HS21
Mechanical
- Sep 13, 2023
- 1
Hello,
I have been analyzing a machine base plate for off normal conditions (e.g. seismic events). I am a mechanical engineer by trade, so I wanted to see if my concerns with using AISC Design Guide 1 for reference in this scenario are founded.
The problem is such that a significant uplift and moment may exist in a base plate where the magnitude of the moment is great enough that the resulting upwards force with eccentricity would lie outside of the bolt pattern. This would imply that not all bolts are in tension, and a bearing stress must exist at the steel-concrete interface. However, DG-1's sections either discuss uplift with no moment, or compressive force in the column with moment. From my intuition, this seems most similar to the "Base Plates with Large Moments" section, as one anchor is in tension and the other section is in bearing. However, I wanted to see if anyone here could poke holes in this approach.
What jumps out at me off the bat is that the elastic deformation in the anchor rod in tension could reduce the max possible value of bearing length Y as the base plate lifts off the concrete foundation. Furthermore, I'm not convinced that Y should be taken as Pr/qmax now that Pr acts in the upwards direction, so this would require some reformulation.
I'd greatly appreciate it if anyone could weigh in on this, or suggest a more fitting approach that I could look at.
I have been analyzing a machine base plate for off normal conditions (e.g. seismic events). I am a mechanical engineer by trade, so I wanted to see if my concerns with using AISC Design Guide 1 for reference in this scenario are founded.
The problem is such that a significant uplift and moment may exist in a base plate where the magnitude of the moment is great enough that the resulting upwards force with eccentricity would lie outside of the bolt pattern. This would imply that not all bolts are in tension, and a bearing stress must exist at the steel-concrete interface. However, DG-1's sections either discuss uplift with no moment, or compressive force in the column with moment. From my intuition, this seems most similar to the "Base Plates with Large Moments" section, as one anchor is in tension and the other section is in bearing. However, I wanted to see if anyone here could poke holes in this approach.
What jumps out at me off the bat is that the elastic deformation in the anchor rod in tension could reduce the max possible value of bearing length Y as the base plate lifts off the concrete foundation. Furthermore, I'm not convinced that Y should be taken as Pr/qmax now that Pr acts in the upwards direction, so this would require some reformulation.
I'd greatly appreciate it if anyone could weigh in on this, or suggest a more fitting approach that I could look at.