7 replies to this topic

[Calixico]

I am looking through Ruwan Rajapakse Civil PE Professional Engineer Exam Construction Module Book and came across this problem:
A wall is build using standard brick (L=8", H=2-1/4", Thickness= 3- 3/4 in).

A wall to be built with bricks is one brick thick. How many bricks are required if motar is 1/4 in thick?

The solution states that the new length of the brick (with the motar) 8"+ 1/8" + 1/8" = 8 1/4" and the height of the brick with the motar is 2 -1/4" + 1/8" +1/8" = 2- 1/2".

This is where I am confused: shouldn't the new length and height of the brick be: 8"+ 1/4" + 1/4" = 8 1/2" and 2 -1/4" + 1/4" +1/4" = 2- 3/4" respectively. Or this is standard? It appears that the author is splitting the mortar into two, then adding it (mortar thickness/2) to the brick's length and height.


FYI, The problem in his book is on page 103 for the construction folks who have this book

[palvarez83]

The solution makes sense to me. If you look at a single brick, yes it has 1/4" of mortar all around (top, bottom, left, & right). However, given that there are bricks adjacent to it (top, bottom, left& right), you split the (motar) gap with each of these. So, for example if you have two bricks with a 1/4" of mortar between them, you say that 1/8" belongs to each brick. This is the case on all 4 sides. So for length it is 8" (brick) + 1/8" (left) +1/8" (right). For height it is 2-1/2" (brick)+ 1/8" (top) +1/8"(bottom).

This is the gerneral case for bricks that are not on the boundaries.

[treyjay]

get Walkers Building Estimating book...it will have factors that will get the "most nearly" answer for these types of problems (CMU, brick, structural cly tile, etc.)...as well as other topics related to quantities.

[Jayman_PE]

Calixio,

The Ruwan solution on page 103 is correct. He's simply splitting the dimensions along the centerline perimeter of the mortar.

However, I did notice an issue on page 102, problems 2.4 and 2.5 where Ruwan can't decide to use 1" or 1/2" mortar. He isn't the most refined author. At least the text has some good substance that may actually be useful for the construction PM exam. Here's hoping.

Jason

[Calixico]

Ok. If a problem ask how many blocks are needed in a problem statement, given each sides dimensions, and the wall thickness of a building, I should use the outside perimeter lengths to get the total perimeter of all sides, which is ultimately used and divided by the s.f. of one block. Assuming they say neglect mortar joints. Correct.

[Jayman_PE]

That is not correct. You only need to know the composite area of one mortar surrounded by half the mortar thickness around it's perimeter. Let's call this Area 'A'. Then take the total wall area and divide by Area 'A'. Bingo.

Jason

[treyjay]

really...seriously....you can make this way more complicated than it needs to be....consult a decent estimating reference (I like Walker's) to get the factors and material descriptions/methods you need to answer these types of questions....there are 1000 ways engineers can over think this stuff....

[Jayman_PE]

It's very simple. Just sketch up one typical and you're golden.....