Lag Screws - Compression perpendicular to grain
Lag Screws - Compression perpendicular to grain
(OP)
I want to attach a wood beam to a wood column using a lag screw with 12" embedment into the column, not including the length of the tapered tip. I am expecting the lag screw to resist withdrawal only. The beam is a 6×12 DF #1 and the column is a 6×6 DF #1. The lag screw conforms to A307 grade A, Fy=60,000psi, and has a standard washer conforming to ASTM F844. Based on the table below, I find compression perpendicular to grain in the wood beam is the limiting factor by far.
1/2"Ø 3/4"Ø
Tensile Capacity 3883 lbs 9460 lbs
12" Embed Capacity 3402 lbs 4617 lbs
Comp Perp to Grain Capacity 966 lbs 1951 lbs
If I use a round plate washer from Portland Bolt instead of a standard washer, then I get the following values.
1/2"Ø 3/4"Ø
Comp Perp to Grain Capacity 2772 lbs 4626 lbs
So my question is, is everyone using large plate washers on lag screws or am I missing something? Thanks in advance for all your help.
Additional calculations are below.
Max Capacity of a lag screw at the net (root) section
1/2"Ø - Dr = 0.371" [2005 NDS Appendix L] = (60,000 psi * pi * (0.371"/2)^2)/1.67 = 3883 lbs [Steel Manual, 13th edition, Page 5-2]
3/4" - Dr = 0.579" [2005 NDS Appendix L] = (60,000 psi * pi * (0.579"/2)^2)/1.67 = 9460 lbs [Steel Manual, 13th edition, Page 5-2]
Capacity of a lag screw with 12" embedment
1/2"Ø - 378 lbs/in * 12" * 0.75 (Ceg) = 3402 lbs [2005 NDS Page 68]
3/4"Ø - 513 lbs/in * 12" * 0.75 (Ceg) = 4617 lbs [2005 NDS Page 68]
Compression Perpendicular to grain with a standard washer conforming to ASTM F844
1/2"Ø - 1 3/8"Ø OD and 9/16"Ø ID - 625 psi * pi * ((1.375"/2)^2-(0.5625"/2)^2) * 1.25 (Cb) = 966 lbs [2005 NDS Supplement Page 45] and [2005 NDS Page 68]
3/4"Ø - 2"Ø OD and 13/16"Ø ID - 625 psi * pi * ((2"/2)^2-(0.8125"/2)^2) * 1.19 (Cb) = 1951 lbs [2005 NDS Supplement Page 45] and [2005 NDS Page 68]
Compression Perpendicular to grain with a round plate washer from Portland Bolt
1/2"Ø - 2 1/4"Ø OD and 9/16"Ø ID - 625 psi * pi * ((2.25"/2)^2-(0.5625"/2)^2) * 1.19 (Cb) = 2772 lbs [2005 NDS Supplement Page 45] and [2005 NDS Page 68]
3/4"Ø - 3"Ø OD and 13/16"Ø ID - 625 psi * pi * ((3"/2)^2-(0.8125"/2)^2) * 1.13 (Cb) = 4626 lbs [2005 NDS Supplement Page 45] and [2005 NDS Page 68]
1/2"Ø 3/4"Ø
Tensile Capacity 3883 lbs 9460 lbs
12" Embed Capacity 3402 lbs 4617 lbs
Comp Perp to Grain Capacity 966 lbs 1951 lbs
If I use a round plate washer from Portland Bolt instead of a standard washer, then I get the following values.
1/2"Ø 3/4"Ø
Comp Perp to Grain Capacity 2772 lbs 4626 lbs
So my question is, is everyone using large plate washers on lag screws or am I missing something? Thanks in advance for all your help.
Additional calculations are below.
Max Capacity of a lag screw at the net (root) section
1/2"Ø - Dr = 0.371" [2005 NDS Appendix L] = (60,000 psi * pi * (0.371"/2)^2)/1.67 = 3883 lbs [Steel Manual, 13th edition, Page 5-2]
3/4" - Dr = 0.579" [2005 NDS Appendix L] = (60,000 psi * pi * (0.579"/2)^2)/1.67 = 9460 lbs [Steel Manual, 13th edition, Page 5-2]
Capacity of a lag screw with 12" embedment
1/2"Ø - 378 lbs/in * 12" * 0.75 (Ceg) = 3402 lbs [2005 NDS Page 68]
3/4"Ø - 513 lbs/in * 12" * 0.75 (Ceg) = 4617 lbs [2005 NDS Page 68]
Compression Perpendicular to grain with a standard washer conforming to ASTM F844
1/2"Ø - 1 3/8"Ø OD and 9/16"Ø ID - 625 psi * pi * ((1.375"/2)^2-(0.5625"/2)^2) * 1.25 (Cb) = 966 lbs [2005 NDS Supplement Page 45] and [2005 NDS Page 68]
3/4"Ø - 2"Ø OD and 13/16"Ø ID - 625 psi * pi * ((2"/2)^2-(0.8125"/2)^2) * 1.19 (Cb) = 1951 lbs [2005 NDS Supplement Page 45] and [2005 NDS Page 68]
Compression Perpendicular to grain with a round plate washer from Portland Bolt
1/2"Ø - 2 1/4"Ø OD and 9/16"Ø ID - 625 psi * pi * ((2.25"/2)^2-(0.5625"/2)^2) * 1.19 (Cb) = 2772 lbs [2005 NDS Supplement Page 45] and [2005 NDS Page 68]
3/4"Ø - 3"Ø OD and 13/16"Ø ID - 625 psi * pi * ((3"/2)^2-(0.8125"/2)^2) * 1.13 (Cb) = 4626 lbs [2005 NDS Supplement Page 45] and [2005 NDS Page 68]






RE: Lag Screws - Compression perpendicular to grain
______________
MAP
RE: Lag Screws - Compression perpendicular to grain
BA
RE: Lag Screws - Compression perpendicular to grain
Let's pretend that the 6×6 column is actually a 6×16 wood beam and I have the same question. Do you have any additional thoughts?
RE: Lag Screws - Compression perpendicular to grain
If instead, the column is a 6x16 beam and you are placing the screw normal to the grain, that is an entirely different matter. I still don't like the detail, but if you insist on using it, it could be justified.
BA
RE: Lag Screws - Compression perpendicular to grain
I have attached a detail showing the beam to column connection. My thought in changing the column to a beam was to satisfy your dislike of using a lag screw for withdrawal from end grain and steer you back to my original question about compression perpendicular to grain.
Without distracting from the question of compression perpendicular to grain, my opinion is that the code allows lag screws to be used in withdrawal from end grain and provides a 0.75 adjustment factor while offering no warning or recommendation against this practice. I very much respect your opinion and appreciate the posts you make on this site. Since I don't see any code restriction on or recommendation against withdrawal from end grain with a lag screw, I would appreciate an explanation of why you don't think withdrawal from end grain is good practice. Over your career have you seen failures or other symptoms that worry you from this type of connection? We use the attached detail all the time at covered patios when the architect does not want to see any connectors. If you would avoid this detail then what might you propose to meet the architect's request? Thanks
RE: Lag Screws - Compression perpendicular to grain
BA
RE: Lag Screws - Compression perpendicular to grain
RE: Lag Screws - Compression perpendicular to grain
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Lag Screws - Compression perpendicular to grain
BA
RE: Lag Screws - Compression perpendicular to grain
______________
MAP
RE: Lag Screws - Compression perpendicular to grain
RE: Lag Screws - Compression perpendicular to grain
Here is a link for one such system http://www.timberlinx.com/
______________
MAP
RE: Lag Screws - Compression perpendicular to grain
The device you linked is a little more complicated than what I had in mind, but interesting nonetheless. Your device does the tightening/clamping action by the way the cross dowel device expands as it is tightened. Some sort of wedging and expansion means? And, this limited tightening action will likely require some more accuracy in the location, distance btwn., the cross bores. Mine was just a round bar dowel, drilled and taped, and you screwed the .5" rod into the dowel in its cross bore. This eventually seated the dowel at the top of the cross bore, as the rod bottomed out at the bottom of the cross bore. Then you installed the top nut and washer. There are several slightly different systems, mostly marketed to timber framers, I think. That’s most likely where I’ve seen them.