Lifting Cable Tear Out of Wood Member

[Irontrussman]

I work for a modular builder and we set the mods in place by lifting them with a cable that is 1/2 or 5/8 inch in diameter. The cable is threaded thru a hole drilled in the rim joists at the bottom of the mod. The rim joists are 2 plies of 2x12 SPF #1/#2. I've been charged with determining if we can use SPF(S)#2 without having the cables tear thru the rim joists. Each lifting point has approx. 9100 Lbs on the cable. I looked at the NDS yield limit equation I[sub]m[/sub] from table 11.3.1A, treating the cable like a dowel, but the calculated allowable "Z" value of 705 Lbs is way too low. In the mechanical world this would be a simple shear calculation with two resisting shear areas extending tangentially from the hole, perpendicular to the edge of the joists. But I haven't been able to find any published values on shear strength perpendicular to the grain, other than the crushing stress for bearings, which is limited to .04 inch of deformation (which is probably why the "Z" value was so low). Any suggestions.

 

[Lion06]

Where in the cross section of the 2x12 is the cable? 9100# is a pretty high shear load for wood - especially SPF. I can't picture the exact connection you're talking about, but what I think is more of an issue than the two shear planes you talk about would be a tension-across-the-grain failure of the wood. This really depends on the particulars of your situation - can you post a sketch?

 

[msquared48]

Once through the double rim joist, what does the cable attach to?

Mike McCann
MMC Engineering

 

[Irontrussman]

I've attached a couple of jpg files to illustrate the lifting configuration. The cable goes across the bottom of the mod and out the other side. There are spreaders above the mods to keep the cable away from the sides.

 

[Irontrussman]

There was supposed to be a second jpg with my last post. I've resubmitted it here. I'm new to these forums, can you only link to one file per post?

 

[BAretired]

This raises an interesting question. What is the horizontal force in the cable and can your rim joists take it without failing?

For the vertical load, you could use a steel angle fastened to the underside of rim joist to spread the load. You would need to put a keeper plate on the angle to prevent the cable from slipping.

BA

 

[Irontrussman]

I believe, with a cable, if you have 9100# on one end you have to have 9100# on the other end, disregarding friction and other forces associated with bending around the corner of the box (let's not make this anymore complicated than we have to).

We already use this system for lifting mods, and it works. The problem is I am not able to quantify it for the sake of determining a factor of safety or in making comparisons for different lumber species.

 

[RHTPE]

Just because it has worked doesn't mean the calcs will prove it's okay. Been there too many times with lifting apparatus & rigging.

Just from the minimal contact area against the wood where the wire rope makes the turn from horizontal to vertical at the rim joist I can see a problem with compression perp to the grain. Especially with something as soft as SPF lumber.

How do you thread the wire rope through the rim joist? Do you have to unassemble the eye at the rope's end to thread it through the hole?

I agree with the KISS principle as well, BUT, if it works and simple calcs won't prove it, then either the calcs are too simple or the initial assumptions are not quite correct. Or you've never had the factor of safety that you should have had.

You should also attempt to define what failure really is. In addition, you should look an ASD approach utilizing the duration of loading factor (1.6 for 10 minute duration) and see how it compares (won't help for compression perp though).

Ralph
Structures Consulting
Northeast USA

 

[msquared48]

OK... If you want to make the numbers work, modify the connection with a piece of simple hardware.

Use a 3.5" piece of 1.5" steel pipe welded to a metal backing plate with a 1.5" diameter hole in it for the cable, then weld the back plate to a flat bearing plate that will catch the underside of the double wood rim. If the Z value is not increased enough with the 1.5" hole, the bearing plate will pick up the difference, in fact probably all of the 9 Kip load. The swteel pipe is inserted into a drilled hole in the rim.

Shear should not be a problem with the wall above the rim adding to the shear resistance.

Mike McCann
MMC Engineering

 

[Irontrussman]

I believe we're getting a little off track. I don't want a redesign, I want to quantify what we're doing. Does anyone know of a reference that gives values for across the grain shear? I recently read an article on a website, where some guy is designing an ark, like Noah's, that stated across the grain shear was about 4 times higher than parallel grain shear, but the article didn't give a source for that number. If someone does a lot of wood doweling, such as building arks or covered bridges, it seems that across the grain shear would be a major concern.