Lag Screws - Compression perpendicular to grain 1

[PrescottEngineer]

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]

 

[focuseng]

I try to avoid fastener withdrawl modes if at all possible in wood and use something else when it is critical (especially at the loads you are quoting). But yes, the plate washer may be required and it is proper to evaluate it in your design. I have been specifying 2" square washers for 10 years now just for a typical sill plate anchor (not that anybody installs them...). There is little to no capacity in a standard washer for pull through in wood when it counts. I remember reading a while back about some inspections on houses in high wind events where the typical anchors were nicely sitting in the foundations and the sill plate is somewhere else.

______________
MAP

 

[BAretired]

The use of lag screws in end grain should be avoided.

BA

 

[PrescottEngineer]

BAretired,

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?

 

[BAretired]

Provide a sketch showing both situations. Maybe I am misreading your initial post. The only way you could obtain 12" embedment into a 6x6 column is to place the screw into the end grain of the column. That is a bad detail which should be avoided.

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

 

[PrescottEngineer]

BA Retired,

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

 

[BAretired]

I am using CSA 086-01 "Engineering Design in Wood". It too has a 0.75 end grain factor but it warns "Use of lag screws in end grain should be avoided whenever possible." Thus it is allowed but I would interpret the note to mean it is not considered good practice. I do not recall having seen specific failures as a result of this detail but that is because I do not recall having seen the detail used.

BA

 

[hokie66]

The problem with this type connection is that, in many cases, the bolt corrodes. It only takes a small amount of corrosion for the resulting expansion to split the post. I might allow such a connection with a stainless steel lag bolt, if such a thing exists. A better solution is to shoot the architect. If he doesn't want to see connections, wood is the wrong material.

 

[dicksewerrat]

Are we allowed to shoot architects? I have to go clean my guns.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[BAretired]

I do remember seeing a wood dowel used in this type of situation. The parts were drilled and a hardwood dowel was glued to both parts. I'm not recommending this, but I have seen it used. I much prefer hardware such as produced by Simpson Strong-Ties for this type of connection. But that would mean shooting the Architect which requirea a separate permit.

BA

 

[focuseng]

When is Architect season?

______________
MAP

 

[dhengr]

I would use a square washer right on top of the beam, no counter bore, with a .5" threaded rod and nut and washer. Then you drill down into the column for the threaded rod. About 12" down in the column, you cross bore the column to intersect the longitudinal threaded rod bore. Then there is a piece of hardware, about 1.25" round bar about 4.5" long which is inserted in the cross bore. This round bar is drilled and tapped for the .5" threaded rod. Once the threaded rod is installed the cross bore is plugged. The cross bar acts in compression parallel to the grain and in bearing parallel to the grain, and tries to pull a two surfaced shear block out of the end of the column. I have seen these used and for sale, but off hand I don’t remember where.

 

[focuseng]

dhengr, when you saw these in the field did you talk to the installers by any chance? I have seen these advertised too but never gotten the guts to actually specify one. I always figured the installation would make my ears ring too much. I imagine a jig of some sort would be needed to get the alignment just so.
Here is a link for one such system

http://www.timberlinx.com/

______________
MAP

 

[dhengr]

Focuseng:
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.