Mononpole cross-section and Seismic/Wind issues 2

[adurbin]

I would like to know where I can find design information on seismic/wind issues with regard to tower
installation inside/on top of existing buildings.
Also why monopoles are chosen to have certain type(s) of cross-section (18 sided, 16 sided) to prevent cross-wind excitation.
Any help in the right direction would be most helpful.
Thanks

 

[dlew]

adurbin,

For wind/seismic loads on structures on top of buildings I would use ASCE 7 "Minimum Design Loads for Buildings and Other Structures", a publication of the American Society of Civil Engineers. Or, the applicable Building Code.

I may be wrong, but I think the number of sides on the cross-section of monopoles has to do with the manufacturing process. All poles, polygonal or cylindrical section, may be subject to cross-wind excitations if the conditions producing these vibrations (wind velocity, pole height and natural frequency, etc.) exist.

AEF

 

[RHK]

I am in the process of designing a 66m high tower to go on a 50 storey building, so I hope I know what I am talking about.

Design wind loads will come from your local building codes. Usually these give you wind speeds at differing heights above ground. The drag of the different forms of tower will also be in the Code. If you don't have a local code, then I suggest John Holmes "Wind Loading of Structures".

Earthquake should not govern for a tower on top of a building. The building acts to isolate the tower from the ground vibrations. Also, a tower usually has a low natural frequency which reduces the magnitude of earthquake forces.

Tapered cylindrical poles are sometimes made with 8 or 12 flat sides, simply for ease of fabrication. Note that the drag coefficient of a octagon is about 1.3. A smooth circular cylinder has a drag coefficient of 1.2 at low Reynolds Numbers, but this reduces to 0.6 at higher Re. The typical tower on a building will have the lower figure.

Long cylindrical members or polygons are susceptable to wind-excited vibration. This comes from vortex shedding. The vibration will become unstable if there isn't sufficient damping in the structure.

The frequency of vortex shedding can be upset by adding spiral rings to the tower. A 16mm fibre rope wrapped round a 250mm light post works quite well. Another way of upsetting the vortex shedding frequency is to have a large bank of lights at the top of the tower. For this reason, tall posts for illuminating football fields do not need damping.

The alternative to upsetting the rhythm of vortex shedding is to provide a damper. A relatively cheap damper, such as a tuned chain in a tube, will provide over 2% damping and solve vibration problems for the average tower.

The stiffness requirement for the tower is not well defined in Building Codes. If the tower is to carry microwave antennas, there will be a servicability requirement for the rotation of the antenna. This ensures the transmitting antenna points at the receiver up to (say) the 6-year return wind gust. Russell Keays