taut flat cable parallel to roof to support a roof? 12

[IJR]

Pals

Will a set of cables, parallel to a simple roof, work just like purlins to support a flexible roof material?

For a cable to work I think I need a profile with a sag, so that no further sag can occur under load

But the architect says no sag.

Anyway around this. Seems impossible to me.

respects
ijr

 

[slickdeals]

You can pretension the cables to not have any sag.

 

[MiketheEngineer]

Yes - But once loaded up - the cable tension force will skyrocket

 

[rb1957]

the architect is being an architect ... of course there'll be some sag in any real cable in the real world.

but you say "flexible roof" ... like a tent ?

 

[dik]

as Mikey aludes to... the tension forces can be very high... and shouldn't be taken lightly. If you have an end slope of 1:100, a 100 lb vertical force is transferred as a 10000 lb tension...

Dik

 

[JStephen]

If you take a cable that is theoretically straight across and apply a load to it, you will get some sag, even if that load is only the self-weight of the cable. Adding another load just increases the amount of sag. You can build things like that, whether it's economical is a different issue.

You might also consider how you know what that initial tension is. Telling a construction worker to "tighten that nut until that cable is straight" could lead to highly variable results.

Temperature effects would also affect cable tension.

 

[msquared48]

And I suppose this guy thinks that there is no "sag" with wood or steel purlins under load too? He is being totally unrealistic.

If he wants no sag, tell him to outlaw gravity. (Obvious God complex, so let him do it). You can't do that.

Structural Engineers are from Reality, Architects from Fantasia.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[paddingtongreen]

http://wiki.answers.com/Q/What_is_the_Formula_for_sag_in_a_catenary

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[JAE]

See the attachment. This is the La Villita Assembly Hall in San Antonio, designed by W. E. Simpson Co. it is a "flat" cable roof structure of sorts. The center tension ring was pulled down, concrete planks placed on the cables, and then when the ring was released the roof sprung upward, putting the planks into compression.

 

[IJR]

rb1957 and paddington

yes, a tensile fabric will be hung on cables that act as simple purlins(same way you put purlins on an industrial one story building frame)

paddington, I am not after the equations really, I am after some experience on these issues. I appreciate your immediate response howeve and will appreciate more.

 

[IJR]

JAE

I found that article is inspiring. As usual JAE, you inspire. Can't thank enough

My application involves a cable flat in the sense that it is parallel to the ground with loads suspended on it, and even a small load brings about 10kips(5metric tons) tension and a huge sag.

It hints sensitivity to loading. No temperature analysis yet.

respects

 

[rb1957]

just reading that post, not seeing any calcs, it doesn't sound quite right to me ... a huge tension would indicate (typically) a small deflection. if you've got a huge area loading the cable, maybe the flexible roofing will tear away before the cable breaks ?

 

[paddingtongreen]

IJR, your OP asked if you could use a cable system with no sag, I posted the link as a quick , "No" that included the reason.

The only possibility I can think of is somewhat awkward, it is to used a sagging cable, supporting at intervals, another almost straight cable or pipe, much like a suspension bridge. It is awkward because of the number of penetrations for the supports.

Other than that, I think the architect needs to ask the Great Architect of the Universe to change the laws of physics.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

A cable with low sag forms a catenary which may be approximated by a parabola. If it has load per unit length w, span L and height h, the tension in the cable is wL²/8h. If h = 0, the cable is straight and the tension is infinite.

BA

 

[dhengr]

IJR:

Paddington and BA are just being killjoys, they always want to bring equations and the laws of physics into the discussion. They don’t know that your arch. is on a first name basis with the Great Architect of the Universe. I can just hear him/her saying “and there shalt be no G.. Damn sag in the cable.” JAE’s picture is a special case, quite clever, but I didn’t see much article. They initially stretched the radial cables btwn. an exterior compression ring and the center tension ring structure and then tensioned them to some predetermined level. They pulled the tension ring down, thus prestressing and stretching the cables further, and installed the conc. planks which work in compression when the tension ring is released. Their trick is to calc. the prestress so that any anticipated live loads cause the tension ring to move, up and down, vertically within an acceptable range. This is not ‘no deflection’ due to added load, but you don’t see the deflection in terms of significant added cable sag. This is easier to do in a circular form (tension and compression rings) than in a rectangular building form. Their system is attractive because the reacting force systems make up the roof, are shaped well to do this, and are internal to the roof system, and not an external tie off.

Let’s see if I have the right picture: flat roof structure; beams spaced at X; cables (like purlins) spaced at Y, on top of the beams; flexible fabric stretched over the cables; none of these have any deflection or is it just sag in the cables which must be zero. You’ll want to patent this because it will be a first. Your first gut feeling about cable sag was right on the money, except more vert. load will produce more sag and higher cable stresses; later you say the cables seem sensitive to loading, right again; and you say you haven’t considered temperature effects yet. Don’t forget to consider the rain filled swimming pools, each X long and Y wide, which will form on this roof system.

The arch. seems to have missed the ‘gravity and loads thing’ in school. You must explain this to him/her because you can’t overcome it. Part of the architectural sex appeal of these tension structures is their deflected shape, and you can’t ignore it. You must consider cable and fabric stretch, ponding, load accumulation, drainage of any low spots, etc.

 

[IJR]

dhengr and fellows

Thanks for the detailed posts. I dont want to extend this more than worth your time.

But isnt it true that no amount of prestressing can make a cable flat. Since h=0 means infinite horizontal force, doesnt the contrary mean no sag-free cable is possible? A sag of a half inch(roughly 12mm) would reduce the force significantly

So here is what I am goint to do. I am goint to provide my X cables(X is parallel to roof slope) and then provide stiff Y purlins. This way the two will interact and I will have a safety margin, while pleasing the architect(Sorry I seem to have no real alternative).

respects
ijr

 

[BAretired]

IJR,
You may indeed be sorry if you take that option. Why is pleasing the architect your prime concern?

BA

 

[IJR]

BAretired

You are right I will be sorry. But I have to ensure safety to the level of my knowledge, which simply says cable takes transverse load only if it can take a shape different from a straight line between the ends.

However this architect, a nice man too, insists and he is the one who calls all the shots.

think of calling the shots BA

respects
ijr

 

[BAretired]

I would suggest that the person who "calls the shots" should be the one accepting responsibility for those shots.

BA

 

[MikeHalloran]

Maybe you can talk him into something that _appears_ to be what he wants, but isn't.

E.g. a tubular truss, finished in flat black, with one chord finished bright.

E.g. a cable strung as a catenary, with queenposts holding up the structure, all finished in a way that hides them, and a highly visible, straight, decorative cable, strung where the architect, and the great unwashed, can see it.



Mike Halloran
Pembroke Pines, FL, USA