hconstruct
Structural
- Oct 9, 2006
- 3
I am trying to design a “composite” beam out of 3 logs stacked on top of each other, that are anywhere from 14” to 17” in diameter. The size is dictated by the amount of space there will be in a window wall to hide the support column for the logs. Regardless, with the current loads and deflection criteria, the logs fail when they are looked at as separate acting logs (kind of like a gluelam without the glue) so now I want to design the “glue”, or shear bolts rather, to allow the 3 stacked logs to act as a composite beam and I can utilize the their collective moment of inertia. I am looking for a general design stragegy, as opposed to giving exact span, loads and what not, but I will gladly include complete details of the problem if they are needed.
Here are some strategies that I have tried but well I am not sure if they fundamentally correct.
1) Compare to a composite steel/concrete beam, and designing the shear studs. That is in this case assume the PNA is in the middle of the symmetric section of 3 stacked logs (using 14” logs) and saying that the max horz. shear on the bolts will be that of the fully yielded section of either the compression or tension stress. In other words Fy (wood yield strength) x Aw (Area of wood section above or below PNA). I don’t like because the larger the log it appears the more horiz. shear capacity the bolts would need. Granted the larger the log, the less the shear bolts are needed, because the section will carry the stress. Which leads me to think another strategy is needed………
2) Look at the max moment from the loads and the stress that it creates on the section. That stress is what I base my shear connectors off of, so technically there would be more horiz. shear connectors near the max moment area and less toward the ends, I have less of an understanding of this potential strategy and I feel like something is fundamentally off? ……
That is the idea that the shear diagram is what I should be looking at. But how is horizontal shear the same as vertical shear? Only connection I can see is the fact that the shear failure of a wood beam is assumed to be at a 45 degree angle, which means there are both vertical and horiz. shear components, and then get thoughts of relating this too Mohr’s circle but not sure how. But isn’t it possible that the horiz. shear do to the stress from the moment? Obviously this is where the something is missing in my logic/understanding.
Any help would be very much appreciated.
Here are some strategies that I have tried but well I am not sure if they fundamentally correct.
1) Compare to a composite steel/concrete beam, and designing the shear studs. That is in this case assume the PNA is in the middle of the symmetric section of 3 stacked logs (using 14” logs) and saying that the max horz. shear on the bolts will be that of the fully yielded section of either the compression or tension stress. In other words Fy (wood yield strength) x Aw (Area of wood section above or below PNA). I don’t like because the larger the log it appears the more horiz. shear capacity the bolts would need. Granted the larger the log, the less the shear bolts are needed, because the section will carry the stress. Which leads me to think another strategy is needed………
2) Look at the max moment from the loads and the stress that it creates on the section. That stress is what I base my shear connectors off of, so technically there would be more horiz. shear connectors near the max moment area and less toward the ends, I have less of an understanding of this potential strategy and I feel like something is fundamentally off? ……
That is the idea that the shear diagram is what I should be looking at. But how is horizontal shear the same as vertical shear? Only connection I can see is the fact that the shear failure of a wood beam is assumed to be at a 45 degree angle, which means there are both vertical and horiz. shear components, and then get thoughts of relating this too Mohr’s circle but not sure how. But isn’t it possible that the horiz. shear do to the stress from the moment? Obviously this is where the something is missing in my logic/understanding.
Any help would be very much appreciated.
