. or less.
• Interpolate between table rows: off by default. The Specification does not say whether its tables may be interpolated. Off reads the row the units meet, which is conservative; on interpolates linearly between rows. Your decision.
• Specified f′m and, for grouted masonry, the grout strength.
• Movement joints: wall height, joint spacing, and your criteria: a maximum spacing and a maximum ratio of panel length to height. The Code does not set them. For concrete masonry CMHA's crack control guidance (TEK 10-2C, CMU-TEC-009-23) uses 1.5 and 25 ft with horizontal reinforcement; for clay masonry BIA Technical Note 18A spaces expansion joints at about 25 ft in walls without openings. Those values are prefilled; change them to your own.

The results also list the unit strength each mortar type needs for the specified f′m, the way the Specification's commentary reads the tables backward.

Auto-design (editor toolbar) varies the unit strength over the values of the two tables (and the mortar, if you unlock it). Objective: the weakest units. f′m, grout and the joint criteria are kept.

Checks performed #

The drawing is the wall panel between joints with the strengths noted.

Limitations #

• The unit strength method only. The prism test method (1.4 B.3) and AAC masonry (whose f′AAC is the unit's strength class) are not covered.
• Unit, mortar and grout testing are the project's quality assurance, not part of the calculation.
• Joint spacing is checked against your criteria; shrinkage, thermal and moisture movements are not calculated.
• Table values are those of TMS 602-16 (to confirm against the code text: against 602-22).

Verification examples #

Verification status: 13 values compared in 7 examples (5 published, 2 hand calculations from the cited equations): 13 within tolerance, 0 recorded deviations. Code basis: not yet recorded for this module.

Published: the rows of TMS 602-16 Table 2 (concrete) and Table 1 (clay) read forward and backward: f′m for a unit strength and mortar, and the unit strength needed for an f′m. Hand calculations: the 85 percent of short units and an interpolated value. There is no published worked example of the method beyond the tables themselves.