Analysis of cables 5

[eit09]

I have been requested to design a tower with a conveyor running through the center of this tower. The catch is no bents can be used so I am using cables to support the truss on both sides of the tower. I am using RISA3D as my FEA program to model these cables. I chose a specific cable and created a tension member using its specific parameters. I am just wondering if any of you have worked with cables and have any good references and or modeling/ design suggestions or tips for designing with cables? The cables are crossed so I will be using a spreader bar to keep the cables coming parallel down to the connection points on the conveyor. Not sure how to analyze the spreader bar any suggestions? I will attach a PDF of my structure for a visual. Thanks for your responses!

 

[ishvaaag]

RISA 3-D is a small deflection program, that I know. So even if through P-Delta procedures you wil get some appraisal of geometrical nonlinearity some program with large deflection abilities (I think SAP 2000 does) should do better.

Anyway for your model the shape and general configuration of the cables are unlikely to change much, you could even assume undisplaceable fixity at your conveyor supports and then see what cables at what tension you need to put in place to deliver equilibrium, then see how these affect what without them you had. And also cover for the decided setup what the situation will be at both the loaded and unloaded situations, and in-between.

 

[IJR]

eit09

Not that I know much, but I am interested in the subject.

Like ishvaag pointed out, you dont need large displacement modelling in your case, and you may just go ahead with using frame elements. That was a real great tip.

And like ishvaag pointed out, if your software has some nonlinear modelling options, you can model the cable as a frame element as before, and then try to model the stiffening effect of the loads(typical pretension and gravity loads) by using nonlinear features provided by your sofware. In sap2000 for example, I know you can do that by loading the cable with a special force known as a P-delta force. This basically modifies the stiffness matrix and is useful when the frame slackens a bit due to loading such as that proposed by ishvaag above(alternate span loading or dynamic loads). You will need to know what you are doing then. Cant tell you more because I have only tried with simpler models before.

good luck
respects
ijr

 

[paddingtongreen]

I'm not up-to-date on the programs but I still know structures.

I don't know why you have crossed the wires, I don't see any gain from it, there is no compression counterpart.

I assume that exterior restraints control the positions of the tower columns, even so, I would look into making the support level a transition floor, and aligning the upper tower with the conveyor. That would get rid of the built in horizontal kick from the offset wires.

I actually don't like the wire at all. I see a reaction/support at the left end of the conveyor. That means you will have to provide the steel erector with an installation tension for the wire, to be set with only the read load of the steel, but which will get the correct load when the conveyor is installed and running, plus he will need a means of adjustment.

I think that, instead of running the tower that high, I would try to finish it at the conveyor support level and provide trusses on both sides of the conveyor, sitting on the support floor. Less interesting but more practical.

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

 

[eit09]

paddingtongreen,

1. The reason for crossing the wires is to eliminate as much horizontal deflection as posible during a high wind load. I modeled the wires as tension only members so you are right that there is no compression counterpart.

2. I have provided a turnbuckle which would be means of adjustment for the erector.

3. A truss for this situation would get too large, and to provide horizontal deflection from wind as the cables do would get very interesting with a truss.


Thanks for your input any other suggestions or comments?

 

[paddingtongreen]

eit09, You will need strain gauges on the wires to know what preload you have put in them.

I don't know the dimensions, but I'm not convinced about the truss. You have what amounts to a truss now with the conveyor boxed trusses being the bottom chord and the wire being the top chord. I suppose that using a wire gets you away from kl/r limits for tension members.

I think the wire does virtually nothing for you in a cross wind, look down on it in a plan view, it's angle is very flat, it's stiffness in the lateral direction is so very much less that that of the bottom truss of the conveyor box it will be of negligeable help..

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

 

[eit09]

Paddingtongreen, When I spoke with a couple of steel cable providers they had said it was a standard practice crossing the cables in this type of arrangement. I see your point and will try running the different scenarios to see the difference in deflection. Much less design checks if I dont cross them.

 

[aggman]

eit09,
I have analyzed a lot of cable supported conveyors in RISA and other programs. I think what you have here is going to give you problems. The advice you have been given so far is good. As far as modeling the structure you can do this in RISA and do a good job with it. To analyze this thing you need a very good grasp of your dead loads and their locations. Once the loads are on the structure and are accurate this becomes an iterative problem. If the construction crew does their job correctly they will put the conveyor up so that it is straight. That means that every support point will end up with no deflection or a slight upward deflection. The most typical way of accomplishing this is to string line the structure (or survey it) to set the cable pick points to a straight line elevation from fixed points of support (the tail end and the tower supports). They will not preload the cables to some set load like is done on cable stay bridges. If they did it would be the first time I have ever seen it done on a conveyor. By knowing this you have to make a first guess at the preload at each point. (A start would be to analyze it as fixed supports and translate that into cable forces) Once the preload is known then you apply the preload to the cables and then analyze the structure. You can then analyze the cable pick up point deflections and adjust from their to get the deflection to roughly zero. That means you will have to analyze the structure, review the deflections, determine adjusted cable forces, and analyze again until you are satisfied with the cable forces and truss response. You will likely never get it exact, but you should get pretty close. Remember that the cables will have a different "E" than steel. So they will stretch a lot more than just rods. This has to be accounted for because of the next steps. At this point you will have a nice structure with known cable forces and everything looks real nice (on paper!). Now you start applying your live loads, wind loads, seismic loads, etc. Also remember that your live loads will not always be uniform on the conveyor, as the material starts and stops at times. Remember also that once the structure is up, the cable forces are set and fixed. They can only pick up load as they stretch and they will stretch at a much smaller load than what your truss can carry. What does all that mean? It means that under live loads and seismic and wind loads, etc, that the truss will carry a lot of those forces and will get overloaded quite quickly because of the high flexibility of the cables. To help offset this you have only a few options in my opinion. Really massive cables/ rods, or a truss that can more or less carry a significant portion of the live load by itself in addition to the forces already in the structure due to the cable supported dead load structure. It becomes quite an interesting structure to analyze and make work in reality, but can be done and done well with RISA or any other small deflection program if you know how to manipulate it to what you want. P-Delta is a must though IMO.
I said in the begining I thought that this structure configuration will give you problems and this is why. From what I can tell you have about a 140' approach span and a 100' cantilever, more or less. The cable crossing is good and bad IMO. I would be very concerned that the cables will rub on each other, causing a wear point on them. The wind always blows, the ground always moves a little, and the live load always varies. That means that the cables and truss will always be moving. With the two cables crossing each other they will rub, and likely start breaking cable strands with time. Don't count on anybody to inspect it, they wont. They will just call you when its broken and laying on the ground. I wouldn't want to take that chance. RISA will also give you fits as well because if the members are actually crossing in the model, more than likely it will put a node at the cross point and analyze them like they are attached, which is not what they would be in the field. If you draw a free body diagram as well you know that as the structure sways to the side it will induce an upward load on the truss. So as the truss sways it will raise up as well. This probably isn't so bad. What you will probably find though is like I discussed earlier with the vertical loads. The truss will likely hog the load and fail the chords at the tower. So it will take a hefty truss to cantilever 100' for lateral loads. A better option would be to put sway cables in the truss along the truss plane attached to the tower. Your angles would be better and they will do more work. In addition this will require a fairly stiff tower to support the unbalanced forces, both vertically, horizontally, and torsionally. I could give you more, but I hope that I have given you some general help with what you are getting into. This is not a simple structure to analyze. I would highly recommend that if you don't have much experience with cable supported structures that you work with someone who has on this. Some cable structures are straight forward. This one is not imo. If you don't have a good "feel" for these type of structures you might end up with something that looks really good, and is a disaster in reality.

 

[eit09]

Thanks for all of the posts thus far! To bring this into perspective:
The truss is 4.5’ wide by 3’ deep, Supporting a 42” conveyor system. The cantilevered portion is 80’ and the span between the tail section and tower is 110’. The bottom chords of the box truss are L6x4x3/8 (LLH). The cables I am using in the analysis are 1” Dia. Galvanized 6x19 I.W.R.C. The E for 0-20% of loading from WRTB is 13,500 ksi. I chose to use 10,000 ksi in my analysis to stay on the conservative side.
Aggman,
Do you cross the cables in your designs? When you use the sway cables do you cross these also?

 

[aggman]

I haven't crossed them before. Again from my perspective I think it could cause problems. I also tend to think that the truss will show failure in the chords before the cables would pick up enough force to carry the lateral shears. Typically we just hang the cables from the tower and keep them more or less in line with the truss. The sway cables would ideally attach to the bottom chords of the truss. This should allow them to slide under the walkway. You would have one coming off of the tower going out towards the head section on each side. These cables would not be visible in an elevation view as they would be on the same plane as your bottom chord plan lattice.

 

[hokie66]

eit09,

Aggman's advice is good, but the best part is to get some help from someone who has done this type design before.

 

[eit09]

Yes I will definitely have someone take a look at my design once completed. I plan on connecting the cables to the top of each column in the tower thus creating an angle to the support point to the truss. I plan on designing a spreader bar to keep the cables running parallel at the connection points on the truss.

Aggman or anyone else,

Do you use spreader bars on your designs to keep cables in line?

 

[trainguy]

eit09,

You should show your concept to a senior structural engineer ASAP, and focus less on "How to do it with RISA", and more on "does this make sense".

A preperly designed structure has straightforward load paths, and I can't find those in your sketch.

This should be reviewed before you complete the design.

tg

 

[eit09]

I am comfortable with the structure and know the tower and truss are a good design. I just want to make sure that the redundancy I have in the structure due to supporting with the cables is sufficiant.

Thanks!

 

[IJR]

eit09

Make sure the tower has sufficient torsional strength. Be conservative on lateral loads.

And carefully detail your anchors and foundation

respects
ijr

 

[darkwing888]

"A truss for this situation would get too large"

What do you consider too large?

I doubt the trusses will be prohibitively heavy.

As the structural engineer of record for a few mobile mining equipment structures, ditch the cables and use steel shapes. The only time there are cables is when they are used around pulleys to wench the structure up and down.

 

[hokie66]

Another voice of reason. eit09, I hope you have someone like aggman or darkwing to lead you in this design.