Anyone seen data on maximizing the holding power of large lag screws in
I'm using several different sizes/lengths on a post-beam barn I'm
building. I WISH I knew stuff like:
(1) What's the best diameter and depth (vs total penetration) for pilot
holes in various woods?? Example: I have 1/2" dia lagscrews that
penetrate 4 inches into hemlock. My guess: 5/16" pilot, um, 2 inches
(2) What's the right torque for best holding without getting near the
tearout/stripping point? My guess: What 'feels right' now.
(3) What's a reasonable side (shear) load for, um, 2" thick hemlock with
a 1/2" lag screw?? I've heard of anti-creep washers / strips used in
high sheer loading applications. Anyone know about them?
And pointer, suggestions appreciated! Some midwest barn-building Ag
students must have figured this out, but I can't find it...
Regards, Terry King ...In The Woods In Vermont
Thanks for the help and comments. Jeez, for a minute I thought the
FlameThrower was aimed at me...
Well I tightened a 1/4" lag screw to failure just before I made that
post.. unfortunately I didn't do it with a torque wrench!
So far, I've abstracted some working points for this application (large
hemlock beams): Pilot about 90% of the root diameter, Pilot about 90 %
of the screw-in depth. Use some (beeswax) lubrication. Use large washers.
Torque until it feels right and doesn't break.
The torque is the one I want to define better. I've used lags for many
years, and I've over-torqued some (of some value or other of diameter and
penetration length and wood species) so that they started to strip out
and the torque started to DROP. Too late to go back, not good!
I like the suggestion to do some testing; I've got enough scrap to do
that. So I'm thinking the data looks like:
Number of turns Torque Comments (edited for publication)
---------------- ------ --------------------------------
...with 'end point' being "Broken", "Torque dropping", or "Washer Crush
Sound right? Comments.
I'm downloading the "Wood Handbook" now; maybe I'll find something.
Regards, Terry King ...In The Woods In Vermont
the number of turns will be a decent but not absolute indicator of
torque. in the real world, you don't need to get too fussy about
torque wrenches when bolting together pieces of wood- the wood is
gonna expand and contract anyway. what you doo need to do is not use
so small a fastener that the needed tightness approaches the failure
point of the fastener. give yourself a comfortable margin. if you're
breaking bolts off, go get some bigger bolts <G>
using a pneumatic or electric driver with a torque limiting clutch is
a convenient and fast way to drive a lot of lag bolts, especially if
you're working off of a ladder or climbing the framing.
Optimum pilot hole size is always something "a little bit less" than the
inside diameter of the threads. Thus, it's going to depend on the cut
of the threads. I find a bit that I can hold against the threads, and
'just barely' see the body on _both_ sides of the threading. Then I use
a bit that is one size _smaller_.
I go to within about 1/2" of where the lag screw starts to taper.
_GROSSLY_ "insufficient data". <grin>
A lag screw, or pretty much any other screw-type fastener (including nails,
which, in this context, are simply a 'threadless screw') has the function
of holding piece A tightly against piece B.
_When_ piece A will 'slip' or 'creep', against piece B depends on several
1) the texture of the surfaces of both A and B -- 'rough' surfaces (but
with good mating contact with the opposite surface) provide more shear
resistance than smooth ones, higher coefficient of friction.
2) the surface area in contact -- bigger areas provide more resistance to
slippage than smaller ones do.
3) the pressure forcing the two surfaces together -- the harder they're
jammed together, the more effort it takes to make them 'slip'.
"anti-creep" washers/strips just provide a roughened surface on both sides,
to get an improved 'grip' on the surface they're mating to. Assuming that
that surface has some 'give' in it, you get the benefit of the higher friction
"Everyone has the inalienable right to be wrong.....
sometimes you just have to let them exercise their rights."
I manage to stay out of more fights,
by just letting people exercise their 'inalienable rights'..
That aside, the _challenged_ party has choice of time, place, weapons,
and all other 'terms of contest'.
And I don't *like* flamethrowers. They *stink*. jellied kerosene is
not one of my favorite scents. <grin>
for soft woods drill the pilot hole the root diameter (the bottom of
the threaded section) in the part that you are fastening to, and a
clearance hole the size of the outside of the threads on the part that
is being fastened.
for hard woods the pilot hole should be larger than the root diameter.
for very hard and brittle woods the pilot should be large *and* the
threads should be tapped.
the load limit data of the bolt should be available somewhere online,
but the load limit of the wood is probably too variable to trust
generic data. note that the specs for the bolt will vary according to
the grade of the bolt and the manufacturer. some of the cheap borg
hardware will barely support their own weight....
if in doubt, set up a test connection and torque it to failure. I'd
probably use around 75% of that. however, there's no reason to keep
turning once the two parts are pulled together as tightly as needed
for the application. if 50 Ft/Lbs gets your beams pulled up tight it's
fine to stop there even if the load limit is 300 Ft/Lbs....
I'd think about through bolts where possible, so you can tighten them when
the wood shrinks.
Old factory construction was beautiful, with lots of big iron plates to
distribute the compression. I think big washers are a must if you want to
develop anything like the max compression possible for the wood/hardware.
Multiple smaller lags should be better than one big one. In some wood,
lubrication will get more compression before wringing off the bolt.
If you find guidelines, please post them.
Pay site, Free access for 24 hours. per topic. Design of Wood Structures
ASD (4th Edition) About $ 75. Other editions available, or check at your
Lag screws, like wood screws, require prebored holes of the proper size. The
lead hole for the shank should be the same diameter as the shank. The
diameter of the lead hole for the threaded part varies with the density of
the wood : For low-density softwoods, such as the cedars and whitepines, 40%
to 70% of the shank diameter; for Douglas-fir and Southern Pine, 60% to 75%;
and for dense hardwoods, such as oaks, 65% to 85%. The smaller percentage in
each range applies to lag screws of the smaller diameters and the larger
percentage to lag screws of larger diameters. Soap or similar lubricants
should be used on the screw to facilitate turning,and lead holes slightly
larger than those recommended for maximum efficiency should be used with
long screws.In determining the withdrawal resistance, the allowable tensile
strength of the lag screw at the net (root) section should not be exceeded.
Penetration of the threaded part to a distance about seven times the shank
diameter in the denser species (specific gravity greater than 0.61) and 10
to 12 times the shank diameter in the less dense species (specific gravity
less than 0.42) will develop approximately the ultimate tensile strength of
the lag screw. Penetrations at intermediate densities may be found by
straight-line interpolation.The resistance to withdrawal of a lag screw from
the end-grain surface of a piece of wood is about three-fourths as great as
its resistance to withdrawal from the side-grain surface of the same piece.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.