Wire Rope Factor of Safety 1

[XR250]

I am going to use some wire rope in the place of rafter ties for residential renovation. The working load of wire rope appears to be based on a factor safety of 5.0 – which seems excessive for this application. What do you all think? I assume it is this high due to the uncertainties in rigging applications. FWIW, they are removing the ceiling joists of this space which are running perp. to the rafters so I am not sure what is keeping the roof from spreading!

 

[dik]

It's high... but the connections can be a real problem... you don't want to skimp there.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

Wire rope can also be susceptible to relaxation. I'm not sure how much the FoS mitigates that, but my gut tells me it helps.

 

[JStephen]

This is referenced in IBC 2208:

https://ascelibrary.org/doi/book/10.1061/9780784414392

 

[glass99]

As JStephen says, we use ASCE 19 for all of our cable structures. The FOS is approx 2.2, depending on a few factors.

 

[SlideRuleEra]

Wire rope factor of safety (5 in this thread) seems high since it is the ratio of breaking strength to working load. However, wire rope elastic limit is about 55% of breaking strength. Therefore, the ratio of elastic limit to working load is approximately 2.75. This appears to be in line with glass99's experience that a safety factor of 2.2 is acceptable for static loading... allowable load is limited to 80% of the elastic limit.

Wire rope stretch typically varies for 0.25% to 1% of wire rope length, depending on relative magnitude of the load. A value of 0.5% is commonly used for typical applications.

http://www.SlideRuleEra.net

 

[Sawbux]

Critical factor is how you are going to fasten the cable and pretension it. Hard to get enough blots into wood to develop the strength of the cable and turnbuckles have limited working load compared to the cable.
I have done this a couple of times by using a bracket to bear on the end of the beam or joist. Used swaged rod ends on the cable with nuts through the bracket to tighten. At a church, used a steel tube section to catch the ends of multiple scissor truss ends with and carry the load to cables spaced 10 ft o.c. or so.

 

[glass99]

I think the genesis of the FOS of 5 is where it is used for crane type hoisting applications, which I think is an OSHA requirement. Structural applications are much less conservative.

The formula from ASCE19-16 is given below. The fitting factor and deflector factor are usually 1 if you have good fittings like swages. It's based of the breaking strength, not the yield.

 

[GregLocock]

Bear in mind the 'efficiency' of the termination can be as low as 55%, ie actual breaking load/cable UTS.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[XR250]

Thanks for all the info. My demand is only about 1,200 lbs ASD. Looks like 3/16" 7-19 wire rope with a 3/8" turn buckle should work. Has an ultimate capacity of 4,200lbs. Gonna spec a thimble and (3) saddles at each connection.
It may never see any load except the pre-stress as there is nothing apparent currently keeping the roof from spreading out and it seems to be functioning fine (for 40 years)

 

[glass99]

@XR250, 3/16" wire rope is quite small, though would probably be good strength wise. I would also be interested in its flexibility over the length. Wire rope has an effective E of ~12,000ksi because of the winding. FYI the fitting reduction factor for thimbles is 0.95.

Which ASTM is the wire rope made to? A1023 only goes down to 1/4".

 

[XR250]

Good to know Glass99. Did not check ASTM - just typ. manufactures catalogs. I'll check the elongation and see if it seems reasonable. I can go to 1/4"Ø, however they're going to need to be working inside some joist bays and I figured the 3/16 would be easier to manage.
Thanks

 

[XR250]

OK, so the elongation is about 1" - not so great. Might need 5/16"

 

[XR250]

Contractor decided that installing a structural ridge would be the lesser of the evils.