Hand Calculation 7

[crawdaddy1983]

I'm designing a retrofit for a monopole cell tower (steel pole-ASTM A572) using (3) channels attached to the outside of the pole (bolts at 2 ft on center). I'm considering it a column fixed at the base and free at top. I've analyzed in a program and I've determined a moment and shear diagram along the length of the pole/column. I'm having trouble coming up with a calculation to anaylze bending using the given information below. This is what is
Given:
Mu= ultimate moment
Combined properties (tower + channels) which are
Ix
Iy
Sx
Sy
rx
ry
total area of cross section

I guess i need to find Mn (nominal moment) or required Ix/Sx to withstand Mu...

Any help would be appreciated.

 

[Stillerz]

Fb = M/S

 

[dhengr]

crawdaddy1983:

A computer program does not make an engineer. In fact if you were one (an engineer, that is), you would not need a computer program to determine the moment and shear for your problem, a simple cantilever beam-column. You should get yourself some good strength of materials and theory of elasticity reference books; these are common loads on an engineer’s book shelf; they tell you how to calc. I, S & r, whatever those symbols stand for. You say nothing about the shape of the existing tower. Is it a closed section, then how do you tighten the nuts and bolts? How do you know that some sized bolts should be spaced at 2' o/c, until further into the design? Are channels really the most practical shape to achieve the reinforcement? If the tower needs strengthening, is the foundation and base plate still O.K? Assuming the tower is outside, bolting causes open faying surfaces, which lead to a rusting problem, would continuous welding be better? What are loads you are designing for, where did you get them? What’s causing the need for the strengthening of this tower? You should probably hire a real engineer to help you on this project.

None of us here object to a meaningful, well thought out, honest question, we can even be helpful at times. After all, we all started out with lots of questions and a fair amount of uncertainty and probably still have too many questions. But, your question doesn’t show the vaguest engineering understanding of the problem you are asking about. And, this kind of cookbook approach or the idea that ‘I are an ingner,’ cause I got my hands on a program, is downright dangerous. One of us could be standing near that tower on a windy day.

 

[Ron]

One thing you have not considered (at least from your posting) is the torsion invoked by the additional antennae. Monopoles are more susceptible to torsion than lattice towers, because of the types of antennae that are usually attached to monopoles. I've done lots of them and as dhengr noted....you have to consider more than what your program is telling you.

 

[transmissiontowers]

I'll assume you are trying to find a way to add capacity to a steel monopole cell structure that is a tapered slip jointed tubular pole with probably 16 sides. The steel pole is probably directly embedded into the ground or it might have a base plate and anchor bolts. The cell company wants to add antennas which made the existing pole overloaded.

Before going too deep into finding a solution, you might determine if the existing base plate, anchor bolts and concrete foundation are up to the task of the new loads (if it uses anchor bolts) or if the direct embedded part of the old pole is up to the increased loading.

If the foundation system works, how do you intend to attach channels to the pole? Bolting a channel web to the pole shaft with the 2 flanges sticking out at discreet points poses the question of how to get inside the pole to tighten the bolts. A continuous weld would be a better connection to the pole shaft and ensure that the section acts as a composite. If you just bolt the channels on they could buckle if not bolted close enough together. You will also have to consider the flanges buckling no matter how the channel is attached to the pole.

Welding onto a pole shaft that is galvanized is also a problem because you melt away the zinc coating at the weld and need to re-coat. With all these problems in mind it might be easier to install a larger pole close to the cell ground platform.

If you want to go ahead with the analysis of the strengthened pole, you will have to consider it as a step tapered column and calculate the combined properties for every foot or so along the length, input those properties in most any structural analysis program and determine the allowable bending moment for each 1 foot section of the pole to see if the channel you chose was big enough. You also need to consider that the channels attached to the shaft will increase the drag factor for the wind on the pole.

At this point, the amount of engineering time you will expend may not be worth it. The cell companies are making tons of $$$ and can afford to replace one pole.

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

 

[ash060]

dhengr

You sound a little harsh to crawdaddy, maybe you could lighten up a bit it is the holiday season.

They make blind bolts, but I do agree with everyone else that welding would be better.

 

[crawdaddy1983]

dhengr

I don't know what it is about blogs but it never ceases to amaze me how rude some can be when they don't have to look across at someone in the face when they type these comments. Nevertheless...

Shape of the tower bottom section (TP47.5x39.3138x0.5)- channel size yet to be determined

You connect using Lindapter Bolts. I've used them to connect hss-hss. We would rather not weld considering the costs of welding up high on the tower.

Assume 2 ft on center for now, this can change if need be. This is information taken from other retrofitted monopoles.

I've seen thick plates but a channel gives more flexural strength per cross section area. Plus there is 2 inch clearance from outside tower to the edge of anchor bolts/nuts.

Foundation is okay per structural drawings. Baseplate will be reinforced with gussets.

Loads and location are determined by a "mapper" that climbs the tower and measures needed info. Each antennae/dish weight is given by the client.

This tower needs reinforcing because additional towers have been added since this tower has been erected. This structural analysis is an after thought.

Ron-

What codes would you recommend for "torsion on towers"
I'm using RisaTower and I understand that this program has limitations hence the question for hand calcs on this type of design problems.

Transmissiontowers-

It's actually 18 sides. Lindapter bolts. This analysis is an after thought. The new equipment is on the pole already. I came into the picture after this was already done. I think a new pole is out of the question according to the client.

I've broke the tower up into sections (5 ft to be exact) in order to determine section properties along the tower w/ channel attached. I have the built up (moment of inertia and section modulus). Can you name a program that determines allowable bending moment? I can't input a built-up Section in RisaTower. Honestly i would rather know how to do it by hand.

Ditto on the increased drag factor.

 

[ash060]

Well it depends on the symmetry of the cross-section.

It has to have at least one axis of symmetry for the typical Sx analysis to work.

One thing to make sure of is the compactness of the new cross-section. You will probably not see the shape you have in Chapter B of the AISC spec, but use the closest one applicable. Just make sure that everthing is thick enough so that local buckling is not an issue

As far as torsion since you have a closed shape the channels are not really doing a whole lot for you so I would just count on the original shape for the torsion (Chapter H AISC) also check out their design guide for torsion (DG 9).

Also, you will have two different material yield strengths 50 and 36, so the channels being far from the NA will yield way before the monopole.

Trying using RISA Section to give you all the geometry and properties than check the AISC code to get you Fcr's for bending and axial. Than M/S + P/A

I am not an expert on transmission towers, so I could be totally off base. But with the info i have that is what I would do

 

[cntw1953]

I think dhengr has given a well thought comments based on the opening write-up. After reading your responses above, I sincerely hope you are not working on this alone. Suggest to get support from someone can perform the simple hand-cal, then verify with the computer program.

 

[paddingtongreen]

crawdaddy, I am just as disturbed as dhengr and cntw1953 at the fact that you are trying to perform a structural analysis without understanding the very basics of bending theory. You appear to be using the computer as a black box.

I am perfectly willing to help engineers with less experience, but they have to know the fundamentals.

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

 

[csd72]

This sounds like the classic 2 hour hand calculation that would take a good 4 or 5 hours to do with a computer.

I agree with the masses that you do not know how to do it.

For capacity of the section look up parallel axis theorem and longitudinal shear the rest is just first year fundamentals.

Make sure you check the stress in both the pole and the channels as the pole is probably lower stress.

The critical bending section is the section with the largest M/S once you have determined this it is a simple hand calc.

Good luck, I suggest you google some basics as a refresher.

 

[kelowna]

Agree with everybody else. If you are not capable of doing that hand calculation, in my humble opinion you should not be designing the retrofit of that tower.

You might find out the formula you need to apply and how to apply it. I would be more worried about what else you have missed that you do not know you missed.

 

[transmissiontowers]

Can you name a program that determines allowable bending moment? I can't input a built-up Section in Risa-Tower. Honestly i would rather know how to do it by hand. Ditto on the increased drag factor.

I don't know of a program that handles built up sections so I would write an Excel spreadsheet to calculate the new Moment of Inertia and Section Modulus ( I probably would do it in FORTRAN if I wanted to keep in practice). I would suggest you break up the pole into beam elements with a length of 5% of the total pole height. Once you input all the section properties in RISA for the pole, you will output the moment of each beam element. From your Excel calculations, you should be able to calculate the allowable bending moment for each beam element and compare them to the RISA output. Keep in mind that the channels could buckle and lower the capacity. IIRC, the latest EIA/TIA rev G uses LRFD and may have some tips for built up sections.

If you know who provided the pole originally, you can get them to suggest a fix for the overload unless you work for that company and it's your job to find a fix.

You also might consider contacting the Engineer of Record that allowed the new antennas to be placed on this pole without an analysis showing the pole was adequate and inform him/her that you will be contacting the ethics section of the State Licensing Board for their incompetence.

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

 

[IJR]

Pals,

I think crawdaddy1983 asked a honest question. And I think dhengr was not rude, he just passed a tough advice.

Asked a honest question, we respect the honesty, respond soft or tough. All OK.

Paddingtongreen put it best(and so did Csd72). He is just disturbed.

respects
ijr