Tapered Hollow Section 1

[ars13]

I have a situation where I have to design steel tapered hollow columns which have both axial and bending loads. Does anyone know where I can find some good information on tapered members and their design? Also, can tapered members be entered into Risa? Thanks

 

[transmissiontowers]

AFA oscillations and vortex shedding, I have heard of companies that hang chains with weights on them inside the columns to add some damping and change the natural frequency.

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I have been called "A storehouse of worthless information" many times.

 

[ST04]

I'm encountering a situation close to this topic.

Maybe I'm messing something
But I can't see difference between design of Tapered hollow section and normal hollow section???

Regarding the design, why not simply design the pole by taking several sections through its height, (While going up the pole diameter will reduce leading to reduction of section area and other properties) and then compare the capacity of each section with the internal loads at this section.

Wouldn't this work fine for the design part?

Regarding buckling checks would it really have significant difference from hollow pipe sections (for hollow tapered sections)? (In case of applying what I mentioned above)

For the analysis, STAAD Pro supports Tapered I sections as well as tapered tubes (with several alternatives; Round, square, octagonal, etc).

But the problem of STAAD regarding cable analysis will make it useless if you will create a model of poles connected by cables.

I would recommend SAP2000 for cable analysis as it has a lot of options regarding cables and very strong analysis engine. But I couldn't find a way to create a tapered pole in SAP2000???

If somebody can correct my information I will be thankful.
Regards

 

[transmissiontowers]

The allowable stress at each cross section is dependant on the w/t (flat width to thickness ratio). There are equations in ASCE 48 to determine the allowable stress when the local buckling is too low. PLS-POLE from Power Line Systems can handle these tapered tubes connected by cables. They have a brute force design function where you input the min and max top and bottom pole OD and a range of thicknesses to try. The program analyzes 1000's of pole geometries and remembers the lightest one.

You also have to account for splicing the pole sections. We use a lap splice to connect sections because plate bending machines that form the trapezoids into tapered tubes are limited to around 40 feet as are the galvanizing kettles. For tall poles (we have assembled them to over 200' tall) you stack the sections on top of each other.

Like I observed a few posts above, this is a very specialized part of structural engineering and there are specialized programs that handle the analysis and design. The big pole fab companies are happy to design the poles for free if you give them a load tree.

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I have been called "A storehouse of worthless information" many times.

 

[ST04]

Thanks transmissiontowers for the information

What I wanted is to use a circular hollow section (CHS) for wind fence columns that are nearly 19 m height, Using uniform diameter pipe column will end up with non-economical design.

Because the bottom of the fence is resisting huge moment (6000 to 13000 kN.m), so it will require section with large diameter and large thickness. But as the height increases the bending reduces significantly. And reducing the thickness alone will not solve the economic issue.

So there are two solutions:
1- Using uniform CHS and use smaller diameters each specific segment length along the column and have special connection in between.

2- Use a tapered hollow section with large bottom diameter that reduces uniformly.

I thought that the second choice maybe better for the fencing connections and because it will not require the special connection (kind of an elevated base plate) as in choice 1.

So If I want to use the tapered pipe, there is only difference in the calculation of the buckling?
What about a finite element model?

Regards

 

[transmissiontowers]

I don't deal with metric much so 19m is around 60 feet which is a pretty tall fence. I just fired up PLS-POLE and created a tapered hollow round pole in a couple of minutes. I designed a pole for a 6000 pound transverse load applied 2 feet from the top with a 3000 pound vertical load and a 45 PSF wind load. The pole was 91 feet tall, 8 inches OD at the top and 35 inches OD at the base made from 3 sections lap spliced together. Total pole weight was 16,265 pounds. The section thicknesses were 0.1875, 0.1875, and 0.21875 inches respectively. The max moment at the base was 875 foot-kips.

PLS-POLE will calculate all the properties and the buckling allowable for tapered round, 8 sided, 12 sided, and 16 sided hollow poles. Look at

http://www.powline.com

for some more info about tapered poles.

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I have been called "A storehouse of worthless information" many times.

 

[BAretired]

Most structural engineers, including me, do not know how to consider the effects of vortex shedding, but I have observed, from time to time that such effects are of major concern.

I have watched occasionally, tapered octagonal sections flailing about in a moderate but constant wind...traffic lights bouncing up and down by eight or ten feet. The members did not fail because the wind did not persist, but given time, who knows?

The problem is not so much in knowing what force the member can withstand as in knowing what the forces are and with what frequency they will be repeated. Fatigue, it seems to me, plays an important role in this.

BA

 

[transmissiontowers]

After playing with the metric numbers I designed a pole 19m tall with a load 2 feet from the top of 311kN that gives a base moment of 5728kN-m (no wind on the pole). The pole it selected was 381mm OD at the top, 1498mm OD at the base, with a 0.9525cm thickness. The pole weighs 40,711N.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[ST04]

Thanks a lot transmissiontowers
You have provided excellent details.
I do really appreciate your help.

I will be looking for the software soon. But does it support adding a uniform UDL load along the pole height?

Best regards

 

[transmissiontowers]

You can input a wind pressure and use several different pressure adjustment methods to account for the pressure increase with pole height. I will attach a screen shot of the load input dialog box.

The program calculates the average area for each tapered pole segment and determines the correct pressure to place at its centroid. You mentioned a wind fence. Does this mean you have tapered hollow columns with solid membrane elements between them? If so, the program does not use any mesh or FEs, but you could place wide beams between the columns and apply a wind on the system and it will calculate the forces that load up the pole and do the pole analysis. It also has cable elements and guy wire elements so you can attach cables between poles and guy the poles to the ground.

A couple of weeks ago I designed a wall with wood poles and plywood and I used this program to select the right wood pole.

Contact the PLS-CADD people and ask how they can help you solve your problem. It is used for Transmission Line design which is a very specialized area. Look around their web site to see the types of structures it will handle. A couple of mine are at the bottom of the PLS-POLE page.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[transmissiontowers]

I forgot to mention that the program will use metric or English units and you can start in metric and switch to English and it will do the conversion automatically.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[ST04]

wow
Thanks a lot transmissiontowers
You have provided more than enough details
I'm very grateful for your help.

The fence is made of porous fabric connected to the poles.
Best regards

 

[transmissiontowers]

You are welcome. The pole software allows you to attach equipment areas to pole joints so you can calculate the equivalent solid area of your fabric and put that area on the pole at joints you define. The load file has the input for wind pressure so you can analyze the structure for whatever wind you require.

I assume you are outside the USA and the next task once you analyze and design your pole will to be to have it fabricated. Most fabricators that we deal with have their own pole design programs so you might not need the software at all. The only pole supplier that I have heard of that makes tapered round was a company out of Australia called Transfield or something like that. They opened a plant in Texas but had to close down for some reason. I believe Valmont produces small street light poles that are around 10m tall. 8, 12, and 16 sided tapered poles are more common in our industry especially in the sizes you are talking about. The basic concept is to form the shaft out of flat plate into 2 halves and weld them together with an automatic long seam welder. You then weld the shaft to a base plate (or make the pole longer and directly embed it into the ground). The base plate design is another tricky part worthy of a Doctoral dissertation. If you are still interested, get a copy of ASCE 48 which has all the equations for the pole shafts and base plate thickness.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[ST04]

Dear transmissiontowers
I appreciate your intention to help.
Regarding the base plate, It happened that I worked on several ones before. And totally agree with you about its "tricky part"

Best regards.

 

[ST04]

One last thing
Regarding the variation of number of sides of hollow tapered section is that it makes it more stiff?

Is it possible in practical to have tapered section having uniform circle? for me I have seen it only in STAAD PRO!!!

I will be in contact soon with a local fabricator to see what they can do.

Regards

 

[transmissiontowers]

In order to make tapered hollow shapes in the sizes you will need takes some special equipment and welding equipment to do it right.

Lots of fab shops can roll plate into cylinders because the rollers are all parallel and you force the upper roller down between the 2 bottom rollers to determine the diameter. With the tapered round shape, the bottom 2 rollers are at an angle that determines the taper and the upper roller must be lower on the big end. Not many shops have this capability to produce many poles. Two halves are rolled and welded together.

For 8, 12, 16 sided tapered poles, you just need a big long press brake. Tandem 30 footers are combined to make 60 foot pole sections but a single 40 foot brake is normal. With a press brake, you just cut a big plate into a trapezoid and bend the plate 30° for a 12 sided pole along a bend line.

The number of sides is usually determined by the w/t ratio of the flats because as the pole gets larger in diameter the local buckling becomes a limiting factor. 65 ksi plate is normally used for pole shafts, but with a large w/t the limiting stress can get as low as 30 ksi.

I don't use Staad but we have GTStrudl for a general purpose analysis and design and it will handle tapered hollow sections but you have to break up the pole into many beam members and do a stepwise set of constant shapes to mimic a true tapered beam. They don't have the capability to check for buckling either and I'm not sure if Staad does it either. I have used ANSYS and they have a tapered beam element but that was many years ago.

PLS-POLE has a base plate check and design option as well as the buckling check.

I'm sorry for the long explanations but there are probably less than 100 Engineers in the USA that regularly use or design tapered tubes and you have found one that like to try to educate the rest of the world.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[ST04]

Dear transmissiontowers
Thanks a lot
And have a nice day