Empirical Masonry Design

[bouk715]

I have a question I'm hoping someone can shed some light on...I am designing a small 50'x40' structure with masonry walls and a timber framed roof (plywood sheathing/wood trusses). The wind speed and other factors are such that I am allowed by the IBC to use empirical design for the masonry. This structure has a gable roof with a 10' eave but the height at the gable end extends to almost 21' at the ridge. Using empirical design, I have no trouble getting an 8" block size to work along the eave edges (meeting h/t of 20 or 18). However, at the gable end this requirement leads to a 12" or larger block.

First off, I would assume that the h/t requirements apply to the gable end as well as the sides supporting the truss bearing. What is the typical framing used to get an 8 or 10" block to work at the gable end? If a gable-end truss is used to terminate the masonry at 10', is there anywhere I can find details for connecting the truss to the top of the CMU; as well as how to brace the gable truss back to the roof sheathing for out of plane loading? Is there a different detail that typically be used?

 

[DaveAtkins]

I would reinforce the CMU at the gable end for the entire 21' height.
If you must use a gable end truss, there are two options. You could provide OSB for all or part of the ceiling, thus creating a flat diaphragm that braces the end wall at 10'. Or you could provide diagonal 2 X 6's to brace the top of the CMU up to the roof diaphragm.

DaveAtkins

 

[bouk715]

Thank you very much for your help. I have one other question: If I engineer the masonry at the gable end, do I design for the full ridge height as your answer above seems to suggest? Or is there an intermediate height between the eave and ridge heights it is designed for? I haven't been able to find much literature on this. Thanks again.

 

[miecz]

The literature always seems to avoid the tough questions. I would design to the the eave plus 2/3 the difference. After all, the wall will not fail in tiny strips, but as a whole.

 

[bouk715]

Thanks, that seems to make sense. The local highway department here states that sloped retaining walls should be designed for the height at 3/4 distance from the low point to the hight point along the length of the wall for that very reason. I figured it might be something similar.

 

[DaveAtkins]

I would have designed for the full height, but what jmiec says makes sense.

DaveAtkins