07-27-2012, 10:28 PM
Walter
Found the formula for lag bolt allowable withdrawal load. "Marks Standard Handbook for Mechanical Engineers" Won't say how long ago I bought that book for school. Can't be that long ago its only the 8th edition.
Sorry don't remember how to write formula on the computer but here it is.
P=1800D(to the power of 3/4) G ( to the power of 3/2)
Where D = shank dia of lag
G = specific gravity of wood
Most construction material is SPF spruce pine or fir.
The lowest being pine at .43
So for a 3/8 lag in pine side grain you get 243 Pounds pull out strength per inch of thread.
Think I went 2 1/2 lags to get 2 in of thread into the joists or 486 pounds per lag at 2 per joist 972 pounds on each joist. Spread the load over 3 joist gives 2916. Derate by a factor of safety of 3 and you get close to 1000 pounds if the load is at the wall, worst case scenario.
My ceiling was sheeted with chip board which I wasn't about to remove.
Not sure the 1/2 inch bolts your thinking of using would give much more strength with 2 inches of wood for pullout on the joists. Don't know where you'd get the numbers for that.
Just food for thought.
Found the formula for lag bolt allowable withdrawal load. "Marks Standard Handbook for Mechanical Engineers" Won't say how long ago I bought that book for school. Can't be that long ago its only the 8th edition.
Sorry don't remember how to write formula on the computer but here it is.
P=1800D(to the power of 3/4) G ( to the power of 3/2)
Where D = shank dia of lag
G = specific gravity of wood
Most construction material is SPF spruce pine or fir.
The lowest being pine at .43
So for a 3/8 lag in pine side grain you get 243 Pounds pull out strength per inch of thread.
Think I went 2 1/2 lags to get 2 in of thread into the joists or 486 pounds per lag at 2 per joist 972 pounds on each joist. Spread the load over 3 joist gives 2916. Derate by a factor of safety of 3 and you get close to 1000 pounds if the load is at the wall, worst case scenario.
My ceiling was sheeted with chip board which I wasn't about to remove.
Not sure the 1/2 inch bolts your thinking of using would give much more strength with 2 inches of wood for pullout on the joists. Don't know where you'd get the numbers for that.
Just food for thought.
Free advice is worth exactly what you payed for it.
Greg
Greg