asabender
Structural
- Feb 8, 2013
- 28
The engineers I trained under, and everyone in the area of New England we work in, pin foundations to ledge if in direct bearing... meaning #5 redar embedded 6" or so into crystalline bedrock at maybe 48" on center. I have done so for years, but was recently questioned about it, and realized (to my surprise) that I didn't know enough to give ready answers to specific design questions. I have designed more custom pinning/tendon anchorage to ledge using deep development and kwixset or epoxy for rock tendons, tensile anchors, tensile-shear dowels, and pure shear pinning; but only when I was working in non-standard applications. I tried researching it and came up empty. Does anyone have any thoughts on why this is a standard recommendation on a relatively flat site?
I provided an answer, so this is for my own elucidation. The answer I ended up giving was that it was basically for lateral restraint. Lateral restraint with pins/dowels being preferable since sites tend not to have flat interfaces between the stone and the concrete and the concrete leveling pads/footings tend to pop loose in temperature changes or under deformation. It's also a good idea to get some bite past the potentially cracked rock surface imho, and I correspondingly increase embedment to try to get into the sound crystalline stuff; but that was not included in my response.
I provided an answer, so this is for my own elucidation. The answer I ended up giving was that it was basically for lateral restraint. Lateral restraint with pins/dowels being preferable since sites tend not to have flat interfaces between the stone and the concrete and the concrete leveling pads/footings tend to pop loose in temperature changes or under deformation. It's also a good idea to get some bite past the potentially cracked rock surface imho, and I correspondingly increase embedment to try to get into the sound crystalline stuff; but that was not included in my response.