Lag Screw Connection for Handrail 2

[jdblake]

I need some help. I have a steel handrail post (5'0" o.c.) that needs to be attached to the side of a wood ramp. I can use lag screws in the post's baseplate and attach the lag screws into the end of 2x6 (Doug Fir-Larch). I have calculated the moment in the baseplate to be 875 ft-lbs which results in a tension load of 2625 lbs. Divide by two screws (top two in the baseplate) and I'm left with 1313 lbs of tension in one lag screw. The way I'm calcuating tension for a lag screw, I would need a 1/2" screw 10" long. I know that this is overkill but don't know enough about lag screw connections to figure out what I'm doing wrong. Please help.

 

[archeng59]

Check out this design guide from Simpson Strong-tie. They have some clips to help anchor railing posts to the deck framing. Might be adaptable to steel post.

http://www.strongtie.com/ftp/fliers/F-DECKCODE09.pdf

 

[bdSE]

Hand rails connecting into wood are challenging to meet code. You may need creative detailing to take advantage of the material properties at hand.

According to NDS 2005

Specific gravity (G) of DFL is assumed as 0.50
From table 11.2A
1/2" lag gives...
W (withdrawal in pounds) = 378 lbs. / inch

11.5.2 End Grain Factor, Ceg
11.5.2.1 When lag screws are loaded in withdrawal from end grain, the reference withdrawal design values, W, shall be multiplied by the end grain factor, Ceg = 0.75.

W (0.75) = 283 lbs. / inch

1313# / 283 = 4.6 inches, therefore a 5" lag should do the trick. Make sure to pre-drill to avoid splitting the end grain. If you are uncertain how large (or small) to make the lead hole, then you can read more about it in section 11.1.3. Let me know if you need more specifics.

I think that should do it, unless I misunderstood your problem statement. You could achieve the same load resistance with a 5/8" x 4" lag screw.

 

[CTW]

See if this helps at all.

http://www.structuremag.org/article.aspx?articleID=303

 

[archeng59]

Thanks, CTW. I remembered seeing that article but not when or which magazine.

 

[jdblake]

CTW, damronb and archeng59 - Thanks for the replies. They were really helpful. It looks like I have to use a Simpson connector for this situation.

Damronb - This is exactly how I was proceeding with the calculation and noticed that the T-E length of the lag screw, given in Appendix L Table L2, is the length that is required instead of the overall screw length. It works out that I would have to have a 1/2" lag screw 10 inches long.

 

[bdSE]

Excellent, thanks for following up with the Table L2 comment. I had forgotten how much of the lag screw was a smooth shank and therefore required a longer actual length to provide the minimum threaded shaft length to meet the appropriate withdrawal value.

 

[vandede427]

Simpson just came out with a new connector specifically for this connection.

 

[racookpe1978]

If there is any way possible, I always use a through bolts (galvanized rather than stainless most of the time) over lag screws.

Also, I use an oversized washer on the far side to put the compression forces spread out as much of the wood area as I can.

 

[RHTPE]

I would NOT rely on a lag bolt into end grain for a guardrail post. While one can "design" this condition knowing the size of the bolt, the specific gravity of the wood, etc., there is no assurance that the wood will maintain its properties over time with exposure to the elements. Just go look at the ends of any members used for a deck that has been exposed to the elements.

I would devise an anchorage that will maintain its integrity in spite of the long term drying and cracking that will occur over time at the ends of wood members.

Unless, of course, this connection exists in a controlled environment. But even then I would resist this kind of connection for a guardrail post.


Ralph
Structures Consulting
Northeast USA