Strength Design for Wire Rope?
Strength Design for Wire Rope?
(OP)
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.
Thanks.






RE: Strength Design for Wire Rope?
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.
RE: Strength Design for Wire Rope?
RE: Strength Design for Wire Rope?
Dave
Thaidavid
RE: Strength Design for Wire Rope?
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).
RE: Strength Design for Wire Rope?
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.
RE: Strength Design for Wire Rope?
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
RE: Strength Design for Wire Rope?
RE: Strength Design for Wire Rope?
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.
RE: Strength Design for Wire Rope?
I worked on the bridge but never saw the design calculations. Ralph Somariva, P.E., was Project Engineer.
RE: Strength Design for Wire Rope?
RE: Strength Design for Wire Rope?
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
RE: Strength Design for Wire Rope?