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Tapered Hollow Section

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

AISC and MBMA are coming out with a design guide that is geared towards tapered members.  I haven't looked at it in awhile. It was really based on the type of tapered wide flange members that you use in metal buildings.  However, my impression was that the theory and design procedures presented could be extended into tapered pipes or tubes as well.  

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

In a distant galaxy, distant in time, I divided the column into six segments and entered each with the properties at the middle of the segment into STAAD. I thought that would be close enough.

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

What is driving the use of the tapered HSS columns and what are you intending for the taper detail. Are you providing the industrial welded end plate at every taper location or is it intended on being more "aesthetic" with the detailing.

For the analysis, I would subdivide the member and provide elements with the appropriate section properties for each taper.  

RE: Tapered Hollow Section

(OP)
Maybe I should clarify myself and say they are tapered octahedron members that are about 60' tall.  They will be used for an industrial application to support electrical equipment.  Maybe I should be asking about mono pole design?  Does Risa have mono pole design with tapered members?

RE: Tapered Hollow Section

RISA-3D does not have tapered monopoles.... But, the program formerly known as RISATower (now called TNX Tower) does have tapered monopoles.

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

Ars13:
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

I only used six. The paper I used showed that six was very accurate for a constant section member, so I figured it would be close enough.

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

RE: Tapered Hollow Section

As dhengr has stated, the problem could readily be solved with the use of Newmark's Numerical Methods.  The number of segments chosen is a little arbitrary, but using 10 segments would seem imminently reasonable to me.  Selecting more segments would not alter the results in any significant way.

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

Doug,

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

I suggested 10 or 15 segments because they would give nice segment lengths of 6' or 4', and you use these lengths many times in the calcs., so you don't want some weird a$$ed length like 6.67' because you selected 9 segments.  This clean length selection would be true with the computer solution also.  But, BA is right, in our hand calced. solution, there would not be much improvement in results by going from 10 segments to 15 segments.
 

RE: Tapered Hollow Section

Ars13:

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

The link provided by "oldrunner" to the pole oscillating
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

There was no flag to show wind direction, but I think it was perpendicular to the sway. I think we saw vortex shedding, albeit at a low wind speed, but the pole obviously had a low natural frequency.

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

I agree with Paddington's first paragraph, and disagree with his second paragraph.  I believe constant or near constant wind velocity and direction is often when vortex shedding occurs, particularly around cylindrical shapes.  I think he meant to say if the wind was not constant in velocity and/or direction, the oscillation would have been much less likely.  Although, I do not consider myself the last word in structural dynamics.

RE: Tapered Hollow Section

Well, the problem was that the pole was moving in the same direction as the wind.

Go figure.

RE: Tapered Hollow Section

I agree with you, dhengr, I phrased it badly, I meant if it was simple bending, under a steady wind force, it would not be oscillating.

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

We use tapered tube members all the time in the Transmission line business and also in the design of substation structures.  ASCE 113 is a design guide for substation structures which includes tapered tubular sections for the columns.

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 )

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

RE: Tapered Hollow Section

I would have tried a rope, helically wound around it, that modifies opposite sides and spoils the shedding, but I would have done that from the beginning.

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

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.

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

RE: Tapered Hollow Section

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

RE: Tapered Hollow Section

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.

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

RE: Tapered Hollow Section

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

RE: Tapered Hollow Section

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 www.powline.com for some more info about tapered poles.

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

RE: Tapered Hollow Section

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

RE: Tapered Hollow Section

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.

RE: Tapered Hollow Section

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

RE: Tapered Hollow Section

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.

RE: Tapered Hollow Section

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.

RE: Tapered Hollow Section

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

RE: Tapered Hollow Section

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.

RE: Tapered Hollow Section

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.

RE: Tapered Hollow Section

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

RE: Tapered Hollow Section

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.

RE: Tapered Hollow Section

Dear transmissiontowers
Thanks a lot
And have a nice day :)

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