Masonry Magazine May 1976 Page. 16
Engineered Masonry Construction
In engineered concrete masonry construction, allowable compressive stresses for nonreinforced and reinforced concrete brick masonry are based on f'm the 28-day ultimate compressive strength of concrete masonry. As discussed in TEK 70, either of two methods-one based on unit strength tests and the other based on prism or small masonry assemblage tests-may be used to determine f'm.
Structural design tables for non-reinforced and reinforced walls based on the requirements contained in "Specification for the Design and Construction of Load-Bearing Concrete Masonry," have been prepared and published by NCMA. "Non-reinforced Concrete Masonry Design Tables," 1971, contains allowable load tables for ten different types of composite walls built with 4-in. concrete brick and either solid or hollow concrete block. Total composite wall thicknesses range from 8 to 16 inches and f'm values vary from 1000 to 3500 psi. Table 4 (taken from the previously cited report) contains allowable compressive loads (kips per foot of wall length) which can be supported by an 8-in. composite wall constructed of 4-in. concrete brick and 4-in. hollow concrete block. Allowable compressive load design tables for 10, 12 and 14-in. concrete brick and hollow block cavity walls are also provided in the 1971 NCMA report.
TABLE 2 Strength and Absorption Requirements
Compressive Strength, min, psi (MN/m²)
(Concrete Brick Tested Flatwise)
Average Gross Area
Weight Classification
Grade
Avg of 3
Individual
Water Absorption, max, (Average of 3 brick)
with Oven Dry Weight of Concrete,
lb/ft³ (kg/m³)
Lightweight
less than
Medium Weight Normal weight
less than
125 (2000)
Concrete Brick Concrete Brick 105 (1680)
125 to 105
(2000 to 1680)
or more
N-1
N-11
3500 (24.1)
3000 (20.7)
15 (240)
13 (208)
10 (160)
5-1
2500 (17.3)
2000 (13.8)
18 (288)
15 (240)
13 (208)
5-11
TABLE 3 Allowable Compressive Stress in
Nonreinforced Concrete Brick Masonry
(Based on ANSI A41.1-1953)
Allowable compressive stresses
gross cross-sectional area
(except as noted)
Construction; grade of unit
Type
Type
Type
M
S
N
Mortar
Mortar
Mortar
Solid Masonry:
8,000 plus; psi
400
350
300
4,500 to 8,000, psi
250
225
200
2,500 to 4,500, psi
175
160
140
Grouted Solid Masonry:
4,500 plus, psi
350
275
200
2,500 to 4,500, psi
275
215
155
Hollow walls (cavity or masonry bonded)
Solid units 2,500 psl
140
130
110
On gross cross-sectional area of cavity between wythes. The allowable compressive stresses for cavity walls are based upon the assumption that the floor loads bear upon but one of the two wythes. When hollow walls are loaded concentrically, the allowable stresses may be Increased by 25 percent.
TABLE 1 Moisture-Content Requirements
for Type Concrete Brick
Moisture Consent, max,
percent of Total Absorption
(Average of 3 Concrete Brick)
Linear
Shrinkage,
percent
Humidity Conditiom at Job
Site or Point of Use
Inter-
Humid
Arid
mediate
0.03 or less
45
40
35
From 0.03 to 0.045
40
35
30
0.045 to 0.065, max 35
30
25
Arid-Average annual relative humidity less than 50 percent. Intermediate-Average annual relative humidity 50 to 75 percent. Humid-Average annual relative humidity above 75 percent.