Question on Masonry Design

[PEin2010]

Hi! I am giving the structural PM. I started looking at some of the depth problems today and realized I'm quite inexperienced on Masonry design. One question I have is:

For a concrete masonry wall with reinforcement, and a roof dead load, the question asks to calculate the maximum design moment (lb-ft/ft). The loads are given as roof dead load is 55 psf (includes the snow load) and the effective length contributing is 12 feet. P = 12x55 = 660 lb/ft. The wall is 8" concrete block reinforced with steel reinforcement. The solution cacluates an 'e' eccentricity value as 7.625/2 + 3.5 = 7.31 inches. So M = P x e. However I am not able to understand how and why they calculated 'e'? Any guidance is appreciated!

Thank you in advance!

 

[WoodSlinger]

Hi! I am giving the structural PM. I started looking at some of the depth problems today and realized I'm quite inexperienced on Masonry design. One question I have is:
For a concrete masonry wall with reinforcement, and a roof dead load, the question asks to calculate the maximum design moment (lb-ft/ft). The loads are given as roof dead load is 55 psf (includes the snow load) and the effective length contributing is 12 feet. P = 12x55 = 660 lb/ft. The wall is 8" concrete block reinforced with steel reinforcement. The solution cacluates an 'e' eccentricity value as 7.625/2 + 3.5 = 7.31 inches. So M = P x e. However I am not able to understand how and why they calculated 'e'? Any guidance is appreciated!

Thank you in advance!

An 8" block is 7-5/8" wide. There must be some kind of ledger board on the wall, etc., that pushes the roof member reactions 3.5" of the face of the wall. The eccentricity, or moment arm, is this 3.5" dimension plus 1/2 of the block width. There is your "e".