Question on Structural Masonary Design Structural Depth (PM)

[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!

 

[outatime2002]

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!

I think it would be best for you to post a diagram of the problem so we can see the whole picture of what is going on, but for the time being I will take a stab at it. In typical masonry construction, there is a ledger that the roof beams frame into. I'm guessing that your ledger is 3.5" from the face of the wall. Therefore your eccentricity, "e", from the centerline of the 8" nominal wall, 7.625" actual width, is equal to 7.625"/2 + 3.5" = 7.31". Again, this is my best guess because you have not provided a diagram of the problem.

 

[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!

I think it would be best for you to post a diagram of the problem so we can see the whole picture of what is going on, but for the time being I will take a stab at it. In typical masonry construction, there is a ledger that the roof beams frame into. I'm guessing that your ledger is 3.5" from the face of the wall. Therefore your eccentricity, "e", from the centerline of the 8" nominal wall, 7.625" actual width, is equal to 7.625"/2 + 3.5" = 7.31". Again, this is my best guess because you have not provided a diagram of the problem.

Thanks for replying. The diagram is showing a 4x12 ledger with 2x12 roof joists going into it. I will scan the diagram and upload tomorrow. Aaah and I just found that the actual dimensions of masonry units are typically 3/8 in less than nominal dimensions, so that the 4 or 8 inch module size is maintained with 3/8 in mortar joints.

Thanks!

 

[outatime2002]

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!

I think it would be best for you to post a diagram of the problem so we can see the whole picture of what is going on, but for the time being I will take a stab at it. In typical masonry construction, there is a ledger that the roof beams frame into. I'm guessing that your ledger is 3.5" from the face of the wall. Therefore your eccentricity, "e", from the centerline of the 8" nominal wall, 7.625" actual width, is equal to 7.625"/2 + 3.5" = 7.31". Again, this is my best guess because you have not provided a diagram of the problem.

Thanks for replying. The diagram is showing a 4x12 ledger with 2x12 roof joists going into it. I will scan the diagram and upload tomorrow. Aaah and I just found that the actual dimensions of masonry units are typically 3/8 in less than nominal dimensions, so that the 4 or 8 inch module size is maintained with 3/8 in mortar joints.

Thanks!

The 4x12 ledger would explain the additional 3.5" of eccentricity. You would subtract 1/2" from the nominal width of the timber beam to get actual width. And yes, you are correct in saying that actual dimensions of masonry are 3/8" less than nominal, which would explain a 7.625" thick masonry wall. I can see the diagram in my head now. No need to post. I hope I helped.