Wind load effect on Lattice Tower in MS Tower. Your thoughts. 1

[cyphos168]

Hi All,

I am working on a lattice tower that is 65m tall and using MS Tower to design. I have created the load tower profile and generated the wind load combinations.

I proceeded to analyse the tower and this is what I noticed.

1. Tower alone without any ancillaries, the deflection of the tower is generally vertical.
2. When I increased the wind speed from a Vb of say 36m/s to say 200m/s just to see how the tower will response. There is very little difference compared to the 36m/s wind speed.
3. I proceeded to add an antenna dish on the tower and analyse. The deflection is now lateral as well as vertical. Getting around 3mm deflection laterally (Wind speed at 56m/s).
4. Placing more antennas causes the deflection to go as high as 500mm lateral deflection.

So am I correct to say that wind load effect on lattice structure is actually very little? If anything, the faster the wind speed, the effect of the wind load will cause the lattice structure
to vibrate as oppose to deflect more? Thanks in advance.

Regards,

Al

 

[JStephen]

That's not my field, but that sounds pretty flaky. I would think a lattice tower would have negligible vertical deflection and noticeable wind deflection, not the other way around. I would think you're missing a load factor or a load combination or missing some area or something along the way.
If the projected area of that antenna is comparable to that of the tower, I would think the deflections due to each would be of the same order of magnitude.

 

[msquared48]

I agree there is something grossly wrong with at least the input, if not the program.

Antenna sise and elevation placement on the tower will affect the deflection... the lower the placement, and the less the size, the less the lateral deflection.

And wind loads used seem extremely high under TIA-222(H now?)guidelines. Here we normally use about 100 mph max (about 45 m/sec), but at 200 m/sec, you are talking over 400 mph! Where is this tower? Jupiter?

Normally for good antenna performance, the lateral deflection has to be limited by the stiffness of the tower structure.

I used TNX Tower when I was doing these towers several years ago under TIA 222-G at the time.

Mike McCann, PE, SE (WA)

 

[cyphos168]

Hi Guys,

Thanks for your input on this matter. What I meant to say as far as item 1 is concern is that there is very little to no lateral deflection on the tower when I check again wind load combination.
I mentioned vertical deflection as I am looking at the tower deflected shape on the screen. But looking closely at the values, its practically 0 deflection vertically or laterally.

Which comes back to my observation about the wind load effect on the tower if there is no ancillary equipment on the tower. The wind load does nothing to the tower. As pointed out by JStephen,
I think I may be missing some combination or load factor. Bear in mind this is my first time using this software to analyse so still trying to get familiarise with the ins and outs of the program.

I am also using Staadpro to design the same tower so I am not relying solely on MStower. MStower is just something I wanted to try and seeing the results its kinda odd to me that wind load has little
effect. Hence my post.

@msquared48, the wind speed of 56m/s was assigned by the client so I have to use this figure to design.

Below is my load tower file.Maybe I am missing something in the script.

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

PARAMETERS
ANGN 90.0
CODE BS8100
VB 56.00 MEAN $ hourly mean
PSF-V 1.20
PSF-M 1.10
RPSERV 3 $ return period, service
OVERLAP 0
END


$ Complete terrain block by supplying data for different wind angles
$ NB wind angle is bearing measured east-of-north.
$ Refer to manual for meaning of parameters

TERRAIN
ANGLE 0 TCAT 2 $ HH hh BETAH betah XLEE xlee $
END

LOADS
CASE 100 Dead Load
DL
$ additional node loads if required
$ NDLD node-list FX fx FY fy FZ fz

$ CASE 200 Miscellaneaous
$ MI
$ NDLD node-list FX fx FY fy FZ fz

CASE 1000 Wind at 0.0 to X axis
WL ANGLX 0.0 NOICE

CASE 1020 Wind at 45.0 to X axis
WL ANGLX 45.0 NOICE

CASE 1040 Wind at 90.0 to X axis
WL ANGLX 90.0 NOICE

CASE 1060 Wind at 135.0 to X axis
WL ANGLX 135.0 NOICE

CASE 1080 Wind at 180.0 to X axis
WL ANGLX 180.0 NOICE

CASE 1100 Wind at 225.0 to X axis
WL ANGLX 225.0 NOICE

CASE 1120 Wind at 270.0 to X axis
WL ANGLX 270.0 NOICE

CASE 1140 Wind at 315.0 to X axis
WL ANGLX 315.0 NOICE


$ Check that the factors used in load combinations
$ are consistent with the code to be used in checking
$ member strength and tower displacements.

CASE 3000 Max DL
COMBIN 100 1.05

CASE 4000 Min DL + Wind at 0.0 to X NOICE
COMBIN 100 0.90
COMBIN 1000 1.00

CASE 4020 Max DL + Wind at 0.0 to X NOICE
COMBIN 100 1.05
COMBIN 1000 1.00

CASE 4040 Min DL + Wind at 45.0 to X NOICE
COMBIN 100 0.90
COMBIN 1020 1.00

CASE 4060 Max DL + Wind at 45.0 to X NOICE
COMBIN 100 1.05
COMBIN 1020 1.00

CASE 4080 Min DL + Wind at 90.0 to X NOICE
COMBIN 100 0.90
COMBIN 1040 1.00

CASE 4100 Max DL + Wind at 90.0 to X NOICE
COMBIN 100 1.05
COMBIN 1040 1.00

CASE 4120 Min DL + Wind at 135.0 to X NOICE
COMBIN 100 0.90
COMBIN 1060 1.00

CASE 4140 Max DL + Wind at 135.0 to X NOICE
COMBIN 100 1.05
COMBIN 1060 1.00

CASE 4160 Min DL + Wind at 180.0 to X NOICE
COMBIN 100 0.90
COMBIN 1080 1.00

CASE 4180 Max DL + Wind at 180.0 to X NOICE
COMBIN 100 1.05
COMBIN 1080 1.00

CASE 4200 Min DL + Wind at 225.0 to X NOICE
COMBIN 100 0.90
COMBIN 1100 1.00

CASE 4220 Max DL + Wind at 225.0 to X NOICE
COMBIN 100 1.05
COMBIN 1100 1.00

CASE 4240 Min DL + Wind at 270.0 to X NOICE
COMBIN 100 0.90
COMBIN 1120 1.00

CASE 4260 Max DL + Wind at 270.0 to X NOICE
COMBIN 100 1.05
COMBIN 1120 1.00

CASE 4280 Min DL + Wind at 315.0 to X NOICE
COMBIN 100 0.90
COMBIN 1140 1.00

CASE 4300 Max DL + Wind at 315.0 to X NOICE
COMBIN 100 1.05
COMBIN 1140 1.00

END
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Al

 

[msquared48]

56mps is reasonable if you are in hurricane country or have wind speed up increases due to the terrain.

There may be a toggle switch in the program to consider wind load to the tower members themselves, aside from the wind.

Mike McCann, PE, SE (WA)

 

[hokie66]

Did you input the section properties into the analysis? Perhaps the program just thinks the members are lines.

 

[WARose]

[blue](cyphos168)[/blue]

I am also using Staadpro to design the same tower so I am not relying solely on MStower.

Any way you could post the edit file from STAAD? (Or a link to the file itself.)

I have STAAD on my end so I could look at it that way.

 

[cyphos168]

@msquared48 Ya maybe there is one but I haven't found it yet. Haha... Will try to research some more on this.

@hokie66 In the script I have assigned the correct member to each member and when I switched the toggle for 3D view, I can see the different member sizes.

@WARose Please see attached file

 

[WARose]

@WARose Please see attached file

Got it. Thanks. To go back to your OP:

1. Tower alone without any ancillaries, the deflection of the tower is generally vertical.

I think that is due to the arrangement of the bracing. You have chevron bracing in that top portion. I'm not sure how much it is contributing: but you also have no members designated as truss members. (I'd (at least) have the diagonals designated as such.)

2. When I increased the wind speed from a Vb of say 36m/s to say 200m/s just to see how the tower will response. There is very little difference compared to the 36m/s wind speed.

No difference in deflections or forces? With the wind load you have in there, you are getting a pretty good deflection. (7.8 inches in some load combinations.) So obviously it is having some effect.

3. I proceeded to add an antenna dish on the tower and analyse. The deflection is now lateral as well as vertical. Getting around 3mm deflection laterally (Wind speed at 56m/s).
4. Placing more antennas causes the deflection to go as high as 500mm lateral deflection.

With loads on one side of a frame with chevron bracing: that doesn't surprise me.

So am I correct to say that wind load effect on lattice structure is actually very little?

Depends on what you want to call "very little". The applied wind load is 65 kN in one direction and 59 in the other.

If anything, the faster the wind speed, the effect of the wind load will cause the lattice structure
to vibrate as oppose to deflect more? Thanks in advance.

Most of the tower codes have criteria for vibration control from wind (i.e. vortex shedding). We talk about it on this thread:

http://www.eng-tips.com/viewthread.cfm?qid=426344

 

[cyphos168]

@WARose,

Thanks for your input on this topic. My initial post and observations were referring to MSTower model. Not the Staadpro model. Sorry if my post may have been confusing.

With Staadpro, the model is working as I would expect it to. In fact I have designed and successfully built a 27m light tower using Staadpro. Just wanted to try using MSTower to do the design of the 65m tower but not getting the result I was hoping for.

Thanks for the link. Will have read on this.

Al