huangyhg

tension rod sag
we have some big cantilevered architectural canopy, where we have 1-1/2" diameter tension rods splaying out from a vertical column (similar to a cable stay bridge). well, the rod that goes to the end of the canopy is about 31' long at a 45 degree angled slope, doing a beam deflection calculation (for beam on a slope) gives a deflection of 12.8" just under its own weight.
this sag seems too excessive . . . is there any problem with using a tension rod with this much sag in it? the roof of the canopy is going to be poured concrete. i am just having trouble envisioning this rod with that much of a deflection. it would seem like it would start straightening out when they start pouring concrete on the roof and that it would be impossible to get the roof set level and at correct elevation.
we will take a long time to look at this, so i am just posing this question to see if someone has come across this situation before, and if my fears are founded in how the sag would affect the construction of the roof.
on a side note: i've been asking a lot of questions lately, so thank you to everybody who has taken the time to respond to me. i've been getting into a lot of "one of a kind" and "new to me" type projects, and also find out that a lot of them are new to my bosses as well. just running things by you fine folks on the more difficult issues.
you can't use beam theory to calculate the deflection of a rod like this. classical beam theory only works when deflections are small compared to the beam depth. since 12" deflection is much much greater than the 1.5" rod, you will have to use something else, such as cable theory perhaps or large deflection theory. if you remember some assumptions made in classical beam theory, such as sin(theta) = theta for small angles, that will tell you why the equations for classical beam theory are not valid for large deflections (theta is no longer small). there are others assumptions also. using the proper theory for your deflection calc will also allow you to account for how much the tension will help keep the rod straightened out.
how are your axial deflections working out for you? will the rod stretch much and cause the roof to sag down? are these rods exposed to wind? you might have some vibration problems or encounter some vortex shedding from the wind. rain water on the cables may also exacerbate the problem by increasing drag. i believe aisc recommends limiting tension only
usfse is correct in that the beam theory doesn't apply here.
you might however also want to look at things like icing, creep and reversal due to wind as well as vibration and harmonics which can be a problem for tension structures. you should also seriously consider inspection and maintenance which i think are often overlooked by architects on low budget stuff.
how thick is the roof by the way? why do you need to use a concrete roof? since it will be a poured concrete roof, why dont you try to add a cantilever beam (steel or concrete) to reduce the deflection?
you can not treat it as a simple beam, you have to model it using any design software.
ucfse is also correct in stating that tension-only
thanks a lot, everyone. i can see now that i was using the wrong theory to get this deflection. what steered me that way was risa 3d was giving me a really high deflection as well, 12.5 inches and a moment was being applied to the middle of the tension rod (which goes against what the cable theory assumes).
a couple of points:
1) the code says: "for members in which the design is based on tension, the slenderness ratio for l/r preferably should not exceed 300. the above limitation does not apply to rods in tension." we had a 1-1/2" diameter solid rod and were taking this exception to make this work. is there any problem with using this exception?
2) using risa 3d, the tension rod, even as marked as a tension only member, showed this 12" deflection and a bending moment. i'll have to read up with risa 3d to see how to model those members as actual cables. any thoughts?
3) as for the canopy itself, we have steel framing supporting concrete deck spanning between the steel
what about uplift from wind on the canopy?
del2000,
i may argue that your rod is not only in tension but with bending as well. so you are not complying the code to the letter. i will interpret the code to mean vertical rods for lifting purposes.
ciao.
kl/r less than 300 does not apply to rods in tension. see aisc asd section b7 page 5-37.
a construction idea: have the gc shore the ends of the cantilever until the concrete has set up. tension your rods, accounting for the deflection you will get from the initial load and any additional live load "camber" you need--and then gradually pull the shoring. i would also get on the phone to the best erector in town and ask them how they would do it.
good luck.
kl/r<300 does not apply to rods in tension is correct. kl/r<300 does not strictly apply to anything since it is a recommendation for tension members and not a requirement.
a 1.5" diameter rod 31 feet long has a kl/r near 1000. i would expect this to be highly sensitive to dynamic effects from wind. it's going to require a much more involved analysis than just sizing rods for static loading. limiting the slenderness ratio helps alleviate problems you can get with such slender members, though the limit is of course not required.
as you see you have to be a little careful how you model things in software. risa uses beam elements for everything (except plates of course). checking the t/c box does not change the element to a bar type but rather only reduces or eliminates its ability to carry load in a certain direction. this is a great example demonstrating that you have to be aware of what the software is doing. you can get any answer you want but that doesn't make it right i checked briefly in the risa help file and didn't find anyway to model bars as bars, much less having

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