Table 4-3.—Average Concrete Masonry Units and Mortar Per 100 Sq Ft of wallrequired for every 100 square feet of a concretelength by the number of block in height, which, in thismasonry wall.NOTE: Actual wall length is measured from theoutside edge to outside edge of units and equals thenominal length minus 3/8” (one mortar joint).NOTE: For concrete masonry units 7 5/8” and 35/8” in height laid with 3/8” mortar joints. Height ismeasured from center to center of mortar joints.NOTE: Mortar is based on 3/8” joint with aface-shell mortar bed and 10% allowance for waste.As a Builder, you might find yourself in the fieldwithout the tables handy. To solve that problem, wewill cover two methods for estimating concretemasonry units (CMUs) without the tables. The firstmethod is CHASING THE BOND, which is using the3/4 rule and the 3/2 rule. Remember when estimating,example, is 75 CMUs in length times 15 courses high,which equals 1,125 (FB). Let’s take another example:Given: A building 20-feet long by 8-feet wide by8-feet high.75 x 20 x 2 (sides) = 30 (8" x 8“ x 16" block).75 x 8 x 2 (sides) = 12 (8" x 8“ x 16" block)Or you can find the total linear feet (LF) of thebuilding and multiply by .75.20 x 2 (sides) + 8 x 2 (sides) = 56 LF56 x .75 =42 FB1.5 x 8 = 12 courses high42 FB x 12 courses = 504 total FBalways use OUTSIDE measurements to calculate theThe second method of estimating CMU is thenumber of blocks required per course. In most SeabeeSQUARE FOOT METHOD. It is usually the quickestconstruction, 8-inch by 8-inch by 16-inch block isused. Using the 3/4 rule (three full block per 4 feet inlength)or .75, multiply the length of the wall by .75.For example, a retaining wall that is 10 feet high by100 feet in length (1,000 sf) will require 75 block forthe first course.Length of course in feet x rule 3/4 = number of CMUper courseUsing the 3/2 rule (three full block per 2 feet in height),multiply the height of the wall by 1.5. For example, theheight of the retaining wall is 10 feet. Multiply (10) bythe rule 3/2 (1.5) which will equal 15 block high(courses high). See the following formula:Height of wall in feet x rule 3/2 = courses highThen, to find the total number of full block (FB)in the retaining wall, multiply the number of block inand simplest method but NOT the most accurate.However, you, the estimator, will use this methodquite frequently. Remember in the first example, theretaining wall was 10 feet high and 100 feet in length.All you do is multiply L x H = SF in this example; theanswer is 1,000 square feet (SF). To find the numberof 8" x 8" x 16" block required, you must determinethe square footage of one CMU which is .89 SF perblock. Next, you divide 1,000 SF by .89 SF/CMUwhich equals 1,124 FB. You calculated the block for1,000 SF and the difference was (1) less block figuringby the SF method. See the following formula:Total SF divided by SF/CMU = total number ofCMUNow calculate the 20 ft x 20 ft x 8 ft building:20 x 8 = 160 SF x 2 (sides) = 320 SF4-3

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