Avoiding Cracks in Stack Bond Toronto ON

I have heard that masonry laid in stack bond is more likely to crack than masonry laid in running bond. Why? Assuming walls were built using the same mortar and brick in Toronto, why wouldn't the vertical joints in stack bond be as strong as the horizontal joints in both stack bond and running bond?

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Avoiding Cracks in Stack Bond

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Source: MASONRY CONSTRUCTION MAGAZINE
Publication date: August 1, 1994

I have heard that masonry laid in stack bond is more likely to crack than masonry laid in running bond. Why? Assuming walls were built using the same mortar and brick, why wouldn't the vertical joints in stack bond be as strong as the horizontal joints in both stack bond and running bond?

Unreinforced stack bond walls are more likely to crack vertically than walls built with running bond. That is why ACI 530 requires any masonry laid in other than running bond to have horizontal joint reinforcement spaced no further than 4 feet apart in order to control cracking.In a running bond wall, any vertical crack must either pass through a brick at every other course or follow the mortar through head and bed joints. Usually the weakest path is through the units. Therefore the wall's strength is determined mainly by the strength of the units, not the mortar bond strength at head joints. In stack bond walls that are not reinforced, resistance to vertical cracking depends solely on mortar-to-brick bond, which is weaker in vertical joints than in horizontal joints. Better bond is developed in horizontal joints because the weight of the masonry keeps the joints under pressure throughout construction. If cracks do develop, they generally are very small because of this compression.

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