Strength Design for Wire Rope? 2

[phamENG]

Hello, everyone. I'm working on a retrofit of an existing building to resist progressive collapse. Due to space constraints, I'm interested in using high strength wire rope for tension members, but all of the literature I've found references safe working loads. This strikes me as more of an allowable stress approach than a strength design or LRFD approach. In this application, the rope will sit "dormant" in the wall until an extraordinary event, at which point it will prevent collapse while the occupants evacuate. I understand it's typical to apply a FS of 4 or 5 to the wire rope breaking strength, but this seems excessive for the application and I'd like to take advantage of the additional capacities often achieved by using strength design (that and it's required by the government standards to which I am designing). Does anyone have a reference or know of research applying LRFD principles to wire rope design?

Thanks.

 

[dhengr]

PhamODU:
Wire rope/cable has two forms of stretch, namely elastic stretch and mechanical stretch. The latter is a function of the lay of the wire rope and its manufacturing methods. Wire rope generally has too much stretch/elongation per unit load to be used effectively for bracing in buildings, because you can’t tolerate the P/Delta movement of the structure or its surface finishes and the like can’t tolerate this movement. Thus, it is generally better to use round or rectangular solid steel bars, or angles, channels and the like for diag. bracing in bldgs. Cable works fine for tethering or bracing something like a crane boom or any other structure or machine which can work around the cable stretch under loading. The FoS of 4 or 5 on wire rope is there because in most uses the wire rope sees much more abuse, bending, fatigue, wear, etc. I think a FoS of about 2 is pretty common in regular structural use. The problems of end fittings and length adjustment, and then getting the wire rope item into a regular framed building structure under the right amount of tension is going to be a problem. You can’t have the bldg. moving a couple inches before the cable starts to tighten up. Think of that wire rope diag. brace, the bldg. has moved 2-3" btwn. floors, but the diag. length has only changed a small percentage in total length, not even starting to take up the mech. stretch in the cable. What you want is a solid bar, or some such, which starts to absorb energy as soon as the bldg. starts to move laterally. And, finally it will yield in tension or buckle in compression, and have to be replaced, but it has slowed the bldg. movement, from the start, and that’s what you want.

 

[hokie66]

Agree with dhengr, for the reasons he has given. Wire rope is not a suitable material for the application.

 

[thaidavid40]

A good many of the older PEMB structures in our area (SE USA) used wire cables for lateral bracing. Many of these structures have taken on substantial historical wind loads without much damage to the envelope materials. The theoretical discussion of the reasons for not using cables make logical sense, but the practical experience of seeing how they have held up indicates that for smaller or less important structures, they have apparently worked well in the past.
Dave

Thaidavid

 

[Lomarandil]

Just to add my experience to what dhengr put out, I often use wire rope for temporary construction applications -- in that case, an FS of 2.0 can be appropriate.

If you're able to overcome the issues he mentioned and do use it in a more permanent application like a building, I'd use an FS of 2.5-3.0 (and potentially additional reductions in capacity depending on the fittings/connections).

 

[hokie66]

thaidavid40,

I will guess that the structures you have observed performed reasonably well due to the lateral forces taken by the cladding rather than by the wire rope.

 

[thaidavid40]

@hokie66,
Exactly! I didn't go into the mechanics of it, but you are are most probably correct. Except for lateral loads near or beyond the design loads, tension-only bracing of this sort seems almost redundant, doesn't it?

Thaidavid

 

[hokie66]

Except if the discrete bracing is stiff enough, it prevents damage to the cladding, which hopefully will remain watertight. I don't think wire rope will do that.

 

[phamENG]

Thanks for the input, everyone. This a progressive collapse prevention retrofit for a government modification to the ASCE 7 Extraordinary Events combination for residual capacity (2.5.2.2), so the building can move like a wet noodle for all I care - as long as the occupants can get out and it can be shored/demolished with relative safety. The cable will only see load if one of these events happen; otherwise the load will be supported on the existing bearing walls and laterally restrained by shear walls.

My primary question was whether or not there's a strength design method in accepted literature somewhere, or if it's all about factors of safety and working load/stress designs. I'm guessing the answer is no, since nobody mentioned one.

Thanks again.

 

[BUGGAR]

Check out Caltrans design for the Dumbarton Bridge. To keep the girders from falling off the piers in the really Big One, we tied each girder to the pier cap with 2 inch plus diameter galvanized wire rope. The wire rope was wrapped around a Styrofoam core that would crush (controllably?) and absorb motion until the girder went off the bearing pad. Design motion was three feet laterally. Then the cable would keep the girder from falling into the bay below.
I worked on the bridge but never saw the design calculations. Ralph Somariva, P.E., was Project Engineer.

 

[phamENG]

Thanks, Buggar. I'll have to look into that. I've had the opportunity to work on a couple of small buildings on the west coast, and I must say I have a lot of respect for you guys out there.

 

[hawkaz]

Check out Lexco Cables web site also

http://www.lexcocable.com

I believe they list ASTMs that may help. Personally, I use a FS of 4. I sleep better that way.
Keep in mind your connections may be the weak point

 

[phamENG]

Thanks again to everyone who answered. Just got an email from ASCE Publications advertising ASCE 19-16, the newly revised "Structural Applications of Steel Cables For Buildings." Haven't purchased it yet, but I probably will soon.