Tapered Hollow Section
Tapered Hollow Section
(OP)
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






RE: Tapered Hollow Section
At the current time, RISA does not have tapered tubes or pipes. Only tapered I-beams. You can always create a model for your tapered column by breaking it into a number of smaller untapered sections.
RE: Tapered Hollow Section
Before that, I used a similar process to find stiffness's at each end of the column for use in moment distribution or in slope deflection analysis.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Tapered Hollow Section
For the analysis, I would subdivide the member and provide elements with the appropriate section properties for each taper.
RE: Tapered Hollow Section
RE: Tapered Hollow Section
That program breaks the tapered monopole into about 20 segments of prismatic members to approximate the taper.
Twenty sections is probably overkill. I think something like 10 segments would probably be enough. However, structural stability effects (i.e. 2nd order / P-Delta effects) usually control these types of slender monopoles. So, overkill on the analysis is one of those "better safe than sorry" type of things.
RE: Tapered Hollow Section
I suspect BAretired would agree with me, that this sounds like a good problem for Newmark's Method of Numerical Integration. If you are using TNX Tower as JoshPlum suggests, I would go with the 20 segments, for the improved accuracy of representing the cross section, since you are not doing the actual number crunching. BA and I probably would have gone with 10 or 15 segments when we were doing that problem by hand, although it's just a bookkeeping problem, requiring a wider piece of graph paper, to hold more columns of calcs. for each segment, once you decide to tackle it.
RE: Tapered Hollow Section
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Tapered Hollow Section
A closed form solution using a continuous function for moment of inertia, bending and axial load may be possible but prone to mathematical error and not in the least attractive for the majority of practicing engineers, including me.
BA
RE: Tapered Hollow Section
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but if you want to analyse a framework with tapered members, rather than a single column, the subdivided memebers method should be fine.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Tapered Hollow Section
If you want to analyze a framework with tapered members, I suppose you could determine stiffness and carry-over factors for each member in the frame and carry out a Hardy Cross Moment Distribution. Alternatively, I believe you could carry out a slope deflection analysis with this much information in which your results would be theoretically, correct.
I have never done it because I have never had occasion to analyze a framework with tapered members.
BA
RE: Tapered Hollow Section
RE: Tapered Hollow Section
The Guide Specifications for Design of Metal Flagpoles is a good place to start. (ANSI/NAAMM FP 1001-07). This guide has sample calculations for designing a 55 foot tapered pole. In this sample calculations there is also has a procedure of how to analysis the pole in tabular form such as a spreadsheet. Note that this method also takes into account the P-delta forces by estimating the second order effects. Starting with this guide you can follow the Calculation Procedure in Section 7.
Of course there's more to this than just initial sizing. We know that you have some additional loads from equipment. At this time I assume that these are cantilever poles. The second step then would be to use AASHTO's Highway Signs, Luminaires and Traffic Signals standard specifications. Here you will find the rest of the parameters that have to be considered such as vortex shedding, buckling, fatigue etc. This standard also provides width-thickness ratios for octagonal sections when checking whether the section is compact, non-compact and a maximum limit. It also provides the allowable bending stress for octagonal members. Note that this code uses ASD and not LRFD. One nice thing about both the Guide and the Specification is that they contain excellent commentary.
One nice thing about using a spreadsheet and dividing the tube into sections is that for a 60 foot member, you can use 10 – 6 foots section or 6 – 10 foot sections. I like to vary the thickness of the sections, thickest at the bottom and thinnest at the top. With a well organized spreadsheet you can play with the input and "tune" the total member.
One or two sections? If you are going to have two sections, then you'll have to design mating flanges somewhere. For this area you will have to be paying close attention to local buckling near the flanges.
If you have never done a pole like this before, make sure it doesn't end up like this: http://www.youtube.com/watch?v=tB82mqEyBBg. This pole was only up about 24 hours before they cut it down.
If you want to see a pole that's been up for a few years: http://en.wikipedia.org/wiki/Aqaba_Flagpole.
This pole has mating flanges using large A354 bolts that had to be tightened with tension meters.
Good luck.
RE: Tapered Hollow Section
is really interesting.It brings up a few questions.
This occurred with wind at 15mph which results in a steady
state loading of about 0.5 #/sq. ft....
If the pole was designed for say a design wind of 90mph, it
would result in a loading of about 16#/sq.ft and say an
allow. tip deflection of 6" per 100ft=3.30x6= approx. 20"
Mag. factor would be 16/0.5=32 to get a def. of 20" due
to oscillation at 15mph.From the video it looked like
a lot more than that.
I have never encountered that high a mag. factor and leaves
me reassessing what I thought I knew about this problem.
I would not have expected such a large deflection from such a low driving force.I am sure there are alot engineers out there who are more knowledgeable in vibration than I am and
look forward to their input.
RE: Tapered Hollow Section
If it had been in a near constant wind, it would not have been oscillating, especially not in a harmonic mode.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Tapered Hollow Section
RE: Tapered Hollow Section
Go figure.
RE: Tapered Hollow Section
Vortex shedding does require a steady wind.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Tapered Hollow Section
There are several specialty programs for analysis but we use PLS-POLE from Power Line Systems which will handle free standing single poles and frames that include tapered poles for columns. I also use GTStrudl which has a command for generating a tapered pole but you cannot use it for a nonlinear analysis (the are working on making it work). You can also generate the properties in a spreadsheet and make the step tapered prismatic members.
Also take a look at ASCE 48 which is the design guide for the tapered tubular poles.
All the big manufacturers of tapered poles like Thomas & Betts, Valmont, FWT and a few others I forget right now have their own programs and will design the poles for free.
I would get ASCE 113 and read up on the process. ( I was on the committee that wrote the guide )
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RE: Tapered Hollow Section
Oldrunner, that wind velocity must have been oscillating at or near the lowest natural frequency of the pole for it to bend with the wind in that manner.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Tapered Hollow Section
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I have been called "A storehouse of worthless information" many times.
RE: Tapered Hollow Section
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
RE: Tapered Hollow Section
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.
RE: Tapered Hollow Section
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
RE: Tapered Hollow Section
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 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.
RE: Tapered Hollow Section
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
RE: Tapered Hollow Section
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I have been called "A storehouse of worthless information" many times.
RE: Tapered Hollow Section
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
RE: Tapered Hollow Section
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.
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I have been called "A storehouse of worthless information" many times.
RE: Tapered Hollow Section
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I have been called "A storehouse of worthless information" many times.
RE: Tapered Hollow Section
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
RE: Tapered Hollow Section
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.
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I have been called "A storehouse of worthless information" many times.
RE: Tapered Hollow Section
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.
RE: Tapered Hollow Section
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
RE: Tapered Hollow Section
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. ;)
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I have been called "A storehouse of worthless information" many times.
RE: Tapered Hollow Section
Thanks a lot
And have a nice day :)